RU2673013C1 - Coherent lighting device with light recuperation - Google Patents

Abstract

FIELD: optics.SUBSTANCE: invention relates to optical technique. Technical result is achieved due to at least one light guide plate (LGP), LGP has two flat surfaces and has a first end and a second end opposite to the first end; one or more introductory diffraction gratings placed on one of these two surfaces, and these one or more introductory diffraction gratings are located in the region adjacent to the first end; at least one coherent light source configured to supply coherent light to the LGP through the input diffraction gratings, and the input diffraction gratings are adapted to direct the light into the LGP at the angle to ensure complete internal reflection of the light inside the LGP; one or more output diffraction gratings placed on one of these two surfaces, the output diffraction gratings are adapted to partially diffract the light incident on the output diffraction gratings from inside the LGP, outside the LGP.EFFECT: technical result is to increase the efficiency of the backlight provided by the backlight device (BLU).48 cl, 1 tbl, 18 dwg

Description

FIELD OF THE INVENTION

The present invention relates, in general, to optical technology and, more specifically, to backlight devices providing uniform and coherent output lighting. In particular, the present invention relates to a coherent backlight device with light recovery, as well as to a display device comprising this backlight device. The proposed backlight devices can be used for backlighting in mobile devices such as smartphones and visualization systems such as 3D or holographic displays.

BACKGROUND OF THE INVENTION

The most promising approach at the present stage applied to the design of backlight devices (BLU), taking into account the need to minimize their thickness, is the edge backlight. In the case of an edge backlight, the light sources are located at the edges of the waveguide, as a rule, perpendicular to the lighting area. The waveguide is typically a glass light guide plate (LGP). As a rule, laser diodes are used as light sources for edge backlighting. Laser diodes are located at the edges of the LGP and have a limited width (usually 5-6 mm). In known solutions, the thickness of the BLU depends mainly on the width of the laser diode, and not on the thickness of the waveguide.

A difficult task is to ensure high uniformity and high collimation of light at the output and increase the efficiency of its use.

In FIG. 1 schematically shows the propagation of light in an LGP 100 of a BLU according to the prior art. LGP 100 has two flat surfaces, one of which has input diffraction gratings 101 and output diffraction gratings 102. In this case, this surface is a light-emitting surface. As can be seen from FIG. 1, the introductory diffraction gratings 101 are located in the region of the light emitting surface directly adjacent to the end face of the LGP. A coherent light source (not shown) is installed near this end face, i.e. The case of edge backlighting is considered. The light 103 from the light source enters the diffraction gratings 101, diffracts onto them and is directed into the LGP 100 at an angle that provides full internal reflection of light in the LGP 100. The light propagating inside the LGP 100 falls on the output diffraction gratings 102 and, as a result of the diffraction, partially is directed to the observer 104, in the case under consideration, practically perpendicular to the light-emitting surface (diffraction of the 1st order) and partially directed in the opposite direction (diffraction + 1st order), leaving the LGP 100 through the top opposite to its light-emitting surface, i.e. essentially lost.

As can be seen from the described operating principle of the BLU, after each incident on the output diffraction gratings 102, the light propagating in the LGP 100 weakens. Thus, with each subsequent drop, the energy of light output through the light-emitting surface will be less than with the previous one. This leads to heterogeneity in the presentation of the displayed visual information due to a decrease in light intensity in the direction along the output diffraction gratings 102. Thus, in FIG. Figure 2 shows a graph (upper) showing the calculated spatial intensity distribution obtained by model calculation of the output diffraction gratings, and a graph (lower) illustrating the actual intensity drop with distance.

Although theoretically this effect can be compensated by varying the efficiency of light output, in practice such compensation is unlikely to be achieved due to the spread of losses and incidence angles depending on the wavelength of light. So in FIG. Figure 3 shows graphs illustrating the decrease in light intensity with distance for three different wavelengths, from which it is clear that blue light undergoes the largest drop in intensity and red - the smallest.

Moreover, the efficiency of the output diffraction gratings 102 is usually very low, and with a limited number (usually several tens) of light incidence on them, the output diffraction gratings 102 cannot remove all the light from the LGP 100 — some of the light incident on them will propagate in the direction opposite end of the LGP 100 (see Fig. 1) and scatter outward through it, i.e. get lost again. It should be noted here that in the case under consideration, such scattering is necessary, since if this light were simply reflected at the LGP end and thus returned to the LGP 100 in the form of scattered light, it would become a source of noise, leading to a decrease in image quality.

FIG. 4 illustrates the heterogeneity of the luminance distribution of an exemplary image due to the above reasons. In FIG. 5 light losses are indicated on real BLU equipment.

US 2016/0147003 (SAMSUNG ELECTRONICS CO LTD) proposes a backlight device (BLU) for a binocular holographic display device and a binocular holographic display device including such a BLU. The BLU comprises a light source unit that emits coherent illumination light, and a light guide plate that is transparent and has a light input surface onto which coherent illumination light emitted by the light source unit is incident, and a light emitting surface through which the illumination light is output. The light source unit includes a beam deflector that adjusts the angle of incidence of the backlight incident on the light guide plate.

US 6,580,529 (ELOP ELECTRO-OPTICS INDUSTRIES LTD, YEDA RESEARCH AND DEVELOPMENT CO LTD) proposes a holographic optical device comprising a light guide substrate, a first holographic optical element placed on the substrate, at least one second holographic optical element located on the side of the substrate a first holographic optical element, and at least one third holographic optical element located on the substrate and laterally spaced from the first and second holographic optical elements; wherein the center of at least one of the first, second, and third holographic optical elements is outside a single straight line.

The prior art solutions, including the above two, are characterized by significant disadvantages outlined above, which are the non-use of undiffracted light scattered from the LGP end and the non-use of light diffracted in the opposite direction from the observer, which results in low backlight efficiency. In particular, to stop the above-described inefficiencies and heterogeneity of brightness, more powerful light sources are used, which is extremely undesirable in the context of mobile devices, since it obviously leads to a faster consumption of the battery.

SUMMARY OF THE INVENTION

The aim of the present invention is to increase the efficiency of the backlight provided by the backlight device (BLU), by increasing the uniformity and brightness of the light emitted by him, without increasing the power and / or number / size of light sources.

According to a first aspect of the invention, there is provided a BLU for a display device, comprising: a light guide plate (LGP), wherein the LGP has two flat surfaces and has a first end and a second end opposite the first end; one or more introductory diffraction gratings arranged on one of said two surfaces, these one or more introductory diffraction gratings located in a region adjacent to the first end face; at least one coherent light source configured to supply coherent light to the LGP through the input diffraction gratings, the input diffraction gratings being adapted to direct the light into the LGP at an angle so as to ensure full internal reflection of the light inside the LGP; one or more output diffraction gratings located on one of the two surfaces, wherein the output diffraction gratings are adapted to partially diffract light incident on the output diffraction gratings from within the LGP, to the outside of the LGP; and a light recovery module. The light recovery module comprises: a first recovery element located on one of the two surfaces, the first recovery element comprising a first retroreflective diffraction grating located in a region adjacent to the second end face, and a second recovery element located on the other of the two surfaces, the second recuperation element contains either a second retroreflective diffraction grating or a mirror placed in a region adjacent to the second end face. The output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have the same and constant grating period, while the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have substantially the same orientation with respect to each other. The second recovery element is adapted to partially direct light incident on the second retroreflective diffraction grating or mirror to the first retroreflective diffraction grating or mirror when traveling in the forward direction inside the LGP from the input diffraction gratings. The first retroreflective diffraction grating is adapted to diffract the light incident on it in such a way that the diffracted light returns to the LGP, heading in the opposite direction at a predetermined angle with respect to this incident light. The output diffraction gratings are designed such that the light directed outside the LGP when diffracting said return light on the output diffraction gratings is essentially collinear light directed outside the LGP when diffracting said forward-propagating light on the output diffraction gratings. The LGP further has a dichroic coating for reflecting light that falls substantially normally onto the dichroic coating from the output diffraction gratings, back to the output diffraction gratings.

The light directed outside the LGP is preferably directed towards the observer.

LGP is preferably made of an optical material that is transparent in the visible range.

Each of the retroreflective diffraction gratings preferably has a mirror coating.

The display device may be a holographic display device, wherein the display device is preferably part of a portable computing device.

Introductory diffraction gratings, output diffraction gratings and the first retroreflective diffraction grating and the second retroreflective diffraction grating can be implemented either as phase-relief diffraction gratings or as bulk diffraction gratings.

At least the output diffraction gratings and the first retroreflective diffraction grating are preferably made as phase-relief diffraction gratings, wherein the first retroreflective diffraction grating has a profile optimized for second-order diffraction to maximize the return of light incident on the first retroreflective diffraction grating , in LGP so that the returned light falls on the output diffraction gratings with a predetermined spatial shift relative to the aforementioned rostranyayuschegosya forward light and excretory diffraction gratings have a profile adapted to provide said collinear beam direction outwards LGP.

The dichroic coating is preferably optimized for specific wavelengths.

A dichroic coating can be applied to the surface of the LGP, which is opposite to the surface on which the output diffraction gratings are placed,

The dichroic coating can be applied directly to the output diffraction gratings.

The dichroic coating can be directly applied to the same LGP surface as the output diffraction gratings in such a way that the dichroic coating is located under the output diffraction gratings relative to this surface, while the output diffraction gratings are coated externally with an immersion polymer.

The output diffraction gratings can be coated externally with an immersion polymer, and a dichroic coating can be applied to the immersion polymer.

Preferably, the diffraction efficiency of the output diffraction gratings is optimized, depending on the distance from the input diffraction gratings, for a particular wavelength, wherein the diffraction efficiency of the retroreflective diffraction gratings is optimized for this particular wavelength.

According to a second aspect of the invention, there is provided a BLU for a display device, comprising: a light guide plate (LGP), wherein the LGP has two flat surfaces and has a first end and a second end opposite the first end; one or more introductory diffraction gratings arranged on one of said two surfaces, these one or more introductory diffraction gratings located in a region adjacent to the first end face; at least one coherent light source configured to supply coherent light to the LGP through the input diffraction gratings, the input diffraction gratings being adapted to direct the light into the LGP at an angle so as to ensure full internal reflection of the light inside the LGP; one or more output diffraction gratings located on one of the two surfaces, wherein the output diffraction gratings are adapted to partially diffract light incident on the output diffraction gratings from within the LGP, to the outside of the LGP; and a light recovery module. The light recovery module comprises: a first recovery element located on one of the two surfaces, the first recovery element comprising a first retroreflective diffraction grating located in a region adjacent to the second end face, and a second recovery element located on the other of the two surfaces, the second recuperation element contains either a second retroreflective diffraction grating or a mirror placed in a region adjacent to the second end face. The output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have the same and constant grating period, while the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have substantially the same orientation with respect to each other. The second recovery element is adapted to partially direct light incident on the second retroreflective diffraction grating or mirror to the first retroreflective diffraction grating or mirror when traveling in the forward direction inside the LGP from the input diffraction gratings. The first retroreflective diffraction grating is adapted to diffract the light incident on it in such a way that the diffracted light returns to the LGP, heading in the opposite direction at a predetermined angle with respect to this incident light. The output diffraction gratings are designed such that the light directed outside the LGP when diffracting said return light on the output diffraction gratings is essentially collinear light directed outside the LGP when diffracting said forward-propagating light on the output diffraction gratings.

According to a third aspect of the invention, there is provided a BLU for a display device, comprising: a light guide plate (LGP), wherein the LGP has two flat surfaces and has a first end and a second end opposite the first end; one or more introductory diffraction gratings arranged on one of said two surfaces, these one or more introductory diffraction gratings located in a region adjacent to the first end face; at least one coherent light source configured to supply coherent light to the LGP through the input diffraction gratings, the input diffraction gratings being adapted to direct the light into the LGP at an angle so as to ensure full internal reflection of the light inside the LGP; and one or more output diffraction gratings located on one of the two surfaces, wherein the output diffraction gratings are adapted to partially diffract the light incident on the output diffraction gratings from within the LGP, to the outside of the LGP. In this case, the LGP has a dichroic coating for reflecting light, which falls essentially normally on the dichroic coating from the output diffraction gratings, back to the output diffraction gratings.

According to a fourth aspect of the invention, there is provided a BLU for a display device, comprising: a plurality of light guide plates (LGPs) stacked on top of each other, each LGP of a plurality of LGPs having two flat surfaces and having a first end and a second end opposite the first end; and at least one coherent light source configured to supply multiple wavelength coherent light to a plurality of LGPs. Each of the many LGPs has: one or more introductory diffraction gratings located on one of the two surfaces, these one or more introductory diffraction gratings located in a region adjacent to the first end of the LGP, while the introductory diffraction gratings are adapted to direct the light supplied said coherent light source, in the LGP at an angle such that full internal reflection of light within the LGP is ensured; one or more output diffraction gratings located on one of the two surfaces, wherein the output diffraction gratings are adapted to partially diffract light incident on the output diffraction gratings from within the LGP, to the outside of the LGP; and a light recovery module. The light recovery module comprises: a first recovery element located on one of the two surfaces, the first recovery element comprising a first retroreflective diffraction grating located in a region adjacent to the second end face, and a second recovery element located on the other of the two surfaces, the second recuperation element contains either a second retroreflective diffraction grating or a mirror placed in a region adjacent to the second end face. The output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have the same and constant grating period, while the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have substantially the same orientation with respect to each other. The second recovery element is adapted to partially direct light incident on the second retroreflective diffraction grating or mirror to the first retroreflective diffraction grating or mirror when traveling in the forward direction inside the LGP from the input diffraction gratings. The first retroreflective diffraction grating is adapted to diffract the light incident on it in such a way that the diffracted light returns to the LGP, heading in the opposite direction at a predetermined angle with respect to this incident light. The output diffraction gratings are designed such that the light directed outside the LGP when diffracting said return light on the output diffraction gratings is essentially collinear light directed outside the LGP when diffracting said forward-propagating light on the output diffraction gratings. The LGP further has a dichroic coating for reflecting light that falls substantially normally onto the dichroic coating from the output diffraction gratings, back to the output diffraction gratings. The output diffraction gratings, the first retroreflective diffraction grating, the second retroreflective diffraction grating and the dichroic coating located on each LGP of the plurality of LGPs are optimized for a specific wavelength of a plurality of different wavelengths.

Said plurality of wavelengths preferably consists of three wavelengths corresponding to red, green, and blue, and said plurality of LGPs comprises three LGPs.

At least the output diffraction gratings and the first retroreflective diffraction grating of each LGP of the plurality of LGPs are preferably made as phase-relief diffraction gratings, while in each LGP of the plurality of LGPs: the first retroreflecting diffraction grating has a profile optimized for second-order diffraction light of the corresponding specific wavelength to maximize the return of light incident on the first retroreflective diffraction grating to the LGP so that the returned light falls on the output ifraktsionnye lattice with a predetermined spatial offset with respect to said propagating light in the forward direction; output diffraction gratings have a profile adapted to provide the collinear direction of light of this particular wavelength outside the LGP; and the dichroic coating is adapted to reflect only the light of said specific wavelength, while being transparent to light of other wavelengths.

In each LGP of the plurality of LGPs, the diffraction efficiency of the output diffraction gratings is preferably optimized depending on the distance from the input diffraction gratings.

Compared with prior art technologies, the present invention provides an increase in the efficiency of use of light in a waveguide and an increase in spatial uniformity of light emitted in the direction of the observer, which in turn results in an increase in backlight brightness and an increase in image quality without increasing the power / number of light sources which result is tangible on the side of the observer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of light propagation in an LGP according to the prior art;

FIG. 2 are graphs showing the calculated spatial distribution of light intensity and the actual decrease in light intensity with distance according to the prior art;

FIG. 3 is a graph illustrating a drop in light intensity with distance for different wavelengths according to the prior art;

FIG. 4 is an illustration of the heterogeneity of the brightness distribution of an exemplary image according to the prior art;

FIG. 5 is an illustration of light loss on BLU equipment according to the prior art;

FIG. 6 is a schematic illustration of light propagation in an LGP according to the present invention;

FIG. 7 is an enlarged schematic illustration of the coordinated operation of the light recovery module and output diffraction gratings of FIG. 6 according to the present invention;

FIG. 8 is an embodiment of the light recovery module of FIG. 6 with a mirror;

FIG. 9 is an embodiment of the light recovery module of FIG. 6 with a second retroreflective diffraction grating;

FIG. 10 is an illustration of multipath interference;

FIG. 11a-11d are possible uses of a dichroic coating in a BLU according to the present invention;

FIG. 12 is an illustration of a multilayer waveguide according to the present invention;

FIG. 13 is a general diagram of testing equipment of the present invention.

FIG. 14 are graphs showing a calculated spatial distribution of light intensity using light recovery and without light recovery.

FIG. 15 is an experimental graph showing the dependence of the energy of the light output when using the dichroic coating according to the present invention and without it.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following is a reference to exemplary embodiments of the present invention, which are illustrated in the accompanying drawings, where the same reference numbers indicate similar elements. It should be understood that the embodiments of the invention may take various forms and should not be construed as limited by the descriptions herein. Accordingly, illustrative embodiments are described below with reference to the figures of the drawings to explain the essence of aspects of the present invention.

In FIG. 6 schematically shows the propagation of light in an LGP 600 of a BLU of the present invention. The LGP 600 is preferably made of an optical material that is transparent in the visible range (e.g., glass). LGP 600 has two flat surfaces and has two opposite ends. Here, for purposes of clarity, but not limitation, and with reference to FIG. 6, these surfaces will be respectively referred to as the upper surface and the lower surface, and the ends will be respectively designated as the left end and the right end. In the present exemplary embodiment, the bottom surface of the LGP 600 is a light emitting surface.

One or more input diffraction gratings 601 are located on the lower surface, in the region adjacent to the left end face. One or more output diffraction gratings 602 are located on the same surface, and they extend almost to the right end of the LGP 600. A coherent one is installed near the left end a light source (not shown), which can be either a single source or a collection of sources. As in the prior art, coherent light 603 from the light source enters the input diffraction gratings 601, diffracts onto them, and is directed into the LGP 600 at an angle providing full internal reflection of the light in the LGP 600. The light propagating inside the LGP 100 is incident on the output diffraction gratings 602 and, as a result of diffraction on them, partially directed to the observer 604, in this case, actually perpendicular to the light-emitting surface, and partially directed in the opposite direction, towards the upper surface LGP 600. As noted earlier in the presentation of FIG. 1, with each incident of light on the output diffraction gratings 602, a portion thereof will continue to propagate towards the right end of the LGP 600 (non-diffracted light, 0th-order diffraction), as illustrated in FIG. 6.

The BLU of the present invention also includes a light recovery module 605, consisting of two components that are located opposite each other on opposite surfaces of the LGP 600 and each of which is placed in direct contact with its right end. These components of the light recovery module 605, for purposes of illustration, but not limitation, and with reference to FIG. 6 are designated as an upper recovery element and a lower recovery element.

The upper recuperation element is represented by the first retroreflective diffraction grating 606, and the lower recuperation element is represented by the second retroreflective diffraction grating or mirror 607. Below, the implementation of the lower recuperation element in the form of a mirror and in the form of a diffraction grating will be described in more detail below.

The basic purpose of the lower recovery element is to direct the main part of the light, which, propagating inside the LGP 600 in the forward direction from the input diffraction gratings 601, falls on the second retroreflective diffraction grating or mirror 607, on the first retroreflective diffraction grating 606.

The light incident on the first retroreflective diffraction grating 606 from the lower recovery element diffracts on it. Moreover, according to the present invention, the first retroreflective diffraction grating 606 is configured so that the light diffracted thereon mainly returns to the LGP 600, while traveling in the opposite direction at a predetermined angle with respect to the incident light. This embodiment of the first retroreflective diffraction grating 606 is preferably realized by optimizing its structure for second-order diffraction so that the light incident on the first retroreflective diffraction grating 606 is returned to the LGP 600 at a predetermined angle to the incident light. Returning the diffracted light at a predetermined angle to the incident light ensures that this returned light falls on the output diffraction gratings 602 with the required spatial shift relative to the light propagating in the forward direction. At the same time, noise is not brought in in this way.

The output diffraction gratings 602 are manufactured according to the present invention so as to take into account the light recovered in the LGP 600 by the light recovery unit 605, as described above. Namely, the output diffraction gratings 602 are configured so that the light directed outside the LGP when diffracting the returned light thereon is substantially colinear light directed outside the LGP when diffracting light on them traveling in the forward direction from the input diffraction gratings 601 Such collimation of light in the direction of the observer 604 is preferably achieved through specialized optimization of the structure of the output diffraction gratings 602.

In addition, as can be seen from FIG. 3, for different wavelengths, the light emission intensity decreases with distance unequally for different wavelengths. As a result, the distribution of the diffraction efficiency of the output gratings 602 is calculated and implemented in such a way as to more compensate for the decrease in the intensity of blue and green light with increasing distance from the light entrance into the waveguide (i.e., it is optimized depending on the distance for specific light wavelengths) .

The above coordinated operation of the light recovery unit 605 and the output diffraction gratings 602 according to the present invention is illustrated in FIG. 7. It must be emphasized that for such coordinated operation, the output diffraction gratings 602, the first retroreflective diffraction grating 606 and the second retroreflective diffraction grating 607 (if any) must have (a) the same and constant grating period and (b) essentially , the same orientation with respect to each other. Once again, the consistency of the light recovery module 605 and the output diffraction gratings 602 in this application is understood to mean their implementation in such a way that undiffracted light radiation that remains in the LGP 600 (i.e., that which was not removed by the output diffraction gratings 602), it was redirected by the second retroreflective diffraction grating or mirror 607 and the first retroreflective diffraction grating 606 in the opposite direction along practically the same path as with its direct propagation, and was output from the LGP 600 output diffraction gratings 602 in the direction that is in accord (essentially colinear) to the direction of light emission output from the LGP 600 hatchers diffraction gratings 602 in the direct spreading.

The described light recovery provides an increase in the uniformity of the spatial distribution of the light intensity emitted from the LGP through its light emitting surface, and also provides an increase in its brightness. What is significant here is that with the light recovery proposed in this application, the backlight coherence is maintained, which is a critically important requirement, for example, for 3D displays or holographic displays.

It should be emphasized that the above consistency cannot be ensured by replacing the proposed light recovery module with known mirrors, corner reflectors, or the like. to return undiffracted light inside the LGP, since the light returned in this way will inevitably be a source of noise. Accordingly, the beneficial effects of increasing uniformity and increasing brightness noted above will not be achieved.

Aspects of the manufacture and operation of light sources, waveguides, and introductory diffraction gratings are widely known in the art (see, for example, Nikonorov N.V., Shandarov S.M. “Waveguide photonics. Study guide”, Ministry of Education and Science of the Russian Federation, Federal Agency by education, St. Petersburg State University of Information Technologies, Mechanics and Optics, St. Petersburg 2008, http://books.ifmo.ru/file/pdf/766.pdf) and do not directly relate to the essence of the present invention. Additional components that may be involved in the operation of the BLU are indicated, in particular, in US 2016/0147003.

As the diffraction gratings used in the present invention, phase diffraction gratings are preferably used, which, in turn, can be realized in the form of phase-relief gratings, when the required change in the phase incursion of the wave (due to which the shape of the wavefront changes - diverging, collimated, converging, etc., and the propagation direction) is provided by changing the surface profile of the grating), or in the form of bulk (Bragg) diffraction gratings (VBG), when required The change in the phase incursion of the wave is ensured by changing the refractive index of the material, in which case both liquid crystals and polymers can be used.

So, in the case of using phase-relief diffraction gratings, the above-mentioned optimization of the structure of the first retroreflective diffraction grating 606 and optimization of the structure of the output diffraction gratings 602 can be realized by the corresponding calculation and configuration of the profiles of these gratings, for which one of the known technologies can be used, for example, lithography, holographic recording. Thus, the profile of the first retroreflective diffraction grating, in order to optimize for diffraction of the second order, has a larger depth and a specially calculated value of the duty cycle parameter, which shows the relationship between the convexity and the cavity of the profile, and for output diffraction gratings these parameters can be changed to obtaining a special diffraction efficiency (DE) profile. For the second retroreflective diffraction grating, a profile similar to the profile of the output diffraction gratings can be used, i.e. with optimization for diffraction ± 1st order.

In addition, each of the first retroreflective diffraction grating 606 and the second retroreflective diffraction grating 607 (if any) has a mirror coating. In particular, the light resulting from the diffraction of the 1st order on the second retroreflective diffraction grating will be reflected from the mirror coating and thereby. It will be directed to the first retroreflective diffraction grating along with the light resulting from the diffraction of the 1st order.

In the above example, we used the specific arrangement of the input diffraction gratings, output diffraction gratings, and the first retroreflective diffraction grating on the LGP surfaces. It must be emphasized that their mutual arrangement is not the only possible one and, therefore, does not impose a restriction on the possible implementation of the invention. So, the placement of the input diffraction gratings is mainly dictated by the location of the light source, and the main requirement determining the location of the output diffraction gratings and the first retroreflective diffraction grating on the LGP surfaces is to ensure the aforementioned consistency.

In FIG. 8, 9 show, respectively, Embodiment 1 of the light recovery module 605 of FIG. 6, in which a mirror is used as the lower recovery element, and embodiment 2 of the light recovery module 605 of FIG. 6, in which a second retroreflective diffraction grating is used as the lower recovery element.

A comparative analysis of options 1 and 2 implementation, performed on the basis of experimental data, are summarized below in Table 1:

Color Efficiency without recovery Embodiment 1 Embodiment 2 Recovery Efficiency Recovery efficiency Recovery Efficiency Recovery efficiency R ~ 5 ... 7% ~ 60% ~ 10 ... 15% ~ 70% ~ 15% G ~ 5 ... 7% ~ 40% ~ 10% ~ 40% ~ 10% B ~ 5 ... 7% ~ 20% ~ 10% ~ 20% ~ 10%

Table 1

As can be seen from Table 1, for the experiment, a more expensive implementation based on two diffraction gratings (option 2 implementation) gives only a small increase in efficiency compared to the implementation based on the mirror (option 1 implementation).

As noted above when considering the prior art with reference to FIG. 1, as a result of the diffraction of the light radiation propagating inside the LGP, on the output gratings, a part of the diffracted light is directed in the opposite direction from the observer, causing losses. Accordingly, one of the basic ideas underlying the present invention is the use of this potentially lost light for illumination. To this end, it is proposed to use a dichroic coating in BLU to reflect light that actually falls normally from the output diffraction gratings (see Figs. 1, 6) onto the dichroic coating and back onto the output diffraction gratings for its subsequent output outward as a backlight.

Below will be discussed in detail the application aspect of the dichroic coating according to the present invention.

The operation of any dichroic coating is based on the phenomenon of multipath interference, which is illustrated on the multilayer optical structure of FIG. 10, where layers with a high refractive index (n _high ) and a low refractive index (n _low ) alternate. The thicknesses of the layers are selected so that the light rays reflected at the boundaries between the layers always come in phase and, thereby, reinforce each other. This method of operation of interference mirrors is widely known in the technical field to which the present invention relates.

Naturally, a mirror can be used instead of a dichroic coating for the light reflection under consideration. However, any mirror is characterized by losses associated with absorption at the metal-insulator interface; therefore, its use will inevitably lead to a violation of the conditions of total internal reflection, which is the basis for the functioning of the waveguide, and, accordingly, will reduce the efficiency of the backlight device.

In the proposed dichroic coating, for this reason, there are no mirror layers, i.e. Absorption is absent and total internal reflection is maintained. In other words, the application of the proposed dichroic coating does not adversely affect the efficiency of the waveguide.

Further, the characteristics of the layers of the proposed dichroic coating are calculated in such a way that it reflects only such light radiation that has a specific wavelength or length and which falls on the dichroic coating at a specific angle. Relatively speaking, the dichroic coating works on reflection only for light radiation, which should be displayed outside the LGP as a backlight towards the observer. Such an implementation of the desired dichroic coating can be implemented using technologies widely known in the art, such as, for example, chemical deposition, ion beam sputtering, magnetron sputtering.

So, in the illustrative example considered with reference to FIG. 6, the dichroic coating 608 applied to the upper surface of the LGP 600 will only reflect light incident on it perpendicularly as a result of 1st-order diffraction on the output gratings 602, thereby directing it back through the light-emitting surface of the LGP 600 collinear to the light resulting from + 1st order diffraction on the output gratings 602. Light incident on the dichroic coating 608 at other angles and / or having different wavelengths will not be reflected, i.e. for him, the dichroic coating will, in fact, be transparent. Thus, the efficiency of the use of light is almost doubled, the specific result of which is a significant increase in the brightness of the backlight.

Below with reference to FIG. 11a-11d illustrate possible, but not the only possible, use cases of a dichroic coating in a BLU.

So in FIG. 11a, an embodiment of a BLU is shown where a dichroic coating is applied to an LGP surface that is opposite to the surface on which the output diffraction gratings are placed. In FIG. 11b shows an embodiment of a BLU, where an embodiment of a BLU is shown, where the output diffraction gratings are coated externally with an immersion polymer and a dichroic coating is applied to the immersion polymer. In this embodiment, the output diffraction gratings operate as a phase element at the boundary of two materials (and not at the material-air interface, as in the case of Fig. 11a). Immersion is used to make the surface flat (i.e. planarize it), and a dichroic coating is already applied to this flat surface. Such planarization can be implemented in practice by applying UV curable adhesive to the diffraction grating profile and leveling the surface with a stamp, which is widely known and has wide application. Moreover, for the embodiment of FIG. 11b, the temporal coherence of radiation is preserved due to the small difference between direct and reflected radiation.

Further, in FIG. 11c, an embodiment of a BLU is shown where a dichroic coating is applied directly to the profile of the output diffraction gratings. Such an embodiment also retains temporal coherence, but may prove difficult in terms of production. In FIG. 11d, a dichroic coating is applied to the same LGP surface as the output diffraction gratings, such that the dichroic coating is below the output diffraction gratings with respect to the surface. That is, first a dichroic coating is applied to the surface, and then to it - output diffraction gratings. Moreover, the output diffraction gratings can be coated externally with an immersion polymer for planarization. For this implementation, the preservation of temporal coherence is also characteristic, as well as for the embodiments of FIG. 11b, 11s.

A further development of the above approach is the creation of a multilayer stack-like waveguide, including superimposed structures according to the present invention, each of which is similar to that described above with reference to FIG. 6 and, at the same time, optimized for a specific wavelength of light radiation. In FIG. 12 illustrates a preferred implementation of such a stack-like waveguide consisting of three LGPs. As shown in FIG. 12, the input diffraction gratings, the output diffraction gratings, the light recovery module and the dichroic coating are located on each of the component LGPs, as described above, while in the case under consideration, each of these three LGPs has the output diffraction gratings, retroreflective diffraction gratings, and dichroic coating, respectively, optimized for a specific wavelength of wavelengths of red (R), green (G) and blue (B) colors. In particular, as illustrated in FIG. 12, the lower component structure is optimized for the blue wavelength, the middle component structure is optimized for the green wavelength, and the upper component structure is optimized for the red wavelength. As noted above, at the level of diffraction gratings, such optimization can be realized by configuring their profile, and the dichroic coating is made in a known manner so as to reflect only the light radiation of the required wavelength (i.e., red, green or blue in this example) and be transparent to radiation of other wavelengths.

Such a multilayer implementation provides an additional increase in efficiency and uniformity in the sense that is understood in this application, due to the ability to more finely optimize the characteristics of diffraction gratings and dichroic coatings individually on each layer for different light wavelengths.

It should be understood that the example of FIG. 12 is not limiting in terms of the number of layers. For example, a larger number of such structural layers (say, 5) can be used with optimization for other and / or additional light wavelengths.

The following are the test results of the present invention, obtained in experiments conducted on real equipment.

In FIG. 13 shows a general diagram of the equipment used for testing, and the corresponding type of real equipment. As can be seen from FIG. 13, in the direction of light from three R-G-B coherent sources, the BLU proposed in this application is appropriately preceded by a system of three means for directing / increasing the size of the light beam. The BLU of the present invention is followed by a Fresnel lens, followed by a holographic panel (typically an LCD), on which a digital holographic image is directly formed. At the same time, it must be emphasized that the main general requirement in this context is to ensure a high degree of coherence of the light incident on the holographic panel - only in this case the observer will see a three-dimensional image (otherwise, he will see a blurred cloud). Additional information regarding these and other components of such equipment can be found, for example, in US 2016/0147003.

By using the approach corresponding to the present invention, namely, the light recovery module and the dichroic coating, it was possible to increase the efficiency of the use of light in the waveguide by about two times, resulting in, in particular, increasing the brightness and increasing the spatial uniformity of the output light radiation by approximately 12.5%.

In FIG. Figure 14 shows the graphs obtained from the simulation results, which show the dependence of the output light intensity on the distance. Moreover, the lower graph in FIG. 14 corresponds to modeling without light recovery, i.e. essentially corresponds to the similar graph shown in FIG. 2 when considering the prior art. The upper graph in FIG. 14 corresponds to the light recovery simulation according to the present invention, as disclosed above. From FIG. 14 (see, for example, a window in the form of a rectangle) you can see an almost 2-fold increase in intensity and a noticeable increase in spatial uniformity (less sharp peaks and troughs in the graph) when light recovery is involved - for example, the uniformity index in this case reaches 90%.

In FIG. 15 shows graphs obtained from experimental results showing the dependence of the energy of the light emitted along the direction of direct light propagation in the waveguide on the coordinate counted from the end of the input grating for the following three cases: using the dichroic coating according to the present invention (upper graph), using a mirror ( namely, silver) coating instead of dichroic coating (average graph) and without the use of dichroic or mirror coating (lower graph). From FIG. Figure 15 shows that even the use of a mirror coating gives an increase in intensity of approximately two times; the use of the dichroic coating according to the present invention gives an additional increase in intensity of approximately 21% compared with a mirror coating, which, in accordance with the foregoing, is due to the absorption of light by the mirror coating, leading to a violation of the total internal reflection inside the waveguide. The effect of using the dichroic coating according to the present invention is to increase the brightness of light radiation to 300 nits.

It should be understood that the shown embodiments are only preferred, but not the only possible examples of the present invention. On the contrary, the scope of the invention is defined by the following claims and their equivalents.

Claims (105)

1. A backlight device (BLU) for a display device, comprising: a light guide plate (LGP), wherein the LGP has two flat surfaces and has a first end and a second end opposite the first end; one or more introductory diffraction gratings arranged on one of said two surfaces, these one or more introductory diffraction gratings located in a region adjacent to the first end face; at least one coherent light source configured to supply coherent light to the LGP through the input diffraction gratings, the input diffraction gratings being adapted to direct the light into the LGP at an angle so as to ensure full internal reflection of the light inside the LGP; one or more output diffraction gratings located on one of the two surfaces, wherein the output diffraction gratings are adapted to partially diffract light incident on the output diffraction gratings from within the LGP, to the outside of the LGP; and a light recovery module comprising: a first recovery element arranged on one of the two surfaces, the first recovery element comprising a first retroreflective diffraction grating located in a region adjacent to the second end face, and a second recovery element located on the other of the two surfaces, the second recovery element comprising either a second retroreflective diffraction grating or a mirror placed in a region adjacent to the second end, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have the same and constant period of the grating, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have substantially the same orientation with respect to each other, wherein the second recovery element is adapted to partially direct light incident on the second retroreflective diffraction grating or mirror to the first retroreflective diffraction grating or mirror when traveling in the forward direction inside the LGP from the input diffraction gratings, wherein the first retroreflective diffraction grating is adapted to diffract the light incident on it in such a way that the diffracted light returns to the LGP, heading in the opposite direction at a predetermined angle with respect to this incident light, and wherein the output diffraction gratings are configured such that the light directed outwardly by the LGP when diffracting said return light on the output diffraction gratings is essentially collinear light directed outwardly by the LGP upon diffraction of said forward-propagating light on the output diffraction gratings; and however, LGP additionally has a dichroic coating for reflecting light, which falls essentially normally on the dichroic coating from the output diffraction gratings, back to the output diffraction gratings. 2. BLU according to claim 1, wherein the light directed outside the LGP is directed towards the observer. 3. The BLU of claim 1, wherein the LGP is made of an optical material that is transparent in the visible range. 4. BLU according to claim 1, in which each of the retroreflective diffraction gratings has a mirror coating. 5. BLU according to claim 1, wherein the display device is a holographic display device. 6. BLU according to claim 5, wherein the display device is part of a portable computing device. 7. BLU according to claim 1, in which the input diffraction gratings, output diffraction gratings and the first retroreflective diffraction grating, and the second retroreflective diffraction grating are made either as phase-relief diffraction gratings, or as bulk diffraction gratings. 8. BLU according to claim 7, in which at least the output diffraction gratings and the first retroreflective diffraction grating are made as phase-relief diffraction gratings, wherein the first retroreflective diffraction grating has a profile optimized for second-order diffraction in order to maximally return the light incident on the first retroreflective grating to the LGP so that the returned light falls on the output diffraction gratings with a predetermined spatial shift relative to the said forward-propagating diffraction grating direction of light; and the output diffraction gratings have a profile adapted to provide said collinear direction of light outside the LGP. 9. BLU according to claim 1, in which the dichroic coating is optimized for specific wavelengths. 10. BLU according to claim 9, in which the dichroic coating is applied to the surface of the LGP, which is opposite to the surface on which the output diffraction gratings are placed, 11. BLU according to claim 9, in which the dichroic coating is applied directly to the output diffraction gratings. 12. BLU according to claim 9, in which the dichroic coating is directly applied to the same LGP surface as the output diffraction gratings in such a way that the dichroic coating is located under the output diffraction gratings relative to this surface, while the output diffraction gratings are coated externally with an immersion polymer . 13. BLU according to claim 9, in which the output diffraction gratings are coated externally with an immersion polymer, and a dichroic coating is applied to the immersion polymer. 14. The BLU of claim 1, wherein the diffraction efficiency of the output diffraction gratings is optimized, depending on the distance from the input diffraction gratings, for a particular wavelength, wherein the diffraction efficiency of the retroreflective diffraction gratings is optimized for that specific wavelength. 15. A backlight device (BLU) for a display device, comprising: a light guide plate (LGP), wherein the LGP has two flat surfaces and has a first end and a second end opposite the first end; one or more introductory diffraction gratings arranged on one of said two surfaces, these one or more introductory diffraction gratings located in a region adjacent to the first end face; at least one coherent light source configured to supply coherent light to the LGP through the input diffraction gratings, the input diffraction gratings being adapted to direct the light into the LGP at an angle so as to ensure full internal reflection of the light inside the LGP; one or more output diffraction gratings located on one of the two surfaces, wherein the output diffraction gratings are adapted to partially diffract light incident on the output diffraction gratings from within the LGP, to the outside of the LGP; a light recovery module comprising: a first recovery element arranged on one of the two surfaces, the first recovery element comprising a first retroreflective diffraction grating located in a region adjacent to the second end face, and a second recovery element located on the other of the two surfaces, the second recovery element comprising either a second retroreflective diffraction grating or a mirror placed in a region adjacent to the second end, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have the same and constant period of the grating, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have substantially the same orientation with respect to each other, wherein the second recovery element is adapted to partially direct light incident on the second retroreflective diffraction grating or mirror to the first retroreflective diffraction grating or mirror when traveling in the forward direction inside the LGP from the input diffraction gratings, wherein the first retroreflective diffraction grating is adapted to diffract the light incident on it in such a way that the diffracted light returns to the LGP, heading in the opposite direction at a predetermined angle with respect to this incident light, and wherein the output diffraction gratings are designed such that the light directed outside the LGP when diffracting said return light on the output diffraction gratings is essentially collinear light directed outside the LGP when diffracting said forward light propagating on the output diffraction gratings. 16. BLU according to clause 15, while the light directed outside the LGP is directed towards the observer. 17. The BLU of claim 15, wherein the LGP is made of an optical material that is transparent in the visible range. 18. BLU according to clause 15, in which each of the retroreflective diffraction gratings has a mirror coating. 19. BLU according to clause 15, wherein the display device is a holographic display device. 20. BLU according to claim 19, wherein the display device is part of a portable computing device. 21. BLU according to clause 15, in which the input diffraction gratings, output diffraction gratings and the first retroreflective diffraction grating and the second retroreflective diffraction grating are made either as phase-relief diffraction gratings, or as bulk diffraction gratings. 22. BLU according to item 21, in which at least the output diffraction gratings and the first retroreflective diffraction grating are made as phase-relief diffraction gratings, wherein the first retroreflective diffraction grating has a profile optimized for second-order diffraction in order to maximally return the light incident on the first retroreflective grating to the LGP so that the returned light falls on the output diffraction gratings with a predetermined spatial shift relative to the said forward-propagating diffraction grating direction of light; and the output diffraction gratings have a profile adapted to provide said collinear direction of light outside the LGP. 23. The BLU of claim 15, wherein the LGP further has a dichroic coating for reflecting light that falls substantially normally onto the dichroic coating from the output diffraction gratings, back onto the output diffraction gratings, the dichroic coating being optimized for specific wavelengths. 24. BLU according to item 23, in which the dichroic coating is applied to the surface of the LGP, which is opposite to the surface on which the output diffraction gratings are placed, 25. BLU according to item 23, in which the dichroic coating is applied directly to the output diffraction gratings. 26. BLU according to item 23, in which the dichroic coating is directly applied to the same LGP surface as the output diffraction gratings in such a way that the dichroic coating is located under the output diffraction gratings relative to this surface, while the output diffraction gratings are coated on the outside with an immersion polymer . 27. BLU according to item 23, in which the output diffraction gratings are coated externally with an immersion polymer, and a dichroic coating is applied to the immersion polymer. 28. BLU according to clause 15, in which the diffraction efficiency of the output diffraction gratings is optimized, depending on the distance from the input diffraction gratings, for a specific wavelength, and the diffraction efficiency of retroreflective diffraction gratings is optimized for this specific wavelength. 29. A backlight device (BLU) for a display device, comprising: a light guide plate (LGP), wherein the LGP has two flat surfaces and has a first end and a second end opposite the first end; one or more introductory diffraction gratings arranged on one of said two surfaces, these one or more introductory diffraction gratings located in a region adjacent to the first end face; at least one coherent light source configured to supply coherent light to the LGP through the input diffraction gratings, the input diffraction gratings being adapted to direct the light into the LGP at an angle so as to ensure full internal reflection of the light inside the LGP; and one or more output diffraction gratings located on one of the two surfaces, wherein the output diffraction gratings are adapted to partially diffract light incident on the output diffraction gratings from within the LGP, to the outside of the LGP; however, LGP has a dichroic coating for reflecting light, which falls essentially normally on the dichroic coating from the output diffraction gratings, back to the output diffraction gratings. 30. BLU according to clause 29, further comprising a light recovery module, comprising: a first recovery element arranged on one of the two surfaces, the first recovery element comprising a first retroreflective diffraction grating located in a region adjacent to the second end face, and a second recovery element located on the other of the two surfaces, the second recovery element comprising either a second retroreflective diffraction grating or a mirror placed in a region adjacent to the second end, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have the same and constant period of the grating, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have substantially the same orientation with respect to each other, wherein the second recovery element is adapted to partially direct light incident on the second retroreflective diffraction grating or mirror to the first retroreflective diffraction grating or mirror when traveling in the forward direction inside the LGP from the input diffraction gratings, wherein the first retroreflective diffraction grating is adapted to diffract the light incident on it in such a way that the diffracted light returns to the LGP, heading in the opposite direction at a predetermined angle with respect to this incident light, and wherein the output diffraction gratings are configured such that the light directed outwardly by the LGP when diffracting said return light on the output diffraction gratings is essentially collinear light directed outwardly by the LGP upon diffraction of said forward-propagating light on the output diffraction gratings; and 31. BLU according to clause 29, wherein the light directed outside the LGP is directed towards the observer. 32. BLU according to clause 29, in which the LGP is made of an optical material that is transparent in the visible range. 33. BLU according to item 30, in which each of the retroreflective diffraction gratings has a mirror coating. 34. BLU according to clause 29, wherein the display device is a holographic display device. 35. BLU according to clause 34, while the display device is part of a portable computing device. 36. BLU according to Claim 30, wherein the input diffraction gratings, output diffraction gratings, and the first retroreflective diffraction grating and the second retroreflective diffraction grating are made either as phase-relief diffraction gratings or as bulk diffraction gratings. 37. BLU according to clause 36, in which at least the output diffraction gratings and the first retroreflective diffraction grating are made as a relief phase diffraction grating, the first retroreflective diffraction grating has a profile optimized for second-order diffraction in order to maximally return the light incident on the first retroreflective grating to the LGP so that the returned light falls on the output diffraction gratings with a predetermined spatial shift relative to the said forward-propagating diffraction grating direction of light; and the output diffraction gratings have a profile adapted to provide said collinear direction of light outside the LGP. 38. BLU according to clause 29, in which the dichroic coating is optimized for specific wavelengths. 39. BLU according to § 38, in which the dichroic coating is applied to the surface of the LGP, which is opposite to the surface on which the output diffraction gratings are placed, 40. BLU according to § 38, in which the dichroic coating is applied directly to the output diffraction gratings. 41. BLU according to § 38, in which the dichroic coating is directly applied to the same LGP surface as the output diffraction gratings in such a way that the dichroic coating is located under the output diffraction gratings relative to this surface, while the output diffraction gratings are coated externally with an immersion polymer . 42. BLU according to § 38, in which the output diffraction gratings are coated externally with an immersion polymer, and a dichroic coating is applied to the immersion polymer. 43. BLU according to clause 29, in which the diffraction efficiency of the output diffraction gratings is optimized, depending on the distance from the input diffraction gratings, for a specific wavelength, and the diffraction efficiency of retroreflective diffraction gratings is optimized for this specific wavelength. 44. A backlight device (BLU) for a display device, comprising: a plurality of light guide plates (LGPs) stacked on top of each other, each LGP of the plurality of LGPs having two flat surfaces and having a first end and a second end opposite the first end; and at least one coherent light source configured to supply coherent light with multiple wavelengths to a plurality of LGPs; each of the many LGPs has: one or more input diffraction gratings located on one of the two surfaces, these one or more input diffraction gratings located in a region adjacent to the first end face of the LGP, while the input diffraction gratings are adapted to direct the light supplied by said coherent light source to LGP at such an angle that provides full internal reflection of light inside the LGP, one or more output diffraction gratings arranged on one of the two surfaces mentioned, wherein the output diffraction gratings are adapted to partially diffract light incident on the output diffraction gratings from within the LGP, to the outside of the LGP, and a light recovery module comprising: a first recovery element arranged on one of the two surfaces, the first recovery element comprising a first retroreflective diffraction grating located in a region adjacent to the second end face, and a second recovery element located on the other of the two surfaces, the second recovery element comprising either a second retroreflective diffraction grating or a mirror placed in a region adjacent to the second end, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have the same and constant period of the grating, wherein the output diffraction gratings, the first retroreflective diffraction grating and the second retroreflective diffraction grating have substantially the same orientation with respect to each other, wherein the second recovery element is adapted to partially direct light incident on the second retroreflective diffraction grating or mirror to the first retroreflective diffraction grating or mirror when traveling in the forward direction inside the LGP from the input diffraction gratings, wherein the first retroreflective diffraction grating is adapted to diffract the light incident on it in such a way that the diffracted light returns to the LGP, heading in the opposite direction at a predetermined angle with respect to this incident light, and wherein the output diffraction gratings are configured such that the light directed outwardly by the LGP when diffracting said return light on the output diffraction gratings is essentially collinear light directed outwardly by the LGP upon diffraction of said forward-propagating light on the output diffraction gratings, and wherein the LGP further has a dichroic coating for reflecting light that falls substantially normally onto the dichroic coating from the output diffraction gratings, back onto the output diffraction gratings, wherein the output diffraction gratings, the first retroreflective diffraction grating, the second retroreflective diffraction grating and the dichroic coating located on each LGP from the plurality of LGPs are optimized for a specific wavelength of the plurality of different wavelengths. 45. BLU according to item 44, wherein said plurality of wavelengths consists of three wavelengths corresponding to red, green, and blue, and said plurality of LGPs comprises three LGPs. 46. BLU according to item 45, in which at least the output diffraction gratings and the first retroreflective diffraction grating of each LGP from the set of LGPs are made as phase-relief diffraction gratings, while in each LGP from the set of LGP: the first retroreflective diffraction grating has a profile optimized for second-order diffraction of light of a particular wavelength in order to maximize the return of light incident on the first retroreflective diffraction grating to LGP so that the returned light falls on the output diffraction gratings with a predetermined spatial shift relative to said forward-propagating light, the output diffraction gratings have a profile adapted to provide the collinear direction of light of this particular wavelength outside the LGP, and the dichroic coating is adapted to reflect only the light of said specific wavelength, while being transparent to light of other wavelengths. 47. BLU according to item 44, in which each of the retroreflective diffraction gratings has a mirror coating. 48. The BLU of claim 46, wherein in each LGP of the plurality of LGPs, the diffraction efficiency of the output diffraction gratings is optimized depending on the distance from the input diffraction gratings.

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