GB2073905A - Lens and optic fibre systems for image projectors - Google Patents

Abstract

The invention provides a lens system for use in image projection systems such as ground-based flight simulators. Image information is transmitted by a fibre optic ribbon to a lens 16 permitting wide-angle display, the prolongation of the axis of each fibre 22 comprising the ribbon intersecting the lens surface 17 at normal incidence. The fibres 22 may be optically coupled to the lens 16 by an index matching medium 24. A method is described for optimising the design of such lens systems to minimise spherical and/ or chromatic aberration to meet desired performance criteria. <IMAGE>

Description

SPECIFICATION Lens systems for image projectors The present invention relates to lens systems for image projectors of the type in for example ground-based flight simulators.

Image projectors are known in which image information is transmitted one line at a time via a fibre optic ribbon and a lens with spherical symmetry to a reciprocating or rotating mirror. The fibre optic ribbon comprises fibres or bundles of fibres, there being a oneto-one correspondence between the fibres at each end of the ribbon. The fibre optic ribbon provides a flexible coupling between a stationary line scan signal generating apparatus and the lens and mirror which are mounted on and move with the helmet of a pilot using the simulator.

The ends of the individual fibres in the ribbon adjacent the lens must be carefully ground flat and the axes of their fibre ends must also be very accurately directed relative to the lens to avoid distortion of the image. In practice this is difficult to achieve. Even if these problems are overcome however spherical aberration in the lens reduces the definition in the projected image. Where a full colour image is required, chromatic aberration further reduces the image quality.

It is an object of the invention to provide a lens system the performance of which enables the quality of the image projected to be significantly improved.

According to the present invention, there is provided a lens system for an image projector comprising a lens and a fibre optic ribbon one end of which is secured in a predetermined orientation relative to the lens, characterised in that the prolongation of the axis of each fibre comprising the ribbon intersects the lens surface at normal incidence.

Preferably, the lens has a plurality of principle axes and optical spherical symmetry.

Preferably the lens and each fibre end in the ribbon are optically coupled together by an index matching medium such that light emanating from the fibre enters the lens without passing through an optical discontinuity.

Preferably the lens comprises two sectorshaped portions bounded by spherical surfaces centred on a common point, one or more fibre optic ribbons being coupled to one or more of said spherical surfaces.

The lens may be multi-layered to minimise spherical and/or chromatic aberration, the mating surfaces of adjacent layers being spherical and centred on a common point.

In order to enable the optimisation of the design of lens system of the above type, the invention also provides a method for selecting the optimum distance between the ends of the fibres and the spherical lens surface. In this method, the maximum acceptable projected spot radius (r2) for light emanating from a single fibre is selected, the relationship between the projected spot radius (r) and the cone half angle (a) is determined for a range of fibre end to lens surface distances (x), the relationship between the fibre end to lens surface distance (x) and the cone half angle (a2) at which the projected spot radius reaches the maximum acceptable radius is determined, the relationship between the fibre end to lens surface distance (x) and the cone half angle (aO) at which the spot radius as a function of the cone half angle reaches a maximum or minimum value (pub) is determined, the relationship between the fibre end to lens surface distance (x) and the spot radius (rO) at the or each said maximum or minimum value is determined, and the optimum fibre end to lens surface distance is selected to meet desired performance criteria (aO, a2, rO) from the determined relationships.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an image projection system of known type; Figure 2 is a side view of a portion of the system of Fig. 1; Figure 3 illustrates a lens system according to the present invention; Figures 4 to 7 illustrate the principles involved in designing a lens in accordance with the present invention; Figures 8 to 11 illustrate characteristics of multi-layer lens in accordance with the present invention; Figure 12 illustrates characteristics of a particular two-layer lens; and Figure 13 illustrates the characteristics of a preferred embodiment of the present invention.

Referring to Fig. 1 a laser beam source 1 provides an output laser beam 2 which is directed through a full colour modulator 3.

Both the laser beam source 1 and the modulator 3 are of known form. The full colour modulated beam output is indicated by numeral 4 in Fig. 1. For the sake of simplicity intermediate beam splitters are not shown.

A line scanner comprises a synchronously driven polygonal section mirror drum 5 which rotates continuously in the direction shown by the arrow 6 to sweep the beam 4 over a scan path 7. One pass occurs for the movement of each mirror facet of the mirror drum 5 past the beam 4.

A fibre optic ribbon 8 is formed with planar ends 9, 1 0. There is a one-to-one correspondence between the position of individual fibres at each end 9, 10 of the ribbon 8. The fibres at the input end 9 of the ribbon 8 are arranged in an arc the length of which corresponds to the width of the scan path 7 so that the modulated beam 4 is scanned along the arc-for each line of the image to be projected.

At the output end 10 of the ribbon 8 the individual fibres are similarly formed into an arc, the fibres being positioned in the same order at the two ends 9 and 10, so that the scani.ed image line at the input end 9 is exactly reproduced at the output end 10.

The end of each fibre in the ribbon output end 10 is spaced from and directly radially with respect to a spherical projection lens 11.

The ends of all the individual fibres are equidistant from the centre of the lens 11. Thus the output end 10 of the ribbon 8 must describe an arc of a circle and the individual fibre ends must be perpendicular to that arc. The ends of the individual fibres are arranged coplanar, so that light emitted along the fibre axis at the output end of each fibre is projected through the centre of the sphere. Light emitted from the fibres will be focussed to an image by the lens 11.

The emergent rays from the output end 10 of the ribbon 8 are focussed by the spherical lens 11 and strike the face of a frame scanning mirror 1 2. Each scanned image line is required to subtend a large angle (e.g. 90 ) from the frame scanning mirror 1 2 and this is achieved using the spherical projection lens 11. The frame scanning mirror 1 2 is oscillated by motors 1 3 and 14.

The emergent rays are reflected from the mirror 12, as represented by beam 1 5 to form a single line of the image. As the mirror 1 2 is moved successive lines of the image are projected to form the entire projected image.

Fig. 2 shows, in side view, the output end 10 of ribbon 8, the spherical lens 11, the mirror 1 2 and the reflected beam 1 5 as described above with reference to Fig. 1.

In the abovedescribed system, chromatic aberration in the lens 11 is sufficiently severe to make the projection of high definition colour images extremely difficult. In addition it is difficult to ensure that the ends of the fibres in the ribbon 8 are flat and perpendicular to the fibre axes, that the fibres axes are directed radially with respect to the lens 11, and that the fibre ends are at the correct distance from the lens. Thus even with monochromatic light it is difficult to project high quality images.

Fig. 3 shows an embodiment of the invention which seeks to overcome the above problems. In Fig. 3, a lens 16 comprises an upper part-spherical surface 17, a lower part-spherical surface 1 8 and intermediate part-spherical surfaces 1 9 and 20. All the surfaces 1 7 to 20 are centred on point 21, the general shape of the lens being that of two sectors connected by a waist about point 21. The surfaces 1 9 and 20 form boundaries between three regions of differing refractive index. As described below, a multi-layer lens arrangement of this general type enables lens performance to be optimised.

One end of a fibre optic ribbon comprising a row of optic fibres 22 bonded into a member 23 is secured by means not shown against the upper surface 1 7. For the sake of clarity only a few of the fibres 22 are shown.

Sandwiched between the fibres 22 and the surface 1 7 is an index matching medium 24 such as a liquid or an adhesive. The index matching medium optically couples the fibres to the lens and as a result the surface 1 8 is the only glass/air interface in the lens system.

Imperfections in the end faces of the fibres do not affect the optical quality of the system, and there is no optical discontinuity at the fibre/lens interface.

The narrow waist has to be large enough to accept each of the narrow cones of light emanating from the fibres 22. If this was not the case the waist would appear as an oblique aperture to the fibres at the ends of the member 23. The angle subtended by surface 1 8 must be equal to the angle subtended by the row of fibres 22 plus the divergence angle of the light emanating from the fibres 22.

The components shown in Fig. 3 can be used to replace the components 10 and 11 in Fig. 2. The overall operation of the image projection system is thereby improved.

Referring to Figs. 4 to 7, the principles involved in designing a single layer lens similar to the lens of Fig. 3 will be described.

Referring specifically to Fig. 4, a lens 25 is shown with a ray of light 26 emanating from a single fibre 27 and being refracted to a reflective screen 26. The fibre 27 is optically coupled to the lens by an index matching medium 29. The lens 25 is fabricated from a single piece of glass but is otherwise similar to the lens 1 6 of Fig. 3. The lens, fibre and index matching media all have the same refractive index n. The axis of the fibre 27 is indicated by line 30. There will of course be many fibres each having its own optical axis but only one is shown in the interests of clarity.

If we assume that the screen 28 is at a distance from the lens to give optimum focus to rays nearly parallel to the axis 30, then as the angle a between the ray 26 and the axis increases, the distance r between the axis 30 and the point of incidence of the ray on the screen 28 increases as shown in Fig. 5.

If the end of the fibre 27 is then moved along the axis 30 the rversus a relationship changes to that shown in Fig. 6. ris still zero when a is zero, but as a increases rpasses through a minimum at a= aO, returns to zero at a= a1, and then increases to r2 at a= a2.

With optical fibres, the intensity of light eventually decreases with increase in a. The greater the range of a over which r can be maintained within acceptable limits, the better the visibly apparent focussing. The quality of focus is indicated by the values rO, a0 and a2, a2 being the angle at which defocussing becomes unacceptable given the light intensity at this angle.

For example, in a flight simulator projector where light is projected onto a spherical screen by a reciprocating mirror where the effective screen distance is 180cm, it might be decided that defocussing must not exceed 1 mm, i.e. r2 = 1 mm. For a given position of the fibre relative to the lens, the maximum cone half angle of the fibres (i.e. angle a2) could then be derived. The intensity at a= a0 is of course much greater than at a= a2 and therefore in practice the tolerable maximum value of rio is considerably less than r2.

To enable optimum quality of focus to be achieved, the quality of focus parameters rO, a0 and a2 may be plotted against x, where x is the distance from the lens surface 31 to the fibre. A plot for a lens of index n = 1.5, radius (of surface 31) R = 1cm (i.e. focal length = nR/(n - 1) = 3) screen distance 180cm, and maximum defocus (r2) of 1 mm, is shown in Fig. 7. If we take the case where the lens to fibre distance x is 1 % larger than the focal length, the lens is operable from a= 0 up to a= 0.06 radian and could produce a spot radius r0 of 0.05mm if all light at an angle greater than 0.01 radian to the fibre axis were excluded.Any desired sharpness of focus (r0) may be selected and the limiting angles a0 and a2 read from the respective curves for the value of defocus 100 (x3)/3 at which the desired sharpness of focus r0 is obtained.

The above procedure may be carried out for any lens of the general type shown in Fig. 4.

It should be noted that smaller lenses exhibit more spherical aberration and thus provide greater potential for compromise between the quality of focus parameters.

The principles described above also apply to multi-layer lenses. For example Fig. 8 shows a two layer lens having region 32, index n, (equal to the index of fibre 33 and matching medium 34) and region 35, index n2 (n2 > n,).

The rversus a curve for the lens of Fig. 8 is shown in Fig. 9. Fig. 10 shows a three layer lens having region 36, index n1 (equal to the index of fibre 37 and matching medium 38), region 39 of index n2, and region 40 of index n3. The rversus a curve for the lens of Fig.

10 is shown in Fig. 11. Thus by increasing the number of layers of the lens, the maximum tolerable angle a2 can be extended, each added layer of the lens adding a further maximum or minimum to the curve. As before, the quality of focus parameters can be plotted and evaluated graphically. Each layer of the lens introduces an rO, a0 pair of parameters but this does not alter the principle of evaluation in any way.

It will be appreciated from the above that, for monochromatic light, a lens system as described can be obtained which is capable of meeting a wide range of desired performance characteristics.

When it is desired to project a full colour image, the simple relationships described above are no longer valid due to chromatic aberration. The focal length of a simple single layer lens is dependent upon the colour of light to be passed through it. Thus for a given lens system the effect of changing the colour of the light is similar to changing the fibre-tolens distance.

In Fig. 7 the quality of focus parameters are plotted as functions of x where x is the fibreto-lens distance. Changing the colour of light being passed through the system effectively shifts the abscissa value of x. Thus the same plot can be used to evaluate chromatic aberration.

For example, looking specifically at the plot of Fig. 7, there would be a refractive index change of i between red and blue light, so a defocus setting optimised for green light would be 2/3% too close for red light and 2/3% to distant for blue light. Spot radii of greater than 1 mm would result even for angles as small as 0.05 radian.

Lenses can be designed which give little chromatic aberration. For example a lens of the type shown in Fig. 8 and having a first layer of radius .448cm of a glass having an index of 1.5170 to green light and a second layer of radius 1cm of a glass having an index of 1.6725 to green light, provides a performance in blue light which is virtually the same for red light and very similar for green light. Unfortunately spot sizes of less than 1 mm diameter are hard to achieve with such lenses due to excessive spherical aberration.

This aberration is caused by the high curvature interface surface between the two layers of the lens. In the selected example glasses of very different refractive index were chosen to limit this problem but nevertheless the lens performance is still not adequate.

Fig. 1 2 shows the relationship between r and and a for the above-described achromatic lens. It will be seen that the spot size rises to a maximum, falls to zero and then rapidly increases to a negative excessive value.

This is the inverse of the relationship for the single layer lens as shown in Fig. 6. By achieving a compromise between the two designs the spherical aberration of the achromatic lens can be reduced at the cost of increasing the chromatic aberration relatively slightly.

In fact if the ratio of the radii of the layers of the achromatic lens is increased from 0.448 to 0.47, a particularly flat rversus a relationship can be obtained as shown in Fig. 1 3.

Thus a lens of the type shown in Fig. 8 comprising a first layer of radius 0.47cm, index to green light 1.5170, and a second layer of radius 1cm, index to green light 1.6725, has the following focal lengths.

Green light 2.42626cm Blue light 2.42451cm Red light 2.42865cm This lens can produce spot diameters of less than 0.2mm on a screen 1 80 cm distant for fibre output cone angles up to 0.08 radian when focussed for green light by selection of the optimum fibre to lens distance. With this same fibre to lens distance, the spot size increase by only about 50% for red or blue light. Thus the lens permits the projection of high definition full colour images.

The use of an index matching medium to couple the fibre optic ribbon to the lens results essentially in a lens system where the image represented by the end of any one fibre within the lens. There is no refractive index discontinuity between the fibre and the lens.

This greatly simplifies lens design.

In the described embodiments of the invention, only one fibre optic ribbon per lens is maintained. In fact however two or more fibre optic ribbons may be associated with a single lens. Such a double ribbon arrangement is useful where it is desired to project a composite image which is made up from two superimposed or adjacent images.

Claims (7)

1. A lens system for an image projector comprising a lens and a fibre optic ribbon one end of which is secured in a predetermined orientation relative to the lens, characteristic in that the prolongation of the axis of each fibre comprising the ribbon intersects the lens surface at normal incidence.

2. A lens system as in Claim 1, characterised in that the lens has a plurality of principal axes.

3. A lens system as in Claim 1 or 2, characterised in that the lens has optical spherical symmetry.

4. A lens system according to Claim 1, 2 or 3, characterised in that the lens and each fibre end in the ribbon are optically coupled together by an index matching medium such that light emanating from the fibre enters the lens without passing through an optical discontinuity.

5. A lens system according to any preceding claim, characterised in that the lens comprises sector-shaped portions bounded by spherical surfaces centred on a common point, one or more fibre optic ribbons being coupled to one or more of said spherical surfaces.

6. A lens system according to claim 3, characterised in that the lens is multi-layered to minimise spherical and/or chromatic aberration, the mating surfaces of adjacent layers being spherical and centred on a common point.

7. A lens system for an image projector substantially as hereinbefore described with reference to Fig. 3 to 1 3 of the accompanying drawings.