JPH0576013B2 - - Google Patents

Description

Claim 1: A visible light filter for a head-up display, comprising a display means for forming a display information image, and an optical system having a lens means and for directing the display information image toward the observer's eyes. , a single optical filter means is provided between the display means and the lens means in or near the focal plane of the rear focal length of the lens means, and the optical filter means reversibly responsive to focused visible light traveling back through the lens means toward the display means to block the focused visible light from converging between the lens means and the display means; A visible light filter for a head-up display, characterized in that it is transparent to display information except at local locations where the focused visible light is blocked. 2. A visible light filter for a head-up display as claimed in claim 1, wherein said optical filter means is constructed from a photochromic material. 3. A visible light filter for a head-up display as claimed in claim 1, wherein said optical filter means is a plate of photochromic material. 4. A visible light filter for a head-up display as claimed in claim 1, wherein said optical filter means is a prism optical element made of a photochromic material. 5. A visible light filter for a head-up display as claimed in claim 1, wherein said optical filter means is a transmissive liquid crystal detector matrix. TECHNICAL BACKGROUND The present invention relates to diffractive optical displays that filter out solar radiation. BACKGROUND OF THE INVENTION The present invention embodies a head-up display device for eliminating solar radiation that would wash out display information. For example, a head-up display (hereinafter referred to as HUD) of the type described in US Patent No. 3940204, "Optical Display System Using Holographic Lenses," published February 24, 1976 by RJ Withrington et al. The person views the scene through a holographic grating lens. Display information images from light sources such as cathode ray tubes located outside the line of sight within the airplane are combined or superimposed by optical systems including relay optics, folding reflectors, and holographic lenses. In operation, the optical system sends an image of the display information to the holographic lens where it is diffracted or reflected toward the observer's eye. and are combined in the observer's eyes. As a result of this combining function, the holographic lens is hereinafter referred to as a combiner. This type of head-up display is exposed to solar noise from solar radiation. When the sun is within a certain critical angle of incidence, the optical path is reversed and the display information on the cathode ray tube is erased. A number of solutions to this solar noise problem have been investigated, but most are effective only when the incident radiation exceeds a limited angle of incidence. For example, narrowband reflection or transmission filters. Typical narrowband equivalent filters are made of thin films of materials with high refractive indices, such as metals and dielectrics, and exhibit a narrowband pass with high transmittance. This type of filter preferably crosses the optical axis obliquely in front of the relay optics, and the cathode ray tube is placed behind the relay optics. In operation, this filter reduces the amount of solar radiation incident on the surface of the cathode ray tube at solar incidence angles above a predetermined range. Another solution is to use narrowband reflective mirrors, somewhat similar to diagonally inserted transmission filters, but operating in a reflection mode rather than a transmission mode. Such mirrors often have an absorbing glass layer in front of the reflective surface of the dichroic mirror. The disadvantage of this method is again that the effective angular range is limited. Other methods that have been investigated but have not yet been successful include mobile mechanical blocking devices or Venetian blind blocking elements. SUMMARY OF THE INVENTION The present invention relates to a head-up display solar filter that prevents incident solar radiation from erasing display information. For example, the optical system may have a photochromic element installed at or near the back focal plane of the optical system, and be configured so that the solar radiation that has passed through the optical path back toward the display device is focused on a minute position of the photochromic element. There is. For this reason, the photochromic element turns black at the focal point, and solar energy is blocked. The remaining areas of the solar filter that do not collect solar radiation allow low-intensity display information to pass through. Specifically, in a diffractive head-up display, the cathode ray tube display information travels forward along an optical path through a solar filter, a relay lens, a folding reflector, and is reflected from the reflector toward a combiner lens. The combiner lens has a diffraction grating that allows only narrow bandwidth light having the wavelength of the display information to be transmitted by diffraction. This diffraction image is directed toward the exit pupil for visual viewing. When solar radiation with a broad spectrum illuminates the other side of the combiner lens, the combiner diffracts out the components of the sunlight that are within the narrow bandwidth display information. However, the combiner transmits the remainder of the broad spectrum solar energy. If the sun is within the overhead angle, this remaining solar energy travels back down the optical path to the cathode ray tube and washes out the display information. However, the filter of the present invention prevents this removal. This filter is located at the back focal plane of the relay optic so that any solar radiation traveling back in the optical path is focused onto a very small spot on the filter. The photochromic material of the filter blackens at the focal point with high energy density, effectively preventing solar energy from reaching the surface of the cathode ray tube. This filter has the advantage of blocking solar energy incident on the head-up display at any angle. Since the angle of incidence of sunlight changes continuously, the focal point moves on the solar filter accordingly. The blocking state of solar energy depends on the degree of blackening of the photochromic material. Once the concentrated solar energy is removed from the predetermined filter area, the reversible photochromic material begins to glow again. It is another advantage of the present invention that the scotoma of the photochromic material is so small that it typically does not result in blocking display information. For example, an observer with two eyes will see the scotoma becoming a display point with at least one eye, so he will not lose sight of the display information. Furthermore, even if the scotoma obscures critical visual fields for a given eye, if the observer moves his or her head or eyes laterally relative to the visual axis, he or she may see previously blocked display information. be able to. Yet another advantage of the invention is that the solar filter element can be placed behind the relay optic along the optical axis without increasing the optical path length. Therefore, the overall dimensions of the head-up display can be kept short and can be kept within the limited space of an airplane cockpit.

[Brief explanation of the drawing]

FIG. 1 is a schematic side view of a head-up display, and the optical system shows the trajectory of light rays that converge the display information from the cathode ray tube (CRT) to the observer's eyes. FIG. 2 is a schematic side view of the system of FIG. 1, showing ray trajectories that focus solar radiation back toward display information on a CRT surface. FIG. 3 is a schematic side view showing a preferred embodiment of the HUD optical system in more detail. FIG. 4 shows a second embodiment of the solar filter.

[Detailed description of the invention]

Examples will be described in detail with reference to the drawings. 1st
The figure schematically shows a HUD optical system in which information displayed on an image plane, such as the surface of a cathode tube, is focused or directed towards an exit pupil, which can be considered a region of space. Hereinafter, the term ``above'' or ``front'' will refer to the direction of the light rays traveling from the cathode ray tube toward the exit pupil.''
The terms ``down'' or ``backward'' are used to refer to the opposite direction of movement toward the cathode ray tube. An observer, such as a pilot, positions his or her eyes within this exit pupil and views the image from the cathode ray tube 10. The cathode ray tube 10 includes a screen having a phosphor that produces an image of a narrow band of light centered at a wavelength λ ₀ of 543 nm. at the edges of the light spectrum. Although tilting the screen of cathode ray tube 10 does not form part of the present invention, tilting is commonly known to improve the performance of optical systems. The relatively low intensity display light is transmitted through a solar filter 12. In a preferred embodiment, the solar filter 12 is, for example, a photochromic material plate of the type described in detail with reference to FIG. is preferably located at the back focal plane of the relay optical system 14. The relay optical system receives the display information light transmitted through the solar filter.The relay optical system 14 partially eliminates the aberrations of the holographic lens system. The display light from the relay optical system travels toward the folding reflector 16. The folding reflector 16 serves to partially collect the intermediate image and shorten the total length of the optical system. The intermediate image received through the folding reflector 16 is reflected towards a coupler 18 located between the windscreen 20 and the observer's eye. The coupler 18 is a fluorographic diffractive optical lens that reflects a narrow bandwidth of light and transmits substantially all other wavelengths of light according to the well-known Bragg reflection law. The method is described in detail in US Patent No. 3940204 and will not be repeated here. In operation, coupler 18 reflects a narrow bandwidth of light from the display toward the exit pupil. In this way, the display image is , the observer uses windscreen 2
It is combined and superimposed with the scene in front of the vehicle seen through 0. Since the combiner 18 is tuned to reflect a preferred bandwidth of light centered at λ ₀ , the broadband portion of the light from the external scene is filtered out. Refer now to FIG. If the sun is within a predetermined critical angle, solar noise in the form of solar radiation having a wide spectrum is transmitted through the combiner 18;
The light travels backward through the optical system and reaches the surface of the cathode ray tube 10.
However, the solar filter 12 effectively blocks solar radiation, so that the solar radiation is transmitted to the cathode ray tube 10.
The display information on the screen will not be erased. In particular, the broad spectrum solar radiation that shines at the surface of the coupler 18 has a narrow bandwidth carved out by being reflected off the coupler 18. As previously mentioned, the reflection capability of coupler 18 is tuned to a bandwidth that includes wavelength λ ₀ . Therefore, this part of the solar radiation spectrum is reflected off the surface of the combiner 18 and the rest is transmitted back through the optical system to the folding reflector 16.
and reaches the solar filter 12 along the optical axis through the relay optical system 14. Of course, it is also possible that the angle of incidence of the solar radiation exceeds the criticality of the coupler 18 and still shines back through the optical path. The solar filter 12 is preferably arranged in the back focal plane of the relay optics 14. Solar energy is therefore concentrated to a point or spot within the solar filter 12. As a result, the photochromic material of the solar filter 12 darkens at the focal point, creating a dark area that effectively blocks solar energy. Otherwise, the display information would have been erased, but as a result, no solar energy can reach the cathode ray tube 10 surface. Furthermore, as the relative angle of the sun moves, the focal point of solar energy also moves. As a result, the dark spot moves through the solar filter at the rate of reaction of the photochromic material, and when the concentrated solar radiation is removed from the spot, it becomes brighter from its previous dark state at the rate of recovery of the photochromic material. Therefore, correction continues for the relative movement of solar energy within the entire range of critical solar angles. Of course, the scotoma image forms an image in front and is visible from the exit pupil as a spatial point that obstructs the display information. This spatial point is so small (eg, has a diameter of 0.08 inches) that an observer viewing the scene with both eyes can normally see around the spatial point. Furthermore, the spot image is at a critical viewing angle,
In cases where the ``comet tail'' effect caused by the finite recovery time of photochromic materials prevents the observer from correcting for subtle obstructions even with both eyes, the observer simply moves his or her head. If you position your eyes laterally to the optical axis, you can overlook the scotoma image. FIG. 3 shows in detail a preferred embodiment of a specially constructed optical system. Of course, it is possible to create variations of this configuration for other purposes, and the solar filter 12 is equivalently applicable. As previously mentioned, the image viewing surface 10 is preferably the surface of a cathode ray tube and includes phosphors that produce a very narrow bandwidth image centered at a wavelength of approximately 543 nm. Of course, other types of displays may be used to form the image. For example, a liquid crystal display or other display device may be used that produces light of sufficient brightness and a suitable wavelength that is collected by an optical system and diffracted toward the exit pupil. The housing 25 encloses the screen of the cathode ray tube 10, the solar filter 12, the relay optics 14, and the folding reflector 16, and has an opening for directing the intermediate image rays from the folding reflector towards the combiner 18. There is. Furthermore, the housing 25 holds the optical elements therein. The solar filter 12 consists of a plate of photochromic material that is substantially transparent to the display information image, such that the intensity of the narrow bandwidth display information is substantially greater than the intensity of the broad spectrum solar energy focused onto a spot within the solar filter 12. low. Although solar filter 12 is shown as a thin flat plate, it must have sufficient thickness and shape so that the solar radiation that passes through relay optics 14 and is concentrated behind it is concentrated in a small portion of solar filter 12. Must be. The solar filter 12 must therefore be located such that the focal point of the solar radiation is located within the material of the solar filter or at least in close proximity. Therefore, the back focal plane of the relay optic is a preferred location for positioning the solar filter. The material of the solar filter 12 is preferably photochromic glass, which changes its transmittance reversibly under the action of light. The main material of a particular glass is silicon glass, and the active substance can be a silver halide formed by crystallization from the glassy state during initial cooling or subsequent heat treatment. The silver content is less than 0.5%, and the crystal size is small compared to the wavelength of light, so scattering is not a problem. This type of photochromic glass
GKMegla’s “Exploitation of Photochromic”
Glass”, Optics and Laser Technology, April
1974, pp. 61-68. For special materials, the blackening time is relatively fast for high-intensity, broad-spectrum, concentrated solar radiation. Recovery time is also relatively fast in regions where the focused light has been removed to reduce the "comet tail" effect. Relay optical system 14 includes a series of lenses arranged along the optical axis. The set of lenses used is that disclosed with respect to FIG. 16 of US Patent No. 3940204. In particular, the relay lens 14 includes positive spherical lens elements 30, 32, 36, 4.
0, a negative spherical lens element 34, and a negative cylindrical lens element 38. Although negative cylindrical lens curvature is illustrated in the side view of FIG. 3, positive lenses are also within the concept of the present invention. Cylindrical lens 38 corrects the remaining axial astigmatism of coupler 18. Instead of this lens element 38, one or more positive spherical lenses of the relay optics 14 can be replaced by cylindrical surfaces. Axial coma is best corrected in the illustrated arrangement by off-centering the negative spherical lens. Axial coma is corrected by off-centering any suitable lens of relay optic 14. The folding reflector 16 is a prism that partially corrects coma in the axial direction. However, this is not its main purpose. Prism 16 is a wedge with a mirror on the back. The rays forming the top of the intermediate image pass laterally through a thicker portion of the glass wedge than the rays forming the bottom of the image. Therefore, the tilt of the focal plane of the intermediate image is considerably reduced, and the tilt of the object plane is also reduced. The final tilt of the object plane in the configuration shown is 15°,
If no prism is used it will be more than 30°. The intermediate image rays pass from prism 16 to combiner 18 . The coupler 18 has a grating pattern constructed by means of fluorographic optics, such as that disclosed in US Patent No. 3,940,204. Coupler 18 has a layer 40 of a photosensitive organic material, such as dichromate gelatin or photographic emulsion, with a diffraction grating pattern recorded on its surface. A detailed discussion of such materials and holographic recording can be found in Optical Holography by Robert J. Collier et al., New York:
Academic Press, 1971), p.293 onwards. This thin layer 40 is provided for added structural strength and is sandwiched between two glass layers 42, 44 that protect against physical impact. Coupler 18 has a generally spherical or concave surface so that an intermediate image of display information is reflected from the concave surface toward the exit pupil. Functionally, the grating pattern diffracts or reflects light in a narrow band with high optical efficiency according to the well-known Bragg diffraction principle. However, because the coupler 18 has a high transmittance to light other than this narrow band of reflected light, the pilot can view the external scene while viewing the reflected display information. Thus, the display information appears combined and superimposed with the external scene. Furthermore, the spherical shape of the coupler 18 is such that it is compatible with other optical elements of the optical system so that axial coma or axial astigmatism of the display information image reflected into the exit pupil is corrected. are doing. The coupler 18 is described as having a reflective diffraction pattern, but in 1975 EFOpitek et al.
It is possible to use a transmission type that redirects light waves using the method described in US Patent No. 3915548, published October 28th. As previously mentioned, by placing the solar filter 12 between the cathode ray tube 10 and the relay lens 14, the optical axis length can be shortened, and the optical system can therefore be made more compact. As a result, the optical system can be kept outside the exit line indicated by the dashed line in FIG. This eliminates certain spatial constraints associated with cramped cockpits. Although solar filter 12 has been described as a photochromic type material, it may be constructed from other types of materials. It can also be made from a sensor type device, for example a transmissive liquid crystal photodetector matrix, either of the absorptive or reflective type. If it is a reflective type, the direction of reflection is away from the normal optical path. An example of a transmissive liquid crystal device was published in TDBeard et al. on July 10, 1973.
Disclosed in US Patent No. 3744879 and US Patent No. 3811180 issued May 21, 1974 to M. Braunstein. In FIG. 4, the filter 12 is configured in a shape different from that of a flat plate. Here solar filter 1
2 is constructed as an optical prism of photochromic material. The solar energy that travels backwards and is concentrated through the relay optical system 14 is sent to the optical prism 50.
Since it is concentrated as a spot, it darkens a small volume of material within the prism, thus blocking solar radiation and preventing it from impinging on the image surface 10 containing the display information, erasing the image. or otherwise affect the statue. Although the invention has been described with reference to particular embodiments, various modifications that will be obvious to those skilled in the art are possible without departing from the invention.