JPH0488312A - Video superposing device - Google Patents

Abstract

PURPOSE:To increase the width of a display angle by forming both 1st and 2nd diffraction elements of holograms multiple-recorded with plural pieces of plane diffraction gratings. CONSTITUTION:The 1st and 2nd diffraction elements 12A, 12B are respectively constituted of the elements multiple-recorded with the transmission type total reflection holograms. Then, the luminous fluxes of video information emitted from the point P1 of the display surface of a display means 4 are advanced like solid lines by the 1st holograms to the point Q1 and the luminous fluxes of the video information from the point P2 are advanced in the route of dotted lines by the 2nd holograms. These luminous fluxes respectively form virtual images at the point Q2. The videos superposed in such a manner are divergent light to an observer and, therefore, the range of eyes' positions is widened and the degree of freedom is increased. Namely, the display angle range is increased.

Description

[Detailed Description of the Invention] [Summary] Regarding a video superimposing device that superimposes and displays second video information on scenery, video, etc. within the field of view, the present invention aims to increase the display angle width in the video superimposing device, an optical element that converts the information image from the display means into parallel light; a first diffraction element that diffracts the parallel light; and a first diffraction element that diffracts the parallel light at the end of the transparent parallel plate that changes the propagation direction by reflection or diffraction; a deflection means formed with a second diffraction element that diffracts and extracts the propagated diffracted light of the first diffraction element; In an image superimposing device sandwiched between a focusing means for focusing a light beam of a background image, and a diverging means for superimposing and extracting the information image extracted from the second diffraction element and the background image from the focusing means, Both the first and second diffraction elements are constructed from multiple recording holograms or reflection holograms.

[Industrial application field]

The present invention relates to a video superimposing device that superimposes and displays second video information on scenery, video, etc. within a field of view.

Displaying secondary video information superimposed on scenery, images, etc. within the field of view has great utility value in that it allows more information to be recognized almost simultaneously without a large movement of the line of sight, and has traditionally been used in combat. It has been put into practical use as a head-up display for aircraft cockpits. Also,
Recently, application to automobiles has been considered in order to improve maneuverability, safety, and convenience by making it possible to visually obtain control and driving information without taking your eyes off the field of view when maneuvering or driving. ing.

[Conventional technology]

Conventionally proposed image superimposition methods use flat or curved optical elements (with appropriately set reflectance and transmittance).
(referred to as an image combiner) is used. For example, as shown in Fig. 15, a reflective element image combiner such as a half mirror 1 is used to superimpose the background and the displayed image at an appropriate intensity ratio, or, although not particularly shown, a wavelength-selective image combiner is used. There is a combination of a reflection hologram and a display device with a narrow wavelength band. Although the former is simple, it is necessary to increase the reflectance of the combiner in order to make the display easier to see, which has the disadvantage that the transmittance decreases and the background becomes dark. Also, the latter is
Although this is an excellent method in terms of the amount of light, it requires the use of a display device with a narrow wavelength band and high brightness, otherwise there will be no advantage in terms of display brightness. However, display devices with a narrow wavelength band and high brightness are generally expensive, and although they are acceptable in terms of price for fighter jets, aircraft, etc., they are not acceptable for automobiles and consumer products.

Head-up displays for fighter jets and aircraft are required to keep distortion to an extremely low level in the displayed images, and strict accuracy conditions are required of lens systems and combiners for this purpose.

However, depending on the use of the video superimposing device, some distortions may be tolerated without requiring such high accuracy. Therefore, there has been a need for a simple and inexpensive image combiner and video superimposing device.

An example of such a simple video superimposition device (J, Apatn
ieks, 5PIE Proc, Vol, 883.
pp. 171-176, 1988) is shown in FIG. As shown in FIG. 17, transmission holograms 2A2B (a hologram medium 2a formed on a substrate 2b) that diffracts incident light at a critical angle or more are formed on a portion of both surfaces of a transparent parallel plate 3. [: The image of RT4 is converted into parallel light by the collimator lens 5, the image is propagated by total reflection inside the transparent parallel plate 3 via the first hologram 2A, and the image is taken out by the second hologram 2B. Most of the background light is transmitted except for light rays having an incident angle near the Black angle of the second hologram 2B. Diffraction elements such as holograms have wavelength dispersion, but when a pair of plane grating holograms 2A and 2B with the same spatial frequency are formed on a parallel plate 3, as shown in FIG. The difference is compensated by the second hologram 2B, and finally the light beam has exactly the same direction as before entering the hologram system, and no chromatic aberration occurs.

[Problem to be solved by the invention]

Problems with the conventional example shown in Figures 16 to 18 are that since the beam that enters the observer's eye is parallel light, the position of the virtual image is limited to infinity; The position (range) is limited to an extremely narrow area, and in order to make the image visible with both eyes,
The collimator lens 5 and the holograms 2A and 2B are required to have a diameter that is at least the distance between the two eyes, and the farther the eye position is from the combiner (parallel plate 3), the more severely the eye position is restricted. It will be done. If these shortcomings are resolved, the method will have even more applications.

Therefore, the applicant has proposed at least two gratings (holograms > 2
In a device that enables image superimposition using a transparent parallel plate 3 on which A' and 2B' are formed, a grating (second grating 2) for extracting light from the parallel plate is used.
I would like to devise a structure in which the region in which B') is formed is sandwiched between means 6 having a concave lens function (on the observer's side) and means 7 having a convex lens function (on the opposite side) (FIG. 8). With this configuration, the superimposed image (virtual image) can be displayed at a point Q at a finite distance from the viewer, and the light directed toward the viewer becomes a diverging wave. The range of eye positions from which images can be seen can be expanded. Since the background passes through both the concave and convex lenses 6 and 7, by appropriately selecting the focal length of both lenses,
It is possible to cancel out the actual power and make it appear as if it were simply seen through a transparent plate.

Note that 8 is an antireflection film.

Through the above, the drawbacks of the conventional method can be improved, but
In terms of widening the display area (the vertical display area of the CRT 4 in FIG. 8), the sharp angle selectivity of the transmission holograms 2A' and 2B'
subject to limitations. The angle selectivity of a total reflection hologram as shown in FIG. 17 depends on the film thickness of the hologram and the degree of refractive index modulation, but as shown in FIG. 9, the angle selectivity is about 2 degrees in full width at half maximum. That is, looking at the display surface from the center of the collimator lens 5,
This means that only a 2 degree height display can be used. Therefore,
This video superimposition device is not useful if a wide area is required.

An object of the present invention is to increase the display angle width in the above-mentioned video superimposing device.

[Means to solve the problem]

In order to achieve the above object, according to the present invention, an optical element that converts an information image from a display means into parallel light, and an end portion of a transparent parallel plate that changes the propagation direction by reflection, diffract the parallel light. a first diffraction element, and a deflection means formed with a second diffraction element that diffracts and extracts the diffracted light of the first diffraction element propagated by the transparent parallel plate; A second diffraction element is used as a focusing means for converging the light beam of the background image, and a diverging means for superimposing the information image extracted from the second diffraction element and the background image from the focusing means. and a video image superimposition device that extracts the background image light beam that passes through the deflection means and both the focusing means and the divergence means together with the information image light flux that passes through the deflection means and either the focus means or the divergence means. There is provided an image superimposing device characterized in that both the first and second diffraction elements are formed of holograms in which a plurality of planar diffraction gratings are multiplexed.

According to the second aspect of the present invention, an optical element that converts an information image from a display means into parallel light, and a first diffraction device that diffracts the parallel light at the end of a transparent parallel plate that changes the propagation direction by reflection. element, and a deflection means formed with a second diffraction element that diffracts and extracts the diffracted light of the first diffraction element propagated by the transparent parallel plate, and together with the transparent parallel plate, the second
The diffraction element is held between a focusing means for focusing the light beam of the background image, and a diverging means for superimposing and extracting the forward rotation information image extracted from the second diffraction element and the background image from the focusing means. and extracts the background image light beam that passes through both the deflection means and the focusing means and the divergence means together with the information image light flux that passes through the deflection means and either the focus means or the divergence means,
Both the first and second diffraction elements are formed by reflection holograms.

[For production]

As shown in FIG. 1A, the information image on the display means 4 is converted into parallel light by the conversion means (collimator lens, etc.) 5, and the parallel light is diffracted by the first diffraction element 12A and transmitted to the transparent parallel plate 3. incident. The propagation direction changes within the transparent parallel plate 3 due to reflection, and the light is diffracted by the second diffraction element 12B and output.

The eight emitted lights (information images) at this time are parallel lights, but due to the diverging means 6, they become divergent lights for the observer 9.

Therefore, the information image has a finite virtual image position Q (QIQ,)
will be confirmed. Further, the background image is recognized at the same size through the focusing means 17 and the diverging means 18, and the information image is superimposed on this background image.

Each of the first and second diffraction elements 12A and 12B is constituted by an element in which transmission type total reflection holograms are recorded multiplexed.

Therefore, the point P1 on the display surface of the display means 4 (FIG. 1A,
The light beam of the information image emitted from the point P2 advances as shown by the solid line by, for example, the first hologram H1 of the multiplex recording holograms 12A and 12B, and the light beam of the information image from the point P2 travels to the second hologram. H2, the robot travels along the dotted line path and forms a virtual image at point Q2. In this way, since the superimposed image is of diverging light for the observer, it is possible to widen the range of eye positions and increase the degree of freedom, that is, to enlarge the display angle range.

That is, as shown in FIG. 10, there is a plane grating hologram (Hυ) that diffracts a light beam at an incident angle θ1 (in air) at an angle θ2 (inside a transparent substrate), and a light beam at an incident angle 01' (in air) is diffracted at an angle 02. Holograms (12A, 12
B) of the optical system shown in Figure 8.
Used as 2A' and 2B'. Angle θ1 and 01'
By appropriately setting , the multiplexed planar gratings function independently, making it possible to display a 2 degree height display in two rows, ie, a 4 degree display. The display area can be expanded by increasing the number of multiplexed grids.

Further, in the second invention, a reflection hologram element is used instead of a transmission type total reflection hologram as a means to solve the problem of a narrow display angle width. As shown in FIG. 3, on the opposite side of the transparent flat plate 3 from the collimator means 5, there is a first diffraction element 22 for guiding light into the transparent flat plate.
A (reflection type hologram) is formed, and the propagating light is transmitted to the transparent flat plate 3.
A second diffraction element 22B (reflection type hologram) to be taken out from the transparent plate 3 is formed on either side of the transparent flat plate 3. As a result, the light beam emitted from the point P on the display surface of the display means 4 is extracted from the transparent flat plate as a parallel light beam. Next, the parallel beam is converted into a diverging wave by the diverging means 18 having a concave lens function, and a virtual image is formed on the opposite side Ω of the transparent flat plate.

Since a reflection hologram has weaker angle selectivity than a transmission hologram, it is possible to propagate light over a wide display area using a transparent flat plate. By the way, in the case of a transmission type hologram, the angle range in which light is diffracted is about 2 degrees (full width at half maximum) as mentioned above, but in the case of a reflection type hologram, it is about 1 degree.
Wide range from 0 to 15 degrees.

The focusing means 17 having a convex lens function is arranged at a distance t between the converging means 17 having a convex lens function and the diverging means 18 having a concave lens function so as to cancel out the power as described above.

The reflection hologram has wavelength selectivity as shown in FIG. It has a high reflectance only in a specific wavelength range, and is almost transparent in other wavelength ranges (the figure includes data that includes surface reflection).

Therefore, the width that can be used as display light is 20 to 3Q.
It can be said that the wavelength is only nm. Since the emission band of general display means is wider than 20 to 30 nm, it is difficult to use display light effectively, but even if there is some light loss, if there is sufficient brightness in the end, it can be used as a display device. Very useful. The feature of the second invention can be said to be that the display area is expanded by utilizing the weak angle selectivity of the reflection hologram.

〔Example〕

First, a first embodiment of the present invention will be described.

The angular selectivity of diffraction efficiency in the multiple recording hologram shown in FIG. 10 is such that if the difference in Bragg angle of each diffraction grating is approximately 4 degrees or more, each grating functions almost independently; [R, Alfernes
s and S, K, Ca5e, J, opt,
Soc, AmVol, 65. No, 6. P,
730 (1975) (7) 1 sentence). A solid line 1 represents diffraction by the first hologram H1, and a broken line represents diffraction by the second hologram H2. Each of the curves exhibits characteristics that are approximately the same as the curves for the single diffraction grating shown in FIG. FIG. 12 shows different positions P, , P on the display surface of the display means 4 in the vertical direction.
, is converted into a parallel light beam by the collimator means 5, and taken into the transparent flat plate 3 by the multiple recording hologram 12A (only the optical axis is shown). In reality, P+ and P2 are lines extending perpendicular to the plane of the paper (FIG. 18). Therefore, when the information image is a character, it corresponds to a line.

When the difference in Bragg angles in multiple recording holograms is smaller than 4 degrees, an effect occurs in which incident light is diffracted by both gratings (cross-coupling effect). 1st
Figure 3 shows the situation. Although θ is the Bragg angle of Hl, it is also partially diffracted by H2. Also, θ1' is H
The Bragg angle is 2, but it is also partially diffracted by Hl. In such cases, ghosts occur. 14th
As shown in the figure, the light emitted from one point on the display surface is split into a beam propagating in two directions by the first multiplex recording hologram 12A, and then by the second multiplex recording hologram 12B, which is the part that takes out the light from the transparent flat plate 3. , each luminous flux is further divided into 2
divided into. When a virtual image is formed by the diverging means 18 having a concave lens function (FIG. 1A), these do not converge to the - point. Therefore, if you want to superimpose clear images, set the Bragg angle of multiple holograms to 4
It is preferable to design them so that they differ by more than a degree.

FIG. 1A shows the overall configuration of an optical system of an image superimposing device according to the present invention. As the means 18 having a concave lens function and the means 17 having a convex lens function, for example, a Fresnel concave lens or a Fresnel convex lens can be used. Display 4
After being collimated by the collimator lens 5, the light is diffracted by the first multiplex recording hologram 12A (H,) by a critical angle or more, propagates through the parallel plate 3 by total reflection, and is transmitted to the second multiplex recording hologram 12A (H,). It is taken out into the air again by hologram 12B (H2). The traveling direction of the light beam at this time is the same as that immediately before entering the first multiple recording hologram 12A. Next, the same light beam is converted into a diverging wave by the concave lens 18, but virtual images Q+ and Q2 are formed on the opposite side of the parallel plate to the direction in which the light travels. images can be seen within the field of view. Since it is a diverging wave, you can see the image even if you slightly change the position of your eyes. Due to the canceling effect of the combination of concave and convex lenses 18 and 17, light from the background passes through the transparent curved plate of equal thickness and has almost no optical power. Therefore, a normal field of view can be obtained without changing the magnification. Note that anti-reflection means 8 are provided on the side surfaces of the parallel plate 3 to prevent part of the totally reflected propagating light from being reflected or scattered on the side surfaces and becoming noise light. Specifically, a known technique such as applying a light-absorbing substance may be used.

FIG. 2 shows the external appearance of an image superimposing device created using the optical system shown in FIG. 12, in which the display 4 and collimator system 5 are housed in a lower box (main body) 50. 1st
, the transparent parallel plate 3 on which the second holograms Hr and H2 are formed, and the concave-convex Fresnel lens 1817 are in an upright shape.

In the above embodiment, the first multiplex recording hologram 1
2A (Hl) and the second multiple recording hologram 12B (
H2) are formed on different sides of parallel substrates, but by forming them on the same side, the display 4 and collimator system 5 can be placed on the same side of the image combiner 3 as the observer. can.

As the display means 4, in addition to CRT, a fluorescent display tube with high brightness, a light emitting diode (LED), etc. are advantageous, but it is also possible to use a liquid crystal display or the like.

The collimator means 5 is preferably a lens or lens system with good off-axis performance. Although the display on the display surface near the optical axis of the lens system is imaged without distortion, if the off-axis performance of the lens system is poor, distortion occurs at the periphery of the display. Select a collimator lens depending on the quality required for the image to be displayed.

Next, a second embodiment of the present invention will be described.

As described above, the main feature of the second invention is that the reflection holograms 22A and 22B are used as the first and second diffraction means.

The method of creating a reflection hologram itself is well known.

If the wavelength band used for display is a long wavelength (red), it can be created by irradiating a short wavelength laser from both sides of the hologram recording medium 12H at an appropriate angle and causing interference. However, when displaying green or blue, it is not possible to create a hologram in which the diffracted light propagates through total internal reflection within the transparent flat plate 3 using the above method. In fact, considering human visibility, the maximum value is around 540 nm (green), so green is preferable as the display color. Therefore, when creating a reflection hologram, the prism may be optically brought into close contact with the substrate surface, and one side of the hologram creation light may be irradiated at a large incident angle. The method itself is a well-known technique and therefore will not be described in detail. The reflection type hologram may be formed directly on the transparent flat plate 3 used for light propagation, but it is also created on a separate thin substrate 12b (Fig. 8) and adhered to the transparent flat plate (with a photocurable adhesive). You can take other methods. The same is true for the second reflection hologram that takes out the light beam outside the transparent flat plate 3.

As a specific example of the elements 17 and 18 having concave and convex lens functions, plano-concave and plano-convex lenses may be used, but if the thickness of the lens impairs the compactness and lightness of the device, it is possible to use plastic as described above. A Fresnel concave and convex lens (15ED) may be used. Fresnel lenses have concentric lines, but since they are not in the focal plane of the person viewing the image, they are often not noticeable. Concave and convex lenses 17.18
may be adhered to the transparent flat plate 3, but before the light beam propagating through total internal reflection within the transparent flat plate is diffracted by the second reflective hologram 22B, it is unified in the air by the concave and convex lenses adhered to it. In the case of partial emission, a method is used in which a slight air layer is provided using a spacer or the like and the concave and convex lenses are fixed in a state that does not impede total reflection.

In FIG. 3, the light from the display 4 is collimated by the collimator lens 5, then taken into the transparent flat plate 3 by the first reflection hologram 22A, and propagated by total reflection.
It is taken out of the transparent flat plate by the second reflection type hologram 22B. Next, the light beam is converted into a diverging wave by the concave lens 18, but a virtual image Q is formed on the opposite side of the parallel plate 3 to the direction in which the light travels. can be seen within the field of view. Even if the position of the t1 eye is slightly changed due to the diverging wave, the image can be seen. Light from the background passes through the transparent curved plate of equal thickness due to the canceling effect of the combination of concave and convex lenses 17 and 18, and has almost no optical power. Therefore, a normal field of view can be obtained without changing the magnification. Furthermore, the brightness of the background is such that light with a wavelength bandwidth of 20 to 3 Qnrn is missing due to the reflection hologram, and there is a slight tint (complementary color), but the brightness does not change visually. Note that anti-reflection means 8 are provided on the side surfaces of the parallel plate 3 in the same manner as in the case of FIG. 1A to prevent part of the totally reflected propagating light from being reflected or scattered by the side surfaces and becoming noise light.

Incidentally, the appearance of the image superimposing device created using the optical system shown in FIG. 3 is exactly the same as that shown in FIG. 2.

In the above embodiment, the first hologram 22A and the second hologram 22B are formed on different surfaces of the parallel substrate 3, but by forming them on the same side, the display and collimator system can be It is also possible to arrange the image combiner on the viewer's side.

Further, the holograms 22A and 22B may be multiple holograms as in the case of FIG. 1A.

In that case, color display may be performed.

As mentioned above, Figure 4 shows the wavelength selectivity of the reflection hologram, and as is well known, the value of the wavelength band (peak wavelength) where the reflectance is high can be determined by changing the method of making the reflection hologram. It can be changed by Therefore,
For example, as shown in Figure 4, a reflection hologram whose reflectance is maximum at 530 nm and a reflection hologram whose reflectance is maximum at 630 nm is multiple recorded, or two types of reflection holograms are bonded together. By using these as the reflection holograms 22A and 22B shown in FIG. 3, green and red light can be displayed. Furthermore, 4
Color display (R
GB display) is also possible.

Figures 6 and 7 show two colors (585nm, 65nm), respectively.
5nm), 3 colors (445nm, 530nm, 63
0 nm) is a graph showing wavelength selection characteristics of a reflection hologram created for display.

In the present invention, a "multiple recording hologram" refers not only to a case in which a plurality of holograms are recorded in the same hologram medium layer, but also to a case in which holograms are separately formed in each hologram medium and stacked in multiple layers. Including things.

〔Effect of the invention〕

The configuration described above increases the degree of freedom regarding the position of the eyes. Even when viewing with both eyes, since the light emitted from the virtual image is diverging light, it is easy to place both eyes within the virtual cone. It also becomes possible to display images at a finite distance from the viewer. Although concave and convex lens elements are added, if a plastic Fresnel lens is used, the device can be made inexpensive. and,
The display area can be expanded by using a multiple recording hologram or a reflection hologram.

[Brief explanation of drawings]

FIG. 1A is a diagram showing the basic configuration of the video superimposing device according to the present invention, FIG. 1B is a diagram showing the display position on the display surface of the display, and FIG. 2 is an external view of the video superimposing device shown in FIG. FIG. 3 is a diagram showing the basic configuration of the second invention, FIG. 4 is a diagram showing the efficiency characteristics of a reflection hologram, FIG. 5 is a diagram showing an embodiment using a Fresnel convex-concave lens, and FIG. and FIG. 7 are diagrams showing the efficiency characteristics of a reflective hologram in the case of two-color and three-color display, respectively, and FIG. 8 is a diagram showing the schematic configuration of the image superimposition device proposed by the applicant in the earlier application. Figure 9 is a diagram showing the incidence angle dependence of diffraction efficiency in a transmission hologram, Figure 10 is a diagram explaining the expansion of the selected incident angle by a multiple recording hologram, and Figure 11 is a diagram showing the incidence angle dependence of diffraction efficiency in a multiple recording hologram. A diagram showing the angle dependence, Figure 12 is a diagram explaining the expansion of the display area when multiple recording holograms are used, and Figure 13 is a diagram showing the effect when the Bragg angles of each hologram in multiple holograms are too close. Diagrams for explaining, FIG. 14 is a diagram for explaining the occurrence of ghosts due to cross-coupling, FIG. 15 is a diagram for explaining a conventional video superimposing method, and FIG. 16 is an example of another conventional video superimposing device. 17 is a diagram for explaining the characteristics of a transmission hologram, and FIG. 18 is a diagram for explaining compensation for wavelength dispersion by a hologram pair. 3...Transparent parallel plate, 4...Display device, 12A,
12B... Diffraction grating, 17... Focusing means, 18...
・Variation means. Image superimposition device Fig. 1A Display position on the display surface Fig. 1B Efficiency characteristics of reflection hologram Fig. 4 Example of implementation Figure Hata Fig. 500 °C Wavelength (nm) 7 mark Efficiency characteristics of reflection hologram (2 colors) Fig. 6 Efficiency characteristics of reflection hologram (3 colors) Fig. 8J, a configuration proposed in a prior application that improves the drawbacks of the conventional example
3J Incident angle θ Incident angle dependence of diffraction efficiency in transmission hologram Figure 10 W! Figure 111 Image combiner Conventional video superimposition method Figure 152 Figure 16

Claims (1)

[Claims] 1. An optical element that converts the information image from the display means (4) into parallel light, and a transparent parallel plate (3) that changes the propagation direction by reflection.
) is provided with a first diffraction element (1) that diffracts the parallel light.
2A), and a deflection means formed with a second diffraction element (12B) that diffracts and extracts the diffracted light of the first diffraction element (12A) propagated by the transparent parallel plate (3), The second diffraction element (1) together with the transparent parallel plate (3)
2B) is a focusing means (
17) and a diverging means (18) for superimposing and extracting the information image extracted from the second diffraction element (12B) and the background image from the focusing means (17),
In the image superimposing device for extracting the light beam of the background image that passes through both the deflection means and the focusing means and the diverging means, together with the light flux of the information image passing through the deflecting means and either the focusing means or the diverging means, 1. An image superimposing device characterized in that both the second diffraction elements are formed by holograms (H_1, H_2) in which a plurality of plane diffraction gratings are multiplex recorded. 2. An optical element that converts the information image from the display means (4) into parallel light, and a transparent parallel plate (3) that changes the propagation direction by reflection.
) is provided with a first diffraction element (1) that diffracts the parallel light.
2A), and a deflection means formed with a second diffraction element (12B) that diffracts and extracts the diffracted light of the first diffraction element (12A) propagated by the transparent parallel plate (3), The second diffraction element (1) together with the transparent parallel plate (3)
2B) is a focusing means (
17) and a diverging means (18) for superimposing and extracting the information image extracted from the second diffraction element (12B) and the background image from the focusing means (17),
In the image superimposing device for extracting the light beam of the background image that passes through both the deflection means and the focusing means and the diverging means, together with the light flux of the information image passing through the deflecting means and either the focusing means or the diverging means, 1. An image superimposing device characterized in that both the second diffraction elements are formed of reflection holograms. 3. The image superimposing device according to claim 1 or 2, wherein the diverging means is formed by a concave lens made of glass or plastic. 4. The image superimposing device according to claim 1 or 2, wherein the focusing means is formed by a convex lens made of glass or plastic. 5. The image superimposing device according to claim 2, wherein both the first and second diffraction elements are formed by a hologram in which a plurality of reflection holograms are multiplexed.