Holographic display element

The invention relates to a holographic display element which includes a reflection hologram and is attached to, for example, an automobile windshield so as to project light rays carrying images of display information onto the display element from a luminous source in the car. When external light such as sunlight coming is diffracted by the reflection hologram there is a possibility that visible light ranging from 570 nm (greenish yellow) to 780 nm (red) is diffracted into a certain range of angles in a horizontal plane and makes a strange impression on persons viewing the windshield from the outside. According to the invention another reflection hologram having different diffraction characteristics is superposed on the first hologram on the outer side in order that diffraction of visible light into the aforementioned range of angles may become inconspicuous. In one embodiment the second hologram diffracts light of 570-780 nm at angles outside the aforementioned range of angles thereby to prevent undesirable diffraction by the first hologram. In another embodiment the second hologram diffracts different wavelengths into the range of angles so that additive color mixing with visible light diffracted from the first hologram results in whitening of diffracted light or reduction in the excitation purity of diffracted light as viewed outside the vehicle.

BACKGROUND OF THE INVENTION 
This invention relates to a holographic display element which includes a 
reflection hologram onto which light rays carrying images of display 
information are to be projected from a luminous image source. The 
holographic display element is useful, for example, as the combiner of a 
head-up display system in an automobile or as a display in a building 
window. 
Holographic head-up display systems are already in practical use in 
aircraft cockpits. Recently efforts have been directed to the development 
of holographic head-up display systems for automobiles since holographic 
displays have various merits such as large freedom of layout, highness of 
wavelength selectivity and possibility of affording lens characteristics. 
In most of hitherto developed or proposed head-up display systems for 
automobiles a reflection hologram formed on a transparent substrate is 
incorporated in the windshield, and light rays carrying images of display 
information are projected onto the hologram from a luminous display such 
as a cathode-ray tube positioned beneath the windshield. The driver or 
another observer on the vehicle can view the images of display information 
reflected by the hologram while viewing the forward outside real world 
through the windshield. 
However, there is a problem about diffraction of external light such as 
sunlight by the hologram in the windshield. As external light such as 
sunlight is incident on the hologram from the outboard side of the 
windshield the hologram diffracts the incident light into various 
wavelengths of light at various angles. When wavelengths in the range from 
560 nm (greenish yellow) to 780 nm (red) are diffracted into a certain 
range of angle with the road surface, the diffracted light is visible to 
persons viewing the windshield from the outside, such as pedestrians and 
drivers or passengers on cars running in the opposite direction, as a 
glaring and uncomfortable color and hence gives them a strange impression. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a holographic display 
element of the above described category, which is improved so that the 
diffraction of external light such as sunlight by the display element may 
not make a strange or uncomfortable impression on persons viewing the 
article incorporating the display element from outside the vehicle, i.e. 
the side opposite to the luminous source for holographic displaying. 
According to the invention a holographic display element using a reflection 
hologram having the above described diffraction characteristics is 
improved by adding another reflection hologram having different 
diffraction characteristics. The additional (second) reflection hologram 
is superposed on the usual (first) reflection hologram on the side 
opposite to the luminous source for holographic displaying, and the second 
hologram has such diffraction characteristics that, when collimated 
external light is incident on the display element from the side opposite 
to the luminous source and diffracted by the display element, diffraction 
of light in the visible region of wavelength into a predetermined range of 
angles in a predetermined plane such as, for example, a horizontal plane 
becomes inconspicuous to a person viewing the display element from the 
outside. 
More definitely, the present invention provides a holographic display 
element attached to a transparent plate member for projecting light rays 
carrying images of display information onto the display element from a 
luminous image source positioned on a predetermined first side of the 
display element, the display element comprising a first reflection 
hologram onto which said light rays are to be projected and a second 
reflection hologram which is superposed on the first hologram on the side 
opposite to the first side and is different from the first hologram in 
diffraction characteristics. The first hologram has such diffraction 
characteristics that, when collimated external light such as sunlight is 
incident thereon from the second side opposite to said first side and 
diffracted, visible light such as light of wavelengths in the range from 
about 570 nm to about 780 nm is diffracted into a predetermined range of 
angles in a predetermined horizontal plane. 
In a preferred embodiment of the invention, the second reflection hologram 
has such diffraction characteristics that, when collimated external light 
is incident on the display element from said second side under such 
conditions that the first hologram would diffract the external light as 
visible light of wavelengths in the range from about 570 nm to about 780 
nm into the predetermined range of angles if the second hologram were 
absent, the second hologram diffracts visible light of wavelengths in the 
570-780 nm range at angles outside the predetermined range of angle and 
diffracts light of wavelengths shorter than about 570 nm into that range 
of angles. 
In another preferred embodiment of the invention the second reflection 
hologram has such diffraction characteristics that, when external light is 
incident on the display element from said second side under such 
conditions that the first hologram diffracts the external light as visible 
light into the predetermined range of angles, the second hologram 
diffracts visible light of different wavelengths into the predetermined 
range of angles so as to reduce excitation purity of color appearing in 
the predetermined range of angles by additive color mixing of light 
diffracted by the first and second holograms. 
In the present invention each of the first and second reflection holograms 
is usually a hologram sheet essentially consisting of a transparent 
substrate and a record layer in which a pattern of interference fringes is 
formed. In either of the above stated first and second embodiments the 
second reflection hologram may be placed directly on the first reflection 
hologram or superposed on the first hologram with interposition of at 
least one transparent film between the two holograms. 
In the above stated second embodiment it is also possible to integrate the 
first and second reflection holograms into a single hologram sheet by 
forming a pattern of interference fringes and another pattern of 
interference fringes in a single record layer by a multiple exposure 
process. 
A holographic display element according to the invention can be 
incorporated in an automobile windshield as the combiner of a head-up 
display system. In that case the incidence of external light such as 
sunlight on the display element from the outside does not result in 
diffraction of glaringly visible light of wavelengths of 570-780 nm within 
the range, for example, from 0 to 40 degrees or from 0 to 60 degrees with 
a horizontal plane or road surface, so that the diffracted light does not 
give a strange or uncomfortable impression to persons viewing the 
windshield from the outside. The performance of the display element is not 
adversely affected by the addition of the second reflection hologram. 
It is also possible to use a holographic display element according to the 
invention in a building window or a partition. In such a case the 
aforementioned predetermined range of angles may suitably be set with 
respect to a plane normal to the window or partition at the level of the 
display element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an automobile windshield 10 which incorporates a holographic 
display element 16 according to the invention as a combiner of a head-up 
display system. The windshield 10 uses a laminated glass made up of two 
glass plates 12 and 12' and a transparent plastic interlayer 14 such as a 
polyvinyl butyral film. The holographic display element 16 consists of a 
first reflection hologram 18 in sheet form and a second reflection 
hologram 20 in sheet form which is superposed on the first hologram sheet 
18. The element 16 is confined in the laminated glass such that the first 
hologram 18 is in contact with the glass plate 12' on the inboard side, 
whereby the second hologram 20 faces the glass plate 12 on the outboard 
side through the transparent interlayer 14. The first and second 
reflection holograms 18 and 20 are different in diffraction 
characteristics as will be described hereinafter. The first reflection 
hologram 18 serves as the principal part of the combiner, and the second 
reflection hologram 20 serves an auxiliary purpose. The windshield 10 
including the holographic display element 16 makes an angle .psi. with a 
horizontal plane H. 
In the head-up display system a light emitting display 22 such as a 
cathode-ray tube is the source of an image of information pertaining to 
the operational status of the vehicle such as, for example, vehicle speed 
and engine RPM. From the display 22 information-carrying light rays of a 
given wavelength are projected onto the first reflection hologram 18 as 
represented by arrow C.sub.1 in FIG. 1, and the light rays are diffracted 
by the reflection hologram 18 at a given angle so that an image of the 
information is presented to the eyes 24 of the driver, as represented by 
arrow C.sub.2, while the driver is viewing the external real world scene 
through the combiner 16 in the windshield 10. 
Referring to FIG. 2, the first and second reflection hologram sheets 18 and 
20 are each produced in the following way. 
In FIG. 2 numeral 28 indicates an optically transparent polyester film used 
as the base of the hologram sheet 18 or 20. In advance a photosensitive 
material is coated on the polyester film 28 and dried. Usually dichromated 
gelatin is used as the photosensitive material, but it is also possible to 
use a different material such as a silver halide base composition, a 
photosensitive polymer or a photoresist. For exposure of the 
photosensitive coating layer on the film 28 to light, a laser beam emitted 
from a suitable laser oscillator 30 such as an argon gas laser oscillator 
is divided into two beams by a beam splitter 32. One of the divided beams 
is directed to a convex lens 36 by a reflective mirror 34. Through the 
lens 36 the laser beam diverges into a spherical wave, which illuminates 
the photosensitive coating layer on the film 28. The other beam is 
directed to another convex lens 40 by a reflective mirror 38 to diverge 
into a spherical wave, which illuminates the photosensitive coating layer 
from the opposite side. After that the photosensitive layer on the film 28 
is subjected to usual developing and fixing treatments. By this process 
fine interference fringes are created in the photosensitive layer on the 
film 28, and the processed film 28 can be used as a reflection hologram 
sheet. 
In the above described exposure operation for producing the first 
reflection hologram 18 in FIG. 1, the wavelength of the laser beam (type 
of the laser oscillator 30) and the angle of incidence of the laser beam 
on each side of the film 28 (angle of the axis of the lens 36 or 40 with 
the normal V to the film 28) are determined by using the Bragg's equation 
with consideration of the holographic reconstruction conditions 
(wavelength of the light source 22 and angular positions of the light 
source 22 and the driver's eyes 24) and the amount of a change in the 
thickness of the photosensitive layer on the film 28 during the hologram 
producing process. In producing the second reflection hologram 20 the 
exposure conditions are determined such that the second reflection 
hologram 20 in FIG. 1 diffracts external light of wavelengths ranging from 
570 to 780 nm at angles outside a predetermined range of angles (.theta.) 
with the horizontal plane H. 
EXAMPLE 1 
This is an example of the holographic combiner 16 shown in FIG. 1. The 
display or light source 22 for holographic reconstruction emits green 
light of 545 nm wavelength, and the light source 30 for producing the 
first and second reflection holograms 18, 20 is an argon gas laser 
oscillator which emits light of 488 nm wavelength. In the exposure 
operation for producing the first reflection hologram 18 the angle of 
incidence through the convex lens 36 was 22.5 degrees, and the angle of 
incidence through the convex lens 40 was 38 degrees. 
When the obtained first reflection hologram 18 was oriented as shown in 
FIG. 1 without superposing the second reflection hologram 20, the hologram 
18 diffracted sunlight coming from the outside (opposite to the light 
source 22 for holographic reconstruction) with the characteristic shown in 
FIG. 3. That is, within the range of angle .theta. from 0 to 40 degress 
the diffracted light contained wavelengths ranging from 570 nm (greenish 
yellow) to 630 nm (reddish orange). Light of such wavelengths make a 
strange impression on pedestrians or persons in oncoming cars. 
The chart of FIG. 3 shows that the first reflection hologram 18 diffracts 
external light of 600 nm wavelength at an angle of 20 degrees with the 
horizontal plane H (i.e. the middle of the 0 to 40 degrees range of angle 
.theta.). In producing the second reflection hologram 20 the angle of 
incidence of external light of this wavelength was determined by the 
Bragg's equation, and the exposure conditions (angles of incidence) were 
determined so as to diffract light of this wavelength in a direction 
outside the 0 to 40 degree range of angles, for example at an angle of 60 
degrees with the horizontal plane H. That is, the angle of incidence 
through the convex lens 36 was 25 degrees, and the angle of incidence 
through the convex lens 40 was 8 degrees. When the obtained second 
reflection hologram 20 was oriented as shown in FIG. 1, the hologram 20 
diffracted sunlight coming from the outside with the characteristic shown 
in FIG. 4. It is seen that light of wavelengths longer than 570 nm was 
diffracted at angles greater than 40 degrees with the horizontal plane. 
The second reflection hologram 20 was superposed on the first reflection 
hologram 18 to complete the combiner 16 in the windshield 10 in FIG. 1. 
Referring to FIG. 5, in this case the second reflection hologram 20 
diffracted the incident light S.sub.1 such that wavelengths longer than 
570 nm were diffracted, as represented by arrow D.sub.2, at angles outside 
the predetermined range (0-40 degrees) of angle .theta. with the 
horizontal plane H. That is, the second hologram 20 prevented the first 
hologram 18 from diffracting light of 570-630 nm at angles of 0-40 degrees 
with the horizontal plane. As will be understood from FIG. 4 the second 
hologram 20 diffracted light of wavelengths ranging from 570 nm (greenish 
yellow) to 475 nm (blue) at angles of 0-40 degrees with the horizontal 
plane, so that the diffracted light was not conspicuous and did not give a 
strange impression. A fraction of the incident light of 570-780 nm passes 
through the second hologram 20 and is diffracted by the first hologram 18 
at angles in the 0-40 degree range, as represented by arrow D.sub.1 in 
FIG. 5, but the diffracted light D.sub.1 is neglibile when the diffraction 
efficiency of the second hologram 20 is made sufficiently high, e.g. 95% 
or above. 
While external daylight was diffracted in the above described manner, the 
information-carrying light C.sub.1 of 545 nm (green) was mostly diffracted 
by the first reflection hologram 18 toward the eyes 24 of the driver so 
that an image of the display information was clearly visible to the 
driver. 
EXAMPLE 2 
This is another example of the combiner 16 shown in FIG. 1. The display or 
light source 22 for holographic reconstruction emits red light of 610 nm 
wavelength, and the light source 30 for producing the first and second 
reflection holograms 18, 20 is an argon gas laser oscillator which emits 
light of 514.5 nm wavelength. In the exposure operation for producing the 
first reflection hologram 18 the angle of incidence through the convex 
lens 36 was 38.5 degrees, and the angle of incidence through the convex 
lens 40 was 58 degrees. 
When the obtained first reflection hologram 18 was oriented as shown in 
FIG. 1 without superposing the second reflection hologram 20, the hologram 
18 diffracted sunlight coming from the outside with the characteristic 
shown in FIG. 6. That is, within the range of angle .theta. from 0 to 40 
degrees, the diffracted light contained wavelengths ranging from 620 nm to 
710 nm (red). 
In producing the second reflection hologram 20 the angle of incidence 
through the convex lens 36 was 52 degrees, and the angle of incidence 
through the convex lens 40 was 23.5 degrees. When the obtained second 
reflection hologram 20 was oriented as shown in FIG. 1, the hologram 20 
diffracted sunlight coming from the outside with the characteristic shown 
in FIG. 7. It is seen that light of wavelengths longer than 620 nm was 
diffracted at angles greater than 40 degrees with the horizontal plane. 
The second hologram 20 was superposed on the first hologram 18 to complete 
the combiner 16 in the windshield 10 in FIG. 1. In this case external 
light incident on the combiner 16 was diffracted by the second hologram 20 
such that light of wavelengths longer than 620 nm was diffracted at angles 
greater than 40 degrees with the horizontal plane. Therefore, in the 0-40 
degree range of angle .theta., the diffracted light did not give a strange 
or uncomfortable impression to persons outside the car. FIG. 7 shows that 
the second hologram 20 of this example diffracts light of wavelengths 
ranging from 570 nm (yellowish green) to 620 nm (red) at angles of 25-40 
degrees with the horizontal plane, but actually this offers little problem 
because the diffraction of concern occurs only when the angle of incidence 
of daylight is greater than 110 degrees in terms of angle with the 
horizontal plane. However, according to the angle .psi. of the windshield 
10 there is a possibility that uncomfortable colors of the diffracted 
light become visible to persons outside the car. In such a case the 
problem is solved by superposing another reflection hologram, which is 
identical with the second reflection hologram produced in Example 1, on 
the second hologram 20 in Example 2. 
The information-carrying light C.sub.1 of 610 nm (red) was mostly 
diffracted by the first reflection hologram 18 toward the eyes 24 of the 
driver as represented by arrow C.sub.2 so that an image of the display 
information was clearly visible to the driver. 
In the foregoing examples the first and second holograms 18 and 20 were in 
direct contact with each other, but this is not essential. Alternatively, 
the plastic interlayer 14 in FIG. 1 may be interposed between the first 
and second holograms 18 and 20 by attaching the first hologram 18 to the 
inside surface of the glass plate 12' on the inboard side and the second 
hologram 20 to the inside surface of the glass plate 12 on the outboard 
side. 
In general a hologram sheet is liable to deteriorate by absorption of 
moisture. Therefore, it is rather desirable to cover each side of the 
laminated holographic display element 16 with a transparent protective 
film or the like. In this regard, it is favorable to modify the laminated 
glass of the windshield in FIG. 1 by additionally using another plastic 
interlayer such that the laminated element 16 (18+20) is sandwiched 
between the illustrated interlayer 14 and the additional interlayer. 
Besides, the holographic display element 16 can be attached to a single 
unlaminated) glass plate on condition that the outer surface of the 
display element 16 is covered with a transparent protective film. 
FIG. 8 illustrates an embodiment of another aspect of the present 
invention. More particularly, FIG. 8 shows an automobile windshield 10 
which incorporates a holographic display element 16A according to the 
invention as a combiner of a head-up display system. The windshield 10 
uses a laminated glass which was already described with reference to FIG. 
1. The holographic display element 16A consists of a first reflection 
hologram 18 in sheet form and a second reflection hologram 20A in sheet 
form which is superposed on the first hologram sheet 18, and the element 
16 is confined in the laminated glass such that the first hologram 18 is 
in contact with the glass plate 12' on the inboard side, whereby the 
second hologram 20A faces the glass plate 12 on the outboard side through 
the transparent interlayer 14. The windshield 10 including the display 
element 16 makes an angle .psi. with a horizontal plane H. The first 
reflection hologram 18 serves as the principal part of the combiner 16 and 
does not differ from the counterpart in FIG. 1. The head-up display system 
includes a light emitting display 22 such as a cathode-ray tube and 
operates in the manner already described with reference to FIG. 1. 
When external light such as sunlight is incident on the holographic 
combiner 16 from the outside, as represented by arrow S.sub.1, the first 
reflection hologram 18 diffracts the incident light. Within a 
predetermined range of angle .theta. with the horizontal plane H the 
diffracted light (arrow D.sub.1) will include wavelengths ranging from 560 
nm (yellowish green) to 780 nm (red) which offer discomfort to persons 
viewing the windshield 10 from the outside. The second reflection hologram 
20A serves the purpose of reducing the exciting purity of the light 
diffracted by the first hologram 18 within the aforementioned range of 
angle .theta.. More particularly, the second hologram 20A diffracts 
complementary colors to the yellowish green to red colors diffracted by 
the first hologram 18 within the aforementioned range of angle. 
The second reflection hologram 20A can be produced by using the exposure 
method already described with reference to FIG. 2. 
EXAMPLE 3 
This is an example of the combiner 16A shown in FIG. 8. The angle .psi. of 
the windshield 10 is 30 degrees. The display or light source 22 for 
holographic reconstruction emits green light of 545 nm wavelength, and the 
light source 30 for producing the first reflection hologram 18 is an argon 
gas laser oscillator which emits light of 514.5 nm wavelength. In the 
exposure operation for producing the first reflection hologram 18 the 
angle of incidence through the convex lens 36 was 10.0 degrees, and the 
angle of incidence through the convex lens 40 was 25.0 degrees. 
When the obtained first reflection hologram 18 was oriented as shown in 
FIG. 8 the hologram 18 diffracted sunlight coming from the outside with 
the characteristic represented by curve I in FIG. 9. That is, within the 
range of angles .theta. from 0 to 60 degrees, the diffracted light 
(D.sub.1) contained wavelengths ranging from 550 nm (yellowish green) to 
630 nm (red). 
To described every color by numerical values the most widely used system of 
colorimetry is the XYZ colorimetric system of CIE (Commission 
Internationale de 1'Eclairage). For quantitative description of any part 
of the visible spectrum this colorimetric system uses tristimulus values, 
viz. a stimulus value X, a stimulus value Y and a stimulus value Z. 
Assuming that X+Y+Z=S (constant), the chromaticity of the wavelength in 
question is determined by chromaticity coordinates x (x=X/S) and y 
(y=Y/S). FIG. 11 is the CIE chromaticity diagram. Using this diagram any 
color can be identified as a point within the region enclosed by the 
spectral locus (the curve marked with wavelength values) and the locus of 
pure purples (the straight line joining the ends of the spectral locus). 
On the diagram the point W is the white point (standard light source, 
D.sub.65 :x=0.3127, y=0.3290). The white point W can be taken as the point 
of zero purity of color excitation, and maximum excitation purity can be 
found at all points on the spectral locus and on the locus of pure 
purples. The excitation purity of any one point on the diagram is given by 
the ratio of the distance from that point to the white point W to the 
length of a straight line drawn from W through the point in question to 
the spectral locus or the locus of pure purples. 
In FIG. 11, the chromaticity points indicated by black circle marks are 
representatives of uncomfortable colors which appear within the 0-60 
degrees range of angle .theta. by diffraction by the first reflection 
hologram 18 of Example 3. If a straight line drawn from any one point 
indicated by a black circle mark through the white point W intersects the 
spectral locus on the opposite side, the point of intersection (indicated 
by a white circle mark) is approximately the complementary color to the 
color indicated by the black circle mark. 
In FIG. 9 the curve C represents an ideal diffraction characteristic to 
provide a complementary color to any one of the uncomfortable colors 
diffracted by the first reflection hologram 18 of Example 3. It is 
desirable that the second reflection hologram 20A has the diffraction 
characteristic represented by the curve C, but actually it is difficult to 
fully meet this desire. To approximate the desirable diffraction 
characteristic the exposure conditions for producing the second hologram 
20A were determined in the following way. 
In this case the predetermined range of angle .theta. is from 0.degree. to 
60.degree.. Referring to FIG. 8, with respect to the second hologram 20A 
the angle of incidence of external light (S.sub.2) is assumed to be .phi., 
and the angle of diffraction (indicated by arrow D.sub.2) to be .alpha.. 
At an angle close to (.phi.+.alpha.)/2 the dependence of diffracted 
wavelength on angle becomes small (decrease of a change in wavelength with 
a change in diffraction angle), and at that angle the wavelength of 
diffracted light becomes maximal. Therefore, it is suitable to adjust the 
angle of incidence .phi. and the angle of diffraction .alpha. such that 
the direction of diffracted light given by angle (.phi.+.alpha.)/2 
approximately coincides with 1/2 of the predetermined range of angle 
.theta., viz. at an angle of (60.degree.-0.degree.)/2 (=30.degree.) with 
the horizontal plane H. That is, the conditions of exposure operation for 
producing the second reflection hologram 20A are determined such that 
.phi. and .alpha. satisfy the following equation: 
EQU (.phi.+.alpha.)/2=90.degree.-.psi.(30.degree.)-30.degree.=30.degree. 
Further, the exposure conditions are determined so as to make the angle 
(.phi.+.alpha.) as small as possible since the dependence of diffracted 
wavelength on angle reduces as the angle (.phi.+.alpha.) becomes smaller. 
In determining the exposure conditions it is also taken into consideration 
that a suitable range of the angle of incidence .phi. of external light 
(S.sub.2) is from +30.degree. (on the upside of the normal to the element 
16A: angle of 90.degree. with the horizontal plane) to -10.degree. 
(downside of the normal to the element 16A: angle of 50.degree. with the 
horizontal plane) because outside this range the quantity of incident 
light is relatively small. 
In producing the second reflection hologram 20A by using the optical system 
of FIG. 2, the light source 30 was an argon gas laser oscillator which 
emits light of 488.0 nm wavelength. In the exposure operation the angle of 
incidence through the lens 36 was 45.5 degrees, and the angle of incidence 
through the lens 40 was 0.0 degree. 
When the obtained second hologram 20A was oriented as shown in FIG. 8 the 
hologram 20A diffracted external light (S.sub.2) with the characteristic 
represented by curve II in FIG. 9. That is, the diffraction characteristic 
of the second hologram 20A was successfully approximated to the ideal 
characteristic represented by curve C. Table 1 shows excerpts of the data 
shown in FIG. 9. 
TABLE 1 
______________________________________ 
Wavelength Wavelength 
diffracted Wavelength of 
diffracted 
by the 1st Complementary 
by the 2nd 
hologram 18 Color hologram 20A 
Angle .theta. 
(nm) (nm) (nm) 
______________________________________ 
0.degree. 
554 --*.sup.) 461 
10.degree. 
577 478 474 
20.degree. 
598 489 485 
30.degree. 
616 492 493 
40.degree. 
629 493 495 
50.degree. 
634 493 491 
60.degree. 
630 493 480 
______________________________________ 
*.sup.) No complementary color exists 
The second hologram 20A was superposed on the first hologram 18 to complete 
the combiner 16A in the windshield 10 of FIG. 8. In the 0 to 60 degrees 
range of angle .theta., additive color mixing of diffracted light D.sub.1 
attributed to the first reflection hologram 18 with diffracted light 
D.sub.2 attributed to the second reflection hologram 20A resulted in 
whitening of diffracted light or approximating of excitation purities to 0 
(zero) as represented by cross marks on the diagram of FIG. 11. 
Accordingly the diffracted light (D.sub.1, D.sub.2) did not offer 
discomfort to persons viewing the windshield 10 from the outside. 
The information-carrying light C.sub.1 of 545 nm (green) was mostly 
diffracted by the first hologram 18 toward the eyes 24 of the driver as 
represented by arrow C.sub.2 so that an image of the display information 
was clearly visible to the driver. 
EXAMPLE 4 
This is another example of the combiner 16A shown in FIG. 8. In this case 
the display 22 emits red light of 610 nm wavelength. 
For producing the first hologram 18 the light source 30 was an argon gas 
laser oscillator which emits light of 514.5 nm. In the exposure operation 
of the angle of incidence through the lens 36 was 37.5 degrees, and the 
angle of incidence through the lens 40 was 56.5 degrees. When the obtained 
first hologram 18 was oriented as shown in FIG. 8 the diffraction 
characteristic of the hologram 18 for external light (S.sub.1) was as 
represented by curve I in FIG. 10. In FIG. 10, curve C represents an ideal 
diffraction characteristic to provide a complementary color to any one of 
the uncomfortable colors diffracted by the first hologram 18 of Example 4. 
For producing the second hologram 20A the light source 30 was an argon gas 
laser oscillator which emits light of 488.0 nm. In the exposure operation 
the angle of incidence through the lens 36 was 53.5 degrees, and the angle 
of incidence through the lens 40 was -4.5 degrees. (The negative sign 
means that the convex lens 40 was positioned on the right-hand side of the 
central normal V in FIG. 8.) When the obtained second hologram 20A was 
oriented as shown in FIG. 8 the diffraction characteristic of the hologram 
20A for external light (S.sub.2) was as represented by curve II in FIG. 
10. That is, the diffraction characteristic of the second hologram 20A was 
successfully approximated to the ideal characteristic represented by the 
curve C. Table 2 shows excerpts of the data shown in FIG. 10. 
TABLE 2 
______________________________________ 
Wavelength Wavelength 
diffracted Wavelength of 
diffracted 
by the 1st Complementary 
by the 2nd 
hologram 18 Color hologram 20A 
Angle .theta. 
(nm) (nm) (nm) 
______________________________________ 
0.degree. 
621 492 473 
10.degree. 
648 494 483 
20.degree. 
673 494 492 
30.degree. 
694 494 496 
40.degree. 
708 494 495 
50.degree. 
714 494 488 
60.degree. 
710 494 474 
______________________________________ 
The second hologram 20A was superposed on the first hologram 18 to complete 
the combiner 16A in the windshield 10 of FIG. 8. Also in this example 
additive color mixing of diffracted light occurred in the 0 to 60 degrees 
range of angle .theta., and the effect of color mixing was as described in 
Example 3. The information-carrying light of 610 nm (reddish orange) was 
mostly diffracted by the first hologram 18 toward the eyes 24 of the 
driver so that an image of the display information was clearly visible to 
the driver. 
EXAMPLE 5 
In this example the first and second reflection holograms 18 and 20A of 
Example 3 were integrated into a single hologram sheet by using a multiple 
exposure method. 
FIG. 12 shows an optical system used for the multiple exposure method. This 
optical system is a combination of two sets of optical systems each of 
which is fundamentally similar to the system of FIG. 2. There are two 
laser oscillators 30 and 50. The laser beam from the first laser 30 is 
divided into two beams by a beam splitter 32. One of the divided beams is 
directed to a convex lens 36 by a reflective mirror 34, and through the 
lens 36 the laser beam diverges into a spherical wave which illuminates 
the photosensitive layer on a hologram sheet base 28 such as a polyester 
film. The other beam is directed to a convex lens 40 by a reflective 
mirror 38, and through the lens 40 the laser beam diverges into a 
spherical wave which illuminates the photosensitive layer on the film 28 
from the opposite side. The laser beam from the second laser 50 is divided 
into two beams by a beam splitter 52. By reflective mirrors 54 and 55 one 
of the divided beams is directed to a convex lens 56 which was on the same 
side of the film 28 as the lens 36, and through the lens 56 the laser beam 
diverges into a spherical wave which illuminates the photosensitive layer 
on the film 28. The other beam is directed to a convex lens 60 on the 
opposite side by refractive mirrors 58 and 59, and through the lens 60 the 
laser beam diverges into a spherical wave which illuminates the 
photosensitive layer. After the multiple exposure operation the 
photosentive layer on the film 28 is subjected to usual developing and 
fixing treatments. 
In Example 5 the first laser 30 was an argon gas laser which emits light of 
488.0 nm wave-length, and the second laser 50 was an argon gas laser which 
emits light of 514.5 nm wavelength. With respect to light emitted from the 
first laser 30 the angle of incidence through the lens 36 (angle of the 
lens axis with normal to the film 28) was 10.0 degrees, and the angle of 
incidence through the lens 40 was 25.0 degrees. With respect to light 
emitted from the second layer 50 the angle of incidence through the lens 
56 was 45.5 degrees, and the angle of incidence through the lens 60 was 
0.0 degree. As shown in this example, for the multiple exposure operation 
it is preferable to use two laser oscillators different in oscillation 
wavelength thereby to prevent formation of unwanted hologram (array of 
interference fringes) other than the aimed first and second holograms. 
The hologram sheet produced by the above multiple exposure method proved to 
be functionally identical with the display element 16A of Example 3 
constructed of the first hologram sheet 18 and the second hologram sheet 
20A. 
In Examples 3 to 5 the second reflection hologram 20A was afforded with 
such diffraction characteristics that the additive color mixing of the 
light diffracted by the first hologram and the light diffracted by the 
second hologram results in whitening of the color mixture. However, it is 
an option to modify the characteristic of the second hologram so as to 
produce an arbitrary pale color by the additive color mixing effect on 
condition that the excitation purity of the produced color is not higher 
than about 0.4.