1. Field of the Invention
The present invention relates to a method of fabricating a hologram screen for displaying an image by projecting the image light on a hologram and a hologram imaging apparatus used for the method.
2. Description of the Related Art
In a conventional method, a hologram is fabricated by exposing a photosensitive member using an exposure optical system as shown in FIG. 14 (Japanese Unexamined Patent Publication No. 11-102153).
According to this conventional method, as shown in FIG. 14, a laser beam 86 generated by a laser oscillator 81 is separated into two beams by a beam splitter 82 to produce split light beams 87, 88.
The split beam 87 is diverged through reflectors 831, 832 and an objective lens 841, and this divergent beam is radiated as a reference beam 941 on a photosensitive member 95 obliquely from an upper position.
The other split beam 88 is diverged through a reflector 833 and an objective lens 842, and this divergent beam is radiated on a photosensitive member 95 as an object beam 942 through a light diffuser 92.
As a result, the reference beam 941 and the object beam 942 interfere with each other on the photosensitive member 95 thereby to produce a hologram. Using this hologram, a hologram screen is fabricated.
In an exposure optical system used for this method of fabricating a hologram screen, as shown in FIG. 14, mirrors 93 are arranged to extend toward the photosensitive member 95 at the end portions 921, respectively, of the light diffuser 92 (FIG. 2). In this way, as shown in FIG. 15, the object beam 942 is reflected on the mirror 93 and can enter the photosensitive member 95, thereby producing the same effect as if a large light diffuser is recorded on a hologram.
Specifically, as shown in FIG. 18, the light diffuser 920 reproduced by projecting the white light 430 on the whole hologram screen 950 at an angle and from a distance similar to the reference beam 941 is larger than the light diffuser 92 used for picking up an image.
The hologram screen 950, as shown in FIG. 16, for example, is used by being attached on a glass plate 61 in such a place as a show window or the reception windows of banks or hospitals. The image light 43 is projected by the projector 62 obliquely from an upper position on the hologram screen 950 thus attached on the glass plate 61 thereby to display an image. Specifically, the image light 43 is scattered and transmitted through the hologram screen 950 so that a viewer E1 located on the other side of the glass plate 61 with respect to the projector 62 recognizes an image through the glass plate 61.
The method of fabricating the hologram screen described above, however, poses the following problem. Specifically, as shown in FIG. 16, the viewer E1 standing on the front of the hologram screen 950 fabricated by the method described above can watch an image from the upper end 951 to the lower end 959. In the case where the viewer watches the hologram screen 950 obliquely from a lower position or at a position nearer to the hologram screen 950, however, the image at the upper end portion 952 of the hologram screen 950 is not visible, as shown in FIG. 17.
The reason why this phenomenon occurs will be explained below with reference to FIG. 18.
As described above, the mirrors 93 are arranged at the end portions 921 of the light diffuser 92 of the hologram screen 950 to expose the image, and therefore, as shown in FIG. 18, the light diffuser 92 reproduced when the white light 430 is projected is substantially coincident with a light diffuser (hereinafter referred to as “the pseudo light diffuser”) 920 enlarged by the mirrors 93.
When using the hologram screen 950, therefore, the image can be watched in a visual range in which the pseudo diffuser 920 is visible at a position thereof relative to the hologram screen 950 coincident with the position of the light diffuser 92 relative to the photosensitive member 95 at the time of exposure.
Specifically, as shown in FIG. 18, the angle θh, which the straight line connecting the upper end portion 951 of the hologram screen 950 and the upper end portion 926 of the pseudo diffuser 920 forms with the normal to the hologram screen 950, is substantially coincident with the visual angle at the upper end portion 952 of the hologram screen 950. This angle θh is substantially equal to the angle θH which the straight line connecting the photosensitive member 95 and the forward end 931 of the upper mirror 93 forms with the normal to the photosensitive member 95 at the time of exposure.
On the other hand, the angle θ1 which the straight line connecting the lower end 959 of the hologram screen 950 and the lower end 927 of the pseudo diffuser 920 forms with the normal to the hologram screen 950 provides a visual angle at the lower end portion 958 of the hologram screen 950. This angle θ1 is substantially equal to the angle θL which the straight line connecting the photosensitive member 95 and the forward end 931 of the lower mirror 93 forms with the normal to the photosensitive member 95 at the time of exposure.
Specifically, the visual range of the hologram screen 950 is considered to exist from θH at upper side to θL at lower side.
In the case where the viewer E1 watches the image displayed on the hologram screen 950 at an appropriate distance from the front of the hologram screen 950, therefore, the upper end portion 952 and the lower end portion 958 of the hologram screen 950 exist within the visual range. Thus, the viewer can watch the image on the whole hologram screen 950 (FIG. 18).
In the case where the viewer E2 watches the hologram screen 950 obliquely from a lower position, however, the angle which the straight line connecting the viewer E2 and the upper end portion 951 of the hologram screen 950 forms with the normal to the hologram screen 950 is larger than the visual angle θH, and therefore the viewer E2 cannot watch the image at the upper end portion 952 which fails to function as a part of the screen. Specifically, the upper limit of the range in which the viewer E2 can recognize the image on the hologram screen 950 is the portion lower than the crossing point P between the straight light connecting the viewer E2 and the upper end portion 926 of the pseudo light diffuser 920 and the hologram screen 950. Thus, the portion higher than the crossing point P (upper end portion 952) cannot be recognized by the viewer E2 as an image, as shown in FIG. 17.
A similar phenomenon occurs also in the case where the viewer E3 approaches excessively close to the hologram screen 950, as shown in FIG. 18. In such a case, the angle which the straight line connecting the viewer E3 and the upper end 951 of the hologram screen 950 forms with the normal to the hologram screen 950 is larger than the visual angle θH.
The above-mentioned phenomenon rarely occurs at the lower end portion 952 of the hologram screen 950. This is by reason of the fact that the mirror 93 arranged at the lower end of the light diffuser 92 at the time of exposure of the hologram is sufficiently long and, therefore, as shown in FIG. 18, the pseudo diffuser 920 is sufficiently long downward.
The mirror 93 arranged at the upper end of the light diffuser 92, however, which is arranged so as not to block the incidence path of the reference beam 941, is comparatively short. Therefore, the pseudo light diffuser 920 is not sufficiently long upward.
As a result, in spite of the fact that the visual angle (≈θL≈θl) at the lower end portion 952 of the hologram screen 950 is sufficiently large, the visual angle (≈θH≈θh) at the upper end portion 951 is comparatively small. Therefore, the problem is posed that the visual range at the upper end portion 951 of the hologram screen 950 is narrow.
Assume, on the other hand, that the hologram screen 950 is fabricated in such a manner that the reference beam 941 is radiated obliquely from a lower position and the image light 43 is also radiated obliquely from a lower position in operation. Then, unlike in the aforementioned case, the problem is posed that the visual range at the lower end portion 952 is narrow.
Specifically, the visual range is narrow at the end portion of the hologram screen 950 nearer to the light source of the reference beam 941, i.e. the objective lens 841. The problem, therefore, is how to enlarge the visual range at the end portion of the hologram screen 950 nearer to the light source of the reference beam 941.