A three-dimensional image displaying apparatus using a hologram can, by causing illumination light to enter the hologram, produce reconstruction light from the hologram and display a three-dimensional reconstruction image expressed by means of the reconstruction light. The hologram can be created through computation in addition to being made by causing interference, on a photographic plate, between object light and illumination light. A hologram created through computation is known as a ‘computer hologram’. A three-dimensional image displaying apparatus using a computer hologram provides a spatial light modulation element with a computer hologram, produces reconstruction light from the spatial light modulation element by causing illumination light to enter the spatial light modulation element, and displays a reconstruction image by means of the reconstruction light.
The technology disclosed in Patent Document 1 is known as three-dimensional image display technology using a computer hologram. In the technology disclosed in Patent Document 1, the reconstruction image to be displayed (hereinafter referred to as ‘target reconstruction image’) is regarded as an aggregate of a plurality of bright points and it is assumed that spherical waves produced by each of the plurality of bright points reach the hologram plane. A computer hologram is created by convolving spherical waves and planar transmission waves that reach the pixel positions from each of the plurality of bright points on the target reconstruction image, in the respective pixel positions on the hologram plane. When illumination light is made to enter the spatial light modulation element that expresses the computer hologram, at least one of the amplitude and phase of the illumination light is modulated by each pixel of the spatial light modulation element, whereby reconstruction light is produced by the spatial light modulation element. Then, a reconstruction image is displayed by a reconstruction image converting optical system (more specifically, a convex lens) producing a virtual image or real image wavefront-converted from the reconstruction light.
Also, in Patent Document 1, when the spatial light modulation element can modulate only one of the amplitude and phase, a mask is provided on the back focal plane of the reconstruction image converting optical system. Further, by the mask that blocks zero-order light (light emitted in the same emission direction as the incident direction among the illumination light that enters the spatial light modulation element), and diffracted waves and conjugate waves of an unnecessary order, and that transmits reconstruction light obtained through diffraction of a specified order, the observer can observe the reconstruction image obtained by the transmitted reconstruction light.
In Non-Patent Document 1, a random phase shifter method is described. Object light is generated by causing light to enter a spatial light modulation element that expresses an image and an optical Fourier transform of the object light is made to interfere with reference light on a photographic plate. When, as a result, a Fourier transform hologram is recorded, the dynamic range of the light sensitivity of the light-sensitive material is exceeded because the object light enters focused on one point of the photographic plate, whereby spectral noise appears prominently in the reconstruction image. The random phase shifter method is intended to resolve such a problem and, by randomizing the degree of phase modulation of the respective pixels in the spatial light modulation element that expresses an image, the distribution of object light falling incident on the photographic plate is widened so that the light intensity at the photographic plate does not exceed the dynamic range of the light sensitivity of the light-sensitive material. As a result, a reconstruction image with reduced spectral noise can be obtained.
In Patent Document 1, Patent Document 2, or Non-Patent Document 2, a stereo displaying apparatus that displays different images onto both eyes of the observer is described. Generally, in a stereo displaying apparatus, the image formation position on the retina is fixed and there is no match between the sense of distance due to the parallax of the two eyes and the interval between the image locations of the two eyes. Hence, the observer feels a large amount of fatigue due to the mismatch between the focusing and convergence of the eyes. The technology described in Patent Document 1, Patent Document 2, or Non-Patent Document 2, is intended to resolve this problem.
A device using the Maxwell effect, which is described in Patent Document 2 or Non-Patent Document 2, causes the formation of an image on the retina by means of only the light transmitted through the middle of the pupil of the observer's eye. As a result, a blurring of the image observed by the observer becomes smaller, irrespective of the focusing of the observer's eyes. On the other hand, a device satisfying the super multieye condition, which is described in Patent Document 3, causes a parallax image comprising a plurality of luminous fluxes to enter the pupil of the observer's eye. As a result, the focusing of the observer's eyes is guided close to a stereo image and the sense of distance caused by the parallax of the two eyes and the interval between the image formation positions of the two eyes are matched each other.    Patent Document 1: Japanese Patent Application Laid-Open No. 09-258643    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-277822    Patent Document 3: Japanese Patent Application Laid-Open No. 2002-228978    Non-Patent Document 1: Yoshito TSUNODA, ‘Holographic high-density image recording’, Optics, Vol. 2, No. 6, pages 329 to 346, 1973Non-Patent Document 2: Takahisa ANDO, et al. ‘A See-through display using the Maxwell effect by means of a holographic optical element’, Video Information Media Academic Journal, Vol. 54, No. 10, pages 1466 to 1473, 2000