Full-color holographic printer for replicating reflective holograms

The stereo effect is felt most vividly when the viewer sees the smallest details of texture and a wide range of brightness in the scene. This is especially true for color images. When recording a holographic 3D stereogram, it is very important that all the details are in the area of ​​sharply transmitted space. Very often, when shooting a deep scene, the foreground or background are not sharp enough. This circumstance significantly worsens the quality of stereo perception.

In order to increase the depth of sharply transmitted space during shooting, the maximum aperture of the lens is used, or a small-sized light-sensitive matrix together with short-focus optics. But this is often not enough. A stereogram can be recorded using two matrices. One of them focuses on the foreground of the scene, and the second on the background. The stacking method when processing the angle series can double the depth of sharply transmitted space and improve the quality of perception of the stereo image. Due to the disappearance of the smallest details of a three-dimensional scene in the process of holographic printing using the method proposed by GEOLA in 1999, volumetric forms are only guessed when the viewer moves relative to the stereo display, but are not perceived as naturally as in real life and when viewing analog holograms.

For those interested in the question of how the perception of space on a multiplex hologram depends on the resolution of details and textures, I will explain:

If the interocular distance is expressed through (Bgl ≈ 65 mm), the accommodation of the eyes (A) is in diopters, and the distance to the scene (Za) is in millimeters, then we can derive a formula that expresses the natural relationship between the convergence angle in radians and the accommodation of the eyes:

β = (Vgl)*(A)/1000 (1)

By differentiating formula (1) with respect to the depth coordinate (Z), we obtain an expression that allows us to calculate the maximum number of planes distinguishable by the eye in the depth of the scene.

dZ = -dα*Z²/(Bgl), (2)

Using equation (2) and the values ​​of the eye's maximum angular resolution, the size of the stereo base, and the optical power of the lens, we find that the average observer with the naked eye is actually able to distinguish about 250 planes.

The reason for the unsatisfactory quality of stereoopsis when observing digital holograms is the size of the holopixel, or as it is also called, the hogel, which is noticeable to the viewer's eye. Even the extremely small hogel size of 0.25×0.25 mm destroys the stereoopsis process. You can substitute the angular size of the hogel into formula (2) when viewing the display from the best viewing distance and calculate the number of planes distinguishable by the eye.

Holographic stereograms obtained by a two-stage process have an area of ​​a two-dimensional Fourier element on a master hologram of about 200 mm2. This is quite a decent area for reproducing a high-quality image of each angle. The area of ​​the recorded Fourier image of the integral hologram significantly affects the clarity of the picture and the width of the transmitted tones, which is very important! As a result of the two-stage synthesis of the multiplex hologram, the viewer will not see the original hogels, since the “angle stripes” of the first hologram are located in the plane of the observer's pupils.

Two-step process for synthesizing a color multiplex hologram.

An original two-stage method for synthesizing color holographic stereograms is proposed. The advantage of the method is the precise automatic alignment of color-separated images during the sequential synthesis of a color reflective holographic stereogram and the independence of the transmitting masters from emulsion shrinkage during long-term storage. The disadvantage of the method is the limitation in the size of the synthesized stereo display by the aperture and luminosity of the Fourier transform optics.

Coherent laser light is expanded by a cylindrical lens (1) and illuminates a vertical strip of a diffuser (2). The coherent light scattered by the diffuser (2) passes through a collimation lens (3), forming a wave front illuminating a spatial modulator (4) measuring 24×36 mm. The light modulated by the TFT matrix, passing through a high-aperture (f 24 mm. A 1: 0.7) lens (5), forms a narrow strip of the Fourier image recorded using the reference flat front (R) of the angle in the plane of the photosensitive plate. The recording of a series of Fourier images of the parallaxogram occurs sequentially in the form of strips measuring 0.5×100 mm. Thus, the size of the image carrier of each angle is 50 mm2. This is sufficient for the high resolution restored by the second inverse Fourier transform using a high-aperture (f 150 mm. A 1:0.7) objective (7). The synthesis of a color-separated reflection hologram occurs with the simultaneous restoration of Fourier images using a collimated beam (R_) with an angle of incidence equal to the angle of incidence of the reference beam when recording each individual angle on the carrier (6).

Due to the main properties of the Fourier hologram, the position of the flat reconstructed image depends only on the mutual orientation of the reconstructing beam and the axis of the lens used in the process of the inverse Fourier transform. In other words, the position of the reconstructed image does not depend on the movements of the transmitting master hologram (6) in its own plane. This property is the basis for high-quality sequential color-separated synthesis of a reflective stereogram from three (RGB) transmitting transparencies measuring 60×100 mm with high-precision automatic alignment of color-separated angles.

The size of reflective full-color holographic stereograms is no more than 150×150 mm. In mass circulation, holographic color 3D stereograms can become an affordable and popular holographic product.