According to the characteristics of holography, holographic plate making is characterized by recording the real image of three-dimensional objects through the interference of reference and object light. However, in the holographic recording process, the light scattered and reflected from the surface of different parts of the three-dimensional object has different optical paths when reaching the holographic dry plate due to the different depth of the object. When it interferes with the reference light wave, the optical path difference between the two will change in a large range. Therefore, the recording light source is required to have a longer coherence length, that is, better temporal coherence; In order to record the objects with a large spatial distribution range, the illumination beam is required to be coherent among the rays in a large cross-section, that is, the beam is required to have better spatial coherence; In addition, in order to meet the time and space coherence required by holographic recording, the beam is often slightly expanded after splitting the beam in the holographic recording system, which is used as the reference light wave and the illumination light wave of the object, which requires the energy of the light source to have a high concentration in space. At present, only lasers can meet these requirements, and other ordinary light sources cannot meet these conditions. So far, holograms of three-dimensional objects can only be recorded with lasers, and holograms recorded with white light are limited to two-dimensional objects.

He-Ne laser is generally used to record holograms of three-dimensional objects with holographic plates. A long dry length He-Ne laser can be used to shoot objects with a large depth of field. When dichromate gelatin is used as recording medium, it is best to use Ar+ laser with output wavelength of 400nm, and the selection of photoresist should choose He-Cd laser with output wavelength of 441nm or Ar+ laser with output wavelength of 457.9 nm. If you take a hologram of a moving object, you need a pulsed laser.

Although the color of objects in nature is colorful and varied, according to the visual theory of color, there are three kinds of color-sensitive cone cells in the retina, respectively, red, green, blue (R, G, B) three colors, we call these three colors the three primary colors. The research shows that the various colors in nature can be mixed in principle with the three primary colors in different proportions. This different proportion is reflected in the photographic material, which is different gray levels. Color offset printing is to convert the recording medium of different gray levels into a printing plate composed of dots of different sizes and uneven density, and then overprint with four primary colors or multiple colors to reproduce the level and tone of the original manuscript. Although color holographic printing in the reproduction of the original color and color printing has a very different way, the former is through yellow, magenta, blue, black ink with color subtraction method to reproduce the original, while the latter is red, green, blue three primary color light additive color method to reproduce the original color, but in addition to their specific process technology in the reproduction process is very different, The principle of decomposing the original with three primary colors is the same as that of reproducing the original color with three primary colors superimposed. Therefore, white lasers should be used when recording true color holograms of objects. Studies have shown that the three primary colors can correspond to the three pure spectra, for which the International Commission on Lighting has developed the three primary color standard. The 1931CIE-RGB real three primary color table color system specifies that the wavelength of the red primary color is=700.0nm, the wavelength of the green primary color is=546.1 nm, and the wavelength of the blue primary color is=435.8 nm. 1964CIERGB system provisions: red, green, blue primary colors are 645.2 nm, 526.3 nm, 444.4 nm.

Holographic recording requires the use of lasers, the existing laser output wavelength is generally inconsistent with the wavelength of the three primary colors, so only the relatively close wavelength can be selected. For example, a K+ white laser or an Ar+-Cd laser can be used as a three-color laser source.

The principle of selection is mainly from the realistic consideration of color, and the triangle area of the three wavelengths should be the largest in the x-y chroma diagram, while other factors such as the output power of the laser should be taken into account.

When recording with a three-primary laser, there are three holograms superimposed on a dry plate. If the hologram is still illuminated with a three-color laser in the direction of the reference light, there are nine diffracted light waves that make up the original image.