Introduction of Color Holographic Recording Materials (I)

Holographic recording materials can be divided into two major categories of inorganic and organic. There are silver halide emulsions, dichromated gelatins, photosensitive polymers, photoconductive plastics, anisotropic materials, photorefractive materials, and the like, of which the most widely used are silver halide latex and dichromated gelatin. Photosensitive material. According to the type of change of material properties after the recording material absorbs light, it can be roughly divided into four types: transmissive, refractive, embossed, and hybrid. In the holographic printing, transmissive silver halide, refractive dichromic acid gelatin and an embossed photoresist are mainly used, and a silver halide material used in a bleaching treatment mixed with a refraction and relief type is used.

The selection of recording materials should be considered from two aspects. One is the recording wavelength, and there are two cases of single wavelength recording and multiple wavelength recording. The other type of hologram is the difference between a transmission hologram and a reflection hologram. The former should consider the sensitive wavelength of the recording material, and the latter should consider the thickness of the recording material. Single-wavelength recorded transmissive holograms with red sensitive recording materials such as domestically produced Tianjin holographic type I and II , Agfa8E75 , Ilford SP696T , SP673 and Russian PRG-03 etc. silver salt dry plates, and photoconductive thermoplastic films, etc. ; Lan Min's use of Tianjin holographic type III , 8E56 silver salt in the version and photoresist and so on.

Single-wavelength recording reflection holograms are commonly used in DCG ( Blue Min ) , photopolymers ( Red or Blue ) and 8E56HD , 8E75HD silver salts, etc. Multi-wavelength recording transmission hologram commonly used value = "649" unitname = "F"> 649F , 8E56 silver salt dry version. Reflection holograms are commonly used in 8E56HD and 8E75HD recording interlayer holograms and Russian PRG- 03 full - color silver salt dry version; two-color DCG version; emerging photopolymers are the DuPont HRF series and Ommi Dex holographic film series. In order to facilitate the detailed introduction of the characteristics of the holographic recording materials, the following briefly describes the main characteristics of the characteristics of the recording material.

　　 Characteristic parameters of holographic recording material

1, sensitivity

Sensitivity is the sensitivity of the recording material after it receives light.

Because the recording process is a photochemical effect, the photon energy is related to the wavelength, the longer the wavelength is, the smaller the energy of the photon, and each recording material has a red limit of one wavelength. Light with a wavelength greater than the red limit can not be used for the material. Photochemical reactions. In addition, each material has its own absorption band, and only the wavelength within the absorption band can act as a photochemical reaction. This is the reason why the recording medium has a different degree of spectral sensitivity.

2 , spectral sensitivity

Because the recording process is a photochemical process, the photon energy is related to the wavelength, the longer the wavelength is, the smaller the energy of the photon is. Generally, each kind of recording medium has a red limit of one wavelength. Light with a wavelength greater than the red limit cannot act photochemically. In addition, each recording medium has its absorption band, and the wavelength in the absorption band can play a role of photochemistry. This is the spectral sensitivity. Must have absorption bands in this band. Figure 5-8 shows the characteristic curves of domestically produced Type I and Type I holographic dry plates Kador value = "649" unitname = "F"> 649F , Ilford and Agfa holographic dry plate.

3, diffraction efficiency

Diffraction efficiency is defined as the ratio of the flux of the hologram diffracted image to the total luminous flux of the reconstructed illumination light. The diffraction efficiency depends not only on the nature of the recording medium but also on the type of hologram and the degree of modulation of the fringes. In general, the phase-type recording material has higher diffraction efficiency than the amplitude type. The degree of fringe modulation is related to the ratio of the reference to the object beam. For the same recording material, the diffraction efficiency is also related to the spatial frequency of the hologram. The table lists the theoretical diffraction efficiencies of untyped ideal holograms.

The theoretical value of the diffraction efficiency of the ideal recording medium

4, resolution

The resolution of the recording material is usually expressed as the number of line pairs that can be identified in the value="1" unitname="mm"> 1mm length, that is, the maximum spatial frequency of the spatial modulation of the exposure intensity that the recording material can record, in units of 1/mm . The resolution of an ordinary camera film requires only 200 lines /mm . When recording a hologram, the resolution of the recording material is related to the angle of the reference beam.

5, characteristic curve

The characteristic curve represents the relationship between some parameters related to the recording material. The ordinary photographing curve is the optical density, also called the black density, which is equal to the logarithm of the reciprocal of the transmittance, is the amplitude transmission coefficient, and H is the recording material exposure amount. . More suitable H- curves are commonly used in holography. The ideal amplitude recording material curve should be linear.

For phase-recorded materials, the change in phase in the transmission function changes with exposure. The ideal phase recording material- H curve should be linear. The actual phase-recording material has a linear region within a suitable exposure range.

The hologram diffraction efficiency is a very important parameter in holographic platemaking. The diffraction efficiency is related to the recording material, the intensity of exposure intensity modulation, and the amount of exposure. In order to improve the quality of the holographic original and optimize the platemaking process, the relationship between the diffraction efficiency of the recording material and the modulation degree V of the exposure intensity at different exposure conditions should be analyzed, ie the curve with H0 as the parameter; or the diffraction efficiency with different modulation degrees The relationship between the exposure amount, that is, the curve in which V is a parameter.

6, noise

The noise of the recording material refers to the scattered light caused by the unevenness of the grain size of the material itself and the roughness of the surface. After the exposure and development process, the noise will increase, such as noise generated by non-linear recording, interference fringes or speckles generated between the reference light and the scattered light of the object light wave. For silver-salt dry plates, the speed of development and the bleaching process will increase the particle size and produce noise.

　　 Commonly used holographic recording materials

The ideal recording material should have high sensitivity, high resolution, low noise, and a linear curve for the wavelength used for exposure. From the perspective of production, it should also have the advantages of being reusable and cheap. In fact, it is impossible for a certain recording material to meet these conditions at the same time.

1, silver halide latex

Silver halide photographic materials are the most commonly used holographic materials. They have the advantages of high sensitivity, wide spectral response, high resolution and versatility, but low diffraction efficiency and signal-to-noise ratio. Silver halide milk film is divided into film (film) and dry version (hard film), the main structure of the photosensitive layer and film base, photosensitive layer, also known as the latex layer, ultrafine silver halide (the main component of AgBr and AgCl ) The grains are suspended in gelatin, plus a certain sensitizer made of latex, its thickness is generally 0 . 01 ~ 5μm , latex attached to the film base, customarily used as the glass substrate is called holographic dry plate, and the film base is acetate and polyester films and other films are called holographic film. As a holographic silver halide emulsion, the particle size requirement is 0 . 03 ~ 0 . 09μm .

Because the smaller the particle size is, the lower the photosensitivity is, the exposure of the holographic dry plate is much larger than that of the ordinary photolithography plate. The standard developer for the holographic dry plate is D19 , and this developer gives higher values. The developed silver particle size is 0 . 1 to 0 . 2μm . The fine particle developer was D76 and the developed silver particles were filamentous. The finer the silver particles, the higher the diffraction efficiency of the hologram and the lower the noise. If the silver particle size is less than the wavelength, Rayleigh scattering can be applied. Ultra-fine particle dry plate developed with D76 diluted developer, dry version was brown-red, silver particle size was 0 . 005 to 0 . 025μm , the hologram diffraction efficiency can reach 40 %. Another method to improve the diffraction efficiency is to use D76 development and F5 fixation, and then use R10 ( NaCl ) bleaching, and then with strong light, so that the hologram on the exposed part of the photolysis of silver, its particle size is 0 . 01μm . Holographic diffraction can also be improved.

The holographic dry plate is subjected to an exposure and development process to obtain an amplitude-type hologram, and if it is bleached, it becomes a phase-type hologram. Bleaching is the reduction of metallic silver to a transparent silver salt with an oxidizing agent. As a result, the refractive index of the gelatin in the exposed portion of the hologram is different. The improvement of the diffraction efficiency is related to the difference between the refractive indices of the above two, but it is not that the larger the difference is, the better, but there is an optimum value. There are many kinds of oxidants for bleaching, commonly used mercury chloride ( HgCl2 ), ferric chloride ( FeCl3 ), potassium ferricyanide [K3Fe(CN)6] , ammonium dichromate [(NH4)Cr2O7] , and bromination. Copper (CuBr2) etc. The above-mentioned oxidants and metallic silver are shown in the table.

    Table bleached silver salt and its refractive index

In order to obtain a high-quality hologram, there are some technical issues to be aware of when using and processing the holographic plate. The holographic dry plate tends to generate stress during the drying process after applying the latex. To eliminate this stress, the dry plate can be placed in a container with a certain humidity for one night before use. The flatness of the surface of the base glass of the holographic dry plate is generally not good, and a wood grain-like interference fringe is generated due to the influence of the back surface reflected light at the time of recording. The way to eliminate this kind of fringe is to cover a neutral glass on the back of the dry base glass during the exposure, and add a refraction liquid whose refractive index matches the glass, and remove it when developing. If a reflection hologram is recorded, the dry plate can be placed in a refractive index matching bath. If the light beam is obliquely incident on the dry plate at a relatively large angle during exposure, the light enters the glass from the side and multiple reflections will occur between the two surfaces to form a small hologram, which will affect the quality of the hologram; The solution is to use a beam to limit the beam so that its cross-section is smaller than the holographic plate, or to block the edges of the plate.