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Photosensitive



Introduction

The process of corresponding changes in photographic materials when irradiated by light. The photosensitive process of ordinary silver halide emulsion is basically: the silver halide particles absorb photons during exposure to generate photoelectrons, which are absorbed by the photosensitive nucleus to form a negatively charged core; positively charged free silver ions are attracted by the negative charge and gather at the photosensitive core. , And neutralized by electrons, thereby reducing to silver atoms. The accumulation of silver atoms (usually tens or hundreds) constitutes the latent image nucleus. The sensitivity of silver-free photosensitive materials and electrophotographic and thermophotographic materials are different, and basically they all use photosensitive materials to produce a latent image that can be developed by photochemical or photoelectric effect.

Sensitivity determination

Purpose

Sensitivity determination is a method to study the photochemical action of light on photographic emulsions. Photographic emulsions are generally used to check the ridges. Fallen! The school fell on the heavy ditches. To be more specific, sensitometric measurement is the study of the degree of delicacy in the emulsion layer, and this blackness or image is due to direct exposure of silver salt, or more commonly due to appropriately controlled exposure and subsequent development. And the result of reduction to silver. The same applies to the non-silver-salt method of exposure to light or heat.

Because this relationship can be accurately measured, many practical problems are simplified and solved. The greater advantage of sensitometric measurement is that the result of photochemical action can be measured. For example, the ambiguous adjectives "hard tone", "standard" and "soft tone" used to describe the contrast of photographic paper can be replaced by precise contrast numbers, and these numbers have the same meaning in all countries in the world. However, deciding on the correct measurement and standard for a particular practical problem is very important and sometimes quite difficult.

Steps

Before determining the law of emulsion blackening as the basis of sensitometric measurement, we can say that all sensitometric research can be easily divided into three different operations The procedure, namely:

(1) Expose a series of different negatives under one light source.

(2) Through processing procedures such as developing, fixing, washing and drying, a visible image is obtained.

(3) Measure the blackness of each exposure film.

The above three operating procedures must be carried out under strictly controlled and known conditions.

The photosensitive element

The photosensitive element is equivalent to the film of a film camera. It is used to receive the light signal from the lens and convert it into an electrical signal. Therefore, the photosensitive element determines The final image quality of the photo. The size of the photosensitive element, the number of pixels per unit area, and its sensitivity to light are the three major indicators for judging the performance of the photosensitive element. CMOS is currently a widely used photosensitive element material.

Sensitive materials

Photographic imaging mainly uses the sensitivity of silver halide to light. Silver halide is prepared by mixing soluble silver salt such as nitrate with alkali metal halide such as sodium halide or potassium bromide.

Silver halide photosensitive material is based on silver halides including silver chloride, silver bromide and silver iodide as photosensitive materials. Their microcrystals are dispersed in a gelatin medium to form a photosensitive emulsion and coated on a support Body (film or paper base). Silver halide photosensitive materials used to be one of the areas with the largest use of silver. At present, several photosensitive materials with the largest production and sales are photographic film, photographic paper, medical X-ray film, industrial X-ray film, microfilm, fluorescent information recording film, electron microscope photographic film and printing film. In the 1990s, the world's photographic industry used about 6000-6500 tons of silver. The output of medical X-ray film (including CT film) was 10 times larger than that of industrial X-ray film, and the amount of microfilm was also greatly increased. Since then, due to the development of electronic imaging, digital imaging, non-contact printing and other technologies, the traditional silver halide imaging technology has been impacted and challenged. At present, the use of silver in photosensitive materials has been decreasing year by year.

Sensitive glue

Currently, photosensitive emulsion plate making is still the main method of plate making. Photosensitive adhesive is an important consumable material in the plate-making process. The selection of photosensitive glue has an important influence on the quality of plate making, printing effect and printing products. The photosensitive glue used as a flat screen and a round screen to make a screen should meet the following requirements: good water dispersibility, easy development, economical and safe; its film-forming film layer should have high adhesion to the mesh and mesh, and Certain mechanical strength will not peel off and separate from the net blank; the photosensitive glue must be acid-resistant, alkali-resistant, and chemical-resistant. With the advent of ink-jet and wax-jet plate making technology, photosensitive glue is required to have good compatibility with ink-jet and wax-spray materials and washable effects; photosensitive glue is required to have a higher resolution, so that the printing effect can reach fine, Clearly, the main components of photosensitive adhesive are film-forming agent, sensitizer (photosensitizer) and auxiliary agent. The photosensitizer initiates and participates in the cross-linking reaction of the film-forming macromolecules under the irradiation of a specific light source, so that the film-forming agent forms a network structure and loses solubility, and is firmly combined with the mesh and the mesh.

The film-forming agents of the early flat screen photosensitive adhesives mainly include water-soluble natural polymer materials such as gelatin and polyvinyl alcohol. Although some improvements have been made to this type of material, the printing durability has been improved to a certain extent, but in general, the resistance of this type of material is poor. In the printing production process, the photosensitive adhesive layer flower film is difficult to withstand the erosion of the acid, alkali, and chemical agents in the printing paste and the tens of millions of scraping of the printing squeegee. For this reason, it is generally necessary to carry out reinforcement treatment. That is, after the photosensitive is developed and dried, the method of post-painting is used for reinforcement, or the screen plate is pre-washed and coated with vinyl chloride before being exposed to light, and the spare pre-painting method is used for reinforcement. After developing and drying, no lacquer will be applied, but butyl acetate should be used to wipe out the clear pattern mesh. The photosensitizer used in the photoresist using the above-mentioned film-forming agent is dichromate. Reinforcing agents are raw lacquer and perchloroethylene.

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