Screen Printing Photographic Plate Making Equipment (3)

3 Photographic performance of the photosensitive sheet.

a. Sensitivity. Sensitivity is used to indicate the sensitivity of the photosensitive sheet to light. High sensitivity allows you to take pictures at darker or higher shutter speeds; low sensitivity requires pictures to be taken in brighter conditions or at lower shutter speeds.

At present, the internationally more universal sensitivity units are customized by Germany and represented by DIN; the American ASA system is represented by ASA; the former Soviet Union's Gostia system is represented by ГОСТ. The international standard system (ISO system) was proposed by the international standardization organization and was implemented in 1980. The ISO system adopts the regulations of the new US ASA system. The expression is as follows: For example, the sensitivity of photosensitive materials is equal to ASA100, equivalent to 21 DIN, It is expressed as ISO 100/21, the value indicated before the "/" sign is ASA, and the value indicated after the sign is custom (DIN). In 1981, China began to implement the new sensitivity standard (GB), which is basically the same as the ISO system. There is no conversion relationship between the various sensitivity units, and only a rough comparison can be made. For example, 21 DIN is equivalent to 100 ASA equivalent to 90 ГОСТ.

In the case of the same developing conditions, if the sensitivity of the photosensitive sheet is high, the required exposure amount is small, and conversely, the exposure amount is large. Therefore, to form a certain image density on the photosensitive sheet, the sensitivity is inversely proportional to the exposure. The following formula shows:

S=K/H

Where: S - sensitivity;

H - exposure;

K - constant.

b. Density. After exposure and development of the photosensitive sheet, how much black silver particles are produced is called the density and is represented by “D”. The density value of the plate-making photosensitive sheet is generally 0.05-2.2 and may not exceed 3.0.

c. gamma. Contrast refers to the difference between the maximum density and the minimum density in the original or master image. The image contrast on the photographic plate is often influenced by the performance of the photographic film, the shooting conditions, and the development processing conditions. The contrast with the original document is often different. In order to quantitatively represent the relationship between them, the gamma gamma is used to distinguish:

γ = image contrast / original contrast

If you use γ>1 photographic film to shoot, you can get a master plate with a larger contrast than the original. If you use γ <1 photosensitive film to shoot, you can get a master plate with less contrast than the original.

Contrast and gamma are two different concepts and cannot be confused. Both the platemaking original and the image on the base plate have their own contrast, and the gamma is the inherent performance of the photosensitive plate.

d. Tolerance. The size of a document's ability to record an original's contrast ratio in proportion to it is called latitude, and latitude is an inherent characteristic of a photosensitive film. If the photosensitive sheet can record the original with a large contrast, the latitude is large; if the photosensitive sheet can only record the original with a small contrast, the latitude is small.

Photographic preparation of the bottom plate, you should use a larger degree of latent film to obtain good quality images.

e. Graininess. The thickness of the silver grains on the photosensitive sheet after exposure, development, and the like is called graininess. With a small particle size, the resulting image is clear. Granularity and granularity are two different concepts. Granularity is the concept that people see the particles in the image when they are observing the image of the photographic film. It is a certain concept, and the granularity is an objective measure of the non-uniformity of the particles. The particle size not only depends on the average size of the particle diameter but also on the distribution of the particles. Particle size In addition to the film itself, the development conditions are also closely related to the particle size. The composition of the developer, the development temperature, etc. all affect the particle size to varying degrees.

f. Chromaticity. Sensitive material sensitivity to different wavelengths of light, known as color sensitivity. In general, the photosensitive sheet is classified into a color blind sheet (photosensitive range of 330 to 480 nm), a positive color sheet (photosensitive range of 330 to 600 nm), and a full-color sheet (photosensitive range of 330 to 700 nm) with its photosensitive range.

g. Resolution and clarity. Resolving power refers to the ability of a photosensitive film to record the details of an image, and resolution is also called resolution. The resolution of the photosensitive material is usually calculated as the maximum number of recognizable parallel lines clearly recorded per millimeter of emulsion layer, in units of lines/mm. At present, the resolving power of homemade filmmaking is about 50-100 lines/mm.

Sharpness refers to the degree of sharpness of an image on a photosensitive sheet, which is the sharpness of the image edge. For example, the black characters on the film are clearly defined in black and white. If the edges of the characters have a gray transition, the resolution is low.

h. Photographic characteristics. Photosensitive films receive different exposures and develop different densities after development. If the logarithm of exposure is used as the abscissa and the density obtained at each exposure is used as the ordinate, a curve can be obtained. We call this curve a photosensitive characteristic curve. Different photographic materials have different sensitometric characteristics, but any one characteristic curve has four parts. As shown in Fig. 2-94, the toe portion AB, the straight portion BC, the shoulder portion CD, and the reverse portion DE. The shape of the characteristic curve changes as the flushing conditions change.

Toe (AB section): The slope of the curve in this section is very small, almost parallel to the abscissa. There is no direct proportional relationship between the darkening density and the exposure logarithm after exposure. The exposure value increases much more than the sensitivity value, and the density value increases little. This part of the curve is also called the underexposed part. The unexposed part of the photosensitive material, after development processing, also produces a density, as shown in Figure A.

This density is called fog density.

Straight line section (BC section): Since the exposure value logarithm is directly proportional to the density value, the characteristic curve is expressed as a straight line in this section. The straight part is also called the correct exposure part. This part can correctly show the different brightness of the scene. The light and shade levels of the image are displayed in full scale. So the straight part is the main part of the entire characteristic curve and must be fully utilized.

Shoulder (CD): When the exposure increases to a certain value, the exposure amount continues to increase, while the density increases less.

This part is also called the overexposed part.

Reversal part (CD part): As the exposure increases, the density value does not rise, but instead tends to decrease, causing the characteristic curve to bend downwards. This part of the curve is called the reversal part and is a special kind of sensitometry. phenomenon.

As the exposure increases, the maximum density is called the maximum density, expressed as Dmax. The maximum density reflects the top of the crucible (as shown in Figure D).

Different kinds of photosensitive materials have different requirements for Dmax. Failure to meet the specified technical specifications will directly affect the quality of the image.