The development of black-and-white photosensitive materials can be reflected in four aspects: sensitivity, shape, technology and photosensitive speed (the arrow indicates the direction of increase).

1. Sensitivity: color-blind film (only sensitive to blue and purple) → color separation → full color → infrared film.

2. Form: metal plate → wet plate → dry plate → soft plate → rolled plate.

3. Process: single layer-high analysis rate, low sensitivity and small tolerance.

↓?

Multilayer-low analysis rate, high sensitivity and large tolerance

↓

Multi-layer and thin layer-coated with emulsion with low, medium and high sensitivity can improve the definition and tolerance, and the sensitivity can be greatly improved through strong development.

↓

Dye imaging-no silver salt image, fine particles, large tolerance and variable sensitivity (ASA 125 ~ ASA 1600).

4. Photosensitive speed: slow film → medium film → fast film → fast film.

The development trend of photosensitive materials is that the output of color film greatly exceeds that of black and white film; The coating of photosensitive emulsion develops to thin layer, multi-layer and high sensitivity; The research of non-silver salt photographic system (dye imaging, diazo film, magnetic video recording) is actively carried out, and the processing technology of photosensitive materials is developing rapidly to high temperature; The coated fiber substrate and triacetate fiber substrate coexist simultaneously.

Second, the structure of black and white photographic film

Tablet matrix carrier of 1. emulsion

(1) Requirements for Film Substrate

① High light transmittance (above 90%); ② High mechanical strength (good elastic strength; High breaking strength and impact strength, more folding times); (3) high geometric stability (small temperature content and shrinkage, not easy to deform); (4) Good chemical stability, light resistance, heat resistance, acid and alkali resistance, and no chemical effect on the latex layer.

(2) Film base type

① Nitrocellulose ester tablet base. Flammable, use less. ② Cellulose triacetate tablet base. Non-flammable, poor dimensional stability. ③ Polyester nylon chip base. Non-flammable, stable, expensive, conductive and difficult to coat. ④ Polycarbonate tablet base: Great potential.

2. Latex layer

The main components of emulsion layer are photosensitizer, proppant, sensitizer and additive.

(1) photosensitizer

The main component of photosensitizer is silver halide. Among them, silver bromide is the fastest sensitive and silver iodide is the slowest. Silver bromide usually adds 1% ~ 3% silver iodide to the negative film. Silver chloride is commonly used in positive films, and then some silver bromide is added. Silver halide exists in the form of tiny crystals, which are equiaxed and generally imperfect. However, some of its defects and dislocations and the moving silver ions in the lattice can play an important role in the formation and exposure of the photosensitive center. The presence of impurities may also be a photosensitive center. The crystal diameter of high-speed wafer is large (1. 1 micron); The diameter of silver halide crystal in the negative is 0.9 micron, and that in the positive is 0.3 micron. The larger the diameter, the higher the sensitivity, the greater the particle uniformity and contrast, and the smaller the tolerance; On the contrary, the contrast is small and the tolerance is large.

(2) Proppant-gelatin (animal bone glue)

Silver halide is insoluble in water and must be separated without contact to produce a clear image, so gelatin is used. The function of gelatin is

① protection. Silver halide crystals are isolated from each other and evenly distributed, so that unexposed silver halide crystals are not infected and developed during development, thus ensuring clear images.

② Photography activities. Colloid contains a very small amount of sulfide, which can produce photosensitive center and improve sensitivity. It can also be combined with bromine and chlorine generated during exposure (Brˉ+hυ→Br+e)(g++ e→Ag)(hυ photon; E: free electron) prevents halogen from combining with silver to form silver halide (i.e. dehalogenation). In fact, this also improves the photosensitive efficiency of the emulsion.

③ Gelatin is easy to change from one state to another, which is convenient for preparation and processing. The disadvantage of gelatin is that its trace components change with the change of animal life history, so its photographic activity is very inconsistent. Some people want to use polymer compounds instead, so far there is no major breakthrough.

(3) Sensitizer

Sensitizers can be divided into chemical sensitizers (gold thiocyanate, sodium thiosulfate) and spectral sensitizers (spectral sensitizers).

Spectral sensitization can not only obtain photosensitive materials with different color sensitivities, but also make it possible to obtain various artistic effects by using color filters in photography, which lays an essential foundation for the manufacture of color films and color photography.

(4) additives

It mainly includes antifogging agent, hardener, preservative, antioxidant and surfactant (reducing the surface tension of emulsion and facilitating uniform coating on the film base).

3. Protective layer

Gelatin with a thickness of about 1 micron prevents the latex layer from being rubbed to produce friction fog. This will affect the resolution and sensitivity to a certain extent, so now a more solid method is used.

4. Bottom layer-adhesive layer

Gelatin and a small amount of film-based solvent are dissolved in water or solvent and coated on the film base, which corrodes the surface of the film base and makes it uneven, so that the latex layer is firmly attached to the film base through the action of the bottom layer.

Below the emulsion layer of photographic paper is a barium bottom layer, which consists of barium sulfate crystals and gelatin aqueous solution. It can fill the gap on the paper base and prevent the emulsion from infiltrating into the paper base. At the same time, due to the high reflectivity of barium sulfate, the reflection density of the part without shadow in the photo is very small, so the contrast of the photo can be improved.

The polyethylene resin layer coated with plastic photographic paper can prevent the liquid medicine from infiltrating into the paper base, save the liquid medicine and do not need polishing when drying. Titanium dioxide can also be added to the plastic coating under the latex layer to improve the reflectivity of the paper base and the contrast of the image, marked as RC. Kodak products can be developed by adding reducing agent outside the plastic coating (only in 2% NaOH solution for 30 ~ 50 seconds), and the general development time is 1 ~ 3 minutes; Fixed 1 ~ 2 points; Was with water for 4-5 minute; Dry for 2 ~ 3 minutes.

5. Photographic back layer

The back layer has the functions of anti-halo, anti-static and anti-curl.

① Anti-halo

② When the film is exposed, the light passes through the emulsion layer to reach the film base, and then passes through the interface between the film base and the air, and some light (strong light) will be reflected back to the emulsion layer, resulting in exposure. After the film is developed, a halo will be formed around the image of a bright object, so it must be coated with anti-halo substances (silver, dye, carbon black) for absorption.

(2) Anti-static

The film base is an insulating material, which is prone to the accumulation of electrostatic charges and will produce dendritic shadows on the emulsion layer. Moreover, due to the attraction of static electricity, the film base is easy to absorb dust and sundries, which will scratch the negative film, so antistatic substances (salts of polymers with good conductivity and specially produced organic electrostatic agents) are needed.

(3) Anti-curling

In order to prevent the substrate from curling to this side of the latex layer after coating, an anti-curling layer must be coated on the other side of the latex layer to balance the stress on both sides of the film. Therefore, film-forming substances (such as ethyl fiber resin and polystyrene) or a suitable mixture (such as a mixture of acetone and formaldehyde) are used to treat the back of the film base, so that it expands under the action of solvent and shrinks during drying. The stress caused by this shrinkage can make the coating emulsion curl to the emulsion layer to achieve equilibrium.

Third, the performance of black and white photosensitive materials

1. Sensitivity:

Sensitivity S is an index indicating the photosensitive speed of photosensitive materials, and it is the main basis for determining the exposure combination and taking high-quality photos during photography.

The international standard sensitivity is marked as ISO, such as ISO 100/2 1, ASA 100, and DIN/2 1(ASA is the American national sensitivity standard). In ASA sensitivity standard, every time the number is doubled, the sensitivity is one level worse. In DIN sensitivity standard, it is logarithmic, where ASA 100 is consistent with DIN2 1, ASA50 is consistent with DIN 18, ASA200 is consistent with DIN24, and so on.

2. Density

Density (d) refers to the amount of silver salt reduced or dye formed per unit area after exposure of photosensitive materials, and also refers to the degree of image blackening.

Density is expressed by logarithm of light blocking rate, and light blocking rate = projected luminous flux/transmitted luminous flux.

D=lgO=lgF？ 0/F

Where o is the light blocking rate =F? 0/F(F？ 0= projected light quantity, F= transmitted light quantity)

It is expressed by lg because the value of F0/F is sometimes very large, and the perception of light and shade by human eyes changes according to logarithmic law.

3. smog d? o

Haze d refers to the density of the unexposed part of the film after development (including the density of the film base). The factors that affect the fog density are the development conditions (time, formula and temperature) of the film itself; Preservation conditions; Conditions of use; Especially, after the developer is polluted by the fixing agent, there will be a dichroic fog (brown when transparent and blue-green when reflective).

4. Granularity and image resolution

The thickness of silver particles forming an image is called particle size, which is an objective measure of particle size. Generally, the granularity of photographic film is between 5 and 20, and the larger the value, the coarser the particle. Particle size is affected by the following conditions:

Particle size and coating uniformity of (1) emulsion.

(2) Overexposure will make it thicker.

(3) The development time is longer and becomes thicker. ? It thickens when the temperature is high. ? The higher the PH value, the stronger it becomes.

Resolution, also known as analysis rate, resolution and discrimination rate, refers to the ability of photographic emulsion to record the details (quantity) of the scene. It is often expressed by R (line/mm). No100/21film R=70 lines/mm, positive film R= 100 lines/mm, low-speed particle film R= 1800 lines/mm.

The main factors affecting the resolution are the thickness of silver particles and production technology, and the properties of developer, development conditions, exposure and resolution of lens will also affect the resolution of photosensitive film.

5. Clarity:

Sharpness refers to the clarity of the boundary of every detail in an image, and the image with completely separated boundaries has high clarity.

Generally, it is observed by human eyes to make a general evaluation.

Main factors affecting clarity:

(1) Film itself: particle size, thickness and dyeing of emulsion.

(2) Development conditions: When developing, boundaries with different densities will produce adjacency effect (edge effect), which can increase the density difference on both sides of the boundary, that is, increase the contrast of screen fineness, thus improving the definition.

(3) Exposure time: HD when appropriate.

6. Color sensitivity

Refers to the sensitivity and range of photosensitive materials to various colors of light. If the color sensitivity is good, there are many layers. Color sensitivity is related to darkroom safety lights. The safety standard of the safety lamp is 1 m from the photosensitive material, and the safety lamp will not fog up after exposure for 30 seconds.

Commonly used safety lights are:

Color-blind film-red light;

Color separation-dark red light;

Full-color film-full black (developed to a certain extent, it can be viewed in dark green light, because human eyes are sensitive to yellow-green light in strong light, most sensitive to green light in dark light, and full-color film reacts slowly to green light).

7. Tolerance (also known as extensiveness or flexibility)

Refers to the ability of photosensitive materials to show the brightness interval, which is the allowable range of photosensitive materials during exposure.

The tolerance of black and white film can reach 1: 128. The brightness range of a scene without foreground under direct sunlight is 1: 10? 1:30, so a slight error in exposure can be recorded correctly.

8. Contrast and contrast coefficient

Contrast is density difference, also called hardness and contrast.

The key problem of black-and-white photography is to control the contrast. Because the perception of light and shade by human eyes is logarithmic, the deficit of the scene should be expressed by the ratio or logarithmic difference between the larger brightness and the smaller brightness in the scene, not by the brightness difference.

For example, the brightness of A, B, C and D are 100, 50, 10 and 5 respectively, then

lgA-lgB = LG 100-lg50 = 2.0- 1.7 = 0.3

lgC-lgD = LG 10-lg5 = 1.0-0.7 = 0.3

The factors that affect the contrast are the brightness contrast of the scene itself, the contrast of the film, the degree of development, the shooting conditions, the glare of the camera, the use of filters and the exposure.

Contrast coefficient: γ

Refers to the ratio of image contrast to scene contrast.

γ = image contrast/scene contrast panchromatic γ = 0.8 ~1; Remake version γ=4

Portrait film γ=0.8 Cinematographic film γ=0.65

9. Characteristic curve

γ=tgX

Four, black and white photosensitive materials:

1. Morphologically, it can be divided into

① hard flat glass substrate

② Film-paging film and film.

2. According to the degree of light sensitivity, it can be divided into three types.

(1) Color-blind film-film that can only feel blue and purple, with low film speed, poor color sensitivity and small tolerance. Fine silver particles, high resolution and large contrast. Suitable for feature films, slides and remakes.

(2) color separation-sensitive to blue, purple, yellow and green light, neutral, suitable for shooting objects without red.

(3) Full-color film-can feel all colors of light, with high film speed, large tolerance, good color sensitivity, coarse silver particles, flat contrast and low resolution.

(4) Infrared film-a photosensitive film that uses infrared rays to shoot. It can feel invisible light in the infrared region and is sensitive to blue short-wave light and ultraviolet rays in visible light, so it is necessary to deepen the red filter when shooting. The development process is the same as that of ordinary black and white films.

3. According to the sensitivity, it can be divided into three types.

① Low-speed film-the sensitivity is lower than ISO50/ 18.

② Medium speed film-the sensitivity is between ISO 64/ 19 ~ 125/22.

③ Fast film-the sensitivity is above ISO400/27.

4. New photographic film:

(1) Bora one-step imaging film.

② microbubble (diazo) tablets.

Verb (abbreviation of verb) Selection of black and white photosensitive materials

1. Medium and low speed films should be selected for making large-size films.

2. High-speed film can be used for indoor, sports scenes or stage photos.

3. High speed film should be used for zoom and telephoto lenses.

4. Take black and white words, charts, print slides, and copy the positive film for low-speed color blindness.

5. High-speed film for flash photography is suitable for shooting large scenes.

6. The film can't expire.

7. Don't roll the film too fast to prevent electrostatic reaction.

Six, black and white photosensitive material washing

1. black and white film development

After exposure, the film needs to be treated with some chemicals to get a visible image.

① latent image

The latent image is also composed of silver, which is some silver atoms produced by the exposure of silver halide crystals. They exist on the surface or inside of silver halide crystals and concentrate in the photosensitive center to form the development center.

optical energy

AgBr→Ag+Br

Latent image → pre-latent image → secondary latent image → development center, multiple development centers → latent image (photosensitive center)

② development

Development is the process of producing visible silver by chemical method. The silver obtained by reduction during development is several billion times more than that directly generated by light.

Silver bromide+developer = bromide+silver particles (black)

① Composition and function of developer. General developer consists of developer, protective agent, accelerator and inhibitor.

A. Developers

Mittal: The development speed is fast, the contrast is small, and it is less affected by temperature.

Hydroquinone: mild development, large contrast, greatly affected by temperature.

Phenidone: low loss, small dosage, easy storage and easy fogging.

B. Protective agent: anhydrous sodium sulfite

Na among developers? 2SO？ 3. It first combines with oxygen to prevent the oxidation of the developer, which has the effect of dissolving silver salt and particle effect.

C. accelerator: anhydrous sodium carbonate.

Acidic hydrobromic acid can be produced in the developer, which plays a role in hindering the developer, so Na? 2SO？ 3 increase alkalinity.

D. inhibitor: potassium bromide. It can prevent the unexposed silver salt from being reduced to silver and generating fog.

② Preparation of developer.

A portion of distilled water 750/ 1000ml, 52? c？

B. Weigh with a balance to ensure the quality of drugs.

C. Add one by one (add 10%Na first? 2SO？ 3)。

D. stirring → filtering.

(3) See the book Photographic Chemistry for the formula (omitted).

(4) The methods include basin expansion, box expansion and box expansion.

③ Stop filming: generally neutralize the developing components with weak acid liquid medicine.

④ Fixing: After exposure and development, only 25% of silver salt is reduced to silver particles, and an image is formed on the negative. Other silver halides also have photosensitivity, so they need to be fixed.

① Formula:

A. fixing agent-sodium thiosulfate

B, the pH value of developer-acetic acid is 4.0 ~ 5.0.

C. protective agent -na? 2SO？ 3 prevent hypo decomposition

D. curing agent-potassium alum

E. Buffer-boric acid

② Fixing time: 6 minutes for negative film and 2 minutes for positive film; temperature: 15 ~ 20? c？

Every 1000ml fixer can fix 20 ~ 25 rolls of film; 10× 12 inch image paper 20 ~ 25 sheets.

⑤ Rinse with water for 20 ~ 30 minutes.

⑥ Drying

⑦ Refining, thickening and dyeing (omitted, see the book Photography Formula).

Step 2 make black and white photos

① printing

Printing is a processing method to obtain positive film with the same size as negative film. Printing is completed by the print box, and the general structure of the print box is shown in Figure 3-5.

The specific method is:

① Place the negative film on the exposed glass plate of the printing box, with the drug side of the negative film facing upward.

(2) Press the positive film (photographic paper) face down and press it tightly on the negative film.

(3) Expose the sample (usually within 5 seconds).

④ According to the development of test strips. The development of test strip refers to the change of light intensity (exposure time) and filter density (the change of filter density between light source and exposed window glass) in a specified developer. ) to determine the accurate exposure time.

⑤ After the sample is tested, operate exposure → development → stop development → fixing → cleaning → polishing according to the following process.

② amplification

The principle of magnification is projection exposure.

① Type and structure of amplifier

Among them, the light collecting amplifier (as shown in Figure 3? 6) The light is concentrated, the brightness is high, the contrast is enhanced, and the projected image is clear. However, this kind of magnifying glass is easy to produce negative silver particles, fine dust or spots, or uneven light because of its strong light and concentrated light. This kind of enlarger is often used to enlarge black and white negatives.

Figure 3-8 Astigmatic amplifier (as shown in Figure 3? As shown in figure 7), it is easy to use, with uniform and soft light and weak contrast, and is often used for color negative amplification or portrait negative amplification.

Semi-concentrated magnifying glasses are rarely used.

Now there is a kind of "dual-purpose" amplifier (such as Beselet), which can be easily converted from condensing type to astigmatism type, simplifying the operation process.

② Main equipment for enlarging photos.

In addition to a suitable magnifying glass, you need some other equipment to enlarge photos, including:

A. enlarger lens

Enlarger lens is the most important equipment, which determines the clarity and uniformity of enlarged images. The magnifying lens is generally composed of two groups of lenses with an aperture structure in the middle. The aperture of the magnifying lens is generally F/4 ~ F/ 16. When zooming in, the maximum aperture is usually used to narrow the two-stage aperture, which not only overcomes the shortcomings caused by the spherical aberration of the lens, but also makes up for the diffraction deficiency caused by too many narrow apertures.

Enlarger lens also has different focal lengths. When operating, we should choose the magnifying lens with different focal lengths according to different negatives. Generally, the focal length of the lens is required to be equal to or slightly larger than the diagonal length of the negative. When using large negatives (such as 6-inch negatives) for small magnification amplification or especially negative amplification with clear peripheral images, the exposure time should be appropriately extended to facilitate operation. In order to prevent the brightness of the surrounding light from decreasing, enlarger lens is often used, and its lens focal length is slightly longer than the diagonal of the negative.

B. negative editing

The negative clamp is a device for placing the negative when enlarging. There are two kinds with and without glass plates. This kind of film clip with glass sheet can ensure that the film is flat and the image is clear, and can prevent the film from curling and deforming due to overheating of the lamp holder. But in operation, the possibility of dust pollution increases. When cleaning, it is necessary to ensure that six surfaces (two surfaces of the film and four surfaces of two pieces of glass) are dust-free at the same time, which undoubtedly increases the workload. In addition, Newton's ring may appear when magnifying the negative clamp with glass sheet. Newton's ring is a kind of interference image of light, which is a concentric ring of light and dark caused by repeated reflection and interference when light passes through two very close reflecting surfaces. The base of the negative film is quite smooth, with good planarity and high reflectivity, which forms face-to-face reflection with the glass clamping surface, thus forming the conditions for generating Newton's rings.

When the photo is enlarged with black and white photographic paper, Newton's rings are white stripes, and when enlarged with colored paper, spectral Newton's rings will be produced. In order to prevent the appearance of Newton's rings, it is necessary to fully dry the glass clips and negatives, or use special glass with slightly rough surface to prevent Newton's rings.

C. exposure timer

In order to ensure the accuracy of the exposure time of photographic paper, it is necessary to set an exposure timer, which is divided into three steps: 0 ~ 0.9 seconds, 1 ~ 9 seconds and 10 ~ 99 seconds. The "focusing and timing" actions are controlled by switches.

At present, the time display of some timers is realized by digital tube or luminous digital tube, which is very convenient to use in darkroom.

D. Frame for amplification

There are three types of photo frames used to enlarge photos: standard size photo frames, adjustable photo frames and borderless photo frames. Usually, adjustable frames are the most widely used, as shown in Figure 3? Nine performances. E. Housing tools

When zooming in, you need some special tools of various shapes to do local shading. ③ Amplification process.

Step one. Cleaning the magnifying glass and the dust and dirt on the condenser and lens of the magnifying glass will affect the quality of enlarged photos. When you start work, clean it with blowing balloons, brushes, lens paper, etc.

Step 2, clean and place the negative. Carefully clean the negative to be enlarged with a blow ball and an antistatic brush, and put the medicine face down and the top outward into the negative holder (if a glass splint is used, also carefully clean the four surfaces of the upper and lower glass).

The third step is to select the picture and focus on it. Turn on the safety light, turn off the darkroom lighting, adjust the aperture of enlarger lens to the maximum, turn the timing switch to the "focus" position, lift the magnifying head, select the picture size and magnifying part according to the projection on the magnifying shelf, and then adjust the focusing knob to make the image clear.

Step four, sampling. Turn the timing switch to the "timing" position, turn the timing tape switch to a certain position (i.e. the trial release time), reduce the aperture of the lens of the magnifier by two steps, put a small piece of photographic paper at the selected position of the magnifying frame (usually several positions where the photographic film faces upwards), and press the timing button to expose the photographic paper tape of the sample.

Step 5, develop the sample phase band. Put the sample phase tape into the developer at 20℃. c？ When the normal development time is 2 ~ 3 minutes, (for D? 72 developer) shorter than this time indicates overexposure, and the exposure time should be shortened appropriately, otherwise the exposure time should be extended appropriately.

Step six, enlarge the photo. After the exposure time of the sample photo strip is calibrated, the photo paper to be enlarged can be pressed on the magnifying frame for formal exposure, and then the exposed photo paper goes through a series of processes such as development, fixing, washing, drying (or drying) and mounting, thus completing the whole process of enlarging the negative into photos.

④ Some problems that should be paid attention to when enlarging.

A. The diameter of the condenser of the amplifier is larger than the diagonal of the negative electrode. The focal length of enlarger lens should be equal to or greater than the diagonal of the negative. F50mm lens is suitable for enlarging 135 film, F75mm lens is suitable for enlarging 120 film, and F 105mm lens is suitable for enlarging 6×9 film.

B in order to prevent the interference of stray light, black paper should be used to cover the periphery of the negative. The existing negative clamp is equipped with an adjustable baffle, which is very convenient to use.

C. When focusing, adjust the aperture of enlarger lens to the maximum to obtain the maximum brightness. Focusing should be based on the clarity of human hair, eyes, words and buildings. When zooming in, enlarger lens usually narrows the second to third aperture.

D. prevent vibration during the whole amplification and exposure process.

⑤ Method of adjusting image contrast

There are two ways to adjust the image contrast:

A. select photographic paper with different contrast. The more photographic paper, the greater the contrast. At present, the photographic paper used in China is generally 1 ~ 4, and the contrast increases with the increase of numbers, of which No.2 is neutral contrast and No.4 is ultra-hard contrast, which can be selected according to needs when enlarging.

B. Use different developers (refer to the book "Photographic Formulation").

⑥ Expand processing skills.

The processing skills of amplification include soft light amplification, local light and shade addition, deformation correction, cloud addition, overlap, light and shade separation, relief effect, virtual light and so on. (Quoted from some chapters of Dark Room Encyclopedia).