The imaging principle of film

1. The principle of formation of black and white images

Among chemical elements, due to similar chemical properties of chlorine, bromine and iodine, they are classified into the same family, which is called As a halogen family, their compounds with silver elements are collectively called silver halides.

Silver halide has photosensitive properties and can form an image of silver after being irradiated by light. Silver bromide is mainly used in film emulsions. It is a crystal formed by the combination of bromide ions and silver ions, and is distributed in a granular shape of a certain size.

The photosensitivity process of one silver halide particle is the same, and the same is true for countless silver halide particles. So, when a black-and-white film composed of silver bromide grains waits until the emulsion layer is exposed and developed, a black-and-white image composed of metallic silver will form.

The tone of the image is opposite to that of the original scene, that is, where the light is strong, more metallic silver is produced, and the blackness of the image is deeper; conversely, where the light is weak, little or almost no metallic silver is produced. If not, the image will be faint or basically no image. The two images below illustrate the formation of this black and white image.

When a copy is made from this negative, light passes through the negative and onto the raw film. Therefore, less light is transmitted through areas with greater blackness on the film, while more light is transmitted through areas with lower darkness. This restores the brightness of the scene that was reversed during the film imaging process to an image that is consistent with the brightness of the original scene.

2. Imaging principle of color multi-layer film

The photosensitive mechanism of color images and black-and-white images is the same, both because of the photosensitive properties of silver bromide particles in the emulsion layer And capture the light signal of the scene. The difference is that the final image formed on the film is composed of metallic silver for black and white films, while the color image for color films is composed of dyes.

(1) The structure of the color multi-layer film

The color multi-layer film divides the photosensitive emulsion into three layers, and adds different sensitizers to each layer, so that the three layers It is sensitive to red, green, and blue light, but not sensitive to the other two types of light.

At the same time, a chemical substance called a coupler is also added to the three photosensitive emulsion layers. A cyan coupler is added to the red-sensitive layer, a magenta coupler is added to the green-sensitive layer, and a yellow coupler is added to the blue-sensitive layer.

The actual number of layers of color film is more than three layers, because there are isolation layers in the middle of the three layers. Different film types have different layers. Generally, the number of layers can be five to six, and as many as nine or ten. There are also more chemicals in each layer than mentioned above.

(2) Principle of color image formation

We already know that in the imaging process of black and white film, silver bromide generates the image of metallic silver after exposure and development. In each photosensitive layer of color film.

Silver bromide also plays a photosensitive role, but the red-sensitive layer is formed when it is illuminated by red light; the green-sensitive layer is formed when it is illuminated by green light. When the silver bromide development center is formed, the blue-sensitive layer is formed under blue light irradiation, and the silver bromide development center is formed.

When the silver bromide particles forming the development center encounter the color developer, while the metallic silver is precipitated, the generated color developer oxide will chemically change with the coupler in the layer. Dye precipitated.

Obviously, where the light is strong, more metallic silver is produced, more oxides are produced in the color developer, and more dye is precipitated. On the contrary, in places with weak light, less metallic silver is produced, less color developer oxides are produced, and less dye is precipitated. At this time, two images are generated in the emulsion layer, one is composed of metallic silver, and the other is composed of dye.

When color film is developed, it must also go through a process called bleaching. Its function is to reduce metallic silver to silver bromide so that it can be dissolved in the fixer. This leaves only the image composed of dye.

Extended information

Film is basically composed of two parts, one is the photosensitive emulsion that forms the image, and the other is the support base of the emulsion.

1. Emulsion

The main ingredients are photographic gelatin and silver halide. Silver halide is evenly distributed in the gelatin in the form of extremely small particles. The existence of silver halide is the fundamental reason why film can be sensitive to light. When light hits these tiny silver halide particles, it changes due to photosensitivity and ultimately forms an image.

The shape and particle size of the silver halide greatly affect the photographic properties of the film. Gelatin protects and supports the silver halide particles and makes the entire emulsion layer appear film-like. Different types of film also contain other chemical components in the emulsion layer.

2. Film base

A transparent, soft plastic film with a certain degree of toughness and strength, which plays the role of supporting the emulsion. Because of its existence, the film is in a strip shape and can withstand the tension and wear of various mechanical equipment during the entire process from shooting, printing to projection.

There are two types of film bases currently in use. One is triacetate cellulose film base, which is mainly used for motion picture film (negative film) and photographic film. The other is polyester film base, which is mainly used for movie film (positive film), X-ray film, aviation film, etc. The nitric acid tablet base used in the early days has been eliminated by the world because of its flammability.

3. Other components

In addition to the emulsion layer and film base, film generally also has a gelatin bottom layer that connects the emulsion and gelatin and a coating to prevent static electricity and curling during the photography process. , in color film there is also an anti-halation layer to improve image clarity. With the advancement of manufacturing technology, film quality continues to improve, and coatings become more sophisticated.

Baidu Encyclopedia-Film