How did the application from developing photos to copying machines evolve?

Everyone knows that photos are developed by copying technology. In the darkroom, the photographer covers the developed film on the photographic paper coated with a layer of silver salt photosensitive material, and then uses light to make the photographic paper "photosensitive", that is, the light makes the silver salt in the light-transmitting part of the photographic paper undergo photochemical reaction and reduction, while the silver salt in the light-transmitting part does not react, thus obtaining an image consistent with the film picture. Then put the photographic paper into developer and fixer, and a well-defined and colorful photo will be presented.

Now let's introduce another copying technology, which is an indispensable helper in modern offices-photocopiers.

The copier is a big family. There are many kinds of them. The most representative is the electrostatic copier. The working principle of an electrostatic copier is similar to that of developing an image, which is divided into charging, exposure, development and transfer, but it is more complicated than developing an image. Moreover, the carrier of copy imaging (images and words) is not photographic paper, but ordinary white paper, which is very different from developed images.

Schematic diagram of working principle of copier

The main part of the copier is a drum-shaped photoconductor (selenium drum) coated with a layer of selenium semiconductor, which is characterized by its photosensitive characteristics. In the dark state, it is easy to be charged by external high-voltage electric field, with electrostatic charge, which can be released under illumination. This characteristic is called "photoconductive effect" of semiconductors. Because selenium is a P-type semiconductor, the surface of selenium drum is generally positively charged. This layer of positive charge is somewhat similar to the photosensitive material of photographic paper. "Charging" is the process of "brushing" a layer of "photosensitive material" on the selenium drum. Next, the graphic image of the manuscript is exposed and projected on the selenium drum, so that the charge is released in the light and stored in the dark, thus leaving an electrostatic latent image consistent with the manuscript. This process is called exposure. Because the electrostatic latent image is invisible, the process of changing the invisible latent image into a visible image is "development". In this process, it is necessary to use charged toner with the opposite polarity to the latent image for adsorption, and at the same time let the charged white paper pass through the selenium drum, and the toner adheres to the white paper. Finally, in order to make the image clear and firm, the "fixing" process is completed by high temperature baking or infrared irradiation, and the whole copying process is over. However, in the actual copying process, all the above processes are completed automatically and continuously, and the required time is only a few seconds.