Principle: A photo of a person or thing made by exposing photosensitive paper under a photographic negative and then developing and fixing it. This process is completed in a completely closed darkroom, and it is an image formed by scanning and irradiation of red, green and blue (RGB) laser.

Photosensitive imaging technology magically converts RGB laser into images, and the color of photos grows from the molecular size in photographic paper, instead of digital printing, inkjet printing and other output methods, which are formed by ink accumulation. Photosensitive imaging technology makes the color information of each pixel in the photo complete.

Recognition: The most important difference between photos and other photos is the output mode. The output of photos is made by laser scanning and exposure in a completely closed darkroom (in a large machine), which is different from being sprayed with ink by the machine.

Ba Jin? The first chapter of Deadpool: "There is a big picture frame hanging in the middle of this wall, which is a portrait of a kind woman in her forties." ? Wei Wei? Chapter 12 of the fourth part of the Orient: "When I opened it, it was all in foreign languages, accompanied by a photo of a young western woman."