Lightsensitive films and sensors

We have now almost invented the photographic camera, but need some way of recording the image without actually having to trace it by hand. There are many materials that are sensitive to light. Curtains and carpets and paintwork of all kinds gradually fade under strong illumination. Newspaper yellows if left out in the sun. The trouble with these sorts of materials is that they are much too slow in their reaction - exposure times measured in years would be needed to record a visible picture in the camera. For many years, most cameras used film coated with chemical compounds of silver called silver halides to record the scene. The silver halides are extremely light sensitive and change from a creamy color to black when exposed to light. To construct the film, the silver halides are mixed with gelatine and the resulting light sensitive emulsion is coated onto a plastic backing.

Scientists also discovered that it is not even necessary to wait until the silver halides darken in the camera. You can just let the image light act on it for a fraction of a second, keep the film in the dark and then later place it in a solution of chemicals that develops the silver until the recorded image is strong enough to be visible.

With most films, processing gives us a negative picture on film. Subjects that were white appear as black metallic silver, and dark subjects as clear film. Parts of the subjects that were neither light nor dark are represented as intermediate gray density. The negative is then printed in the darkroom onto paper coated with a similar emulsion containing silver halides. After development, the image on the paper is 'a negative of the negative', i.e. the paper appears white where the original subject was light, black where it was dark and (assuming you are using monochrome materials) a suitable gray tone where it was in between. We have a positive print. The advantage of using negative and positive stages is that many prints can be run off one camera exposure. And by putting the negative in an enlarger (which is rather like a slide projector), enlarged prints can be made. So you don't have to have a big camera to make big photographs.

Figure 7.3 shows, in basic form, the optical and chemical steps in making a black and white photograph. Most pictures of course are shot in color, but the same principles apply. Color films are coated with several emulsion layers, sensitive to blue, green and red. After appropriate processing, color negative film carries images that are reversed in color (blues appear yellow, greens magenta, etc.) as well as in tone. When such a negative is enlarged onto multi-coated color paper the paper responds in a similar way to give a positive print with colors brought back to their original subject hues.

Figure 7.3 Basic stages in making a traditional black and white photograph - from loading and using the camera (top) to processing and printing the film.


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