MadSci Network: Chemistry

Re: Why is film ruined when exposed to yellow light?

Date: Sun Feb 11 09:37:18 2001
Posted By: Gareth Evans, Senior Research Associate
Area of science: Chemistry
ID: 980734681.Ch

Thanks for your question. I'm going to answer it different ways depending on what I will assume is the type of photographic material is being processed. In one, I will assume that the "film" you mention is camera film. Alternatives are colour paper or B&W paper and the answer is different for these.

First let's deal with the film. You described a scene in a movie in which people were looking at an image developing in a dark room lit by red light. If they were really using a modern film, either B&W, colour negative or colour positive, this would not be possible. We'll have to put it down to "artistic licence". Films are designed to record the full visible spectrum and while developing they are sensitive to all visible light including red and yellow. A red light would ruin the image by making the red-sensitive layer develop to its maximum density all over.

If they were developing a B&W print however, they would be able to use red safe-lights. There are exceptions to this but it is a credible situation. This is because the photographic paper is designed to be insensitive to red light so that people can see what they are doing. The B&W film would probably be "Pan Sensitised", that is sensitive to the whole visible spectrum but the image produced in the film is neutral and any light can be used to expose the print paper, as long as the paper is sensitive to that light. B&W paper is usually sensitised with dyes to make it sensitive to the blue and green regions. This allows shorted exposure times or smaller enlarger lens apertures which are better for focussing the image.

If they were developing a colour paper, they could not use red light because the layer producing the red-absorbing dye, the "cyan" dye is red- absorbing and makes that layer red-sensitive. It is the deep red light to which it is most sensitive. This is because some time back, the photofinishers who made colour prints for the public liked to use sodium lights in their safe-lights. Sodium lights can be made so that almost all the light is yellow and confined to a very narrow line (589nm I think). By pushing the sensitivity of the red layer of the paper out to the long red end of the spectrum, a gap of low sensitivity could be opened up between the green and red layer sensitivities. So, interestingly, if your movie actors were using colour paper they should have been more worried about the red light than the yellow! In practice they would have been well advised to work in near total darkness but this would not have made for a very interesting movie scene!

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