Redscale film is a technique of shooting through the back of the film instead of the front therefore exposing the film from the opposite side. This results in an image which has has a strong cast of red/yellow.
Photographic film (C41 colour print) is usually made up of 3 colour emulsion layers, with each layer being sensitive to a different light colour. The bottom layer is red, sensitive to red light, next is green and the top layer is blue. Underneath the blue layer is a yellow filter which removes the blue light and allows the green and red layer underneath to be exposed. There is also an Anti-halation backing which prevents light from bouncing off the back of the pressure plate (camera back) and re-exposing the film.
All of the layers are sensitive to blue light, however the blue light is usually removed by the yellow filer before it hits the green and red layers. So when we reverse the film and shoot through the red emulsion layer first the blue layer is no longer exposed.
(For much more information on film layers for colour print film see wikipedia: http://en.wikipedia.org/wiki/C-41_process).
The Anti-halation backing of the film is very think, in order to penetrate this you will need to expose your shots from at least +2 stops above the the films box speed. E.g. Film rated at 400 iso, shoot it at 100 iso. The more the film is exposed the yellower it will be. There can be substantial differences depending on the brand of film used.
Here are a few of my experiments with redscale film.
The above information is based on my understanding, if anything is incorrect please let me know.
In my next post on redscale film I will cover using colour filters, cross processing redscale, redscale with slide films (E6) and re-shooting exposed redscale the correct way.
How to make redscale film: