Black and white printing paper is relatively insensitive to red light so safelighting usually takes the form of a lamp covered with a coloured filter. The colour will depend on the paper you are using and will be marked on the papers packaging. It will probably be amber/orange or deep red. The deep red filter supplied with many safelights will cover both.
So far so good. Unfortunately you can't just stick a 200 watt red light in your darkroom and wonder what all this " dark" business is about. Because the paper is "relatively" insensitive to red light and because the red dyes in the safelight filter are not 100% efficient at filtering the light you can still fog your paper.
- Your safelight is to bright.
- Your safelight is to close to the paper.
- You expose your paper to "safelighting" for to long.
Check your paper pack for safelight colour and check your safelight handbook for a guide on how close it can be. At least 3 feet/1 metre is a good starting point for small safelights. When you have your safelight or lights set up you will have to carry out a test to check the level of safelight fogging present.