This is very often one of the most over looked aspects of making prints. Some safe-lights or the over powered lamps used in them and or situation can mean that unsafe light is reaching the paper and over a period of time this can affecting the tones on the paper by fogging it. Distinct differences can be caused between paper that is exposed to this unsafe light for longer than other paper, although they might have the same exposure at the time of enlargement. This stray light can lower the grade of the paper and create a greyness over the whole paper or parts of it. To test for safe-light fog, place a piece of photographic paper in the working area and leave for 5 -6 min. When this paper is developed it should show no signs of a circular mark.