Quote Originally Posted by Steve Smith View Post
Some grain focusers come with blue filters to allow better focussing as you are only seeing the light the paper sees. A blue bulb could have the same effect but this may not be the reason it was fitted.
Patrick Gainer did some tests that were published in Photo Techniques a while back. I believe the article was titled "Hazards of the Grain Focuser" or something similar. He found that using blue light is not the best choice for focusing; using either green light or white light works best. Using blue or red light results in misfocusing. This isn't about what the paper sees; it's about what our eyes see; the lenses in the human eye are really pretty bad in many respects, and in particular they focus different wavelengths differently, resulting in focus errors when you focus using light of certain colors.