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 Originally Posted by ic-racer
Blue or cyan filters have no effect on VC or non-VC paper. They block light outside of the paper's sensitivity spectrum. The paper doesn't know or care 
Cyan, yes (in theory; in practice a cyan bulb, especially one made for non-photographic purposes, might not be pure cyan and so might have some effect). Blue, no. VC papers are sensitive to blue and green light, so if you use a blue filter (which blocks green and red light), you end up changing the contrast, since you'll be reducing the amount of green light that hits the paper.
The OP identified the bulb as blue. If that's accurate, I'd expect it would have some effect. Of course, people often misidentify cyan as blue; or the color might be light enough or impure enough or affect only certain frequencies and so have little or no real effect. Testing is the only way to be sure.
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Originally Posted by Steve Smith
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.
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Originally Posted by srs5694
Cyan, yes (in theory; in practice a cyan bulb, especially one made for non-photographic purposes, might not be pure cyan and so might have some effect). Blue, no.
Yes, good point, I was mistaken. A proper blue will add some contrast to each filter grade in a filter set. Its just the non-MG papers that won't care.
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