Why do photocopiers use green light instead of, say, yellow, blue or even white?
I don’t think I’ve ever seen one that used green light.
If you are referring to the greenish-looking light leaking out from the side when the cover is closed, you may be seeing the light refracting out the side of the glass, which itself often has a greenish tint to it (much like you’ll notice if you look at the edges around automobile glass.)
I’ve also never seen a green light. Just normal colour.
Green lights were found on some high end copiers back in the 80’s. Presumably having a more monochromatic light source allowed the optics to achieve better focus than possible with full spectrum light.
OK, I’ve done an ofice wide survey and asked only, “What color is that light?” with the copier top open. So far, 50% say green and the other 50% say white. (I recused myself from the survey.)
I wanted to open the machine and look directly at the light without the glass surface, but it sure looks green to me.
However, I should admit that I hadn’t looked at the light directly before I posted the question.
All new “digital” copiers that I see now have a noticeably green-tinted light. The older copiers I’ve seen used white light. Modern business copiers are more computerized, so maybe the green light is more compatible with digital conversion than white light. The new one at the office I work in is more scanner than copier. It scans the originals into memory then prints it out at whatever speed it is capable of, and will enlarge/reduce in the copier’s memory instead of with the equivalent of a magnifying glass that our old one used. There is a small hard drive built in so frequently used forms can be scanned in and saved to be printed at will. It can be used as a network printer, and it has an option to add fax machine capability.
It has to do with the energy in the lightwave. From here
Although some copiers use white lights, I suspect that a monochome light would be more efficient. A green or blue light would be more efficient than a red light, but I’m nor sure of the exact reasons for a green over a blue, since blue light has more energy than green. It could have to do with particular manufacturing advantages of green bulbs vs. blue ones.
My experience seems to be opposite that. I distinctly remember very bright, very green lights in the copiers in my high school library, and this is from the stone ages when you were hot stuff if your Apple ][+ had the 80-column card in it. Definitely far before the age of digital copiers.
Besides getting a better focus from cheap lenses with semi-monochromatic light, I suspect that green light in particular gave a more natural looking conversion of color images to B&W photocopies.
The equation used for conversion of RGB images nowadays typically emphasizes the Green channel; something like this:
Luminance (B&W) = (0.299 x RED) + (0.587 x GREEN) + (0.114 x BLUE)
Another feature important in old copiers, but perhaps now passe, was their insensitivity to (IIRC) blue marks on the original. This was useful for editorial markup of originals that needed paper and glue modifications, rather than modern digital reprocessing. Using a green source lamp would reduce the intensity of the blue light that needed to be filtered in order to achieve this ‘invisible blue’ effect.
[hijack] Older B/W copiers won’t copy green ink, or type I don’t remember why. It just shows as white space. I don’t know if that has changed in digital copiers.
Copiers have used green light as far back as I can remember. I’m not saying all copiers exclusively used green light; only that green has been in copiers for ages (“ages” being relative to my own age, of course ;))
Here’s a web site that has an image and a description of green copier light. (FIND text “green lights”) It’s not scientific in and of itself, but it does show that copiers do indeed use green lights. (Or lights that appear green when the copier is in use.
I have found others showing white light, but I myself associate green light with copiers in general. I believe refraction is involved, but I see plenty of light filtered through glass that does not look green-- including headlights seen through automotive windsheild glass-- so there must be some connection…
That’s a direct consequence of using green light. Green ink reflects green light and absorbs other colors (i.e. red and blue). So if you shine green light at it, the green ink looks the same as white paper.
As for the reason to use green light, I think Squink nailed it. (I don’t know much about copiers per se, but I do know about optics.)
My WAG is because green ink is the least popular. Red, Blue and Black ink are those most used in modern society so a light of any of those colours would be a PITA due to invisible copy effects.