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These are some really good questions. Here are some of my observations. Scanners typically use either cold cathode lighting or a modified fluorescent tube lighting. The nature of the light generation process in these tubes produces large amounts of UV. What the tube manufacturers try to do is to convert that UV into visible light using the phosphors in the tube. However, that conversion isn't 100%, and a certain amount of UV remains. UV is generally considered to be wavelengths below 400 nm. Looking at the spectral characteristics of the C50 and SPX50 fluorescent tubes, GE makes a good attempt to reduce emissions below 400 nm. However, it doesn't go to 0 until about 375 nm. Fortunately, glass is not a very good transmitter of UV, and I would expect the scanner glass to reduce the UV component even further (but probably not eliminate the higher-frequency UV). So I would expect that a certain small percentage of UV (400 nm - 375 nm) gets through and might cause some small amount of fluorescence of the scanned material. I don't believe UV below 375 nm even gets through the glass. I don't have a UV meter, otherwise I would measure the UV from my scanner. If someone else has one, I would be interested in having them measure the UV. I don't think fluorescence is a big problem, but I haven't tested it. I believe that the spectral characteristics of the scanner light source are fairly well known, and calibrated to a standard color temperataure. When you go to replace the tube, the scanned image will look somewhat different with the new tube than with the old. However, scanner profiles should take the tube spectral characteristics into consideration and compensate for them. The question gets even more interesting because all the 5000K light sources discussed on this list and those I saw at the Seybold show (with the exception of a few halogens) were fluorescent, and these will emit some UV (even the sun emits UV, as those who lay out in the sun too long will testify). Thus, the viewing characteristics under either artificial of natural daylight will probably cause some fluorescence of the inks, and might be similar to the scanner. Viewing though a piece of window glass will probably reduce the UV to what the scanner produces, and "look" the same to the scanner as it does to the viewer. Just my $.02 -----Original Message----- From: email@example.com [mailto:firstname.lastname@example.org]On Behalf Of Laurie Solomon Sent: Friday, February 18, 2000 4:23 PM To: email@example.com Subject: RE: scanner quality when profiling? One issue I haven't seen brought up in relation to scanner quality is the light source used. Since all mixed color printing processes are metemaric to some extent it would seem that the light source and the exact CCD filter responses could have a considerable effect on the accuracy of profiles. For example if the light source contained a large UV component then the fluorescent whiteners in many bright white papers would be activated to a greater extent than they would be under incandescent lights. Even profiling packages that generate scanner profiles from known targets would be affected since the exact dyes and spectral characteristics of the target would be different from the prints being profiled so that a print and a target that looked identical under D50 viewing conditions might appear quite different to the scanner. Do scanner manufacturers specify what viewing conditions the scanner emulates? - Please turn off HTML mail features. Keep quoted material short. Use accurate subject lines. http://www.leben.com/lists for instructions.
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