Quote Originally Posted by gainer
I don't know, for example, what the visible channel or the UV channel are measuring. I ass-u-me that the RGB channels are measuring something related to color separation positives for dye separation or similar processes. Do the specs for a commercially available densitometer provide spectral distribution plots of what these channels measure? If so, what does the visible channel have to do with photosensitive materials, when panchromatic films are more sensitive to red and blue than the eye? How much of the UV that is measured will get through the glass of a printing frame?
I have no specifications at all for what the RGB channels of my X-Rite 810 measure. For the Gretag D-200 there are no spectral distribution plots but I do have information as to the peak wavelength and bandwidth for Visible, Blue, Ortho, Green, Red, UV and Infrared. As I mentioned in another message the peak of the UV reading is 373 nm, and the bandwidth is about 45 nm, which means that nothing will be measured lower than about 350 nm or higher than about 395 nm. Ordinary float glass will pass over 95% of the light in this range and up to 90% at 300 nm. Float glass absorbs virtually all radiation below about 300 nm.

Many alternative processes, including the colloids and metal salts, are highly sensitive to radiation in the 200-300 nm range, but for all practical purposes this light is of absolutely no use unless we devise ways of exposing without using glass between the light and sensitized material. And even if we devised such a means of exposure it would be impractical to use because of the extreme danger posed by UV radiation of this wavelength.

Spectral Power Distribution charts are available for many of the UV light sources used in alternative printing. I have such charts for many of these lights, including BLB tubes and the USHIO metal halide bulb I use in a NuArc 26-Ik plate maker. Most of the radiation in the BLB tubes is between 350 -420 nm. The USHIO metal halide bulb also radiates a lot of energy in this range, but also puts out quite a bit of light above 420 in several spikes even up into the 500 nm range.

So in practice the radiation useful to UV sensitive processes is in the 300 - 420 nm range, with a very rapid drop-off for both colloid and metal salt processes with longer wavelengths.

Sandy