--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Mark" <mark@...> wrote:
> Well, technically speaking, glass-filtered Xenon light sources reproduce theUV/VIW content ...
Oops, "UV/VIW content"should have read UV/VIS content which is simply technical shorthand for the ultraviolet-visible light energy ratio in the spectrum.
Many researchers in the field of fade testing paints, textiles, photographs, etc., get overly insistent on specifying the UV/VIS content must be similar to noon day sunlight, thus insist on Xenon arc/sunlight simulation, and even say we shouldn't be measuring Lux, rather we should be measuring radiometrically in terms of Watts per square meter (W/sq. m) incident on the sample. This is academic posturing IMHO. In any case, one needs to specify the light source spectrum one is using. Once you know that, whether you track the photopic energy band (Lux), or the UV band, or the total energy band with a thermopile is irrelevant. In each case, one can reconstruct what energy was available in each part of the spectrum. But if you don't know the spectrum, them no measuring method will allow you to assess efficacy of the fading source. For example, if you inadvertently used a UV sensor to measure UV filtered light falling on a print, you'd say there was no light at all on the print! Thus, from a practical point of view Lux as a unit of measuremtn is much more accessible to photographers and printmakers. You can buy a Lux meter for as little as $30, and there are even simple ways to estimate lux using a camera meter. So, I specify lux because it's better for photographers, and then the exposure dose becomes Lux hours. When it gets to millions of lux hours in test, it's easier just to write Megalux hours.
Then too, there's the endless debate on the importance of the UV in light fading. Yes, the UV energy is the most potent, photon for photon, and as you go to longer wavelengths, ie. blue to green to red, the fade inducing energy per photon drops significantly. But when you look at the entire body of literature on light fading for fabrics, paints, photographs, etc. you learn that natural daylight has a UV/VIS ratio of about 26%, glass filtered sunlight about 15%, indoor interiors lit by natural daylight through window and bouncing off walls about 10%, whereas glass filtered fluorescent gets down to about 3%, tungsten similar. Then you look at what tests faded what and how much more damaging was total direct sunlight contribution to the result? Well, direct sunlight exposure of UV sensitive objects (some aren't that sensitive) versus totally filtered UV content (using filters to cut off essentially all energy below 400 nanometers) results in fade rates of about 2-5x worse. But for many modern photographic materials that have UV inhibitors and other UV blocking strategies built into the product, to the end-user the benefit of UV blocking glazing is typically about 2-3x factor and in examples like Crystal Archive significantly less than 2x. Now, here's the crux of the matter. The industry has been also been using a standardized illumination level of 450 Lux for 12 hours per day to give you a normalized prediction of years on display. But real world light levels even within just one home let alone in different homes around the world, vary by 2-3 orders of magnitude! In other words, your choice of location for print display is very likely to cause a 10 to 100 times error for better or worse than the prediction. Again, the reason AaI&A reports accumulated exposure dose rather than extraplating to a normalized display year value. The UV or no UV argument with its 2-5x factor of significance begins to get lost in that real world of indoor environments. I'm not saying don't be concerned about UV content in your light source, but I am saying even if you filter all of it out, you need to be highly aware of the general illumination level. I think John Dean was trying to say the very same thing.