Yellow tents and UV (was: outdoor gum demo)
Loris wrote, after I responded to his suggestion that the tent should be yellow by saying that I don't believe the color of the tent makes much difference since it's been demonstrated that the visible color of a thing is not directly related to its UV-blocking capacity:
16 Nisan 2009, Perşembe, 10:29 am tarihinde, Loris Medici yazmış:
> Oh, come on!!?? My understanding of the discussion was that yellow "ink"
> is the most opaque to UV light compared to other colored "ink"s (there
> wasn't named a single inkset which doesn't exhibit this - BTW, black is
> another case and I won't get there...), but it is not necessarily more
> opaque than combination "colors" (mostly; Mark told about R1800 where
> yellow "ink" is the most opaque including combination "color"s in the
> comparison...) for whatever reason (probably because there's more ink on a
> given area then when printing with a single ink). And all above was given
> in the context of many different inksets!
> Most importantly, being highly sensitive to sunlight, I happen to burn
> more under say red, blue, white beach umbrellas compared to yellow ones ->
> empirical evidence to me, bringing us to my original suggestion of a
> yellow tent...
First a comment about the last paragraph: Here you're conflating UVB and UVA. The rays that cause sunburn are UVB, wavelengths 280-320. Those rays don't go through glass or any deeper than the epidermis of the skin, and are of little concern for gum printers. UVA (320-400) is the range we're interested in. UVA is of less concern for sunburn, passes through glass and through the skin deeper into the body, and contains the wavelengths we use to print gum. So it's something of a logical leap to assume that any observation related to sunburn might also relate to the fogging of gum emulsions.
But more than that, I spent much of this weekend searching for any evidence that yellow blocks more UV, even UVB, than other colors, or any evidence to support the general theory that the visible color of a fabric is related in some predictable and lawful way to the UV- blocking property of the fabric, and I just can't find any. I've searched the sites of companies that specialize in selling products to sun-sensitive customers, and the same beach umbrella, available in different colors, will have the same UPF rating (a measurement of transmitted UV) in yellow as it has in red or blue.
I found one site, for a UV-blocking fabric intended for sails and awnings and such (Global Sail) that gave different ratings for colors, but their ratings are not favorable to the yellow-blocks- most theory. Colors that block 95-99% of UV: black, forest green, leaf green, silver grey, desert sand, charcoal, royal blue, bright red, chocolate brown. Colors that block 90-94% of UV: light green, mulberry, medium green, navy blue, marine blue, turquoise. Colors that block 85-89%: rust gold, mist green, terracotta. Then finally, down at the bottom, are sunflower yellow at 81% and white at 78%.
I studied the sites of skin cancer foundations and the American Cancer Society, and although they provide lengthy discussions on their pages about how to protect yourself from harmful rays, and about the relative blocking power of different kinds of clothes and hats, nowhere is there any suggestion that wearing yellow, or using yellow in an umbrella, will provide more protection than other colors. When they say anything about colors, they say that in general, the darker the color the better.
A number of research studies done in the last 15-20 years have established that weave tightness and weave structure are the most important factors in UV protection provided by a fabric, followed by the type of fiber. Dyed fabrics tend to provide more protection than undyed fabrics, everything else held constant, although "chemical UV-absorbers, essentially colorless dyes" provide as much additional protection as a colored dye does. And darker colored dyes, all other things held constant, offer more protection than lighter colored dyes. But one Japanese study demonstrated that dark colors that offer more than 90% UV blocking don't lose that UV- protection if the color fades, so it's not the color per se that's providing the protection, but something in the structure of the dye that doesn't change when the color fades.
So, Laura, if a yellow tent is the only color that's available, then use it, as long as the fabric is heavy and tightly woven. A yellow tent would probably feel more cheerful inside, but is more likely to provide less UV protection than a darker color.
One last thing: while I was doing this search, I found the answer to a question that's puzzled me for years. Conventional wisdom shared among alternative process workers, at least as I've seen it given on this list, that UV varies depending on time of day, season, location. So it's always puzzled me that on the northwest coast of the US, an area not noted for its high UV levels to start with, I could expose a gum print in the sun in less than a minute, same time in summer or in winter. That didn't make sense to me, until this weekend when I learned from a skin cancer foundation site that it's just UVB, the kind that's not useful for gum printing, that varies by season, location and time of day. UVA, the UV we're interested in, is "present with equal intensity during all daylight hours throughout the year."