Re: Yellow tents and UV (was: outdoor gum demo)
On Apr 21, 2009, at 7:36 PM, Paul Viapiano wrote:
Remember when we were talking a little bit re: this on the Hybrid
forum?
Can you point me to a thread?
So far, I've noticed no change in exposure times from UV Index 4
through UV index 9 when exposing via the sun...
Given how the UV index is calculated, this makes perfect sense. The
UV index is heavily weighted toward UVB, being intended as a measure
of the risk factor of overexposure to the skin, and is not at all a
useful measure of UVA. Here's a great site that runs through an
example of how the radiation is weighted by wavelength. The amount
of radiatiion at 295 nm is weighted 1.0 (in other words all of it is
counted in the index, because it is very potentially damaging to the
skin); the amount of radiation at 305 nm is weighted at .22 and the
amount of radiation at 325 is weighted at .03, in other words it is
considered only 3% as damaging to the skin as radiation at 295, and
therefore has almost no weight in the calculation of the index. I
think it would be a reasonable assumption that as the wavelengths
increase into the range that's useful to us, they are given even less
weight in the index.
http://www.serc.si.edu/labs/photobiology/UVIndex_calcutation.jsp
So, yes, it makes complete sense that your sun exposures don't vary
with the UV index; it would be odd if they did.
Katharine
Paul
----- Original Message ----- From: "Loris Medici"
<mail@loris.medici.name>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, April 21, 2009 11:05 AM
Subject: Re: Yellow tents and UV (was: outdoor gum demo)
I don't understand this at all; does the sun change it's irradiation
spectrum (in terms of UVB, UVA, visible and infra-red) through
"our" year?
I don't think so, therefore it's total nonsense to assert (and
believe)
"UVA is present with equal intensity during all daylight hours and
that it
doesn't vary with season". That would mean that in winter (where
we are
farther away from sun) the proportion of UVA in sunlight should
have to
increase actually whereas the proportion of UVB, visible light and
infra-red light all should decrease, as shown with the facts that
we don't
get sunburnt in the winter (except some extreme conditions such as
high
altitude/mountains and highly UV reflecting soil/snow) (UVB),
winter sun
is paler (visible), and winter temperatures are lower (infra-
red)??? How
come the sun knows that it should act that way -> according to our
year/seasons? ;)
I thought to use the UV index for sun exposures, because I was
thinking
UVB and UVA levels are closely related (given there's the same
amnt. of
ozone above and the sky is clear) because the irradiation spectrum
of the
sun is relatively constant (again in terms of UVB, UVA, visible and
infra-red). I still hold that idea...
21 Nisan 2009, Salı, 8:02 pm tarihinde, Katharine Thayer yazmış:
...
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.
...
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."