U of S | Mailing List Archive | alt-photo-process-l | Re: Yellow tents and UV (was: outdoor gum demo)

Re: Yellow tents and UV (was: outdoor gum demo)

I've been searching in vain for a graph of UVA and UVB intensities
across the day, graphed together (I'm not trying to prove anyone
right or wrong here, I just want to know how these look in relation
to each other) but can't find anything.

In the meantime, I came across a great graph of the weightings that
are used to calculate the UV index.


Scroll down to the third graph, the one labeled "Erythmally weighted
spectral irradiances." (It's McKinley and Diffey 1987's "erythmal
action spectrum" that provides the weightings that are used to
calculate the UV Index). This shows rather dramatically how little
UVA figures into the Index.


On Apr 22, 2009, at 9:35 AM, Katharine Thayer wrote:

On Apr 21, 2009, at 7:36 PM, Paul Viapiano wrote:

Remember when we were talking a little bit re: this on the Hybrid

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


So, yes, it makes complete sense that your sun exposures don't vary
with the UV index; it would be odd if they did.


----- Original Message ----- From: "Loris Medici"
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
"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
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
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


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
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
the year."