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)

Okay, I keep looking and looking for data on UVA vs UVB; I want to
see a graph of them together across days and years, but all I've
found is little bits and pieces of data here and there, like this bit
I found as a brief mention in a research paper about damage to some
biological system from the depletion of the ozone layer (sorry, I
didn't save the URL, but the paper itself has little relevance here
except for this sentence): "The ratios of 305 and 320 nm radiation
to total radiation increased from dawn to midday, but those of 340
and 380 nm were constant through the day except for shortly before
sunrise and after sunset when they increased."

And then somewhere else I found this dubious assertion: "The ratio
of UVA/UVB in early morning and late afternoon was 1:500." I assume
they've got that backward, first, and I'd bet maybe too many zeros to
boot. I'd believe 50:1,but 500:1 seems just quite unbelievable.
But without real data, I'm just guessing in the dark.

Otherwise I just keep coming across the same assertions without
data: "UVA is the same strength year round-- the angle of the sun
doesn't matter," "UVA rays are present with equal intensity during
daylight hours throughout the year, and penetrate clouds and glass."

But the more I look and the more I think about it, the more I'm
inclined to agree with Loris, this can't be right. Surely the UVA
must vary some. I'd be willing to believe that UVA varies, but much
less than UVB varies. But without data there's no basis to believe
anything, except of course for what one has observed with own eyes.
I'd also be willing to believe that UVA varies less on the Oregon
coast than it does in most other places, but that doesn't explain
Paul's casual observation of fairly constant exposures in SoCal.
In the meantime, I've spent way too much time searching around the
web for data, and will plan to spend some time at the university
library when I'm next in the city. Surely someone must have some
data on this...


On Apr 23, 2009, at 10:20 AM, Katharine Thayer wrote:

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
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
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
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
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
is "present with equal intensity during all daylight hours
the year."