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)

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?

This is the thread starting at about post 41...

Thanks. Wow, some really dubious advice offered there, such as
recommending the UV index as a "UV lightmeter" for alternative
process printing. (See my previous post for why that's not a good

Also, I should mention that I print in So Cal, under absolutely
clear skies, not a speck of clouds when I print, although there has
been an occasional haze that could affect light as measured in
stops by approx 2/3 to 1 stop, but not enough to put the print into
overcast...more like an f16 to an f11 situation (sharp defined
shadows to less sharply defined shadows).
A haze or even an overcast will affect the amount of visible light,
and on UVB, but will have relatively little effect on the

> 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.


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

> 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
>> 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
>>> 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
>>> 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
>>> throughout
>>> the year."