[pure-silver] Re: UV vs IR photography


----- Original Message ----- From: "Koch, Gerald" <gkoch02@xxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, August 25, 2005 12:00 PM
Subject: [pure-silver] UV vs IR photography


Need input as I am unfamiliar with studio lighting.

I ran across an interesting entry on APUG from a photographer who claims
that as a professional photographer [his emphasis] he spends hundreds of
dollars testing B&W films. He states that one reason for doing this is
that strobe lights generate a lot of UV radiation and this radiation
makes the capillaries in the skin very prominent. He therefore must
determine the UV sensitivity of any film he wishes to use. I find this
very hard to believe for many reasons, among them the amount of UV light
present, transmission cutoff by the glass in the bulbs, in the lens,
....

He erroneously states that UV radiation penetrates "several millimeters"
into the skin and then is reflected back to the camera. He argues that
if he is not careful in his selection of film everyone comes out looking
like W.C. Fields. Now, UV-B radiation (which penetrates the farthest)
is attenuated by over 90% by passing through 2 millimeters of skin.
Therefore the amount reflected back into the camera cannot be more than
1%. Hardly enought to appear in a photograph.

My question, has anyone experienced the problem that this guy states
about capillaries being particulary evident in photos taken with strobe
lighting? Or is this guy full of himself?

The problem he states is true for IR radiation and IR film which makes
human skin appear waxy and translucent. But you shouldn't see this
effect with ordinary B&W film.

BTW, one reads the most appalling statements on APUG.

Jerry

There is enough UV in the output of some strobe lights to cause some dyes or other materials to fluoresce causing a color shift in the recorded colors. For this reason all professional strobe units have either built-in UV filters or accessory filters. The sensitivity of the film is of no importance, the problem is not reflected UV being recorded by the film but rather fluorescence caused by UV from the light. The film testing story is hogwash.
OTOH, IR _will_ show capillaries but this effect requires real IR film and an IR filter than removes all or nearly all visible light. No conventional film has enough IR sensitivity to produce this effect although Technical Pan with a deep red filter might do it. I don't know how much IR comes out of strobe lamps, tungsten or sunlight would be a better choice.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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