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Anti-UV layer in films???



 
 
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  #1  
Old January 24th 04, 05:17 PM
HypoBob
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Default Anti-UV layer in films???

A couple of years ago I read an article about UV photography and learned
that one problem that UV photographers have is that most color films
have a UV blocking layer that keeps UV away from the emulsion.

Does anyone know if any black and white films, particularly the newer
T-grain versions, have built-in UV blockage?

Thanks,
Bob

  #2  
Old January 24th 04, 08:05 PM
Jean-David Beyer
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Default Anti-UV layer in films???

HypoBob wrote:
A couple of years ago I read an article about UV photography and
learned that one problem that UV photographers have is that most
color films have a UV blocking layer that keeps UV away from the
emulsion.

Does anyone know if any black and white films, particularly the newer
T-grain versions, have built-in UV blockage?

I do not know, but I very much doubt it, since there is no particular
need for it. The shorter ultra-violet would be blocked by the glass in
the lenses. The problem with the blue and ultra-violet in color films is
mainly that you do not want them to excite the green and red sensitive
layers, so they put a yellow filter between the blue-sensitive (top)
layer and the others. It is probably mainly colloidal silver that would
be removed in a bleach or fixing step.

I suppose there could be an ultra-violet blocking layer over the top of
the film; I guess this could reduce the apparent sensitivity of the top
layer. The books I have read do not mention that, but the most up to
date one I have is from about 1966 (Theory of the Photographic Process,
Third Edition), and they may have changed since then.

For B&W, anyone who needed it could use a weak (or stronger, of course)
yellow filter such as a #6.

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  #3  
Old January 24th 04, 08:48 PM
Norman Worth
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Default Anti-UV layer in films???

The yellow filter in the emulsion will block UV light from the red and green
sensitive layers. I don't know about the possibility of additional UV
blocking filters over the blue sensitive emulsion, but it could make sense.
You wouldn't use conventional color film for true UV photography anyway.
They make special (and expensive) lenses and thin emulsion films for the
very spcialized area of UV imaging, although you could probably get down to
around 350 nanometers with conventional equipment. Focus shift may be
important.

What many people call UV photography is color photography of fluorescent
objects. If that is what you want to do, you want to block the UV light
that excites the fluorescence. Otherwise, the UV light will overwhelm the
film and give you simply an overexposed, blue image. Generally you need to
use an additional UV blocking filter to capture the fluorescent colors. A 2E
is about the minimum. If you do not have important flourescence in the blue
region, use a No.8 yellow filter.

"HypoBob" wrote in message
...
A couple of years ago I read an article about UV photography and learned
that one problem that UV photographers have is that most color films
have a UV blocking layer that keeps UV away from the emulsion.

Does anyone know if any black and white films, particularly the newer
T-grain versions, have built-in UV blockage?

Thanks,
Bob



  #4  
Old January 24th 04, 08:55 PM
HypoBob
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Posts: n/a
Default Anti-UV layer in films???

Jean-David Beyer wrote:

HypoBob wrote:
A couple of years ago I read an article about UV photography and
learned that one problem that UV photographers have is that most
color films have a UV blocking layer that keeps UV away from the
emulsion.

Does anyone know if any black and white films, particularly the newer
T-grain versions, have built-in UV blockage?

I do not know, but I very much doubt it, since there is no particular
need for it. The shorter ultra-violet would be blocked by the glass in
the lenses. The problem with the blue and ultra-violet in color films
is mainly that you do not want them to excite the green and red
sensitive layers, so they put a yellow filter between the
blue-sensitive (top) layer and the others. It is probably mainly
colloidal silver that would be removed in a bleach or fixing step.

I suppose there could be an ultra-violet blocking layer over the top
of the film; I guess this could reduce the apparent sensitivity of the
top layer. The books I have read do not mention that, but the most up
to date one I have is from about 1966 (Theory of the Photographic
Process, Third Edition), and they may have changed since then.

For B&W, anyone who needed it could use a weak (or stronger, of
course) yellow filter such as a #6.

Jean-David,

One thing that triggered this question is that the Kodak TMax films are
said to have a "reduced blue sensitivity", to the point that some say
the unfiltered films perform as if there were a weak yellow filter on
the lens. It seems that an easy way for Kodak to achieve this effect
would be to transfer an already 'tried and true' technology from their
color lines to the TMax line.

If Kodak is blocking UV for us at the film plane, then that UV filter is
one less piece of glass that needs to be between the subject and the
film in critical applications.

Bob

 




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