No Subject

[John Sparks <sparks@xxxxxxxxxx>]

tom@xxxxxxxxxxxxxxxxxxxxxx (Tom Benedict) writes:
>Here's my understanding of why IR film (Kodak HIE, at least) should be
>loaded in the dark.  I'm bound to have at least some of this wrong, so
>feel free to correct me:
>
>Kodak HIE has no anti-halation backing.  Anti-halation backings are
>typically very good absorbers, and keep light from bouncing off the
>back of the film and re-exposing the emulsion.  Films without these
>backings tend to show halos around bright objects because of this
>effect (hence the term anti-halation).

The anti-halation backing is very effective in preventing light striking
the film during exposure from bouncing back and making halos around
bright objects, but has very little to do with stopping light piping.
Light piping works by total internal reflection (look this up in a
physics book if you are interested).  None of the lightpiped through
the film leaves the plastic film base so it would never get a chance to
be absorbed by the backing.

It's the lack of an anti-lightpipe dye in the film base itself
that is the problem.  In one of Kodak's books (I think it's called
"Professional Black and White Films" or something similar), they
describe the various layers, backings and dyes used in film manufacture.
For roll film, in addition to the anti-halation backing, a gray dye
is added to the plastic of the film base.  For the 0.007" or so
thickness of the film, this dye is essentially transparent (it adds
slightly to the base film density), but looking at the film on end,
the density of this dye adds up to the point where it is opaque after
about 1" of thickness.  

This anti-lightpipe dye has no effect on preventing halos and is not
used in sheet film.  I also wonder about the truth of the felt light
trap leaking light in IR.  Large format film holders also have felt
light traps in the darkslide slot, but I've never had problems with
fogging IR from light entering through there and I've found no leaks in
testing even when having the film holder in sunlight for maybe 15
minutes with the darkslide in place (not all of the felt is blocked by
the darkslide even with the slide in place).  I do cover the camera
back with my darkcloth before removing the darkslide, but I do this for
regular sheet film as well since the darkslide compresses the felt and
may not totally rebound after removing the slide, even regular film 
can be fogged if sunlight is shining directly on the opening.

John Sparks
--
Bye,

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<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 13