Re: HIE and fogging - thoughts

["David J. Romano" <romano@xxxxxxxx>]

Jaap,
Here's my thoughts on the matter. I have not thought to test whether the velvet
light seal is IR safe because we all know you simply can't load/unload in the
light. However, if it were IR safe, I guess the film wouldn't be fogged at all,
unless IR radiation is getting in through the bottom or top of the canister. I
think part of the answer to your question is the angle of incidence and the IR
absorbtion of the surrounding areas:

If the light entered through the velvet slot, it would likely be conducted by
being reflected between the inside of the can, which is probably just anodized
with no special consideration to IR, and the back of the film.

Being that it strikes the film at a very shallow angle, any IR radiation will
probably travel further though the roll of film, getting partially absorbed and
fogging the film along the way, than if it were to strike it at a right angle,
as it would during normal exposure. During normal exposure, IR will scatter as
it penetrates the film, but it is quickly absorbed by the emulsion. It will not
light pipe beyond a microscopic distance.

The idea of the film acting as a light pipe probably sounds sexy and may come
into effect here, but I think you'd agree that the chances of the end of the
film leader being located exactly at the inside entrance to the slit, is slim at
best. I think the only way this can happen is if you rolled the film into the
can and stopped just short of  pulling all the way in. I'm not so sure the
antihalation layer would prevent light piping.

During normal exposure, the film is flat against the pressure plate and the IR
radiation comes in at about 90 degrees to it. In this case, some of the IR is
reflected off the film, some is absorbed by the films dyes and some passes
through to the pressure plate. The pressure plate is black to visible light, but
I would not assume it is black to IR. You can see this when you have a textured
pressure plate, like on some of the Pentax cameras (I had an ME Super). The
light bounces off the pressure plate and re-exposes the film from the back
giving it an overall fog exposure. If the pressure plate is not smooth, the
texture is recorded on the film. The anti-halation layer prevents this. So, the
answer to the audience member's question is YES, lack of the layer DOES cause
fogging during normal exposure! You just don't think about that way!  This would
certainly be one reason why Kodak IR film is so fast. Basically, without the
anti-halation layer, every expoure gets a flash exposure. I would imagine this
fact alone causes a pretty large increase in film speed. If the Maco film did
not have the anti-halation layer, it would probably be alot faster.

Hope this helped more that it confused. It can a fine line sometimes.

David Romano



Jaap Los wrote:

> Hi all,
> When I gave a representation about IR photography last week, I got a
> question from the audience on which I didn't know the exact answer. I said
> that possible fogging of HIE during loading/unloading  in daylight could is
> caused by:
> 1)  the lack of an antihalation layer whereby the film acts like a fibre
> glass cable so that the light spirals from the beginning untill the very end
> of the film within seconds.
> 2) the velvet slot in the cartridge allows IR light to pass.
> Referring to sub 1) somebody in the audience wondered why  the lack of an
> antihalation layer doesn't fog the film during normal exposure (once the
> film has already been loaded in absolute darkness). In my opinion it was a
> good question. During exposure the IR light gets a chance to enter the
> polyester film base and why shouldn't it spiral up and down through the
> whole film?  I said it doesn't because the exposure is very short. I am
> however not sure this was the right answer. Can anybody enlighten me on this
> matter?
> Greetings,
> Jaap Los
> The Netherlands
>
> See also my infrared photography homepage at:
> http://home.wxs.nl/~losjb/hometest.html
> with lots of infrared photographs, some of them lith-printed.
>
> See alo my website
> http://home.wxs.nl/~losjb/JBLhomepage.html
> for those who are interested in company matters
>
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