RE: HIE and fogging

["Brewer, Kevan" <kevan.brewer@xxxxxxxxxxxxxx>]

> -----Original Message-----
> From: Jaap Los [mailto:imagefotografie@xxxxxx]
> Sent: Thursday, December 14, 2000 7:00 PM
> To: Infrared Mailing List
> Subject: HIE and fogging
> 
> 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

Remembering back to my school days, the effect is known as 'total internal 
reflection' and is most easily demonstrated using a tall glass of water (G&T 
would work equally well ;-) If you look down the glass, the sides appear 
mirror-like, however, if you look at the sides near the top (still looking at 
the inside but from a less acute angle) the mirror effect disappears and you 
can see through the glass. I don't remember the physics behind it (Snell's law 
rings a faint bell) but the effect only occurs beyond a critical angle, light 
hitting the film at a greater angle than this is not reflected. Optical fibres 
work in the same way.

Another interesting demonstration of the effect can be seen from the bottom of 
a swimming pool, if you look up at the surface (it helps if the surface is 
calm) you will see a circle through which you can see out of water. Beyond the 
circle will appear as a mirror.

Kevan
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