Re: Pressure Plate Reflectance

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

Andy,
Yes, that's right. This is what an anti-halation backing is for. When
the light reflects off the pressure plate, it basically fogs your film.
This additional exposure increases the apparent speed of the film, but
lowers the sharpness of the image. 

I've always been curious of how much speed it does add. We've got a
spectrophotometer somewhere here at work but it only measures out to 730
nm. I'll try measuring the pressure plate. I suppose I could extrapolate
beyond 730 nm to get a rough estimate. I'll also try measuring some 120
backing papers.

Another way to find out might be to make a pinhole camera but instead of
using a film holder, suspend the film at the image plane with nothing
behind it except some black velvet light baffles. I've found  black
cotton velvetine to be the best "black" background available, especially
in IR.


Dave












Andy Finney wrote:
> 
> If I tape non-ir-reflective tape over the pressure plate in the
> camera I presume that will reduce the light bouncing back through the
> film and therefore reduce the exposure as well as reducing the
> halation? Is that correct.
> 
> If so, assuming a completely ir-black plate, how many stops would I lose?
> 
> Regards
> 
> Andy
> Invisible Light
> www.atsf.co.uk/ilight/
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