Re: About filter reflection factors

[Tom <chowt@xxxxxxxxxxxxxxxxxxx>]

On Wed, 13 Nov 1996 "Willem-Jan Markerink" wrote:

> 
> On 12 Nov 96 at 0:38, Vaughan Bromfield wrote:
> 
> > >So what I need is the spectral reflection coefficient of HIE, as well
> > >as the spectral sensitivity of the OTF flash sensor. That isn't much
> > >to ask, is it? <grin>
> > 
> > If you have the filter between the film rails then the otf sensor won't be
> > seeing the film, it'll be seeing the filter, won't it?
> 
> Yep, but to estimate the difference, I must have the reflection 
> coefficient of both filter and film....;-))
> Only the first doesn't help much, since I have no reference of what 
> it should be with normal film.
> 
> And 'a' reflection coeffient is also not enough, I need it for
> visible and IR, the latter if I use an IR filter in front of the
> flash as well (you can't just pull the BTFR filter off when the
> camera is still loaded with film). The only data known (for EOS) is
> for no BTFR filter (aka bare reading....:-)) and a filter in front
> of the flash. 
> 
> Oh well, I guess I'll just have to bracket and reverse calculate the 
> reflection coeffient difference....8-))

Hi,

Just a thought, but what if you place the IR filter at an angle parallel
to the mirror. Most OTF flash sensors are on the floor of the mirror
chamber, so it would then only see the film. One less variable, and the
OTF sensore will not be seeing any white light!. Ofcourse this probably
would only work on pre-autofocus cameras as it would interfer with the
sub-mirror (which is also present on manual focus cameas like the F3, LX,
OM-4's etc...)

The reason I mentioned this is that the emulsion side of the film is 
somewhat matt, it is designed to absorb photons after all. Resin filters 
are rather shiny, and you would probably get alot of specular reflection 
from the surface, which can adversly affect your OTF sensor. Hey, maybe 
you could get your IR filter anti-reflection coated! ;->



Tom

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Topic No. 3