Re: BTFR question

[Clive Warren <Clive.Warren@xxxxxxxxxxxxx>]

At 9:19 pm +0800 28/4/00, Steve Hodges wrote:

>Clive Warren wrote:
>>
>> >Or reflected off it.
>> >
>> >Steve
>>
>> Yep, good point.
>>
>> Depending on the filter material and how the coating is carried out.  I
>> would bet that 95% of the light hitting the filter makes the trip through
>> to the film and the then 95% of the reflected light from the film makes it
>> back through the filter to the light sensors.
>
>Ah!  95% of the IR goes through.  But what of the visible light?
>
>(The answer you were supposed to give is that the filter material is
>glossy rather than matt, and that reflections are likely to be specular
>rather than diffuse.  As such, reflections from the filter should not
>reach the sensor)
>
>I thought of that after I posted the original.
>
>Steve

Steve,

Sounds good, but wouldn't there be some specular light reaching the light
sensors? The amount of specular reflected light from a BTFR filter must be
very small.

Anyway, re. the visible light answer I was supposed to give - well that may
be the case if the filter resembles a half-silvered mirror (as some do)
however the normal BTFR filters are a polyester base with an infrared
coating which absorbs visible light and converts it to heat.

There are many  other types of filter some of which cancel out certain
wavelengths with diffraction gratings set at certain wavelengths. I do have
a number of very strange scientific (and expensive) filters which reflect
visible light. Will try them out with a camcorder when I get a few moments
to see how they perform with IR.  These filters are typically narrow-band
so may not be that interesting for use with film but could be ideal for
digital camera use.


All the best,

             Clive   http://www.cocam.co.uk
                     Photographic Services, Filters and Equipment, Infrared FAQ


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