Re: Stupid Questions

[m.j.gudzinowicz@xxxxxxxx (Michael J. Gudzinowicz)]

"Daniel Goodale" <goodaled@xxxxxxxxxx> writes:
>Fellow IRDers,
>	I was thinking last night (always a dangerous thing) about 

Not really... you motivated me to check some spectral data for filters,
and I made a few "discoveries" which are outlined below.  I apologize if
I've covered "old" ground.

>filters.  I have two 25 red filters and when I put them together it
looked a whole >lot redder or is it darker red.  Would two medium red
filters add up to a dark 
>red filter?  I'm thinking it doesn't because it would not change the
wavelength
>of the light that got through, just the amount.  But, for the purpose of
IR
>effects on film, will 2 filters block the low wavelengths more and let
the
>same amount of IR light through?  Could I do this with a ND filter or a
>polarizer. Kind of a poor man's 87?  Hope I'm not plowing old ground.

Unfortunately you won't get anywhere near an 87 by combining two #25
filters, but you might find the desired effect by combining the # 25
tri-color red with tri-color blue or tri-color green filters. 

The #25 filter cuts off 99.9%+ of the light below 580, and transmits 79%
at 600 and 89% above 610.  If you use two identical filters, their
optical densities at any wavelength would be doubled.  So the
transmission spectrum would be less than 99.99% below 580, 63% at 600,
and 79+% above 610.  The cutoff for the filter has shifted only a few
nanometers by using two in combination, since the curve is fairly sharp.
The combined effect is similar to a #29 filter. The visible increase in
"red" density of the combination is just that - the combined filters
transmit less light above 610, but the effect is uniform through the IR. 
Therefore one has a speed loss (= density), but exceedingly little
increase in the cutoff.

If you are using Konica film, the #70 filter provides shifts the cutoff
up to 640, with "good" transmission (80%+) above 700 nm and enhances the
IR effect a bit, without much loss of speed.  The #87 results in
excessive speed loss with
Konica without improvement in the "effect" over the #70, but it is fine
for Kodak's film.

Some filters can be combined with a #25 filter (to block UV, blue and
green), and the result is a higher red cutoff since the filters have
significant absorbance in the red and near IR wavelengths. Using a #25
with a 44A (minus red) filter give a broad cutoff in the 700 to 900 nm
region (the #45 is similar). A #25 plus the blue #47(C5), gives a sharper
cutoff from 710 to 780 nm, which approximates the #87 fairly closely.
Although I don't have the IR transmission for the C4 #49 tri-color
series, I'd expect it to be similar to the C5. Also, the green tri-color
#58 series (B2 filters) will push the red cutoff into the 700-750 nm
range, and of course, a tri-color red #25 is required to block visible
wavelengths.

Note that the effect of the tricolor blue and green filters is due to
nearly complete red absorbtion which drops markedly in the near IR.

The absorbtion of most ND filters drops a bit above 700 nm, however, not
enough to be useful in combination with other filters.  Likewise, blue
color correction filters will have little effect since their red
transmission isn't attenuated very much.

 

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