Re: Filter/Speed/Meter

["R. C. Lacovara" <lacovara@xxxxxxxxxxx>]

Many thanks for the replies to my question about metering IR.

Mr. Elliott confirmed something I had suspected: that light meters either 
(a) had to have little or no IR sensitivity inherently (due to the 
selection of light-sensing element) or (b) their sensitivity was destroyed 
by the use of a filter. I am considering "surgery" on my spare Nikon 
body... it's hardly the F series, but it only cost about $200...

I understood that meter response needed to resemble that of the film 
typically used with the meter. A meter with substantial IR sensitivity 
could be fooled by some atypical scene with undue IR content, and would 
hardly yield good results for ordinary film. Hence the filter mentioned by 
Mr. Elliot.

I was surprised this weekend to find that in some inexpensive monochrome 
CCD cameras that an IR filter was in place. It was not a discrete filter, 
such as Mr. Elliott describes in the Nikon metering system, but a coating 
on one element of the lens. I had in the past used such a CCD camera to 
examine landscapes while using the Hoya R-72 (because I got a good deal on 
it, about $25 in the 52mm version). This weekend a friend and I were 
looking at the sun's spectrum through a cheap prism with such a camera, 
because you can see the IR portion at the end of the red with the camera, 
but not, of course, naked eye. Well, you can if  the manufacturer hasn't 
clobbered the IR response!

You can now tell, of course, that my interest in IR is not merely film, but 
a few other applications. Nevertheless, I will experiment with some meters 
to see if I can't get reasonably good results. I have taken note of Cocam's 
IR material, as well as several other sites. I also appreciate the time 
taken by grinch@xxxxxxxxxxxx to share his settings.

I also experimented this week with a type of IR spot meter. However, it was 
based on a circuit used for spectrometry, and it had far too much 
sensitivity for broad daylight in Houston, Texas!

If I make any headway in my meter studies, I'll be happy to share it with 
this group, and apologies in advance for what may, to some, seem like an 
excessively wordy posting.

Regards,

Bob

R. C. Lacovara, Ph. D.				
Electrical Engineer in Computer Science Drag and
Principal Member of Technical Staff

The Charles Stark Draper Laboratory			voice: 281 333 2132
Suite 210
2200 Space Park Drive
Houston, Texas 77058

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