IR response of meters

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

Someone has suggested that metering TTL for IR works because there is a 
good correlation between visible light levels and IR content in normal 
outdoor scenes: this is certainly plausible. Judging from other folks' 
responses, I'd probably put money on this as the explanation for many of 
the useful results that people have with TTL meters.

On the other hand, when you slap an IR filter on the camera, the TTL meter 
is responding to something... perhaps the little bit of red, or some fringe 
IR which you cannot see, but to which the meter has a bit of residual 
sensitivity?

If you think about it, no camera manufacturer is going to leave much IR 
sensitivity in the TTL meter, nor even a hand held. Otherwise, how could 
you ever hope to correctly meter a scene for color? (In an ideal case, the 
meter response would be exactly the same as the film... of course, this is 
only true in an approximate sense.) Any light source which is for some 
reason "rich" in IR is going to upset the total meter reading, and cause 
you to underexpose the visible light portion.

Here's a bit of an experiment for anyone interested. A TV remote control is 
a nice bright source of IR, though I forget the exact wavelength. I know 
that it's visible with my IR converter and my B&W CCD cameras, and it's 
probably on the order of 800 um or there-abouts... It has to be "bright" 
enough to be seen by the TV in a daylit room. I can virtually guarantee you 
that it will fog IR film! You might try metering the business end of the 
controller to see if your light meter is sensitive to the IR from the remote.

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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