From: David J. Romano (romano@agfa.com)
Date: 04/10/00-10:22:31 PM Z
I've been away from the forum for about a week, so I'm behind in this
thread. One thing that I think is causing confusion here is the
assumption that all meters are created equal. Two different meters, by
different manufacturers probably give about the same exposure reading
for a neutral gray scene because the electronics are calibrated that
way. However, why assume that every light meter has the same spectral
response, from UV to IR? That would mean that every meter/camera
manufacturer is using the same sensor manufacturer and part number.
Check out www.hamamatsu.com or www.udt.com. Look at the variety of
sensors available. Also, see that the peak sensitivity for silicon is
not 500 nm. It is in the 850 nm - 950 nm range unless it has been
specifically designed otherwise.
I have a Canon A-1 that I expose Kodak's IR film at about ASA 10000 or
so. I have a digital spot meter that I bought from Adorama that I have
to set to about ASA 3200 to make it agree with my Canon. I also have a
Pentax ME Super. Should I assume that the same ASA number is correct or
if will even work at all? Are the meters really measuring IR? I set up a
TV remote control in a dark room and attempted to read it with both
devices. One problem that I was aware of with this is that the remote
control does not emit a continuous beam of IR, but rather pulsed beams,
of different frequencies and/or duty cycles. This is how the TV knows
what the different commands are. So, I wasn't going to get an accurate
reading of the remote control's LED power, just learn if the meter could
see it. The spot meter gave readings in the 5-7 EV range. The numbers
varied because the sampling frequency of the meter was not the same as
the remote control's output frequency. But, it certainly read it. The
camera had a bit harder time reading the flashing of the LED and gave
odd readings that jumped around quite a bit. But again, the camera
definitely saw the IR energy.
So, whatever kind of meter you happen to have, try this. Don't expect
them all to read the same. Some may not work at at all, I don't know.
I've used an ASA of about 10000 reliably for about 7 years, so I'm not
going to be convinced that it doesn't work.
Dave
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