"The designers of the meter may not have compensated for IR at all, or they may have calibrated to a black body spectrum, or they may have specified materials or coatings for the meter's lens or the photodiode encapsulation window that did a good job of rejecting IR. We don't know. "
I believe the ISO specification for meter calibration uses a 2850K tungsten light source the has been corrected to 4800K. I don't remember seeing any special precautions made for filtering out the IR output of the lamp.
I'm waiting for someone to come out with a multispectral light meter. It should not be diffucult to do these days, as you can buy photodiodes that have built in color filters. Check this out - http://www.taosinc.com/images/product/document/TCS230-E15.pdf "The light-to-frequency converter reads an 8 x 8 array of photodiodes. Sixteen photodiodes have blue filters, 16
photodiodes have green filters, 16 photodiodes have red filters, and 16 photodiodes are clear with no filters." It'a a tiny little IC package about 5 mm square with a 2 mm square for the diode array. Perfect for a light meter.
The neatest part of this idea I think is that this sensor puts out a signal for RGB and "Clear" - it seems to me that a combination color temp meter/ light meter could be made. Even neater, by coupling it with a bit more microprocessor power then is used in todays meters, you could program the computer to "know" the spectral response curves of several colro or B&W films (or at least film types) and then make exposure calculations based on that info. Metering through filters would be easily done and the loss in exposure easily corrected.
And as the RGB diodes also have different responses to IR (see the data sheet link), one could use an external IR filter or a pair of sensors, one IR filtered and one not, to make direct measurements of the amount of IR present and then either eliminate it from the expsoure calculation, or use the spectral response of the RGB filtered diodes in the IR region to match the filters and films that are commonly used in IR photography. You could have a #25, #87, and #87B setting for the meter or match it to your IR film type - extended red, IR or far IR.
Anyway, wish I knew enough about electronics/microprocessor programming to actually work up the design for something like this... I think a lot of people would buy a light meter/color temp meter/IR meter, with both spot and incident in the same meter, of course. I know I would.
Kirk
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