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light meter sensitivity
Light meter sensors are generally constructed out of silicon these days ( my
belief). Silicon is very sensitive to red and near infra-red. What happens
as the wavelength lengthens silicon becomes more transparent to IR. Photon's
go deeper into the silicon, beyond the "detection well" so they are not
converted to electrons therefore no signal and then no sensitivity. There
is a specification for the largest wavelength silicon can see but I forget
it at the moment.
I do not know what wavelengths IR. film is sensitive to or perhaps you can
buy different film for different wavelengths.
Actually silicon ( the metal )was used as lens material for infra-red a
while ago.
So filters just block the other light ( noise relative to infra-red ) that
the sensor can see.
-----Original Message-----
From: Ray Rogers [mailto:earthsoda@yahoo.com]
Sent: Thursday, March 30, 2000 1:31 PM
To: alt-photo-process-l@skyway.usask.ca
Subject: IR measurement trick...ery?
OK! I admit it! I am confused!
It seems that many people are using an 87 series IR
filter and an EI of 3000 - 12800 to get usable
exposure data from visual light meters for infrared
film.
How does the IR filtered Light meter actually work in
practice? Are we certain that it is in fact measuring
IR? Can readings be made in total darkness? (With of
course an IR source present)!
I realize that there are several filters used and they
each have their own ... whatever you call it, and they
may or may not pass a small amount of actinic,
non-infrared light.
I also know that I have been told that ordinary light
meters CAN NOT measure IR because they do not sense
it.
Do ordinary light meters have the ability, even
weakly, to sense IR? Could this commonly repeated here
say have been faulty info?
If light meters can read IR, but are not very
sensitive, then I can understand how this method
works. Otherwise....perhaps it's back to the leaky
filter theory....
Drip, Drip, Drip...
Ray
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