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Re: IR measurement trick...ery?
On Thu, 30 Mar 2000, Ray Rogers wrote:
> I also know that I have been told that ordinary light
> meters CAN NOT measure IR because they do not sense
> it.
The answer here is: "It depends".
The older meters used photo cells that work on the photo electric effect.
These cells generate a current when exposed to sufficiently energetic
light that can be measured by the meter circuit. However, for a current to
be generated by these cells the incident photons of light have to have
enough energy to exceed the work function of the photo cell material. Blue
light is more energetic than red light and as such photo cell based
sensors won't work well, if at all, for measuring IR. Photo cell based
meters often have no batteries and consist primarily of the cell and a
galvanometer to measure the photo current.
The newer meters make use of photodiodes or photo transistors. These
devices are definitely sensitive to IR and it would be possible to use
them as an IR meter. These kinds of meters also require a power supply
(battery) to work. My concern would be that the meter manufacturer
probably would have already installed an IR blocking filter in front of
the sense element to minimize the devices IR sensitivity. Ideally you'd
want to remove any such filters before installing your IR pass filter.
- Wayde
(wallen@boulder.nist.gov)