# Building a light-meter...maybe



## Zelandeth (Dec 9, 2002)

Basically a simple question here. Having found a phototransistor (I think, as the current induced seems to be directly proportionaly ot the light level), would it be possible to actually build my own light-meter. I assume so, as the current reading on the multimeter would be proportional to the light level - in theory. Main thing I'd need would be at least two reference points to create a scale. So, what should I look for as these reference points? i.e. a 60W candescent of a given design having a specific output, and a 100W one a known value as well, and using these vales to "calibrate" the scale. Could this be done?

Zel.


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## Doug S (Dec 9, 2002)

The response function of most photocells and phototransistors as a function of wavelength is very different from the human eye. They are typically most sensitive to wavelengths that are not even light. Photocells that are used to measure light are equipped with special filters to correct their response curves to match that of the human eye.


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## Zelandeth (Dec 12, 2002)

Well, it was at least an idea worth investigating. Even though it's a non-starter; at least unless I can find the datasheets that should have come with the unit.

Thanks for the info,

Zel.


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## INRETECH (Dec 13, 2002)

The device also may not be linear in its sensitivity

Most lightmeters have special sensors and correction hardware/software to correct for this


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## Albany Tom (Dec 14, 2002)

These guys are right. However, with enough time and playing around, you could have something usefull anyway. You could use this to compare two different lights with similar spectral output, like 2 different LED lights. You could play with using photographic filters to tune your sensor to closer to human vision. A UV and IR blocking gel filter would be a good start.

Finally, you could use your sensor as is to measure the brightness vs runtime of almost any light, as Roy has done with his great runtime graphs. To get rough calibrations for brightness, you could use 2 or 3 identical lights superimposed on each other, or even better with some neutral density photo filters.


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