Cheap photodiode light meter for MPE measuring?

[greenalien]

Just found this device :

http://cpc.farnell.com/jsp/search/pr...questid=554935

The spec says that it can handle up to 400,000 LUX max - I wondered how useful this might be for taking MPE - and possibly even laser power - measurements. How could one convert the data from this device into power readings?

[mixedgas]

Its enough of a pain in the neck to go from photonic to radiometric that you would run a spreadsheet on the laptop to do it. Past experience indicates that they saturate at a few 10s of mWs anyways and that the plastic lambertian scatterers on the detector are not good for coherent light.

Steve

[greenalien]

The spreadsheet on the laptop wouldn't be a problem, and I see it could be necessary when converting from photonic to radiometric at different wavelengths. However, if one were just measuring one frequency (i.e. green 532nm) it could be a lot simpler. I'd like to try an 8mm aperture (artificial pupil) in front of the detector to see how much it picked up from a scanned beam; might need a diffuser and some ND filters as well. The recording bar graph could perhaps be useful for showing variable intensity data as a scanned beam passes across the detector.

As they're cheap, I've ordered one of these meters to play with, so I'll be trying a few things out at some point - I have no doubt that I'll get some data, if a laser hit doesn't zap the photodiode, but interpreting the data will be a whole new ball game....

[mccarrot]

It's much easyer to use the simplified MPE calcultations with a static beam.

With a static beam the value is 10mw/cm2 for MPE and 100mw/cm2 for the more usefull 10x MPE

If you search the forum you can find the calculation to convert from 8mm hole (iris) to mw/cm2

for more info readthis article from Greg:
http://www.laserist.org/files/audien...rview_2pt1.pdf

[greenalien]

The meter has now arrived. The detector seems to have a peak sensitivity spot in the middle. So far, I've just tried it with room lighting (120 lux) and given it a quick zap with a 1mW red laser pointer (3500 lux). The response seems quite fast and there is a peak hold function that will be useful.

Next step will be to dig out some ND filter gel, to attenuate the signal - I've got some small pieces in a gel sample booklet that should do the trick.

The response curve for the meter looks very similar to the human perception curve, so this should be ideal for use with a green laser.

My overall aim with this is not to get 'laboratory quality' measurements but as a way of testing relative brightness from a known base - that way it should be possible to calculate maximum MPE by using a static beam, and work back from there to see how much difference a moving beam makes. I think I will also need to use something like another photodiode or an LDR with an oscilloscope to get a measure of pulse length.

[mixedgas]

My overall aim with this is not to get 'laboratory quality' measurements but as a way of testing relative brightness from a known base - that way it should be possible to calculate maximum MPE by using a static beam, and work back from there to see how much difference a moving beam makes. I think I will also need to use something like another photodiode or an LDR with an oscilloscope to get a measure of pulse length.[/QUOTE]

LDRs are not fast enough.

Make sure you check that unit for linearity and saturation if your going to use it for anything audience scanning.

Linearity: use a polarized 532 nm yag and a polarizer, check and see that you get a plot that matches Malus Law', which is a very common first year physics experiment and is all over the web. Your plot should look like 1/4 cycle of a sine wave.

Saturation: blast it till the display stops incementing.

You've got a cosine corrected lens on that thing, watch out for errors from beam position.

Steve