Please help me see the light
April 12, 2012 1:30 PM   Subscribe

Please help me find a component photocell (photoconductor) that works well in the IR range. I'm trying to find a photocell that works in the micrometer wavelength range. The market is full of Cadmium Sulfide (CdS), but those peak at a little less than 1 micrometer wavelengths.

I dusted off the cover of my old optics book and within it I see that there are several types of photoconductors that go into the 10's of micrometers but I am having a hard time finding them for purchase on the Internet. Lots of papers come up, but no retail outlets. It looks like a Germanium based photoconductor may be the best bet, (Ge:Hg, Ge:Cu; or Ge:In).

What I'm trying to do is detect ignition of a really small test sample within a dark chamber. Some samples the CdS photocell I've been using works fine. Other test samples don't seem to emit much visible light, and I suspect that they emit more light in the IR than in the visible.

I'm looking for just the component. (I.e., a photocell with throughhole leads).

As always, Thanks for your input :).
posted by nickerbocker to Science & Nature (9 answers total) 1 user marked this as a favorite
How bright is your source? If photoresistors are not sensitive enough, you might consider using a photodiode. In particular, indium gallium arsenide photodiodes are sensitive to the near-infrared (peak around 1.6 μm).
posted by RichardP at 1:52 PM on April 12, 2012 [1 favorite]

Here's a IR receiver diode. A glance at it's datasheet might shed some light on whether it will be useful for your purposes. you could also use the detector portion of something like this.
posted by ianhattwick at 2:24 PM on April 12, 2012 [1 favorite]

The source is probably not very bright at all. These are teeny tiny samples that are igniting.
posted by nickerbocker at 2:35 PM on April 12, 2012

Depending on your technical savvy and how much you want to invest in this, you can probably get much higher sensitivity if you replace a photoresistor like your CdS photocell with either a photomultiplier tube or an avalanche photodiode. Both can in principle get you to single photon sensitivity.

You could also look at something like this.
posted by pombe at 4:04 PM on April 12, 2012 [1 favorite]

I don't have my Infrared Handbook handy, but one place to get a good sensor, cheap, is a PIR sensor. They have a pyroelectric sensor you can harvest right on the board. It's sensitive in the range of body heat, which is pretty long-wavelength (8-14 micrometers).

The coolest thing about this is that most PIR sensors just have a 5V/logic output or relayed output so dealing with the circuit is trivial if detection is all you want.

Otherwise my go-to is a BPW-34 photodiode with an appropriate cold mirror.
posted by fake at 4:28 PM on April 12, 2012 [1 favorite]

One thing to keep in mind is that CdS sensors are *really* slow. A photodiode or phototransistor will give you much faster response.
posted by HiroProtagonist at 9:16 PM on April 12, 2012 [1 favorite]

Pyroelectrics are slow, too- is your purpose to detect the time of ignition? if so, a PIN diode might work... if the energy is REALLY low, a photomultiplier might be the solution- you can get 'em with responses in the 1.6 micrometer range, and they respond very quickly.
posted by drhydro at 10:05 PM on April 12, 2012 [1 favorite]

Have you searched for InGaAs (indium gallium arsenide) diode? It has much better response in the micron range
posted by Yavsy at 3:28 AM on April 13, 2012 [1 favorite]

Thanks for all the help guy! I'll post back when I get some more parts and see how well they work.
posted by nickerbocker at 7:34 AM on April 13, 2012

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