Ophir 20A thermopile head - info wanted please

[greenalien]

I've just got my hands on an Ophir 20A thermopile head, courtesy of fleabay, and would be grateful for any info that anyone has on this device, as Google hasn't been particularly helpful. Even the manufacturers site doesn't list a datasheet that I could find, all I know is that these were custom-made for SLT for use with CO2 lasers, but I think/hope they're usable for visible light wavelengths. The only connector is a BNC socket on the back.

I'd also be grateful for any pointers towards a suitable circuit to build a Laser Power meter - something along the lines of Robin's Die4thing, sadly not currently available, would be great.

[shrad]

I have several stripped heads, and they are thermopile type with a linear mW/mV curve... high power ones tend to have a circular thermistance-like inclusion around the measurement zone, surely for thermal compensation (I have a 1500W one and this is the only thing I think about it)

[greenalien]

linear mW/mV curve

Does this mean that they put out a voltage that's proportional to the laser power that hits them?

BTW - here's a couple of pics

[shrad]

for those I have, there is direct connection from the leads of the sensor to the BNC, and I have a mV/mW spec sheet which is quite linear, so yeah it seems you read the value directly in mV, but you have to apply offset and gain to get useful readings

[greenalien]

Shrad - thanks for that - so I need some sort of amplified meter to get a useful output, not sure what you mean by 'offset' though.

[shrad]

some thermopiles won't start at 0mV for 0mW, more like one arbitrary value increasing with optical power applied

the offset is useful for adding or substracting a small value to the measure, and then you need a gain to tweak for like 1mV per mW or 10mV per mW... something like that

you can verify that by hooking your sensor to a voltmeter on mV scale and shoot your laser on it, and varying intensity (half intensity should read half the voltage at full intensity, minus value at 0mW)

[greenalien]

I'll try that, at least it'll tell me if the sensor is working or not.

[greenalien]

Tried it last night with a voltmeter, got 0.3mV with approx 1W of laser power and 0.6mV with 2W of laser power, so it does seem to be giving a linear response. Does that seem to be the correct level of output?

Thanks to Robin, it looks like I'm going to be lucky and get my hands on a die4thing board soon as well, looking forward to that.

I've noticed that the front face of the detector has a lot of small silver 'pock marks' , presumably where the black coating has been ablated by multi-watt CO2 laser hits from its previous life. I seem to remember a post a while ago that recommended candle soot and cellulose dope as a coating, I'll have to make some up and try it, as at present it's reflecting a lot of the incident beam!

[lasersbee]

IIRC... a "doctor" member a while back used Carbon Black powder and Shelac
mixture as a coating...

Jerry

[mixedgas]

Does yours have the internal op-amp?
If the damage is not too severe, recoat ting them is usually a moot point and the silver streaks don't hit the accuracy that much.
And smoke from a flame with too much fuel (lighter, match) does a better job then the "mixture" which you really have to work at to get thin enough to work. Ie really fine carbon black is hard to come by these days, unless your in the tire business, as they stopped selling it for poison control at the drugstore.
Steve