Calorimeter (laser power meter) target.... paint???

[300EVIL]

Hey guys,
I just got my Scientech 365 in today and noticed the target disk is a little worn from some serious multi-watt lasers. I painted it black with a sharpie which seems to work pretty damn good. Are there any other methods to get this close to original spec? The sharpie sorta burns off with 300mw. Also, Anybody know of a way I can somewhat effectively calibrate a laser power meter with only one laser power meter?? Probobly not, but I figured it was worth a shot.
Thanks!
Adam

[Laser Ben]

You can if you know the output on a specific laser. It is best if you have 2 meters though one that is precalibrated and another under test. You need to be able to adjust the meter sensitivity though. The best for painting the head would be epoxy with black dye in it. Make sure it is rated to 2000 degrees though!

[300EVIL]

You can if you know the output on a specific laser. It is best if you have 2 meters though one that is precalibrated and another under test. You need to be able to adjust the meter sensitivity though. The best for painting the head would be epoxy with black dye in it. Make sure it is rated to 2000 degrees though!

I could see mixing epoxy with carbon not dye but that would increase the thickness of the target plate and epoxy is usually quite glossy also. I tested the meter with a He-Ne that's marked 5.8mW and it was reading 6.1mW so it shouldn't be too far off.

[The_Doctor]

I posted on this at length:
http://www.photonlexicon.com/forums/...0935#post20935
(Other posts in that thread mention a way to calibrate it.)

You certainly don't need to have it withstand 2000 degrees. If it were at that temperature it would be radiating energy fiercely with a bright orange/yellow light. And carbon is the way to go, not a black dye. The whole point is to conduct the absorbed power to the thermopile. To allow it to get to 2000 degrees, that thermal 'conduction' would have to be truly dismal.

All you need to do is spread the beam so the power density isn't extreme. My DIY coating will give good results with a 200 mW beam 0.8 mm wide giving no hassle. You need to focus that beam to a few micrometers wide before the power density becomes damaging. With a beam 8 mm wide, 30 watts could be measured briefly*, which should answer Buffo's point too, here:
http://www.photonlexicon.com/forums/...6450#post26450

* I advise spreading it to 20 mm wide if you're going to monitor it for more than a minute or so.

[eyceage]

What you need is called lamp black - you should be able to get it at any paint store.

[The_Doctor]

What you need is called lamp black - you should be able to get it at any paint store.

If it was as simple as that I'd have said so.

Lamp black, a deep black pigment consisting of amorphous carbon in a very fine state of division, is obtained by the imperfect combustion of highly carbonaceous substances. When resins, resinous woods, fatty oils and fats, paraffin and paraffin oil, or coal-tar oils, are burnt with an insufficient supply of air, a considerable part of the carbon they contain may be deposited in the form of soot. This soot is not, however, pure carbon, but retains variable proportions of the tarry products of imperfect combustion; these tarry products impart to lamp black a more or less pronounced warm brownish hue, except in the cases in which it has been prepared by processes specially designed to remove the tarry products. Today, lamp black is procured by the imperfect combustion of oils obtained in coal-tar distillation. A fine, light, fluffy powder is derived by collecting the soot from the burning oil. It is the most familiar of the pure carbon black group of pigments.

Source: http://www.naturalpigments.com/detai...UCT_ID=480-50S

Lamp black has two problems, one is the presence of part-burnt fat or oil, the second is a colour that can change the sensitivity of a sensor significantly with wavelength.

If you read that chunk I looted you'll see it mentions amorphous carbon. There are far better sources of that, and the best are food grade. Two specific sources are ideal, both in powder form. One is supplied for medical use, absorbing toxins from the stomach contents, the other, which is where I got mine, is in sachets for fining the extra strong alcohol brew kits sold by firms such as Condessa. It's highly pure, dry, anhydrous in fact, no impurities. And you get to choose your binder and concentrations. And if you follow the method I described you'll get a matt black that rivals the best natural soot deposit.

If you get the pure carbon powder, the limiting threshold for damage will be set by the binder material. Thinned yacht varnish (polyurethane) is pretty good, though if you're going to be measuring 20 watt lasers you'll probably want to find better. Even polyurethane is a LOT better than the fat residue in lamp black.

Edit: Further problem with lamp black is that the binder is so weak that even a low power laser, if the beam is narrow, can have power density enough to burn off the binder material. Even if the carbon remains, you might significantly change its ability to conduct heat to the thermopile, making it inaccurate and possibly useless, especially if the binder material turns to pale ash, increasing reflection. Polyurethane varnish is a relatively good heat conductor, in a thin layer on beryllia ceramic, so it will not likely get hot enough to burn unless you focus a strong laser on it.