He mentions the reason in the video, it’s so the surface is non-reflective. Not because of the black color but non-reflective because it’s aluminum oxide instead of aluminum.
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My thinking as well. By the time you find and buy the part…But this the forum where people were seriously considering ripping thin slices off of 2x4s with a radial arm saw to fit the 1/4" slot in the GF Pro. To some, of course, that is considered fun (and they might save a little $), just like some like the challenge and experience of building their own meter. - Rich
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Well, I just ordered the Mahoney and it looks like it is made in China. It will be interesting to see if it ships from there or from here in the US somewhere.
$111.55 including USPS shipping - that’s about 1/3 the price I found for the brand mentioned in the Sarbar video and they look nearly identical so I’m pretty pleased.
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Mine came from somewhere domestic. It was wrapped and padded and packaged so well I swear they thought UPS would be playing football with it. Even had a little wrench to zero adjust it.
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Are they really a simple design to make? Laserbits sells the meters for 1K and will rent them for $119. Seems REALLY steep for a meter (I’d just buy 3 more K40s for that price)… 
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Isn’t the laser path of the Glowforge fully enclosed? I don’t think you can use one of these where the beam is focused. The unfocused beam is usually around 6mm I think and focuses to say 0.3mm so power density goes up by 400.
Perhaps if you can place it in the bottom of the machine it will be sufficiently out of focus to not burn through the anodising.
The mirrors are enclosed but the beam between mirrors is not.
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Laserbits is either making a killing or providing precision measurement that’s way above our needs. A laser power meter is basically just a calibrated thermometer with a laser absorbing probe. You place the probe in the beam for a specified amount of time (mine is 40.5 seconds) and then wait for the temp to stabilize (25 seconds) and the reading on the scale tells you the power although it could just as easily shown how hot the block got.
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I see. So there must be a few windows in the path as well then.
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Yes, I think the design is to keep us from having to dance with cleaning the mirrors. Just wipe off the windows.
I think the latter. Their $1k device is electronic and claims accuracy to 0.1 watt.
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We’ve experimented with a number of laser meters (including the Mahoney) and find that the accuracy is 10% on a good day. We use very expensive meters. 
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That’s as good as I’d hope for my purposes - baselining and then watching for change over time or to see if new tubes are putting our nearly what they advertise. 
@dan - what sort of meter would you recommend for those of us that want better accuracy for laser power measurements?
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My understanding is that our power meters are $5k a pop… so I wouldn’t recommend buying one at all!
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The Mahoney will have to do for me, then.
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