Inexpensive Mirror Reflectometer

Measuring mirror coating reflectivity can be an expensive proposition especially if you use a commercially manufactured instrument.
I found this ultra-low cost portable method on the web. What do you think of it? Thomas, your input would be quite helpful here:
https://www.sites.google.com/site/photonadroplet/telescope-mirror-reflectivity-meter 

Hi, Rod. It is indeed true that one can ~make~ a very nice specular Reflectance radiometer/photometer with only a light source and detector, and some judicious building of the mount, and one or two simple lenses.

I did a lot of that at work, but always had a nice radiometer head as the detector. (UDT made/makes them and most all are the same and VERY accurate!) Those can be had used for... maybe a hundred bucks? So, I built lots of specialized radiometers. Oh yeah, and filters to select bands. Things to guard against are narcissus [The narcissus effect occurs when the detector, through unintended reflections off internal lens surfaces, etc.], smacking the detector with photons abruptly, and hysteresis. We would typically get accuracies to 0.2%, which is good enough for a lot of stuff. That's 0.2% of the signal, so even high reflectivities are quite accurately measured. That referenced author properly acknowledges that a Reference Mirror is extremely helpful. I built one that looked like a "Doll House" That allowed three paths to the same detector: One direct, one off a reference mirror, and one off the ~test article~, all in rapid succession, without hiccoughing the detector. We used that as a pass/fail QC on coatings, with the reference mirror being one that had been certified as good, using a full-blown spectroradiometer. I have a lot of sketches and pictures of these things, being used. Will look for.

I found my sketch of the Doll House! We then proceeded to (have people) Q/C thousands of mirrors, using the Doll House that I built at home, in my humble shop, on a Sunday. The reason I was surprised to find it... I have hundreds of these sketches, all different configutrations, most then used for specialized radiometric tasks... and most all Quick and CHEAP!

PS also did lots if in-situ measurements of mirrors e.g. inside of telescopes...  FUN!  Tom

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Thomas,
Your "doll-house" concept is quite innovative. The design is highly efficient and user-friendly.
If I recall correctly, I seem to remember an article I read years ago wherein a technician used a "hand held reflectometer" from Cal-Tech to measure reflectivity of a new recoating of the 200" mirror at Mt Palomar. The image in the article showed a tech in a clean suit taking the small gizmo out of his pocket and taking a reading on the mirror surface. I'd sure like to have one of those but I don't even know where you'd find one and I'm assuming they cost a lot, too...

Originally Posted by Rod Kaufman

Thomas,
Your "doll-house" concept is quite innovative. The design is highly efficient and user-friendly.
If I recall correctly, I seem to remember an article I read years ago wherein a technician used a "hand held reflectometer" from Cal-Tech to measure reflectivity of a new recoating of the 200" mirror at Mt Palomar. The image in the article showed a tech in a clean suit taking the small gizmo out of his pocket and taking a reading on the mirror surface. I'd sure like to have one of those but I don't even know where you'd find one and I'm assuming they cost a lot, too...

Just remembered, the context of the Dollhouse was big, flat copier scan-mirrors that we were making for Xerox, tens of thousands. We would do the absolute cal once a day (pantry location) and then use the differential sliding motion platform for the rest of the shift, and one or two more shifts, depending on the work-load. The "hole in the floor" was small, so the two flat mirrors could be ergonomically slid L/R to get Ref/sample1/Ref/sample2/Ref/sample3... without the detector ever seeing broken beam or misalignment. It was a wonderfully-boring job that turned around the measurements as fast as they could go through the trays from coating.

PS: It was FUN having production responsibilities!  Design test-sets, breadboard, build, integrate... and on to the next problem. In a high-volume 24/7 shop, no excuses for procrastination, obfuscation or "not my job." I LOVED it!

Re' In-Situ R measurement. Here, I’ve set up measurement of a GIANT folding-flat. And yes, the light source is a humble laser-pointer. We found that some are stable, and some are terrible. Did the ref/sample1/ref/sample2… as usual, and it worked well. We had considered a little commercial puck thingy… but I looked at their literature and patents, and realized they had built a Lot of assumptions into their design, that would kill traceable BRDF (bi-directional reflectance distribution function) buy-off. So, as usual, we built our own. Nearly all of my concoctions could be used on the production floor, ambient factory lighting. Good design, conjugate baffling, bandpass filters… works wonders!  Tom

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