The View Up Here

Random scribblings about kites, photography, machining, and anything else

SPIE 2016 – Surprises Along The Way

Posted by Tom Benedict on 06/05/2016

I spent the last week scanning all the samples I’ve been prepping for the last several months. We’d scanned some of these materials before, so some of the results didn’t come as a surprise, but a couple did.

The scans I’m running are what’s called “Total Integrated Reflectance”, or TIR. The idea is to illuminate a sample with a particular wavelength of light, collect all the light reflecting off of it at that wavelength, and measure it. By doing this at a bunch of different wavelengths you build up a TIR spectrum. I’m running spectra from 250nm to 2500nm, or from the UV to the near-IR. This covers any instrument that uses a CCD detector along with many instruments that use near-IR detectors like the H2RG from Teledyne.

What you want to see is a flat spectrum around 5% or below from 250nm to 2500nm. That indicates a material that’s good to use to control stray light across that entire range. Unfortunately that’s true for some materials but not for all of them, even if they look black to the human eye. One of the more surprising sets of materials were the black flock papers from Edmund Optical. These are a mainstay for controlling stray light in instruments and amateur telescopes. I was caught off-guard when the reflectance of all three of the flock samples popped up long of 720nm! They’re black to the human eye, but in the near-IR they’re closer to a light gray.

Here’s a look at four of the materials I’m scanning, photographed using an IR-converted CCD camera. The filter in this camera cuts on gradually from about 650nm to around 720nm so it’s not ideal as a measurement tool, but it gives a good idea of how the materials behave from 720nm out to about 1000nm.

SPIE 2016 - Black Samples in the NIR

The blackest material in this set of four is the Industrial Strength Velcro. But only the hook side! The loop side is made from some other material that’s remarkably reflective in the NIR. The black flock paper to the left of the Velcro is considerably more reflective, even though to the human eye it looks much darker. One of the other surprise materials was the Black Tak tape from City Theatrical. Even though it appears not to be very black in this photo, that’s only around 5% reflectance. Even better, it maintains that reflectance across the entire spectral range. It’s neat stuff!

The half-tone bar at the top was printed on a normal laser printer. Toner, which is almost straight carbon, is quite good at absorbing light across the entire range of the tests. (Yes, toner-covered paper is one of the samples I’m running.)

I’ve run into some other surprises along the way, but those will have to wait for the paper. I’ve got another couple of days of scans still ahead of me before the real work begins: analysis and writing. Then I get to design a poster!



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