Tuesday, June 28, 2011

Diffusers – a guest post by Spencer Luster

Light diffusers are wonderful, terrible things. They help provide much desired uniformity to our illumination, but they demand the sacrifice of gazillions of photons.* The plaintive cry of the machine vision engineer might echo forever: "I need more light!"

For back lighting, such may always be the case. For front lighting -- where the camera is not looking directly at the source -- there are options with varying efficiencies. Of course standard "flashed opal" glass and milk white plastic diffusers throw away a lot of light. Ground glass is a step up, but still wasteful.

Holographic diffusers are very efficient and produce excellent uniformity at the target. The problem is that these devices are quite expensive – in the range of $50 to $100 for 1" discs and three times as much for 8" squares.

The various machine vision LED suppliers use some nifty plastic sheet diffuser that seems pretty efficient and obviously can't be too pricey. My problem has been purchasing just the material itself. Few want to sell it, and nobody I've talked to wanted to tell me what it is.

Over the years I've experimented with a lot of materials including textured drafting mylar that works reasonably well, but could be more efficient. The good thing is it's cheap and easily cut. But in ten years will anyone be using drafting mylar? (It's hard enough to find now.) One of my favorite oddball solutions is the plastic wrapping material on individual slices of Kraft American cheese. The plastic has a mild texture that tends to homogenize light beams, without wasting much at all.

A couple of years ago I found an excellent material, perhaps related to what the LED suppliers use. It's textured polycarbonate film, available from McMaster-Carr Supply Company (http://www.mcmaster.com) for under $7.00 per 2ft. square. It comes in thicknesses from 0.005" to 0.040", with "…one side velvet, one side matte." This is really great stuff! You can also tune it to your application by spraying layers of clear lacquer on one or both sides in order to reduce diffusion. For a recent project I used lacquered film with a row of 880nm LEDs. Target uniformity was almost perfect, and efficiency was greater than 90%. It's a very good solution, but sometimes I miss the smell of American cheese.

* Speaking of sacrifices, I've always maintained that a photon is the scream an electron makes when it's tortured.

copyright © 2011 Spencer Luster, LIGHT WORKS LLC


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