Wednesday, December 17, 2014

Lux Life

I just got a priority project dumped on me. I'm trying to figure out if I can build a better stage edge light.

The client goal was to break the existing price point and deliver something cheaper. Unfortunately, that price point is $11 and I'm not sure I can get significantly cheaper.

The one thing I can do is provide a lot better performance at that price point.

The existing commercial product is dead-simple; a 9v battery clip modified to hold a naked 4ma LED. Very simple, small, easy to maintain...but that's the best of it. Undirected and extremely lossy, as 9v "transistor" batteries are much more expensive than AA cells, and since LED forward voltage is under 3v, most of that extra voltage is being wasted as heat in the regulator.

So it is obvious to design around AA/AAA, especially as those already exist in profusion in the theater world, and are better-established in the rechargeable options as well (why I opted for 3xAAA as the standard for my DuckLight).

The LED is a little more of a question. 20ma is easily available in that size, but what is the actual desired intensity? After trying several different estimation models, including illumination, visual astronomy, and f-stop estimation, I've zeroed in to assuming the LED will be behind a diffusor that in turn is tucked into a hood (or otherwise is blocked from direct audience view.

Using a whole bunch of loose assumptions, such as calling typical lux at the front edge of the stage 400-600, calling the albedo of dance marly .1 and ignoring the anisotropic shading model (which will come back to bite me, I know) I end up with a desired lux of around 100 before the self-illuminated surface can be clearly discerned from the background illumination.

So the smaller the surface, the better, right? Well, now we switch to the angular resolution of the human eye, which works out to about half a centimeter from the back of the stage. Except that resolution is actually all over the map when comparing relative magnitudes -- we can see stars, after all, and we don't have the optical power to resolve those.

Still, the numbers are converging around a 1cm^2 diffusor driven by 5-20ma as being "probably" visible from on stage. Omitting glare, of course. Plus the actor or dancer is facing right at sources (the face light) that are several magnitudes greater, and the instantaneous magnitude range of the human eye is only 3-4 f-stops.

This is going to have to be calculated when I work on making a light that can be seen against the glare of the shin-busters to allow a dancer to run off stage. (Well, since they normally navigate by the shin-busters, the real calculation is how much their eye will dark-adapt during the brief moments of a black-out as they race towards a red blinking safety light in the wings).

Unfortunately the battery numbers are not coming out where I'd like them. The competition boasts 125 hours (my calculation confirms this is possible for their setup) and that won't last the week. Running full-out with 20ma, even with the much greater capacity of "penlight" cells I can't stretch to the length of a run.

So the optimal design appears to be 2xAA, which has a slightly larger profile than 2xAAA but will last for ten days. That means the user places fresh cells in during prep for the weekend's performances, and leaves them on over the weekend. That is the best option for mechanical simplicity, cost, and limiting the failure modes (both mechanical and human error).

(I may still go for AAA, which gives a theoretical life at max output of four days. The 4x battery performance I'm getting out of penlight batteries is balanced by the 4x output power I'm aiming for in the design).

For simplicity, the diffusion may be simple cavity reflection; the "hood" is also the diffusor. Which does bring to question, however, if I want to design a tilting head so the same unit could be used for side lights. Actually, though...there's no reason it has to be designed with the light output axial with the batteries. It could be orthogonal to their long axis. This isn't a flashlight, after all!

My best guess at this point is a laser-cut acrylic "housing." The quotes are there because I don't need to close off the battery compartment. I just need to keep the batteries in place. So an open framework with a flat bottom makes the most sense (the flat bottom designed for double-stick tape, screws, or magnets to fasten it in place...provide pockets for supermagnets in the design.)

To get a true picture of the cost, then, I have to upload files to Ponoko. Even if I do intend to laser them myself at TechShop, I am entering into this on a non-profit, open-hardware basis.

Now what I really need is a name (so  I can refer to it in project notes and folder names more efficiently). "mouseLight" works for me. Especially since I am working out the details for this over a production of "Nutcracker."

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