IR path loss estimation...

Feb 05, 2011 2 Replies
F

I have an omnidirectional IR emitter that illuminates a "room". In that room I have say a 10" x 10" diffusive reflective surface that's at distance D1 from the emitter and a sensor at distance D2 from the reflective surface. The reflective surface is diffusive and I'd like to have an estimate of the power density contribution of the reflective surface alone at sensor.



If I count omnidirectional propagation to the target, then again from the target to the sensor, then path loss gets frighteningly huge. But this isn't representative of reality and I miss wall reflection (both paths) and I don't even a rough way to factor that in.



Any hint or pointer?



Thanks, Fred.
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J

On a sunny day (Sun, 06 Feb 2011 00:35:43 +0100) it happened Fred Bartoli wrote in :

Raytracer?

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P

A really diffuse surface has a Lambertian characteristic, i.e. a small patch looks equally bright from all directions. (Think of looking at matte white paint through a drinking straw.) Obliquity makes the total surface radiate power as cos theta, where theta is the angle of incidence. That means that light hitting a Lambertian surface comes back spread uniformly through pi steradians. (The cos theta makes it pi and not 2 pi, as it would be for a really omnidirectional radiator.)

So yes, the path loss is frighteningly large. If you want a SWAG for the contribution from bounces off the rest of the room, the zero-order approximation is 0. The first-order one is to assume that whatever gets back into your detector has made one bounce off a surface with the average reflectance of the ceiling and floor, also Lambertian. You can figure out that contribution by integrating the singly-scattered light over the ceiling and floor, but the result will be "nearly 0".

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
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