Physicspunks

[Steve Schear]

At 10:38 PM 8/16/2001 -0700, Tim May wrote:
>These are not mystical objects. I used them in my lab a few decades ago.
>"Integrating spheres" is the lab supply store name. Looked at from
>another point of view, a hollowed-out sphere with a small hole. Light
>entering the sphere bounces around and is absorbed, reflected, bounced
>around, etc., until it "thermalizes." The integrating sphere thus
>"integrates" the light hitting the hole.

One can perform some interesting experiments using an evacuated 
"integrating" sphere.  Instead of a pinhead sized opening use something a 
bit larger with a quartz window.  Suspend a small high temperature 
refractory ball in the center of the sphere (tungsten) and line the 
interior of the sphere with PV "cells" (e.g., overlapping of standard 
rectangular cells) overcoated to efficiently reflect frequencies above or 
below the band gap.  Sunlight from a concentrator is focused on the sphere 
bringing it to incandescence.

The combination of the ball and the reflective overcoat functions to 
"remix" the solar energy limiting the "leakage" of photons not in the 
bandpass from exiting the system. The vacuum prevents convective coupling 
of the ball and the cells although cooling the exterior of the sphere with 
a water jacket may be advised.  System conversion efficiency can exceed 35%.

steve