Page 218 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

Contamination and Industrial Systems 211

Sample flow
control

Moveable inlet
pipe

LED Modulation
Falling
stream
LEDs 100mm
Photo-
Modulated detector
output
Temp. controlled
block

Machined solid
polymer sphere

Sample outlet
stream to waste
Figure 9.18 Combination of falling stream and the integrating
sphere makes possible noncontact absorption measurement with
reduced sensitivity to attenuation by scatter. Absorption depends
only on the volume of liquid, not the shape. Individual drops work
fine.

9.5.4 Integrating sphere plus falling stream
There is another falling stream system which is useful for highly fouling media.
The integrating sphere of Fig. 9.18 is a hollow cavity made from or internally
coated with a material with high diffuse reflectivity for the light wavelength of
interest. The diffuse character means that after a few reflections, almost any
input light distribution, for instance from an LED, will be transformed into a
uniform optical field, with the power density being the same in all directions
and at all positions throughout the cavity. The detected power-density at any
point on the cavity surface can then be much greater than the input power
divided by the surface area of the cavity. This assumes that there is no local-
ized absorption within the cavity and that power losses through apertures,
cracks, etc. are small.
These spheres are widely used to measure, or integrate, the total light
output from sources such as incandescent bulbs and LEDs, whose total emitted
power is to be determined. In practice they do not need to be even approxi-

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.