AN521 Datasheet, PDF (21/26 Pages) Silicon Laboratories – IRLED SELECTION GUIDE

Totalpowercollectedbysurfaceisconstant
independentofdistance
AN521
Figure 14. irLED Radiant Intensity
4.3.3. Irradiance
Irradiance is the power per unit area incident upon a surface at a specified distance. Since the total power incident
upon the surface is constant and the area increases with distance, the irradiance is then inversely proportional to
the distance from emitter to surface.
Given a point light source, the irradiance and radiant intensity are related by an inverse square law function:
E = I  D²
Figure 15. Inverse Square Law for Point Source
As can be seen, the power falls off as the square of the distance with D=4 having 1/16 the power/area as D=1 (as
shown in Figure 15).
Rev. 0.1
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