[Al Skierkiewicz]

Guys,
As was said earlier in this post, attenuation is a function of the inverse of the distance (for non-coherent light sources) but that only holds true if the emitter and sensor have no other variables that will affect the system. In reality, neither have that great a production control over the directivity of the emissions. Small directional changes produce great variations in received power. Add to that the transmission attenuation, dispersion and refraction variable of the medium (i.e. air) as it varies with density, temperature and pollutants. A slight breeze of air will bring in a variety of particles of varying sizes that will affect the path loss. When you add multiple emitters as we had last year, then there is interference as the multiple sources add in and out of phase, at the receiver. (Think back to wave theory and interference patterns in a ripple tank.) This not only results in varying levels but in distortions as well. Finally add in the variables involved in the emitter for relative output vs. input current, junction temperatures, chages in overall dimensions, aging, and distortions in the molded case of the LED.