A FIELD EXPERIMENT ON CONTRAST REDUCTION LAW

Atmospberic opacity monitoring employs visibility meters based on various physical principles. All the meters currently in use suffer from systematic errors typical of photometric measurements as well as systematic effects due to airborne particle size distribution. Ruhle's luminance contrast meter, which avoids all these sources of error, is not considered a wholly reliable comparison instrument for the other meters because the restrictive conditions on atmosphere and illumination required by the visibility theory on which it relies would not be verifiable during measurements. A re-examination of contrast reduction by the atmosphere in both ideal and non-ideal visibility conditions, and a re-interpretation of Ruhle's hypotheses on errors in apparent luminance measurements, show that Ruhle's instrumental apparatus itself makes it possible to verify the presence of ideal conditions a posteriori of a prolonged measurement period in as broad an opacity range as possible. Field experiment results confirming this fact are reported and discussed.