DEVELOPMENT AND APPLICATIONS OF A STANDARD VISUAL INDEX

A standard visual index appropriate for characterizing visibility thro ugh uniform hazes, is defined in terms of either of the traditional me trics: visual range or extinction coefficient. This index was designed to be linear with respect to perceived visual changes over its entire range in a way that is analogous to the decibel scale for sound. Neit her visual range nor extinction coefficient have this useful property, which has resulted in their misuse for some applications. By assuming the availability of sensitive scenic targets at every distance, it ca n be demonstrated that any specific fractional change in extinction co efficient (or visual range) is equally perceptible regardless of basel ine visibility conditions. The new index is defined so that its scale, which is expressed in deciview (dnu), is linear with respect to fract ional changes in extinction coefficient, b(ext), as 10 ln(b(ext)/0.01 km-1), where extinction coefficient is expressed in km-1. A 1 dnu chan ge is about a 10% change in extinction coefficient, which is a small b ut perceptible scenic change under many circumstances. Since the deciv iew scale is near zero for a pristine atmosphere (dnu = 0 for Rayleigh conditions at about 1.8 km elevation) and increases as visibility is degraded, it measures perceived haziness. Use of the deciview scale in visibility perception and benefits research will promote improved stu dy design and more appropriate use of results. It should also allow th e visibility consequences of alternative air pollution-related decisio ns to be more easily presented and evaluated.