Considerations in the Measurement of Visibility

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Abstract
This report reviews the interpretation and measurement of visibility. The selection of 1) variables that appropriately characterize visibility and 2) instruments that measure these variables must be integrated with our understanding of what happens when a person views distant scenes. The process by which we see distant objects is based on characteristics of the object, its surrounding, the air quality and the illumination of the sight path, and the eye and the brain. Additionally, visibility is an integrative parameter in that the ability to “see” depends on all types of aerosol in the atmosphere as well as on all aerosol contained in the sight path. When establishing a standard, some consideration should be given to choosing a variable which is representative of that quality of the environment that requires protection as well as a variable which can be monitored directly. Classically, visibility has usually been interpreted as visual range, which, roughly speaking, is the distance an observer would have to back away from a target before it disappears. Visual range cannot be measured directly nor is it necessarily representative of what an observer “sees.” A documentation of target contrast (either with the sky or another object) or color and color change may be a better way to characterize visibility. Contrast and color change can be monitored directly and both depend on integrative long-path measurements.

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