The concept of visibility has been defined in numerous ways. One source that is widely used in the United States describes visibility as the greatest distance at which it is just possible to see and identify selected objects or light sources with the unaided eye, or, it may be considered the distance an observer would have to back away from a target before it disappears. The process by which we see distant objects is based on the characteristics of the target, its immediate surroundings, air quality, illumination of the sight path, and the eyes and brain of the observer. Thus, four components of visibility have been noted: (1) target, (2) background, (3) atmosphere, and (4) observer. Most definitions of visibility include the observer as a key element, but it is also possible to measure visibility without the immediate presence of humans by means of instruments such as telephotometers, transmissometers, or nephelometers. Such measurements can be kept in non-dimensional terms and can be compared without human intervention. When used in the field, however, visibility sensors must be calibrated in human terms of visibility measurement.
- Pub Date:
- June 1982
- Optical Measuring Instruments;
- Air Quality;
- Visual Observation;