You searched for: “visibility meter
visibility meter
1. A type of photometer that operates on the principle of artificially reducing the visibility of objects to threshold values (borderline of seeing and not seeing) and measuring the amount of the reduction on a standard scale.
2. An instrument; such as, a transmissometer (device used to measure the transmission of light through a medium), for making direct measurements of a visual range in the atmosphere or of the physical characteristics of the atmosphere which determine the visual range.
3. A photometric device for determining the range of visibility during daylight hours.

The measurement is made visually and the visibility meter is also used in lighting engineering for measuring the values of light (brightness) contrasts between an object and the background against which it is found or projected.

At meteorological stations visibility meters are used to measure the transparency of the atmosphere in a horizontal direction by measuring the contrast of a remote dark object; for example, a forest, against the background of the sky. There will be less contrast as the air transparency decreases.