Visibility Meter

a photometric device for determining the range of visibility during daylight hours. The measurement is made visually. 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.

In the USSR the IDV and M-53 visibility meters are common. Both of these devices are based on the principle of superimposing an artificial haze on the observed natural contrast between a given object and the background in the instrument’s field of vision. To do this the image of the landscape being observed is broken into two parts that partially overlap. Using attachments of various designs (a revolving Polaroid in the M-53 and diaphragms that gradually open up the field of vision in the IDV), the brightness of one image is increased while the brightness of the second is simultaneously decreased. As this is done the increasing brightness of the background (for example, the sky) of the one image is the artificial haze that is superimposed on the other image and brings the observed contrast to a value not perceptible to the eye. The value of the additional brightness of the artificial haze is determined by the reading attachments of the visibility meter (angle of turn of the Polaroid in the M-53 instrument and position of the diaphragm in the IDV). From this the observed contrast is found and the visibility range of an absolutely black object against the background of the sky is determined: this is the measure of the atmosphere’s transparency. A weak point of visibility meters is the subjective nature of these measurements.