Light is transmitted through a section of filter tape before and after ambient air is drawn through it. The optical density of the particle deposit on the tape is determined from the logarithm of the ratio of intensities measured through the filter tape with and without the deposit. A clean portion of the filter tape is periodically (usually hourly) moved into the sampling position to obtain a short-term measurement of optical density. This short-term sampling frequency allows diurnal changes in particulate optical density to be monitored. If the optical properties of the aerosol are constant, the optical density is related to the mass concentration in the atmosphere. This optical density can also be related to atmospheric light absorption and Coefficient of Haze (COH). COH is defined as one hundred times the optical density of the deposit. It is usually normalized to COH/1000 linear meters or feet.