Photoelectric transmittance meter
From Glossary of Meteorology
photoelectric transmittance meter
(Or photoelectric transmissometer.) An instrument for measuring the transmissivity of the atmosphere; a type of transmissometer.
It consists of a constant-intensity collimated light source located at a suitable distance from a photoelectric cell. Variation in the turbidity of the atmosphere causes changes in the intensity of the light received by the photo cell, thereby varying its electrical output.