From AMS Glossary
(Also called ceiling projector) A type of cloud-height indicator that uses a searchlight to project vertically a narrow beam of light onto a cloud base.
The height of the cloud base is determined by using a clinometer, located at a known distance from the ceiling light, to measure the angle included by the illuminated spot on the cloud, the observer, and the ceiling light.
Middleton, W. E. K. 1969. Invention of the Meteorological Instruments. Johns Hopkins Press, Baltimore, . p. 274.