Measurement of Light Intensity.—Methods of measuring light are discussed in detail by Wiesner (1907), Rübel (1922, 1928), Braid  (1923), Lundegårdh (1930), Morton (1927), and Klugh (1927). Physically unimpeachable methods of measuring light are so complicated that they are not available to the plant sociologist. Approximate and comparative measurements can be had by the photographic-paper method of Wiesner, which is still generally used. It depends on the darkening of a sensitive silver chloride paper. Vouk and EderHecht have elaborated the method of Wiesner. The Graukeil photometer of Eder-Hecht, as now used, serves for determining both the momentary light intensity and the summation of light for definite periods of time. It can be had at a reasonable price, with instructions for use and accessory equipment from the Photographische Industriegesellschaft Herlango, Vienna. Lundegårdh has constructed a self-registering apparatus for measuring light. (Braun-Blanquet 1932:103-4)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.