The luminance and luminous intensity characteristics of the leaves of several forest species of eucalypts have been investigated and compared with freshly smoked magnesium oxide. Results indicate that different species of the same genus can have distinctive indicatrices. Inspection of the indicatrices for the leaves showed that, for practical purposes, three types of reflection are recognizable, i.e., diffuse, spread, reflex. It seems important therefore that the research worker should be aware of the type of reflection being measured, since this may influence his conclusions. The cusp in the direction of reflex reflection was found to increase rapidly about the normal (0°) to the leaf’s surface and to attain a maximum at the normal. As the angle of incident light is increased from 0° to 45° or more, the spread reflection component becomes increasingly conspicuous. With normal incidence, the maximum luminous intensity was attained in the direction of reflex reflection, but rapidly decreased toward zero beyond about ±60° to the normal.
J. A. Howard, "Luminance and Luminous Intensity Indicatrices of lsobilateral Leaves," Appl. Opt. 10, 2354-2360 (1971)