© 1959 by Institute of Chartered Foresters
THE USE OF A SPHERICAL RADIATION METER IN WOODLANDS
Department of Forestry, University of Aberdeen
For measurements of the light climate in woodlands, conventional light meters have a limited usefulness. The flat light-receiving surface results in preferential sensitivity to radiation incident in a direction normal to the surface; while barrier-layer cells vary in sensitivity according to the light wavelength, and cannot at present be accurately calibrated to characterize the spectral variation under a tree canopy.
A new type of radiation meter, designed by E. C. Wassink and C. Van der Scheer and comprising two photocells with hemispherical covers, was compared with a flat meter in several woodland environments. It is shown to be less sensitive than the flat meter to changes in direction of light influx, and more sensitive to changes in light intensity due to cover type and ground vegetation. The influence of ground vegetation on the light intensity above it is assumed to be due to differences in light reflection by different vegetation types.
It is suggested that under a tree canopy the measurement of light influx is of more value to the forester than an estimate of illumination at a surface, and that, when barrier-layer cells are calibrated for use in woodlands, the emission spectrum of the standard light source should conform to the spectral curve of photosynthesis.