Studies of the Woodland Light Climate: I. The Photographic Computation of Light Conditions

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Abstract
Effects of a plant canopy on diffuse skylight and direct sunlight must be considered separately. It is difficult to predict the average percentage of diffuse light cut off over a period from instantaneous measurements, which may be biased by uneven external light distribution interacting with irregular canapy distribution and impossible to predict direct light. Hemispherical photographs, taken with a special lens to cover 180", can be used to estimate the average amount of diffuse and direct light. For diffuse light, a grid calculated from the luminance distribution of an overcast sky is placed on the photograph and the unobscured segments counted. For direct light, the sun''s path across the photograph for the desired period is followed and the contribution of unobscured parts, varying with time of day, estimated. The accuracy of these estimates was checked from the partial regression of measured daily totals in a deciduous English wood on the diffuse and direct totals in the open. Barring spectral effects when wavelengths above 700mu were included, there was good agreement between photographic and statistical estimates. In regions with often cloudless skies, the photographic estimate of diffuse light might be unreliable. In general, the photographic method could save much equipment and time when long-term light records are needed from many sites. Because percentage light measurements can represent very different absolute light quantities in different climates and latitudes, final results should wherever possible be given in absolute units.