Hemisperical and Woodland Canopy Photography and the Light Climate

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Abstract
The Hill camera, which photographs a complete hemisphere with a single exposure, provides a rapid method of recording the structure of woodland canopies as seen from a point. By superimposing the track of the sun on the photographs it can be seen whether woodland light measurements have been made under representative conditions and whether direct insolation at a particular time is possible. Canopy photographs taken in temperate and tropical forests demonstrate some of the sources of variation in the fraction of diffuse sky light reaching the forest floor (the ''daylight factor'') and help to elucidate its significance. Seasonal and long-term changes in the canopy can readily be recorded. The technique can also be applied to insolation problems in hilly regions, in greenhouses, and elsewhere. The use of the Hill camera will not replace established methods of investigating light climate for ecological purposes, but it is a valuable adjunct in interpreting the complex data furnished by them. Methods of calculating solar tracks are examined in detail, and it is shown that a mean track can be worked out which is valid for many years with an accuracy sufficient for most purposes.

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