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

When analysing the woodland light climate, the effect of the canopy on diffuse light from the sky and on direct sunlight must be considered separately. Instantaneous measurements can be made to estimate the percentage of diffuse light cut off, the `day-light factor', but problems of spectral composition and instrument response, of unequal distribution of light over the sky, and of short-term fluctuations of light in the open are all liable to bias such estimates. Integrated measurements of light totals at any considerable number of sites are costly, require considerable maintenance, and do not permit prediction of light condition at other times of year. A reasonably accurate estimate of the mean percentage of diffuse and direct light cut off by the canopy can be obtained from hemispherical photographs, and from these percentages, the actual total of light received over any desired period may be calculated. The photographs, taken with a special camera, cover a whole hemisphere. On the circular image grids can be placed to estimate the light conditions, and details of grid construction are given. These photographic estimates compare well with estimates made from the partial regression of daily or hourly totals of light at three sites, in a deciduous wood in east England, on diffuse and direct light over equivalent periods in the open nearby. The evidence suggests that the percentage reduction of diffuse and direct light can be treated as constant over a period of a month, but these are only averages, and over shorter periods discrepancies might appear. Figures for the mean percentage reduction of diffuse light over an hour for the month may be slightly biased by unequal light distribution over the sky. As `daylight factor' has often been misapplied by biologists, and has also an architectural definition, the term `site factor', with appropriate qualifications as to type of light and time, has been adopted instead for the percentage reduction of light. When presenting results, however, these should as far as possible be given as absolute amounts of light, not as percentage reductions. Light conditions in the open vary greatly with climate and latitude, and equal figures for the percentage reduction of light from two different areas may well represent quite different absolute quantities.