solarcalc 7.0: An enhanced version of a program for the analysis of hemispherical canopy photographs

This paper describes solarcalc 7.0, a new implementation of a computer program designed for the analysis of hemispherical photographs. Our aim was to recode, optimize, enhance and recompile the previous version of solarcalc program using Realbasic, a modern, cross-platform BASIC language programming environment. The main improvements of the new version are described as follows: Programming and layout – a revision and enhancement of the original source code in a new, redesigned and user-friendly interface; Calculations – the possibility of allowing calculations for a growing season length, the possibility of defining the number of zenith and azimuth angles used for the calculations, the calculations of direct, indirect and total site factors, the inclusion of two versions of the Standard Overcast (SOC) sky weighting formula, the ability to compute leaf area index using two different methods and to compute the sky view factor, the ability to compute the total daily extraterrestrial radiation, the capacity of handling grayscale and color images and to manually or automatically threshold the images. The performance of the new version of solarcalc was assessed using a series of hemispherical images taken along a light gradient. Comparisons of calculated canopy light and structural indices between the new version of the program and gla (Gap Light Analyser) show highly significant positive correlations (R2=1.00), indicating that solarcalc performs similarly to gla across a variety of forest canopy openings. The incorporated modifications in the new version of the program allows for more detailed and more versatile analyses of hemispherical photographs and as such, represent a significant improvement in comparison with the previous version of the program. Finally, one of the most salient feature of solarcalc compared to other similar software is the possibility of using compiled versions of the program compatible with Macintosh, Windows and Linux/Unix operating systems, localised in English, French and Spanish languages.