A photo-chemical technique was used to measure incident and reflected irradiance over weekly periods, above and at four levels within the crown of four forest stands growing at montane altitude on the eastern side of the Craigieburn Range from February-June, 1982. All the stands were in the steepest part of their sigmoid growth pattern, and were therefore at or near canopy closure. Differences and similarities in the light climate of the four stands have been shown to be related to their architecture. Differences occurred in total surface leaf area (Lt) and its distribution (Lt : Larix decidua 12.5, Nothofagus solandri 17.8, Pseudotsuga menziesii 33.3, Pinus contorta 35.7). Stem-branch geometry and foliage structure and orientation were other factors affecting the absorbance of light in the crown. Horizontal receiving surfaces (P. menziesii, N. solandri) tended to prevent strong illumination from reaching the lower crown, while more vertical receiving surfaces (P. contorta) allowed higher levels of irradiance to reach the lower crown. Seasonal variations in the light climate of the four stands occurred because of changes in earth-sun geometry and forest structure. A simple model of canopy photosynthesis is presented, using light absorbance and ecophysiological characterisitcs of the four stands. The P. contorta canopy was estimated to have the highest photosynthetic rate and L. decidua the lowest. Extinction coefficients (K) for the four stands varied because of differences in their leaf area.