Solar energy absorbed by plants is one the main factors determining photosynthetic and transpiration rates, conditioning the growth and development of crops. Particularly for the absorption of all-wave and photosynthetically active radiation by tree canopies, which is affected by the geometry and the leaf area density of the canopy, the "Whirligig" device originally described by McNaughton et al. (1992), has been shown to be a useful tool. This kind of device was used to measure the amount of all wave radiation absorbed by an orange tree, cv. Pêra do Rio, in an orchard in Piracicaba, São Paulo State, Brazil. Measurements were carried out from October to December 2004 and in May and June 2005. The net radiation absorbed by the tree (Rnl) was correlated with global solar radiation (Rg) and grass net radiation (Rng) measured in a weather station, for 15-min, hourly and daily periods. Regression equations include the effect of different leaf area densities, obtained by progressive tree defoliation, on the radiation balance. Good fittings between Rnl and Rg or Rng were obtained for all time scales. The same was observed for the relationships between Rnl per unit of area projected vertically on the ground by the tree canopy (Rnl/PA) and Rg or Rng. The results showed the possibility to use transfer functions to estimate tree net radiation from common measurements of Rg or Rng, but this functions should be used with caution, because they are only valid for conditions of leaf area range and canopy geometry similar to those observed in the study.