The spectral components analysis identities, LAI/VI × APAR/LAI = APAR/VI and LAI/VI × YIELD/LAI = YIELD/VI, where VI denotes any one of several spectral vegetation indices available, LAI is the leaf area index, APAR is the absorbed photosynthetically active radiation and YIELD is the saleable plant part (grain, fibre or root), express the information conveyed by canopies about their development, response to stresses and yield capability. The rationale includes the concepts that vegetation indices adequately measure the amount of photosynthetic-ally active tissue in plant canopies and that high yields cannot be achieved unless growing conditions permit canopies to develop that effectively intercept the available light during reproduction. For wheat, cotton and maize the coefficients of determination (r2) usually exceeded 0.90 for exponential, power or linear expressions relating the numerator (dependent variable,y) to the denominator (x) variable of each term in the first equation. Results show that APAR can be reliably estimated from VI, and that the relation is nearly linear. The equations help to quantify remote assessments of crop productivity, to unify field-observed interrelations among LAI, APAR and YIELD and to validate remotely observable LAI and APAR inputs for plant process crop growth and yield models, or for growth analysis.