The fraction of absorbed photosynthetically active radiation (fAPAR) for plant canopies is often inferred from top-of-canopy, spectral reflectance, vegetation indices like the normalized difference vegetation index (NDVI). Such measures are derived using passive optical sensors and solar illumination of the canopy. However both the passive sensor-derived NDVI and the accompanying fAPAR measurements are affected by the solar elevation angle (θs). In many cases the effect of θs on both NDVI and fAPAR measurements is similar and the effect of θs is often cancelled out. The new class of active optical sensors (AOS) that contain their own radiant light sources to produce equivalent measurements of NDVI are not influenced by θs even though the accompanying values of fAPAR, as derived using a passive sensor are. This means the fAPAR–NDVIAOS relationship will invariably be sensitive to θs. By way of example, this paper investigates the correlations between the NDVIAOS and fAPAR under conditions of varying solar illumination angle for a tall fescue (Festuca arundinacea) pasture. The NDVIAOS was observed to retain a strong linear correlation with fAPAR (R2 ≥ 0.85) but fAPAR was highly sensitive to θs. Subsequently, simple models can be utilized to predict the fAPAR-NDVIAOS relationship for any solar elevation angle between 30 and 80°.