It is widely recognized that the red:far-red ratio (zeta) acts as a signal that triggers plant morphogenesis. New insights into photomorphogenesis have been gained through experiments in controlled environments. Extrapolation of such results to field conditions requires characterization of the zeta signal perceived by plant organs within canopies. This paper presents a modeling approach to characterize this signal. A wheat (Triticum aestivum) architectural model was coupled with a three-dimensional light model estimating the irradiances of virtual sensors. Architectural parameters and zeta values were measured on two contrasting spring wheat canopies under outdoor conditions. Light simulations were compared with measurements, and an analysis of sensitivity to measurement conditions was carried out. The model results agreed well with measurements and previously published data. The sensitivity analysis showed that zeta strongly depends on canopy development as well as on sky conditions, sensor orientation, and sensor field of view. This paper shows that modeling enables investigation of zeta distribution in a canopy over space and time. It also shows that the characterization of light quality strongly depends on measurement conditions, and that any discrepancies in results are likely attributable to different experimental set-ups. The usefulness of this modeling approach for crop photomorphogenesis studies is discussed.