Light interception and reflection in different cultivars of spring barley (Hordeum vulgare) were measured in a field experiment and used to verify a theoretical relation between the fraction of intercepted photosynthetically active radiation (fPAR) and the relative vegetation index (RVI). The RVI was calculated as the ratio between infrared (790–810 nm) and red (640–660 nm) reflectance. The cultivars form a different canopy structure. However, a regression analysis did not show any cultivar effect on the relation between RVI and fPAR The predicted fPAR from frequently measured RVI was used to calculate the product of daily fPAR and incoming PAR (cumulative PAR interception) in all spring barley cultivars grown in monoculture and in mixture with oil seed rape (Brassica napus). A regression analysis showed that the relation between cumulative intercepted PAR and total above ground biomass was the same in all monocultures and mixtures. The ratio α of incremental dry matter and intercepted PAR was normally 2.4 g MJ−1, but it declined below this value when temperatures fell below 12°C.