These effects are apparent at the leaf as well as the crop level, though they are smaller at crop than at leaf level. In particular, when leaves are reddish the advantage of optimising lamp spectrum is largest. For a rose crop with green leaves optimizing spectrum was estimated to increase instantaneous crop photosynthesis per incident photon up to 12% and for a crop with reddish leaves up to 17%, compared to HPS lamps. However, care is needed to draw conclusions from instantaneous effects. Indeed, whether this can result in comparable benefits in practise depends on the extent at which the short-term (instantaneous) spectral effects on leaf photosynthesis will sustain on the long term and how differences in the spectral emission applied by LEDs further influence crop physiology and morphology under a prolonged illumination (Hogewoning et al., 2007).