Plants respond to the visible light part of the radiant energy spectrum (Figure 6–1). Traditionally, this response has been considered to be maximum in the blue and red regions (called Photosynthesis Action Spectrum I) as compared to maximum visual eye response in the green-yellow region. Recent research indicates that, at higher light levels, plant growth is independent of spectral differences and is proportional to the total radiation received between 400 to 850 nanometers (Figure 6–2, opposite page).