Photometry is the science of measuring visible light in units that are weighted according to the sensitivity of the human eye. It is a quantitative science based on a statistical model of the human visual response to light under carefully controlled conditions. We cannot apply this model to the “perception” of light by algae, because we should substitute the sensitivity of the algal photoreception systems for that of the human eye as quantified by action spectroscopy.

For the human perception, the Commission International d’Eclairage (CIE) photometric curves (photopic and scotopic) provide a weighting function that can be used to convert radiometric into photometric measurements. In scotopic curve, yellowish-green light receives the greatest weight because it stimulates the eye more than blue or red light of equal radiant power (λ_max = 555 nm) (Figure 5.11, Table 5.3); in photopic curve blue-green light receives the greatest weight because it stimulates the eye more than other lights of equal radiant power (λ_max = 507 nm), (Figure 5.11, Table 5.3). For algae, action spectroscopy may be used for a similar purpose even though the spectra so far measured are contradictory, not very accurate, and very often are difficult to interpret (Figure 5.11).