The paper concerns the efficiency of light absorption with different spectral composition by lettuce leaves, and the optimal conditions of light environment and mineral nutrition to increase the productivity and quality of plant products grown under artificial lighting. The greatest positive effect has been achieved on the productivity and quality of the lettuce plants Typhoon variety when been grown on the 1N Knop nutrient solution and illuminated with quasi-continuous spectrum in a limited wavelength range in the yellow area with addition of ～ 10-20% of the total photosynthetically active radiation (PAR) in the near infrared. The most efficient conditions results in the increase on amount of absorbed photon energy by ～ 40% per gram of plant leaf surface over 10 days when rapid biomass growth and plant development take place. It is proposed to use the index of light absorption, describing the absorption increment with biomass growth, as an indicator for the intensity of light energy processing into biomass.
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