This paper reports accurate measurements of the radiation characteristics and optical properties of Nannochloropsis oculata in the photosynthetically active radiation (PAR) region. These marine microalgae were grown in 2 cm thick culture bottles with vented caps exposed, on one side, to either white fluorescent light bulbs or red LEDs emitting at 630 nm. The illuminance varied from 2000 to 10,000 lux. The microalgae average equivalent diameter ranged from 2.52 to 2.63 mu m. Their radiation characteristics and optical properties were statistically identical over most of the PAR region. Other N. oculata grown with 2 vol.% CO2 injection in 1 cm thick flat bottles exposed to light from both sides reached a significantly larger mass concentration and featured lower pigment concentration and smaller absorption cross-sections. This was due to nutrient limited growth conditions. The refraction index was independent of illuminance, spectrum, and growth conditions and featured resonance at wavelengths corresponding to absorption peaks. (C) 2013 Elsevier Ltd. All rights reserved.