Measurement of the Optical Properties of Leaves Under Diffuse Light

Measuring leaf light absorptance is central to many areas of plant biology including photosynthesis and energy balance. Absorptance is calculated from measured values of transmittance and reflectance, and most such measurements have used direct beam light. However, photosynthesis and other processes can differ under direct and diffuse light. Optical properties under diffuse light may be different, but there have been technical difficulties involved in measuring total reflectance of diffuse light. We developed instrumentation to measure this reflectance using a chopped measuring beam delivered alternately to sample and reference integrating spheres, and lock-in detection. We also built instrumentation for measuring transmittance of diffuse light. We developed standards to calibrate our instruments and correct for substitution error, a known systematic error with integrating sphere-based measurements. Helianthus annuus leaves measured under diffuse light reflected 5-10% more and transmitted a few percent less 400-700 nm light than under direct light. Overall absorptance was only a few percent higher under direct light, but leaves may utilize absorbed direct and diffuse light differently. For example, of the light entering the leaf, significantly more direct light than diffuse light is transmitted through the leaf, suggesting differences in localization of absorption within the leaf.