Internal brightness of disk-shaped samples

Abstract

Internal spectral brightness for the front and back layer of planoparallel samples can be measured by using a bipartite ring-shaped cuvette. The inner walls are highly specularly reflecting and enclose samples by means of fluid and windows of the same refractive index as the sample surface. A fibre optic probe, approaching constant spherical responsivity, evaluates the photon fluxes in both compartments of the cuvette. Graphical ray tracing in planoparallel samples serves to derive equations predicting the results of the direct measurements. According to the theory, differently scattering disks of Plexiglass, selected as a series of test materials, show internal photon fluxes for the front layer in the range 1 – 4.6 times that of the externally irradiated flux. Bifacial plant leaves, of Ctenanthe compressa, Philodendron scandens and Kalanchoe marmorata, exhibit internal photon fluxes strongly dependent on leaf morphology and wavelength; in the red at 616 and 672 nm, for the front layer they fall between 1.1 and 1.8 times the irradiated flux, increasing to values between 2.9 and 3.5 times in the far red at 756 nm.