Bidirectional scattering of light from tree leaves

A new spectrogoniophotometer to measure leaf spectral and directional optical properties

“…10 shows the DHRF and DHTF of two leaves at four illumination angles. The DHRF obviously increases while the DHTF decreases with the angle of incidence, as observed in previous studies (Brandt and Tageyeva cited in Brakke et al, 1989;Ross, 1981;WalterShea et al, 1989).…”

“…The maxima range from 0.03 sr − 1 at θ s = 5°to 0.45 sr − 1 at θ s = 65°, i.e., 15 times greater. The BTDF are much more isotropic than the BRDF although one can clearly observe an increase in transmittance in proximity of the principal plane as noticed earlier by Walter-Shea et al (1989), Brakke et al (1989), and Brakke (1994).…”

“…6b) is out of the principal plane besides the specular direction. As observed by Brakke et al (1989), it is difficult to maintain some leaves perfectly flat in the sample holder. Problems of undulation which appeared to increase the reflectance out of Fig.…”

“…In order to better sample the BRDF and/or BTDF, one or several detectors can be mounted on a rotation stage while the light source is fixed in position. Most of the goniophotometers are built this way, which allows measuring scattering in the principal plane (Brakke et al, 1989;Howard, 1971) or full light distributions (Walter-Shea et al, 1989). Sometimes however, it is just the opposite: the detectors are fixed and the light source is mounted on a rotation stage (Okayama, 1996).…”

The effect of soil anisotropy on the radiance field emerging from vegetation canopies

“…Conversely, this assumption is rarely satisfied when the canopy density is low. Leaves have been shown to scatter light anisotropically (e.g., [Brakke et al 1989] and [Govaerts et al 1996]) and to exhibit non-uniform leaf orientation distributions, a fact which directly affects the differential scattering coefficient of the leaves. Yet, light scattering by plant canopies does not exhibit such peculiar effects as can be observed with water droplets and ice crystals, which scatter largely forward and maintain a certain memory of directionality.…”