The pure R version of foursail is included in the package as an easy way to review the code, and to check more optimized versions against. Model originally developed by Wout Verhoef.
r_foursail(rho, tau, bgr, param)input leaf reflectance from 400-2500nm (can be measured or modeled)
input leaf transmittance from 400-2500nm (can be measured or modeled)
background reflectance. Usual input is soil reflectance spectra from 400-2500nm (can be measured or modeled)
A named vector of SAIL parameter values (note: program ignores case):
[1] = Leaf angle distribution function parameter a (LIDFa)
[2] = Leaf angle distribution function parameter b (LIDFb)
[3] = Leaf angle distribution function type (see ?lidfFun)
[4] = Leaf area index (LAI)
[5] = Hot spot effect parameter (hspot) - The foliage hot spot size parameter is equal to the ratio of the correlation length of leaf projections in the horizontal plane and the canopy height (Verhoef & Bach 2007).
[6] = Solar zenith angle (tts)
[7] = Observer zenith angle (tto)
[8] = Sun-sensor azimuth angle (psi)
spectra matrixwith 4 reflectance factors and canopy transmission for wavelengths 400 to 2500nm:
[1] = bi-hemispherical reflectance (rddt). White-sky albedo: the reflectance of the canopy under diffuse illumination. The BRDF integrated over all viewing and illumination directions.
[2] = hemispherical directional reflectance (rsdt). Black-sky albedo: reflectance of a surface under direct light without a diffuse component. It is the integral of the BRDF over all viewing directions.
[3] = directional hemispherical reflectance (rdot). Diffuse reflectance in the vieweing direction.
[4] = bi-directional reflectance (rsot). The ratio of reflected radiance in the viewing direction to the incoming radiant flux in the solar direction.
[5] = Canopy transmission of diffuse light through the canopy (taud).
[6] = transmission of direct light through the canopy (taus).