Theory of PROSPECT model
PROSPECT is a leaf optical model for estimating leaf-level reflectance and transmittance (Jacquemoud and Baret 1990).
The PROSPECT model is based on Allen's multiple layer plate model but adopts a solid angle of incident radiation instead of
an isotropic direction. A leaf is taken as several absorbing plates with rough surfaces giving rise to scattering of light.
The model assumes the leaf is a stack of N identical elementary layers separated by N-1 air spaces. Layers are defined
by their refractive index (n) and absorption coefficient Ki. The number of layers mimics the scattering within the leaf.
Absorption is the linear summation of the concentrations of the biochemicals (chlorophyll, water, dry water etc.) and
the corresponding specific absorption coefficients. Leaf optical spectra from 400 nm to 2500 nm are simulated
through the upward and downward hemispherical radiation flux.
The model can be inverted by numerical iteration to derive the leaf biochemical contents from leaf spectra.
Using an optimization algorithm, leaf biochemical constituents can be numerically iterated by comparing estimated
hemispherical reflectance and transmittance with the measured leaf reflectance and transmittance.
Inversion of PROPECT can estimate the leaf chlorophyll content from the measured leaf reflectance and transmittance.
However, it cannot capture the variations of leaf chlorophyll content across the season and canopy gradient.
Specifically, for understory leaf samples and samples collected in the early and late growing season,
which had low leaf chlorophyll content, the inversion model overestimates the leaf chlorophyll content.
A leaf thickness factor, which is based on observed seasonal and canopy gradient variations in leaf thickness,
is introduced in the PROSPECT model to take into account the effects of changes in leaf structure on light absorption.
With an additional input of leaf thickness, as a surrogate to capture the seasonal and locational variations
in leaf structure and non-chlorophyll light absorption, the PROSPECT model is improved to be capable of deriving
the seasonal and canopy gradient variations in leaf chlorophyll content from leaf reflectance and transmittance spectra
(Zhang et al., 2005a).
The original PROSPECT is developed for estimating leaf optical
properties of broad leaves. For coniferous species, the model
is further improved by considering radiation transfer processes
inside the needle. The side radiation transfer is compensated
which is associated with the measurements of leaf spectra
for conifer needles. The influence of additional biophysical
properties, needle thickness and width, are incorporated in
the model. Compared with measurements, the modified model
improves the estimations on the chlorophyll content of black
spruce needles (Picea mariana) (Zhang et al., 2005b,c).
Jacquemoud, S., and Baret, F., 1990. PROSPECT: a model of
leaf optical properties spectra. Remote Sensing of Environment,
Zhang, Yongqin., Jing M. Chen, Sean C. Thomas, 2005a, Retrieving
seasonal variation in chlorophyll content of overstorey and
understorey sugar maple leaves from leaf-level hyperspectral
data, Canadian Journal of Remote Sensing(in review).
Zhang, Yongqin., Jing M. Chen, Thomas Noland, and John Miller,
2005b. Needleleaf biochemicals retrieval from leaf optical
properties. Remote Sensing of Environment (in preparation).
Zhang, Yongqin., Jing M. Chen, Thomas Noland, John R. Miller,
2005c, Retrieving Black Spruce Leaf Chlorophyll Content from
Hyperspectral Remote Sensing, the 54th Canadian Association
of Geographers annual meeting 2005.