733 
1980 (4)., Thomas et al., 1971 (8). Carlson et al.. 1971 (7). Hunt & Rock. 1989 (12). Cohen. 1991 (31). Jackson & 
Ezra. 1985 (16), Holben et al., 1983 (23). Richardson & Everitt. 1987 (28). Moran et al., 1989 (22). Schutt et al., 
1984 (1). Gausman, 1974 (14). Sinclair et al., 1971 (9)). It is likely that other constituent chemicals may also be 
contributing factors at specific wavelengths (e.g. starch and sugars). 
3.2.2. Non-Lambertian Properties of leaves From literature, the orientation of the sensor and source with respect 
to the viewing area of the leaf affects the leafs reflectance spectrum i.e. leaves are non-lambertian reflectors and in 
a canopy are oriented at many angles. Therefore a variety of illumination conditions exist. However, relatively 
few measurements have been published that present leaf reflectance and transmittance as functions of source 
incidence and view angles and these measurements have been over limited view azimuth angles, (Breece & 
Holmes, 1971 (50), Woolley, 1971 (27)). Other research has mainly been limited to either hemispherical 
reflectance or reflectance at a single source incident and view angle, (Waiter-Shea et al., 1989 (25)). Braake et al, 
1989 (26) investigated the bi-directional reflectance and transmittance of healthy leaves but little research (if any) 
has investigated the bi-directional reflectance and transmittance distributions of individual leaves under stressed 
conditions. The bidirectional scattering properties of individual leaves should be characterised because as Walter 
Shea et al., 1989 (25) stated "leaves are major contributors to the reflectance of vegetation and the scattering 
properties of individual leaves at a variety of view zenith and azimuth angles can be used to develop the scattering 
phase function for leaves used in detailed models of radiative transfer in vegetation". 
3.3. Canopy Reflectance 
The reflectance properties of single leaves are fundamental to understanding the reflectivity of an entire plant or 
vegetation canopy in a field situation, but the single leaf data cannot be applied directly without modifications. 
There are both quantitative and qualitative differences in the two types of spectra. On a percentage basis the 
reflectance from a canopy is considerably less than that from a single leaf because of a general attenuation in 
reflected radiation by variations in illumination angle, leaf orientation, shadows and non-foliage background 
surfaces e.g. soil. (Knipling, 1970 (2)). 
"The visible and near infrared reflectance from a nearly continuous broad-leaved canopy typically might be about 
3%-5% and 35% respectively, whereas the corresponding values for a single leaf are about 10% and 50%. In this 
case the levels of visible and infrared reflectance from the canopy are about 40% and 70% respectively, of the 
levels from a single leaf ", (Knipling, 1970 (2)). The relatively smaller reduction in infrared reflectance is due to a 
compensating factor; much of the incident infrared energy' transmitted through the uppermost leaves is reflected 
from the lower leaves to enhance the overall reflectance. Most vegetation targets are mixtures of different 
components, including leaves, other plant structures, background and shadow. In addition the projected area of 
each component, illuminated and viewed, depends on the atmospheric conditions and the solar & sensor zenith 
angle and the azimuth angle. (Knipling, 1970 (2)). 
Factors which affect a canopy's reflectance are:- 
1 The percentage of vegetation cover. 
2 Soil type (moisture, organic and Iron oxide content, particle size and roughness). 
3 Atmospheric conditions. 
4 The canopy structure or geometry, which is determined by the size and shape and orientation of the plants 
and their leaves 
5. The solar and sensor azimuth and zenith angles relative to the canopy.. 
Guvot. 1990 (41), Heute, 1987 (39), Heute. 1988 (40), Heute et al., 1985 (18), Williamson, 1988 (24), Bowers & 
Hanks, 1965 (20). Curran. 1985 (42), Myers et al., 1983 (37), Wright et al. 1986 (17), Lord et al, 1985 (38), 
Jackson et al. , 1979 (35),.KoUenkark et al. 1982 (36), Pinter et al., 1985 (33). Barnsley- et al, 1984 (15), 
Colewell. 1974 (5), Kuusk. 1991 (32). Heute. 1987 (29), Kimes et al.. 1984 (34) and Egbert & Ulaby, 1972 (19). 
It is necessary to consider the relationship between changes in 'important' target characteristics and changes in the 
level and spectral distribution of the observed radiance.. In agriculture these important characteristics are crop 
type, stage of growth, degree of stress, leaf area index canopy height presence of pests, etc. Unfortunately levels 
of noise exist from 'unimportant' target characteristics. Examples of these are given by Duggin & Whitehead, 
1982 (51)>