749 
DIRECTIONAL INFRARED TEMPERATURE AND EMISSIVITY 
OF VEGETATION: MEASUREMENTS AND MODELS 
J.M. Norman, S. Castello, and L.K. Balick 
Department of Soil Science, Univ. of Wisconsin, Madison, WI 53706 USA 
and E G & G Energy Measurements, Las Vegas, NV 89125 USA 
ABSTRACT 
Directional thermal radiance from vegetation depends on many factors, including the architec 
ture of the plant canopy, thermal irradiance, emissivity of the foliage and soil, view angle, 
slope, and the kinetic temperature distribution within the vegetation-soil system. A one 
dimensional model, which includes the influence of topography, indicates that thermal emis 
sivity of vegetation canopies may remain constant with view angle, or emissivity may increase 
or decrease as view angle from nadir increases. Typically, variations of emissivity with view 
angle are less than 0.01. As view angle increases away from nadir, directional infrared canopy 
temperature usually decreases but may remain nearly constant or even increase. Variations 
in directional temperature with view angle may be 5 C or more. Model predictions of direc 
tional emissivity are compared with field measurements in com canopies and over a bare soil 
using a new method that requires two infrared thermometers. One infrared thermometer is 
sensitive to the 8-14 pm wavelength band and a second to the 14-22 pm band. After correc 
tion for CO, absorption by the atmosphere, a directional canopy emissivity can be obtained 
as a function of view angle in the 8-14 pm band to an accuracy of about 0.005. Modeled and 
measured canopy emissivities for com varied slightly with view angle (0.990 at nadir and 
0.982 at 75° view zenith angle) and did not appear to vary significantly with view angle for 
the bare soil. Canopy emissivity is generally nearer to unity than leaf emissivity. High spec 
tral resolution, canopy thermal emissivity may vary by 0.02 with wavelength even though leaf 
emissivity may vary by 0.07. The one-dimensional model provides reasonably accurate pre 
dictions of infrared temperature and can be used to study the dependence of infrared tempera 
ture on various plant, soil, and environmental factors. 
KEY WORDS: Emissivity, Brightness Temperature, Thermal Infrared, Thermal Radiance 
1 - INTRODUCTION 
Thermal infrared measurements of surface temperature offer a possibility of remotely monitor 
ing quantities related to the surface energy and water budgets. In particular, the difference 
between canopy temperature and air temperature has long been known to be related to évapo 
transpiration. Numerous attempts have been made to relate canopy temperature measurements 
from infrared thermometers to fluxes of heat and water from vegetation. The relationship 
between canopy temperature and évapotranspiration has been well established for more than 
40 years (Penman, 1948). Unfortunately a precise definition of the term "canopy temperature" 
has proven to be difficult. Penman, in fact, eliminated surface temperature from his famous