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BIDIRECTIONAL LEAF REFLECTANCE & TRANSMITTANCE 
OF STRESSED AND UNSTRESSED LEAVES. 
1 remote sensing 
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55-966. 
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'ersity of Arizona. 
3 ^dex (MSAVI), 
19 90, Description 
mote Sensing, vol. 
todeling: the SAIL 
Sonia Rubie. 
University College London, 
Dept of Geography, 
26 Bedford Way, London, WC1H OAP 
England. 
Sira Limited, 
Sira/UCL Postgraduate Research Centre, 
South Hill, Chislehurst, Kent, BR7 5EH, 
England. 
email: soniato'sirapgc.demon.co.uk 
ABSTRACT 
The angular dependence of reflected and transmitted radiation os er the exitant hemisphere for individual stressed 
and unstressed leaves will be studied to determine how the spectral response is affected Investigations will be 
carried out as to whether it is possible to both distinguish sources of stress and identify' them sufficiently early to 
allow corrective action. The reflectance and transmittance will be measured at source incidence angles between 0° 
- 70° and view angles between 0° - 70° in the visible and infrared region to obtain the bi-directional reflectance 
and transmittance distributions of the abaxial and adaxial surfaces of the leaves which have grown under a variety 
of stress conditions in a controlled environment. The leaves will also be analysed for various constituent 
concentrations including water, nutrients and pigments. A limited number of field trials are also proposed to study 
the bidrectional reflectance and transmittance of stressed canopies. 
KEY WORDS: Remote-Sensing, Vegetation, Bidirectional. Stress, Visible. Near-infrared. Water. Nutrient. 
1. INTRODUCTION 
The detection of stressed crops by remote sensing is based on the premise that their reflectivity is qualitatively and 
quantitatively different from that of healthy vegetation before visual signs of stress appear (Richardson & Everitt 
1987 (28)). There is experimental evidence and remote sensing data to support this view but in some cases the 
physical or physiological basis for the differences has been misinterpreted Both the red and infrared reflectance 
are sensitive indicators of physiological stress. Research has shown that the differences in reflectivity that allow 
discrimination of stressed plants can be traced to their leaf and canopy characteristics. 
2. AIMS 
The aim of the work is to investigate the relationship between plant stresses and their spectral responses with the 
long-term objective of developing instrumentation of practical benefit to the agricultural community 7 . Although 
much work has been done in the field of remote sensing, the correlation between plant stress and changes in the 
spectral response is poorly understood Investigating this relationship is an important first step in developing 
instrumentation that can give early warning of plant stress, so that corrective action can be taken promptly. 
Resultant instrumentation is intended for use in low-level sensing i.e. measurements at most a few metres above 
the plant canopy as opposed to satellite or airborne instrumentation. 
3. BACKGROUND REVIEW 
3.1. Vegetation Stress 
Stress may be defined as any 7 factor that reduces the productivity of the plant below its optimal value^ Trie 
productivity of stressed vegetation can be impaired for two reasons. Firstlv the stres canopy may ess