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FIELD MEASUREMENTS OF VEGETATION SIGNATURES USING A TELE-SPECTRO- 
RADIOMETER 
Wolter Arnberg, Dept of Physical Geography, University of Stockholm 
Reflectance measurements on vegetation will contribute to the under- 
standing of the photographic rendition in aerial pictures. Also, such 
measurements will improve the interpretation of aerial photography and 
choice of time, filter and films for aerial photography or corresponding 
parameters of multispectral image data analysis and collecting. 
The rationale for the first Swedish investigation of the reflectance of 
some trees (Bäcksträm and Welander, 1948) was in fact to give recommenda- 
tions for aerial photography and interpretation of trees in aerial photo- 
graphs. These measurements were made in a laboratory. 
However, the reflectance properties of e.g. vegetation must be recorded 
at the locality of the plant - in situ - and in a similar manner as with 
airborne sensors. In Sweden, the first in situ-measurements with visual 
spectro-photometry were made by Rising and Tonnquist (1961). Later on 
Agren (1972) have made in situ-measurements with a spectro-radiometer. 
The discrepancies found in laboratory measurements of reflection compared 
to in situ-measurements often depend on the measurement geometry. The 
reflectance recorded in an aerial photograph correspond to bidirectional 
reflectance. It can be defined as the ratio of radiance from a canopy at 
a particular look angle and the integrated irradiance on a horizontal 
datum (Colwell, 1974, p.176). In this study the datum surface has been a 
transmitting diffusing disc, which closely obeys Lambert's law, or a 
reflectance reference panel. 
The instrument system (manufactured by Gamma Scientific) collects the 
radiation by a telescope with selectable angular acceptance 3". 1°, 20" 
and 6'). The telescope is connected to a grating monochromator with a 
wavelength scanning facility (0.35-0.80 pm) via a fibre optic. Scan rate 
(range 0.4-0.8 ym) in applied measurement is.about 20 sec. The detector 
is a photomultiplier tube.