- 586 - 
In the beginning of August the same wheat field was shortly 
before harvest and looked now equally yellow. The spectral 
response of the crop related to variations in the measuring 
geometry has completely changed as Figure 7 shows. The 
relative changes of R(A) with varied observational zenith 
  
T T T T T 
R 
% ae | Wheat (before harvest) 
e 
o 
F M0" T 40 3 Aug. 1977 7 
30 | 180° + 0° q 
  
   
   
— - 
LA 
ETT 
20 
  
  
R(800) «1 
  
  
  
rss TES R (550) x 2.5 (normalized) 
100 | A 
R(675) x2 2: 
(normalized) 3i *31?- 34? 
Lt L 1 : 1 
45? 30? 15° . o? 15? 30* 5° 
Zenith 5, 
Fig. 7 
angles are always nearly the same for the three selected 
wavelengths. From vertical measurements within three hours 
symmetrical to the solar culmination at 7, - 30,5" no 
significant azimuth dependence of R(A) could be observed, 
Figure 8. 
  
  
  
  
  
  
R 
* Wheat ( before harvest ] Ir =0° 
20 + 3 Aug. 1977 
R (800) 
————" — 
4$ [f 
3-35* NAS 305° 33° 35° 374 
100 | 
ue —-_R1679) 
s L:t--—---—t-—--——7*RIS50 
| 0 A A A A L A À À. ] À A A a À audi 
| -45° -20° 0° + 20° +45° 
Fig. 8 
Figures 9 and 10 demonstrate the effect of a disease on 
the shape of the spectrum. The curves show the spectral 
reflectance factor R of rice plants between 0.4 and 1 um 
determined at three different days in July. These measure- 
ments were made by our group in collaboration with the 
Joint Research Centre at Ispra in 1975. The test site 20 
consisted of a healthy rice canopy while the test site 18