n x 18m 
essed to 
standard 
ixel image 
^OT and 
held and 
print of 
rectional 
lectance 
leviation 
: ZEISS 
is due to 
1e than the 
Manfred Schroeder 
  
  
4. OPTICAL PARAMETERS OF THE ATMOSPHERE 
4.4 Ground irradiance 
To determine the optical parameters of the atmosphere the downwelling total irradiance and the skylight 
irradiance was measured with the Zeiss MCS-Spektrometer. For measuring the skylight the direct sun light was 
covered by a little plate of appr. 5 cm diameter [7]. 
The total irradiance is given by the following formula: 
E = E. cos + E (1) 
E,: Total irradiance 
E,: Direct sun irradiance on a surface vertical to the radiation direction 
Es: Skylight irradiance 
0: Sun zenith angle; can be determined from geographical coordinates and time 
E, and E,,, were measured nearly simultaneously [Fig. 6]. 
  
  
  
  
1800 t i 1 t 1 E 
"total irradiance 11:39 UTC" —— 
1600 + y A An n "skylight 11:39 UTC" na = 
| | io 
1400 - b^ ; 
r= di Mg 
e . ^ 
S poo y e | - 
ë | | F I 
& 1000} M ; | 
x. ij i A 
É 00 - | | | as ] 
3 8 fl ; | h \ 
= 600 + j | 3 
£ | 
400 } ci Ya | j 
hi Te | i 
; "VY ttem itn T A 
200 | ET. : 
0 i 1 1 À i i Re 
300 400 500 600 700 800 900 1000 
wavelength [nm] 
Fig 6: Measurement of total irradiance E ‚and skylight irradiance E sky at 11:39 GMT, 2 
July1998 (sun zentith angle= 20.56°) 
4.2 Atmospheric transmittance 
The direct sun irradiance can also be written as: 
E zuE pts? 
s dues (2) 
Y. Atmospheric Transmittance 
Eo Extraterrestrial irradiance 
Combining (1) and (2) gives the atmospheric transmittance: 
E, e E 
E, - cos € 
cos % 
T 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 283