Image 21

original colour coded

and with an inserted data grid:

(in the middle of tiles, median of raw G-pixels values is given, the bottom value is the tile luminance divided by one candela per square meter). 
# Date/Time    : 2005:08:05 00:55:17
# Resolution   : 2048 x 1536 in jpeg (2064x1540 in pgm/21.pgm)
# Focal length :  8.2mm
# Exposure time:60.120 s 
# Aperture     : f/2.5
# ISO equiv.   : 100

#c li count  min   Yav   max  ovf   min  Cav    max   ovf   min  Gav    max   ovf   min  Mav   max   ovf
# %    Y     C     G     M readings  Y        C        G        M        L / cd/m2
  1   -36   -35   -45   -39      -2.40E-4  -2.47E-4  -3.72E-4  -3.40E-4  -2.00E-4
 10   -13   -13   -17   -15      -8.66E-5  -9.18E-5  -1.40E-4  -1.31E-4  -7.05E-5
 50    19    16    19    17       1.27E-4   1.13E-4   1.57E-4   1.48E-4   1.18E-4
 90   117    89    89    89       7.80E-4   6.29E-4   7.35E-4   7.77E-4   7.93E-4
 99   300   181   189   208       0.00200   0.00128   0.00156   0.00182   0.00213
100 16288 16283 16264 16277       0.109    0.115    0.134    0.142    0.101 
# flux from 6.28 sr ( 1.62E-5 sr/4px) is  0.00202 lx (SQM 8.08E-4 nt, 20.32 mag);
# tilewise luminance  2.64E-4 cd/m2 with rel. SD  154 % (summing pixels below 16380)
# dark frame dark/1m.pgm subtracted from all values
# Yell average=  40.3  minimum= -512  maximum=16288   over 16380:   3
# Cyan average=  28.8  minimum= -433  maximum=16283   over 16380:   3
# Gree average=  29.5  minimum= -512  maximum=16264   over 16380:   2
# Mage average=  29.9  minimum= -512  maximum=16277   over 16380:   2

End of results of "raw2lum" (version 1.22) with parameters:
  -c YCGM -fa -w 7.3 -fc0.214 -o 16380 -d dark/1m.pgm -sm200 -nh -i 21.txt -e 21.eps -n 21.ppm -t156 -c 3412 -s64 pgm/21.pgm
Luminance scale / 1 cd/m<sup>2</sup>
Logarithmic luminance scale (the unit is candela per square meter) – intensities of snow illuminance in lux would be obtained by multiplying the numeric value of luminance by 3.6. For asphalt, such a coefficient is between 20 and 40 (or much less, for specular reflection of distant lamps)