Image 20

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 00:53:55
# Resolution   : 2048 x 1536 in jpeg (2064x1540 in pgm/20.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   -39   -37   -56   -48      -2.60E-4  -2.61E-4  -4.63E-4  -4.19E-4  -1.99E-4
 10   -16   -16   -25   -21      -1.07E-4  -1.13E-4  -2.06E-4  -1.83E-4  -7.66E-5
 50    15    12    12    11      10.00E-5   8.48E-5   9.92E-5   9.60E-5   1.00E-4
 90   100    77    72    72       6.66E-4   5.44E-4   5.95E-4   6.29E-4   6.88E-4
 99   261   161   159   172       0.00174   0.00114   0.00131   0.00150   0.00187
100  6733  8657  7760  6087       0.0449   0.0612   0.0641   0.0531   0.0391
# flux from 6.28 sr ( 1.62E-5 sr/4px) is  0.00158 lx (SQM 6.06E-4 nt, 20.63 mag);
# tilewise luminance  2.19E-4 cd/m2 with rel. SD  161 % (summing pixels below 16380)
# dark frame dark/1m.pgm subtracted from all values
# Yell average=  31.6  minimum= -512  maximum= 6733   over 16380:   0
# Cyan average=  23.0  minimum= -481  maximum= 8657   over 16380:   1
# Gree average=  18.8  minimum= -512  maximum= 7760   over 16380:   0
# Mage average=  19.7  minimum= -512  maximum= 6087   over 16380:   1

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 20.txt -e 20.eps -n 20.ppm -t156 -c 3412 -s64 pgm/20.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)