Each second image in a row shows luminance coded by colours (each colour corresponds to another power of ten, scale is logarithmic; mean red is 1 cd/m2, mean green is 10 cd/m2)
Each third image has a grid overlay. Average luminance of each tile is written at its bottom (in cd/m2). (The middle number in each tile is a median value of green pixels.) Each fourth image has the same grid overlaid on colour-coded lumiances.
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 reflections of distant
lamps)
Westminster bridge over Thames, Parliament and Big Ben are shown. Glare
from the lamps on the bridge becomes prominent already.
A pleasant court (The Temple) illuminated by gas lamps
(glaring, but not too strong).
A beam from a skybeamer behind.
Sky luminance from 0.10 cd/m2 to 0.05 cd/m2,
terrain about 0.02 cd/m2.
Sky with Vega and Deneb within the top of the Northern Cross.
Towards city centre, sky luminance is about 24 mcd/m2.
Sky with Andromeda, Great Square and Aries stars, away from city centre.
Sky luminance about 20 mcd/m2
Sky with Perseus, two Andromeda and Aries stars and some high clouds;
airplane at upper right.
Clear sky luminance about 18 mcd/m2,
clouds have 22 mcd/m2