Lightness Constancy
We tend to perceive the 
lightness of objects veridically, 
even though different degrees 
of illumination fall on them.  
Because the reflectance is 
constant, the entire stair 
appears to be white.

What determines our 
perception of neutral (or 
achromatic) color—–the shades 
of lightness, from white to 
black? The answer that often 
springs most readily to mind is 
that our perception of these 
shades is determined by the 
specific intensity of light 
reflected by a surface to the 
eye, referred to as luminance. 
After all, everyone knows that 
white surfaces reflect more 
light than do black surfaces, 
which is why one wears white 
clothing in the summer to 
reflect sunlight and dark 
clothing in the winter to 
capture the sun’s warmth.

    The luminance of a white 
surface is indeed far greater 
than that of a black one—–at 
least for any given 
illumination. The qualification 
points to the difficulty with the 
absolute-luminance 
explanation. Illumination 
varies from place to place and 
from time to time. A black part 
of a whitewall tire in bright 
sunlight can reflect light to the 
eye that is thousands of times 
stronger than that reflected by 
the tire’s white part in a dimly 
lit garage. Were luminance the 
explanation of lightness 
perception, the black part of 
the tire in sunlight should look 
much lighter than the white 
part indoors. But it doesn’t. 
Constancy prevails.