luminosity
This is the
technical term in optics for brightness. Brightness is relative and
subjective. In this image the grey square is darker than the white
square, but lighter than the black square. This optical illusion then
asks which of the two (identical) grey squares is the darkest (or
brightest). This is where subjectivity and relativity come in.
On the other hand, luminosity is objective and not relative.
It has an absolute value which is measured in Candela. Fortunately we do
not need a good grasp of how bright a candela is to be able to use our exposure
meters or automatic cameras, but it is because luminosity is a standard that
they work.
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