Definition
Lightness is the perceived luminance of an object by humans.
Humans do not perceive luminance of an object linearly with the increase in candela/m^2.
Lightness is a linear prediction of how humans perceive Luminance
How light an object appears when compared to a similarly lit white object.
Different objects on a table can appear more or less “light”, even if lit by the same lamp.
Askip lightness is for objetcs being illuminated, and brightness if for light-emitting objects (lightbulb). But isn't every illuminated object reflecting light, and thus emissing some ? What's the difference when it comes to perceptions ? A fermob lamp looks like a white ball when lit by the sun ?
It's how "light-emitting" it looks to us.
It’s also called value (cf HSL and HSV)
Example
On an image, snow will be lighter than wood
Because of magic color perception shenanigans such as simultaneous contrast, sometimes it’s not really how luminous it is.
Not to be confused with Brightness or Luminance.