Luminosity signifies the total amount of visible light emitted by a light source and is measured in watts or lumens. It is a measure of the raw power output of light.
Luminance, on the other hand, tells us about the perceived brightness of that light as perceived by an observer. It considers multiple factors:
The luminosity of the source
Reflection off the surface receiving the light
The human eye’s sensitivity to different wavelengths
A powerful light source (high luminosity) might shine on a dark surface (low reflection), resulting in a lower perceived brightness (lower luminance).
Conversely, a weaker light source shining on a highly reflective surface could still lead to a higher perceived brightness due to the reflection boosting the received light.