From The Collaborative International Dictionary of English v.0.48 :
Luminosity \Lu`mi*nos"i*ty\, n.
The quality or state of being luminous; luminousness.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
luminosity
n 1: the quality of being luminous; emitting or reflecting
light; "its luminosity is measured relative to that of our
sun" [syn: luminosity, brightness, brightness level,
luminance, luminousness, light]