luminosity distance

An object's luminosity distance (d_L or D_L) from an observer is a distance measure based upon how much the radiant flux is reduced due to its travel:

         L
(dL)² = ———
        4πF

• d_L - luminosity distance.
• L - absolute luminosity of the object.
• F - radiant flux received by the observer.

For non-cosmological distances (i.e., z ≈ 0, certainly including distances to stars within the Milky Way), if there were no reddening, this would correspond to the Euclidean distance. This fact is used as a means of determining the distance to stars (e.g., spectroscopic parallax) when more accurate means are not available, but yields no more than a rough estimate because both the absolute magnitude of the object and reddening would be estimates. In principle, if those two factors are accurately known and taken into account, it would be accurate.

For cosmological distances, the curvature of space and the enlargement of space over the EMR's travel-time (scale factor) complicate matters and luminosity distance generally differs from other concepts of distance. However, the term is commonly used in cosmology, such as in discussions of the factors that make it unreliable. Also, the value is commonly used with equations for cosmological distances and distance-related phenomena, along with correction terms that estimate the effects of these confounding factors.