and Rendering System
Gregory J. Ward / GJWard@lbl.gov
Accuracy is one of the key challenges in physically-based
luminance (or the more general "spectral radiance") is probably the
most versatile unit in lighting.
Photometric units such as luminance are measured in terms of visible
radiation, and radiometric units such as radiance are measured in
terms of power (energy/time).
Luminance represents the quantity of visible radiation passing through a
point in a given direction, measured in
lumens/steradian/meter^2 in SI units.
Radiance is the radiometric equivalent of luminance, measured in
Spectral radiance simply adds a dependence on wavelength to this.
Luminance and spectral radiance are most closely related to a pixel,
which is what the eye actually "sees."
From this single unit, all other lighting metrics can be derived.
Illuminance, for example, is the integral of luminance over a projected
hemisphere (lumens/meter^2 or "lux" in SI units).
Luminous intensity and luminous flux follow similar derivations.
By computing the most basic lighting unit, our simulation will adapt
more readily to new applications.
To assure that a simulation delivers on its promise, it is
essential that the program undergo periodic validation.
In our case, this means comparing luminance values predicted by
to measurements of physical models.
An initial validation was completed in 1989 by Grynberg
, and subsequent validations by ourselves and others confirm
that the values are getting better and not worse .