  Light power spectra are introduced as a new tool for relative light yield
(LY) determinations. Light event spectra have commonly been used for this
purpose. Theoretical background supporting this change is provided. It is shown
that the derivative of a light power spectrum can provide a reliable LY
measurement at levels as low as 2% of those for high-yield liquid
scintillators. Applications to light evolution in the PPO+LAB system and to
water-based liquid scintillators are described.
