A deterministic optics model for predicting measures of light penetration, including Secchi disc transparency (or clarity; SD) and the vertical attenuation coefficient for downward irradiance (Kd), from concentrations of various attenuating substances, is developed, calibrated, and applied for culturally eutrophic Onondaga Lake, NY. The model is an invaluable management tool for establishing the appropriate focus and realistic expectations for improving clarity in the lake. The model is consistent with optical theory, partitioning attenuation according to the processes of absorption and scattering, and materials contributing to these processes. The model is developed from optical measurements made for the lake over the 1987–1990 interval, reported by Perkins and Effler (1996), and attendant estimates of the values of absorption and scattering coefficients presented herein. It is demonstrated that the primary components responsible for low SD and high Kdin the lake are phytoplankton and tripton (other than CaCO3particles). Gelbstoff and CaCO3particles are relatively unimportant. Management efforts to improve clarity should focus on the lake's cultural eutrophication problem. Terrigenous particle inputs received during runoff events would continue to substantially reduce clarity for brief periods if they are not abated.