  The quantum efficiency and reflectivity of thick, back-illuminated CCD's
being fabricated at LBNL for astronomical applications are modeled and compared
with experiment. The treatment differs from standard thin-film optics in that
(a) absorption is permitted in any film, (b) the 200--500~$\mu$m thick silicon
substrate is considered as a thin film in order to observe the fringing
behavior at long wavelengths, and (c) by using approximate boundary conditions,
absorption in the surface films is separated from absorption in the substrate.
For the quantum efficiency measurements the CCD's are normally operated as
CCD's, usually at $T = -140^\circ$C, and at higher temperatures as photodiodes.
They are mounted on mechanical substrates. Reflectivity is measured on
air-backed wafer samples at room temperature. The agreement between model
expectation and quantum efficiency measurement is in general satisfactory.
