The optical data disk consists of a rigid substrate, a thin film multilayer coating, and a transparent overcoat. The optical behavior of the data disk is determined by the thin film trilayer which includes a reflector layer, a non-absorbing dielectric spacer layer, and an absorbing recording layer. The reflector layer is an opaque metal film with high reflectance, typically aluminum, silver, or gold. The spacer layer serves to place the recording layer near a peak in the standing wave that is formed by the interference of forward and reverse flowing waves in front of the reflector. The recording layer is a semitransparent layer of an absorbing material with selected optical and thermal properties. Regions of the recording layer that are heated by absorption and retention of incident energy suffer an irreversible change in physical and optical properties that leads to a written data bit. Discontinuous gold films, frequently referred to as island gold films, offer several advantages in this application. When heated by the absorption of radiation in the trilayer configuration, the islands coalesce, the optical absorption of the island film diminishes, and the trilayer reflectance is changed significantly. The optical behavior of this trilayer is predicted with a particle-size-dependent Maxwell-Garnett model for the island film. This paper begins with a description of the trilayer from the viewpoint of the thin film designer. Then the Maxwell-Garnett model for the optical properties of an island film is introduced. The two theoretical descriptions are then combined to predict the behavior at the helium-neon laser wavelength (633 nm) of a trilayer which contains an island gold recording layer. LaBudde, LaBudde, and Shevlinl presented a comprehensive description of the optical data disk structure which is the subject of this analysis. The optical data disk consists of a rigid substrate, a thin film multilayer coating, and a transparent overcoat. Figure 1 shows the trilayer cross-section and identifies the elements and the optical parameters of interest. Interference within a thick transparent over-coat is not important because the data disk is used with a focused cone of laser light.2 Therefore, the overcoat will be treated as a medium of infinite extent.