Photometry at lambda = 3.5-20-mu-m in 4" and 8" apertures was obtained to measure the flux from the central portion of the beta Pic disk. The 10.1 and 20-mu-m measurements show an excess (relative to the photosphere) which is extended, with significant flux located in the annulus between 4" and 8" diameter centered on the star. Disk models have been calculated to reproduce the ground-based IR data, IRAS fluxes, IRAS 60-mu-m scan profile widths, and optical scattered light distribution. The model results imply: (1) the color temperature and angular scale of the disk are not consistent with emission from large ("blackbody") particles: most of the thermal emission originates from particles which are inefficient radiators, with sizes near 1-mu-m; (2) a simple disk with a single power law spatial distribution is inconsistent with the combined optical and infrared data: models are presented with two structural components, r > 80 AU and r < 80, which are better matches to the available observations; (3) the inner disk component represents a gross deficit of material relative to an inward extrapolation of the outer component; (4) the temperature of typically sized grains at the boundary between the outer and the inner components is in the range 90-140 K, corresponding to the temperature of transition from slow to rapid sublimation of water ice: sublimation would proceed more rapidly than other destruction mechanisms for small ice grains inside the boundary and could be responsible for the relative depletion of grains in the inner disk component; (5) an albedo of approximately 0.35 is deduced for the particles in the outer component; (6) most of the material in the inner component is at temperatures where sublimation of small water ice grains would occur so rapidly that predominance of refractory grains is indicated; (7) the 10-mu-m observations appear to require that the inner component extends inward only to a limit between about 1 and 30 AU from the star, leaving an innermost void.
