The thermal-optical property of an ultralow expansion (ULE^®) 7973 glass wafer was assessed using spectrophotometers and variable angle spectroscopic ellipsometers. By modeling both the spectral transmission and the ellipsometric data, we derived optical dispersion of the glass over a broad spectral region from 180 nm up to 12 μm. Our results indicate that the transparency region of the glass is terminated by two fundamental absorption edges in the ultraviolet (UV) and the infrared (IR). The UV and IR absorption edges are dominated by the titania and silica constituents, respectively. The exponential absorption tail, the Urbach energy, in the UV increases linearly with the temperature ranging from room temperature to 650°C, whereas the optical bandgap decreases linearly over the same temperature range. The temperature-dependent coefficient is 18 μeV / ° C for the Urbach energy and −0.7 meV / ° C for the optical bandgap.