Interest is currently being shown in the role of thermal transport in determining the laser damage threshold of thin film structures. Experimental evidence [1] suggests that the thermal conductivity of materials in thin film form may be significantly different to that in bulk, and that this is probably related to the microstructure of the film. A recent paper by Abraham and Ogilvy [2] presents a linear theory of heat conduction in an etalon filter structure with a weakly absorbing spacer layer. This allows the calculation of temperature distribution at the centre of the spacer for given values of film thermal conductivity. For spacer layers with significant temperature dependences of the refractive index (eg ZnSe near its fundamental absorption edge), the temperature rise induced by laser heating can be followed in principle as the change in transmission produced as the etalon sweeps along the edge of one of its characteristic fringes. When the appropriate cavity conditions are met, optical bistability can occur with incident powers typically in the milliwatt regime with focussed laser beams. This power depends on such parameters as the effective optical nonlinearity of the material, the design of the etalon and wavelength. The heatsinking of the device, normally on a glass substrate, will affect both the switching power and the switching speed. Although the absorption coefficient of the spacer, a, varies rapidly with wavelength near the band gap, the temperature dependence of the refractive index as measured [3] is much less sensitive to wavelength. It may also be noted that thick polycrystalline samples have shown dispersive optical bistability at 476nm, whereby a polished etalon of ZnSe provides feedback [4,5]; bistability due to increasing absorption in which the feedback is inherent in the thermally induced absorption edge shift was also achieved in this type of sample [5].