How are FLIR's microbolometer arrays packaged? Are they under vacuum? Why?

The microbolometer sensing elements are thermistors  that are suspended, as bridge structures, above a readout integrated circuit, or ROIC. The sensing elements need to be able to change temperature individually in response to small amounts of heat energy. To enable this response to small changes in radiant energy, the sensing elements must be thermally isolated from the ROIC. To achieve the thermal isolation, the sensor is housed in a high-quality vacuum package assembly, or VPA, to eliminate the air gap that would otherwise exist between the thermistors and the ROIC. An air gap would enable a conduction path that effectively dampens the sensor responsivity. The VPA also protects the otherwise fragile array of thermistors.