A cavity thermal detector assembly is presented that allows both tunable narrowband and broadband operation. This allows for high light efficiency, low thermal time constant, and flexibility in designing the optical path. The thermal detector/filter layers are part of the top mirror or mirrors of a Gires-Tournois-type optical cavity and provide absorption and reflection that can be adjusted to the desired width and position of the detected band. Tuning, if desired, can be achieved by applying micromechanical methods. Broadband operation may be achieved by bringing the sensor close to the bottom mirror. In this mode, the sensor or its supports may or may not touch over a small area.