Devices and corresponding methods can be provided to monitor or measure temperature of a target or to control a process. Targets can have low, unknown, or variable emissivity. Devices and corresponding methods can be used to measure temperatures of thin film, partially transparent, or opaque targets, as well as targets not filling a sensor's field of view. Temperature measurements can be made independent of emissivity of a target surface by, for example, inserting a target between a thermopile sensor and a background surface maintained at substantially the same temperature as the thermopile sensor. In embodiment devices and methods, a sensor temperature can be controlled to match a target temperature by minimizing or zeroing a net heat flux at the sensor, as derived from a sensor output signal. Alternatively, a target temperature can be controlled to minimize the heat flux.