The wavelength of the light emitted by the object depends on it‘s temperature, with hotter objects generating light at shorter wavelengths.
The response of most detectors used to measure electromagnetic radiation is in some way wavelength dependent.
Thermal detectors have the widest wavelength response with the limiting factor being the transmission window used to protect the sensing element.
Pyroelectric detectors are suitable for this application with high temperature stability, signal to noise and low cost.