Every object emits heat in the form electromagnetic waves and sampling the energy radiated into space allows the measurement of the temperature of the object without ever having to touch it. The wavelength of the light emitted by the object depends on its temperature, with hotter objects generating light at shorter wavelengths. 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, excellent signal to noise and low cost.
A pyroelectric crystal is distinguished by having an atomic structure that will move slightly in response to a change in temperature, rearranging the electric charge within the solid generating a voltage which can be measured using a set of electrodes on the crystal surface.
LASER COMPONENTS is a manufacturer of pyroelectric detectors. The devices are manufactured in standard TO style packages and have long expected lifetimes making them suitable for this fit and forget application.