System for measuring temperature of a filled vessel on a hot plate

In a method for measurement of the temperature of the contents of a filled vessel on a hot plate, of the type including sensing the extent of radiated heat by means of a remote radiation pick-up which is pointed toward and positioned at varying distances from the side of the vessel, said pick-up receiving radiation from a radiation measurement field extending at an angle of taper from said vessel side to said pick-up, and giving a signal corresponding to radiation received from an area on the side of the vessel, in situations in which varying placements of the vessel on the hot plate causes varying extents of spurious radiation available for sensing, the improvement comprising providing a radiation pick-up having a sensor and a lens, adjustably setting the angle of taper of the radiation measurement field to an angle diverging from said vessel side to said pick-up, said setting of said angle of taper being by moving the position of said sensor with respect to the position of said lens interposed between said pick-up and said vessel, said setting of said angle of taper thereby adjusting the magnitude of the area from which radiation is detected.