An apparatus for measuring a temperature of an assembly that is internal to a process chamber. The apparatus may include a light pipe positioned between a lamp radiation filtering window and the assembly, the light pipe has a first end with a bevel configured to redirect infrared radiation emitted from the assembly through the light pipe and has a second end distal to the first end, an optical assembly configured to collimate, filter, and focus infrared radiation from the second end of the light pipe, an optical detector configured to receive an output from the optical assembly and generate at least one signal representative of the infrared radiation, a temperature circuit that transforms the at least one signal into a temperature value, and a controller that is configured to receive the temperature value and to make adjustments to other process parameters of process chamber based on the temperature value.