Abstract: A method of measuring the temperature of a sheet material arranged such that it forms at least one side of a cavity, so as to enhance the effective emissivity of the sheet material in the vicinity of the cavity comprises: generating a thermal image of at least part of the inside of the cavity, the thermal image comprising a plurality of pixels each having a pixel value representative of radiation emitted by a respective region of the cavity; identifying a first subset of the pixels whose pixel values meet predetermined criteria; using the identified first subset to determine a line on the thermal image representative of optimal emissivity enhancement in the cavity; and selecting a second subset of the pixels based on the determined line and generating a temperature profile along the determined line derived from the pixel values associated with each of the second subset of pixels.