A method of measuring the temperature of a sheet material is provided. The sheet material is 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. The method comprises: a) generating a thermal image of at least part of the inside of the cavity using a thermal imaging device to detect radiation emitted by 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; b) identifying a first subset of the plurality of pixels whose pixel values meet predetermined criteria; c) using the identified first subset of pixels to determine a line on the thermal image representative of optimal emissivity enhancement in the cavity; and d) selecting a second subset of the plurality of 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.