Apparatus for processing a wafer, and method of controlling such an apparatus

An apparatus for processing a wafer, the apparatus comprising:

• a rotatable chuck adapted to receive the wafer;
• a heating assembly comprising an array of light-emitting heating elements arranged to illuminate the wafer received by the rotatable chuck to heat the wafer, wherein the array of light-emitting heating elements is arranged to illuminate a first surface of the wafer when the wafer is received by the rotatable chuck, wherein the first surface is on an opposite side of the wafer than a second surface of the wafer, and wherein the second surface is exposed for processing when the wafer is received by the rotatable chuck;
• a liquid dispenser arranged to dispense a liquid onto the second surface of the wafer;
• and
• one or more light sensors arranged to detect light emitted by the array of light-emitting heating elements and configured to output a signal which is related to intensity of light emitted by the array of light-emitting heating elements,
• wherein
• the one or more light sensors comprise a light sensor positioned opposite to the array of light-emitting heating elements and arranged to be on an opposite side of the wafer compared to the array of light-emitting heating elements when the wafer is received by the rotatable chuck:
• or
• the one or more light sensors comprise a light sensor arranged to detect light from the array of light-emitting heating elements that is reflected from the wafer when the wafer is received by the rotatable chuck.