Optical measurement method and sensor apparatus

An optical measurement method using an optical sensor apparatus, the optical sensor apparatus comprising an optical element comprising a mark configured to selectively transmit incident radiation, a photodetector configured to receive radiation transmitted by the mark and provide an output signal that is indicative of the received radiation, and a support Which supports the optical element and is in thermal contact With the optical element. A thermal conductivity of the support is greater than a thermal conductivity of the optical element and a coefficient of thermal expansion of the support is greater than a coefficient of thermal expansion of the optical element. The method comprises performing a first measurement using the optical sensor apparatus, the first measurement including illuminating the mark With radiation. The temperature of the optical element changes during the first measurement. The temperature of the support is substantially constant throughout the first measurement.