Abstract: A method and apparatus make use of data representing changes in wavelength of a radiation source to provide control of focal plane position or to provide correction of sensor data. In the first aspect, the wavelength variation data is provided to control systems that control focus by moving apparatus components including, for example, the mask table, the substrate table or optical elements of the projection optical system. In the second aspect, variation data is used in correcting, e.g., focal plane position data measured by a transmitted image sensor. The two aspects may be combined in a single apparatus or may be used separately.