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.