Multiple wavelength configuration for an optical encoder readhead

A readhead and scale arrangement usable to provide an indication of a position between two members that move relative to one another along a measuring axis direction, the readhead and scale arrangement comprising:

• a readhead comprising a light source configured to output diverging source light comprising a plurality of individually detected wavelengths and at least a first optical signal receiver channel configured to provide a first set of position signals;
• and
• a scale track comprising a scale grating extending along the measuring axis direction on a scale member, the scale track configured to diffract the diverging source light to provide first and second diffracted scale light portions of each of the individually detected wavelengths to the first optical signal receiver channel,
• wherein:
• the readhead is configured such that the first and second diffracted scale light portions of each individually detected wavelength interfere to provide interference fringes proximate to an input of the first optical receiver channel;
• the readhead comprises an optical path difference element comprising a first optical path region and a second optical path region wherein the first and second optical path regions have an optical path length difference, and the optical path difference element is arranged such that the first diffracted scale light portion of each of the individually detected wavelengths is derived from source light that passes through the first optical path region and emerges with a first wavelength-dependent phase shift, and the second diffracted scale light portion of each of the individually detected wavelengths is derived from source light that passes through the second optical path region and emerges with a second wavelength-dependent phase shift, such that for each of the individually detected wavelengths, the interference fringes arising from the first and second scale light portions have a different position-independent spatial phase shift that depends on that wavelength and the optical path length difference of the optical path difference element;
• and
• the input of the first optical signal receiver channel comprises a first spatial filter having a first spatial phase and configured to filter the interference fringes of the individually detected wavelengths such that each member of the first set of position signals comprises a position-dependent periodic signal of one of the individually detected wavelengths, the position-dependent periodic signal having a position-independent phase shift corresponding to the position-independent spatial phase shift of the interference fringes corresponding to that wavelength and the spatial phase of the first spatial filter.