Illumination portion for an adaptable resolution optical encoder

An illumination portion of an optical encoder which comprises a scale grating, an imaging portion, and a detector portion, the illumination portion comprising:

• a light source configured to output light having a wavelength λ;
• a structured illumination generating portion configured to input the light and output structured illumination wherein the structured illumination comprises an illumination fringe pattern oriented transversely to the measuring axis direction;
• a first filtering lens;
• a spatial filtering aperture configuration;
• and
• a second filtering lens, wherein:
• the first filtering lens is configured to focus the structured illumination proximate to a plane of the spatial filter aperture configuration;
• the spatial filtering aperture configuration includes a central portion that is configured to block zero and higher-order light included in the structured illumination and an open aperture portion proximate to the central portion that is configured to output +1 order and −1 order portions of the structured illumination to the second filtering lens;
• and
• the second filtering lens is configured to output the structured illumination to a plane of the scale grating as filtered structured illumination with an illumination fringe pitch PMI along the measuring axis direction at a plane coinciding with the scale grating.