Patent ID: 7265336

Claim:
An encoder comprising: a drum comprising a circular cylindrical surface characterized by an axis and a radius of curvature, said drum having a surface with a normal perpendicular to said axis; a first track comprising a plurality of alternating reflective and non-reflective stripes arranged on said circular cylindrical surface, said reflective stripes comprising a portion of a said circular cylindrical surface, each reflective stripe having a circular cylindrical outer surface having an axis coincident with said axis of said drum; a first light source that illuminates said outer surface of said reflective stripes at an oblique angle relative to said normal; and a first photodetector positioned to receive light from said light source that is reflected from said reflective stripes of said first track when said drum moves relative to said photodetector, an image of said reflective stripes of said first track being formed on said first photodetector, said image having a magnification that depends on said radius of curvature.