Position measuring system with compensation for variable distance between light source and index disk

A measuring system comprising:

• a rotatable index disk located in an index plane having a scale formed by index lines radially extending along a surface of the index disk wherein each index line has a length limited by masked areas;
• a scanning unit positioned in front of the index disk, located in a scanning plane, and having a separation from the index disk, the scanning plate being parallel with the index plane and having scanning fields for scanning the index lines of the index disk, wherein the separation between the index disk and scanning unit can vary from a minimum separation to a maximum separation;
• a light source located in front of the scanning unit wherein the light source emits beams that are oblique to the index plane but parallel to the index lines;
• and
• a plurality of detectors positioned behind the index disk and located in a detector plane parallel with the index plane wherein the detectors are illuminated by light modulated by the index disk, wherein the masked areas of the index disk are positioned to compensate from an increase in the illumination of the detectors which would occur when the separation between the scanning unit and index disk is at its minimum.