Patent ID: 7057160

Claim:
An optical encoder comprising: a light source unit; a scale which has a periodic optical pattern and displaces relatively to the light source unit in a scale moving direction; and a light detector to detect a light beam emitted from a light source of the light source unit and traveled by way of the scale; wherein the light source unit has a light beam exit opening through which a light beam is emitted toward the scale; and assuming that a distance between the light beam exit opening and the scale is z 1 , a distance between the scale and the light detector is z 2 , and a pitch of the periodic optical pattern of the scale is p 1 ; wherein the width W of the light beam exit opening in the scale moving direction is specified as follows: p 1×(2 n− 1.5)×( z 1+ z 2)/(2× z 2)≦ W≦p 1×(2 n− 0.5)×( z 1+ z 2)/(2× z 2) where n is a natural number and the values of z 1 and z 2 are substantially equal to each other.