Optical element exchange unit

An optical element exchange unit, comprising:

• a rotatable wheel, with at least two optical elements in or on the wheel, positioned so that different ones of the optical elements will be moved to intersect an optical axis of the optical element exchange unit by rotating the wheel, the wheel comprising first soft magnetic teeth protruding from a soft magnetic part of the wheel at an even angular offset along at least part of a ring around a rotation axis of the wheel;
• a stator comprising:
• a first soft magnetic yoke, the first soft magnetic yoke having poles with a first group and second group of second soft magnetic teeth respectively, the second soft magnetic teeth of the first and second group protruding from the poles of the first soft magnetic yoke towards the first soft magnetic teeth;
• a second soft magnetic yoke, the second soft magnetic yoke having poles with a third group and fourth group of second soft magnetic teeth respectively, the second soft magnetic teeth of the third and fourth group protruding from the poles of the second soft magnetic yoke towards the first soft magnetic teeth;
• wherein the teeth of the first, second, third and fourth groups are positioned so that, when each second soft magnetic tooth of the first group is aligned with its nearest first soft magnetic tooth, each second soft magnetic tooth of the second group is halfway the angular offset between its nearest first soft magnetic tooth, and the second soft magnetic teeth of the third and fourth group are less than half the angular offset in opposite directions from their nearest, respective, first soft magnetic teeth;
• a permanent magnet, magnetically coupled between parts of the first and second soft magnetic yoke,
• wherein a part of the first soft magnetic teeth having a relatively narrower top width is located in ranges of angular positions where the second soft magnetic teeth face the wheel when respective ones of the optical elements intersect the optical axis respectively, and a further part of the first soft magnetic teeth located in between these ranges of angular positions have a relatively broader top width.