DETERMINATION OF THE ROTATIONAL POSITION WITH AN OPTO-ELECTRONIC ENCODER WITH A BALL LENS

An image based opto-electronic encoder (2) for a joint (1), the encoder (2) having a reading head (5) for the first part (1a) of the joint (1) and an optical system, comprising a ball lens (3) for the second part (1b) of the joint (1). The ball lens (3) has a front surface transparent for measuring light and an at least partially reflective, structured back surface (4). The ball lens (3) has -with respect to the measuring light- a refractive index of at least approximately two or an equal radially varying refractive index, such that a bundle of measuring light rays reflected inside the ball lens (3) from a point (A) of the back surface (4) of the ball lens (3) forms at least approximately a parallel bundle after getting refracted at the front surface of the ball lens (3). The reading head (5) and the ball lens (3) are rotatable relative to each other in at least two degrees of freedom (Rx, Ry, Rz). Such rotational positions of the reading head (5) and ball lens (3) relative to each other are determinable with respect to the at least two degrees of freedom using an image (14) of at least a section of the back surface (4) by image evaluation of the imaged structure of the back surface (4) .