An apparatus in a machine for processing printing material has a register sensor for register measurement between at least two color separations of a printed image printed above one another on a printing material. The register sensor registers optically deviations between the two color separations and forwards the deviations onto a computer. The register sensor contains at least one photodiode having at least two quadrants. The largely rectangular evaluation areas of the photodiode are aligned obliquely in relation to the edges of the conveyed printing material. A register mark to be detected by the register sensor has a plurality of wedge-shaped colored areas pointing in a peripheral direction of the printing material, one of the wedge-shaped colored areas being a reference color used as a reference variable and others of the wedge-shaped colored areas being register colors to be controlled. An alternative register mark has a first field for length calibration; a second field having a wedge of a reference color selected as a reference variable; and a third field containing a register color to be controlled. The may also be provided a measuring field formed as a point-symmetrical field exhibiting a color of a color separation as a full tone.