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 two color
separations lying one above the other on the printing material and
forwards the deviations onto a computer. The apparatus includes register
marks to be measured by the apparatus. The apparatus is distinguished in
that the register sensor contains at least one photodiode having at least
two quadrants, and in that the largely rectangular evaluation areas of
the photodiode are aligned obliquely in relation to the edges of the
conveyed printing material.