Correctly printed address fields permit letter sorting to be accelerated. Pepperl+Fuchs produces the contrast sensor DK12 for this purpose. Integrated into an addressing machine, it triggers the ink jet printer so that printing always is ensured in the address field. The system is triggered by a separate print mark, which intentionally is in weak contrast to the envelope surface. The contrast sensor DK12 even safely recognizes the weak contrast between the two yellow shades.