Apparatus and procedure for the in situ measurement of quantities of polymer printed onto supports A source of radiation 2 sends electromagnetic radiation onto a droplet 9, 10 of polymer solution or polymer dispersion, which is printed onto a support 4. A detector 3 measures the transmitted or re-emitted radiation. Controlled by a comparator device 6 and a control unit 5, the printing parameters are corrected in situ, if necessary. In this manner, a controlled adjustment of the layer thickness of organic light-emitting diodes and therefore luminosity and color perception can be achieved.