Methods and devices relate to radiometric measurement of a thin fluid film (4) using absorption of IR radiation. At least one IR radiator (1) is directed to send signals to an IR detector (2) via a bed (3) on which the fluid film (4) is placed. The IR detector is set to receive IR radiation in a band around a characteristic absorption band of the fluid of the fluid film (4). The present invention is developed for printing presses, where the thickness of fountain solution on the printing plate and the proportion of printing ink in an emulsion of printing ink and fountain solution may be controlled.