A conveying device (11.1, 11.2) of an agricultural harvester conveys crop material (B) along a window (3) which is inserted into a wall (4). A sensor (1, 2, 10.1) measures in a contactless manner a characteristic of the conveyed crop material (B). A fluid ejector (7.1, 7.2, 7.3) ejects a fluid jet (8.1. 8.2, 8.3) onto a surface (FF) formed in the interior of the harvester. The fluid is thereby provided in a space between the window (3) and the conveyed crop material (B).