An in-line optical sensor assembly that measures optical reflection density on a printed sheet horizontally conveyed and supported by a paper transport section of a printer is provided. The sensor assembly includes a densitometer having frame provided with a pair of tapered blades that engage the moving printed sheet, a light source disposed on said frame that illuminates a portion of said printed sheet at a continuous intensity, and a photo-detector mounted on the frame and positioned to receive light from the light source that is reflected off said printed sheet. The optical sensor assembly also includes a mounting that floatably mounts the densitometer in a position over the printed sheet. The mounting can be formed from an opening in a cover plate of the paper transport section that slidably receives the densitometer such that the pair of tapered blades continuously engages the moving sheet in ski-like fashion due to the weight of the densitometer. The floating mounting arrangement maintains a constant, predetermined distance between the photo-detector of the densitometer and the illuminated portion of the moving printed sheet regardless of vertical movement of the printed sheet within said paper transport section.