App note from TT Electronics on reflective assemblies and the surfaces they need to effectively detect/sense.
A reflective sensor provides a major mounting advantage to the designer because only one side of a sensing surface is required. However, there are a number of design variables that need to be considered to assure detection of the object or target. They include the type or size of the reflective surface, the distance to the reflective surface, variation of surface reflectivity, sensor variation and contrast ratio.