A method of "reading" the result of an assay effected by concentrating a detectable material in a comparatively small zone of a carrier in the form of a strip, sheet or layer through the thickness of which electromagnetic radiation such as visible light is transmissible, wherein at least a portion of one face or the carrier is exposed to incident electromagnetic radiation which is substantially uniform across the entire portion, the portion including the small zone, and electromagnetic radiation emerging from the opposite face of the carrier is measured to determine the assay result. Preferably the radiation is diffuse light.