Abstract: A test apparatus is disclosed that has a housing containing a location at which a sample tray containing a plurality of sample wells can be located, a light box positioned to permit irradiation of the sample tray, and an imaging arrangement to permit imaging of the sample tray. The light box has a light source including a light guide in the form of a plate of a transparent material and a plurality of light emitting devices provided adjacent at least one edge of the plate where a surface of the plate is provided with a plurality of transmitting regions and light is transmitted from the plate.