Abstract: A readhead for a photometric diagnostic instrument for illuminating a target area substantially uniformly via only a single light-emitting diode (20) and receiving light from the target area so that reagent tests may be performed is provided with a housing (12, 14, 16) adapted to be incorporated in a photometric diagnostic instrument, first and second light sources (20) mounted in a fixed position relative to the housing (12, 14, 16), a light guide (26) mounted to receive light from each of the light sources (20) which conveys, when only one of the light sources (20) is illuminated, substantially all of the light from the light source (20) to illuminate a target area substantially uniformly, and a light detector (70) coupled to receive light from the target area. Each of the first and second light sources (20) is composed of only a single light-emitting diode (20) for emitting substantially monochromatic light of a different wavelength.