A readhead for use in the determination of the concentration of an analyte in a sample comprises a read area for receiving the sample, a light source including a plurality light emitting elements for outputting light at a plurality of wavelengths, a light guide having an input end and an output end, a lens for receiving light from the output end of the light guide and for illuminating the sample with a substantially collimated beam of light, and a detector for detecting light from the sample in response to illuminating the sample. The input end of the light guide being optically coupled to the light source for receiving light output by the plurality of light emitting elements. The input end of the light guide has a center that is offset from the center of at least one of the plurality of light elements. The light guide guides a substantial portion of the light received from the light source to the output end of the light guide.