A miniature readhead for a photometric diagnostic instrument includes a housing of hand-held form factor, configured for receiving reagent sample media therein. The sample media has a plurality of test areas configured to react with, and change color, according to an amount of an analyte in a sample. The holder is sized and shaped for forming an indexed fit with the sample media and includes an array of light sources coupled to the housing, each of the light sources configured to emit light onto a respective one of the test areas. An array of chambers respectively containing an array of light detectors, is configured to enable each of the light detectors to receive diffuse, non-specular reflections of the light from the test areas, while substantially preventing the light detectors from receiving specular reflections of the light.