A miniature readhead is provided for photometric color detection using ambient light. The readhead is hand-held, including a superposed plate and tray. The tray receives sample media configured to react and to change color according to an amount of analyte in a sample. The plate is optically transmissive, so that ambient light is transmissible therethrough to the sample media indexed within the tray. An array of light detectors is disposed along the plate, including sets of detectors configured to detect light of at least three mutually distinct wavelengths reflected from the test areas. Light shields superposed with each of the light detectors selectively permit ambient light to pass to the tray, while substantially preventing ambient light from reaching the light detectors prior to reflecting from the test areas. The readhead may be incorporated into a photometric diagnostic instrument configured to analyze the reflections and derive a diagnosis value.