Light emitting diodes (LEDs) are mounted in an array to an upper structure overlying a lower structure with a plurality of light detectors thereon. Each LED is configured to overlie a separate detector. Each LED emits light at a frequency relevant for measuring optical density of a specimen. LEDs having different frequencies are included within the LED array. A corresponding array of detectors is also provided, mounted to the lower structure. Spacing between adjacent LEDs and between adjacent detectors match a spacing between wells in a microtiter plate. Spacing between the lower structure and the upper structure supporting the LEDs is sufficient for the microtiter plate to fit between. Circuitry sequentially fires individual LEDs and gathers optical density data through the detectors for specimens in the wells of the microtiter plate. The structures are then moved to a next adjacent well position on the microtiter plate and the process repeated.