A small, portable, hand-sized apparatus for detecting microorganisms or chemicals in liquid samples by fluorescence polarization. The apparatus operates using a low power excitation light source, such as an LED, in order to irradiate a sample with polarized light. Detection of emitted polarized light from the sample is detected in multiple planes simultaneously using low power detectors resulting in an elimination of error caused by drifts in intensity in sequential measurements and in reducing assay time.