An instrument for biological analysis includes a base, an excitation source, an optical sensor, an excitation optical system, and an emission optical system. The base is configured to receive a sample holder comprising a plurality of biological samples. The optical sensor is configured to receive emissions from the biological samples in response to the excitation source. The instrument may additionally include a sensor lens enclosed by a lens case and a focusing mechanism including a gear that engages the lens case, the focusing mechanism being accessible outside the enclosure for adjusting a focus. The may instrument further include a sensor aperture dispose along an emission optical path and a blocking structure disposed to cooperate with the sensor aperture such that none of the reflected radiation from an illuminated surface near the sample holder is received by the optical sensor that does not also reflect off another surface of the instrument.