Embodiments provide an optical sensor head and method of making an optical sensor head. In some cases the sensor head can be used as a fluorometric sensor to measure concentrations of substances within a liquid sample of interest. The sensor head includes a light source window and a detector window that transmit light between the sensor head and an analytical area. In some cases the windows include a ball lens positioned within a channel such that the ball lens and the channel create a seal between the interior and exterior of the sensor head.