Abstract: A liquid level monitor for sensing the level of liquid in a tank or vessel, typically an automobile radiator, employing a probe with a light emitting diode and phototransistor mounted inside an enclosure, the enclosure having a transparent lens through which light can pass, and the lens having a prism configuration which diffuses or alters stray light which occurs by reflection off an interior core wall of the vessel and prevents the diffused or dispersed light from rendering the phototransistor conductive and possibly leading to a false indication or reading of liquid level. The light emitting diode is electrically excited with a high-intensity, short duration pulse, which improves the sensitivity of the monitor.