Abstract: A system for sensing the presence of or level of material within a vessel that includes a light transmitting probe extending into the vessel through a wall of the vessel from externally thereof. The probe has a lens with a closed end within the vessel for contact with the material sought to be detected, and a light source/sensor unit positioned externally of the vessel. The light source/sensor unit directs light energy through the lens of the probe to the closed end of the lens, which is within the vessel. The source/sensor unit detects the presence of material in contact with the closed end of the lens as a function of light energy reflected back to the source/sensor unit. The light source/sensor unit is removably mounted on the lens so that the source/sensor unit may be removed from the lens for service without removing the lens itself from the vessel wall.