Abstract: A device for detecting the liquid level in a tank. The device inlcudes a dioptric element having faces and a base immersible in the liquid. A bundle of optical fibers is operationally arranged with the dioptric element so that the dioptric element is disposed at one plane end of the optical fiber bundle while the other plane end of the optical fiber bundle provides an observation face. The bundle of optical fibers and the dioptric element are arranged and adapted to provide two separate and distinct zones in the observation face which are directly viewable in the surface of the observation face. One zone provides a signal zone which receives a larger quantity of light from a dioptric element when the element is not immersed than when it is immersed. The other zone provides a reference zone which receives a constant quantity of light from the dioptric element whether or not the dioptric element is immersed in the liquid.