A probe for measuring a property of a liquid under test using interrogating radiation at a predetermined wavelength includes a housing member having a window transparent to interrogating radiation mounted at a first end thereof. A partition transparent to interrogating radiation is mounted in spaced relationship to the window. The partition being and the window cooperate to define an air cavity therebetween. The spacing between the partition and the window is such that radiation reflected from a liquid disposed in contact with the second surface of the partition is prevented from evanescently coupling into the window such that the reflected radiation undergoes total internal reflection in the partition rather than in the window.