Patent ID: 7477394

Claim:
A method for measuring a property of a liquid under test, the method comprising the steps of: a) contacting a liquid under test against a substantially planar transparent partition spaced a predetermined distance from a transparent window, the partition having a predetermined index of refraction; b) directing a ray of interrogating radiation having a predetermined wavelength through the transparent window and the partition into the liquid; c) allowing radiation reflected from the liquid to undergo total internal reflection within the partition while simultaneously preventing evanescent coupling of that reflected radiation into the window, the prevention of evanescent coupling itself being accomplished by: i) disposing a medium having an index of refraction less than that of the partition between the window and the partition, and ii) maintaining the spacing between the window and the partition to a distance not less than three times the wavelength of the interrogating radiation; and d) collecting at least some of the radiation passing through the partition, the partition having a maximum thickness dimension sized such that lateral propagation of radiation due to total internal reflection within the partition is minimized so that substantially all of the radiation reflected from a liquid being interrogated exits the partition within a predetermined lateral distance along the plane of the partition.