Patent ID: 7064836

Claim:
An apparatus for measuring light absorption in a sample having a first index of refraction, the apparatus comprising: a ring-down optical cavity formed by at least first and second spaced apart light beam reflectors to receive and reflect a light beam having a selected wavelength width and to establish a resonant mode for the light beam, the light beam being measured after the light beam has been reflected at least once from each reflector to determine the absorption of the light beam by the sample, the optical cavity comprising a medium having a second index of refraction; and a container positioned in the optical cavity for containing the sample, the container comprising: an outer surface having opposing first and second outer faces that are oriented to receive and transmit the light beam, the container including a first portion forming the first outer face having a third index of refraction and a second portion forming the second outer face having a fourth index of refraction; and an inner surface forming a void for containing the sample, the inner surface having opposing first and second inner faces that are oriented to receive and transmit the light beam, wherein the first and second outer faces of the container form first and second cavity/container interfaces, and the first and second inner faces of the container form first and second container/sample interfaces, wherein the first and second outer faces of the outer surface of the container are oriented at first and second angles, respectively, with respect to the light path of the light beam such that the light beam incident upon and exiting the first and second cavity/container interfaces is at approximately the Brewster's angle of the respective first and second cavity/container interfaces, and wherein the first and second inner faces of the inner surface are oriented at third and fourth angles, respectively, with respect to the first and second outer faces of the outer surface such that the light beam incident upon and exiting the first and second container/sample interfaces is at approximately the Brewster's angle of the respective first and second container/sample interfaces.