Gas permeable probe for use in an optical analyzer for an exhaust gas stream flowing through a duct or chimney

A gas permeable probe for use in an optical analyzer for an exhaust gas stream flowing through a duct or chimney has:

• an elongate hollow structure having first and second ends and a side wall, with an optical cavity defined between the first and second ends within the side wall,
• a filter forming part of the elongate hollow structure,
• a mounting structure at the first end and adapted for mounting the elongate hollow structure within the duct or chimney,
• an optical window at the first end permitting a light beam originating from an optical analyzer to enter into the optical cavity to travel from the first end to the second end,
• a retroreflector provided at the second end for returning the light beam to the first end of the hollow structure, and
• a tube disposed within the mounting structure and having first and second opposite ends, the first opposite end being located remote from the first end of the elongate hollow structure and being closed by a first window adapted to transmit the light beam from an optical analyzer into the tube and the second end of the tube being closed by a second window adapted to transmit the beam of light from the tube into the optical cavity and the returned light beam from the optical cavity into the tube, the tube thereby defining a closed cavity, the closed cavity being one of an evacuated cavity and a cavity containing a neutral gas.