|
|---|
|
|---|
A new multiple-pass optical gas measurement cell has been developed that provides improved analytical accuracy and reduced size and cost, compared to older designs.
Optical sampling cells allow the measurement of light absorption in samples of gas, using the Beer-Lambert relation. Single pass cells are not difficult to manufacture, but the light path is too short for practical use with weak gas concentrations or with gases that have low absorption properties. In these situations, multiple-pass cells are used, providing for a longer path for the light to travel. However, accuracy of the light path is more difficult to control in multiple-path cells. Traditional multiple-pass optical cells with analytical levels of accuracy are relatively large, and need a bright, well-collimated light source for best performance.
The new design developed at the NASA Ames Research Center overcomes several of the limitations inherent in older gas cells, by combining a single spherical lens and two or more flat mirrors to extend the length of the light path.
The left part of the figure below illustrates the principles of operation of the new design. A beam, or bundle, of light, which can be divergent, enters the entrance hole, and is reflected by a spherical mirror of radius R and focusing on a top cover on plane P and approximately height R above the spherical mirror. Without additional mirrors, the light beam would thus be focused on the top cover of the device. However, the light is not allowed to strike plane P. Instead, pairs of flat mirrors (one to three pairs would be typical embodiments) are placed near the entrance, and are positioned and oriented to redirect the beam to the spherical mirror. The positions and orientations of the flat mirrors are chosen to make the beam come to a focus between them, so that the central ray of the light beam is reflected back toward the center of the spherical mirror. The last flat mirror is arranged such that, after the final reflection from the spherical mirror, the light beam passes through an exit hole on the top cover. The exit hole is on the same plane as the entrance hole, but at a different azimuthal position on the top cover.
The right part of the figure below illustrates the path followed by the central ray of the beam in an eight-pass cell based on this concept. The ray enters the cell (pass 1), and is reflected by the spherical mirror back toward the first of six flat mirrors (pass 2). Passes 3 through 7 are back and forth between the spherical mirror and the remaining five flat mirrors. After the last reflection from the spherical mirror, the ray leaves through the exit hole in the top cover (pass 8).
Unlike some older multiple-pass absorption cells, it is not necessary to use narrow, well-collimated beams of light with this design when only a few passes are necessary. This is due to the property of the combined spherical and flat mirrors' tendency to reduce stray light paths within the cell, thus reducing errors when calculating gas absorption values.
Another advantage of the new design arises from the need for accurate knowledge of the length of the optical path within the cell in order to compute the light absorption using the Beer-Lambert relation. The configuration of the mirrors in the new design makes it easier to manufacture cells with precisely known light-path lengths.
This Multiple Passes of a Beam of Light through a cell are achieved by reflection between a spherical mirror and multiple pairs of suitably positioned and oriented flat mirrors. In comparison with older multiple-pass configurations, this one is less sensitive to misalignment and better able to accommodate a wide add/or divergent beam of light.
This work was done by Richard Pearson, Dana H. Lynch, and William D. Gunter of Ames Research Center.
This invention has been patented by NASA (U.S. Patent No. 5,459,566).
