Folded path absorption cell gas sensor

• US 4,322,621 A
• Filed: 05/05/1980
• Issued: 03/30/1982
• Est. Priority Date: 05/05/1980
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Aagard, Roger L.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): FIELDS, CAROLYN
• Application Number:
  US06/146,818
• Time in Patent Office: 694 Days
• Field of Search: 250/343, 250/353, 250/373, 250/345, 250/575, 350/293, 350/299, 356/440
• US Class Current: 250/343
• CPC Class Codes: G01N 2021/3181 using LEDs
• CPC Class Codes: G01N 21/031 Multipass arrangements

Abstract

In an optical absorption gas sensor there is a practical problem of obtaining a sufficiently long optical path necessary to provide the required sensitivity within a compact space. In the present invention the path length can be multiplied to about 2 meters in a hemispheric like chamber having a radius about 8 cm. A novel, compact and inexpensive cell structure for increasing the optical path length by multiple reflections is provided.

Claims

1. A folded path optical type absorption cell gas sensor comprising:

an absorption cell gas sensor enclosure;

a spherical shaped reflector forming part of the enclosure of the absorption cell;

said enclosure further including a base section having mounted thereon an annular shaped mirror complex, said annular mirror complex comprising a plurality of mirror pairs, each of said mirror pairs having a first mirror element and a second mirror element, the first mirror element being set at an angle nearly perpendicular to the second mirror element in each of said mirror pairs, and wherein said mirror pairs are each generally directed towards the apex of the spherical reflector;

a source of light;

detector means;

a multifolded light beam directed from said source towards the apex of said spherical reflector and reflected therefrom onto said first mirror element of the first of said mirror pairs, reflected therefrom to the second mirror element thereof, reflected therefrom by way of spherical reflector to the first mirror element of the second mirror pair, the multifolding sequence continuing until the light beam traverses all of said plurality of mirror pairs and is finally reflected to said detector means.

2. Apparatus according to claim 1 wherein said annular mirror complex comprises twelve mirror pairs.

3. Apparatus according to claim 1 wherein the first and second mirror elements of each pair are set at an angle of 90°

-α

'"'"' to each other, where α

'"'"'=TAN^-1 B₂ /L₂, L₂ being the distance from the apex of the spherical mirror and the beam axis interception point on the mirror elements and B₂ being half of the distance between the beam interception points on the first and second mirror elements.

4. Apparatus according to claim 3 wherein a bisector of the angle 90°

-α

'"'"' formed by said mirror pair elements is directed towards the apex of the spherical reflector.

5. Apparatus according to claim 1 wherein said spherical shaped reflector is hemispherical.

6. Apparatus according to claim 1 wherein said spherical shaped reflector is a small sector of a sphere.

7. A folded path absorption cell gas sensor comprising:

a source of infrared light;

a detector of infrared light;

a circle of mirror parts comprising a plurality of mirror pairs, said circle of mirror pairs being mounted on the base of an absorption cell gas sensor chamber, each of said mirror pairs consisting of two planar mirrors, one mirror nearly perpendicular to the other, said two mirrors having a common edge, one mirror being a receiving mirror and the other a reflecting mirror;

a spherical reflector forming an upper portion of said gas sensor chamber, each of said mirror pairs having an axial orientation towards the apex of said spherical reflector;

said light from said source being directed towards said reflector apex and reflected therefrom to the receiving mirror of the first mirror pair of said circle of mirror pairs, the resulting light reflected from the reflecting mirror of said first pair being directed toward said reflector apex and reflected therefrom to the receiving mirror of the second mirror pair of said circle of mirror pairs and continuing in like order to the ultimate of the plurality of mirror pairs whereupon the resulting light reflected from the reflecting mirror of said ultimate pair is directed towards said reflector apex and reflected therefrom to said infrared detector, thereby providing a relatively long light path from source to detector in a compact enclosure.

8. Apparatus according to claim 7 wherein said circle of mirror pairs comprises at least 8 mirror pairs.

9. Apparatus according to claim 7 wherein said circle of mirror pairs comprises twelve mirror pairs.

10. Apparatus according to claim 7 wherein the two planar mirrors of each mirror pair are set at an angle of 90°

-α

'"'"' to each other, where α

'"'"'=TAN^-1 B₂ /L₂, L₂ being the distance from the apex of the spherical mirror to the beam axis interception point on the planar mirror and B₂ being one half the distance between the beam interception points on the two planar mirrors of a pair.

11. Apparatus according to claim 10 wherein a bisector of the angle 90°

-α

'"'"' formed by the two planar mirrors is directed toward the apex of the spherical reflector.