Patent Publication Number: US-4322621-A

Title: Folded path absorption cell gas sensor

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     A long optical path length is often necessary to improve the sensitivity of optical absorption gas sensors. This conflicts with the practical necessity of providing a compact product. In the present invention the path length can be multiplied to about 2 meters in a hemispheric like chamber having a radius of about 8 cm. A novel compact and inexpensive cell structure for increasing the optical path length of multiple relections is disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic side view of the interior of the gas sensor showing the spherical reflector and representative mirror sections; 
     FIG. 2 is a partial top view viewing two pairs of mirror sections and diagramming the reflection path; 
     FIG. 3 is a more complete top view and diagrams the complete reflection path from light source to detector; 
     FIG. 4 is a cross-section of a mirror pair and diagrams the light beam deflection at the mirror facets; and, 
     FIG. 5 is a block diagram of the electronic system in which the sensor is used. 
    
    
     DESCRIPTION 
     Referring now to FIG. 1 there is disclosed an enclosure 10 for a folded path absorption cell gas sensor which includes a reflector or mirror portion 11 in the shape of a sector of a sphere. The spherical sector may be a hemisphere if desired in which case the section 11 of the enclosure directly joins a flat base plate section 12, or sector 11 may be smaller as shown in FIG. 1 in which case a suitable wall section 13 is used. Mounted on the base plate 12 is a center mirror means 14 (the top view of which is best seen in FIG. 3) which is axially aligned around a line L dropped perpendicular from the apex of the spherical reflector. The mirror means 14 is shown in FIG. 3 as having an annular or ring shaped perimeter area divided into 26 equal segments including twelve sets of mirror pairs equally spaced around the perimeter of means 14 together with a source segment S 1  and S 2  and a detector segment D 1  and D 2 . The beam directed from the sources S 1  and S 2  to the apex of reflector 11 makes an angle α with the perpendicular line L. The mirror elements such as A and B or E and F of mirror pairs 1 and 2 are each elevated or canted in two axis. From FIG. 1 it is clear that the mounting surface 15 is generally sloped or elevated at an angle α so that the outer edge 16 of mirror elements A and F is higher than the inner edge 17. Furthermore, the faces of mirror elements (i.e. element A and B) of each mirror pair are set at an angle of 90°-α&#39; (oriented nearly perpendicular) to one another as shown in FIG. 4, where α&#39;=TAN -1  B 2  /L 2 , L 2  being the distance from the apex of the spherical mirror to the beam axis interception point on mirror element A and B 2  being half the distance between interception points on mirror elements A and B. The mirror A further makes a 45° angle to the axis of the light beam 20&#39;. Thus in each of the mirror pairs 1-9, 10&#39;, 11&#39; and 12, the mirror elements face each other at 90°-α&#39; so that the axis of a light beam from the apex of the spherical reflector falling on element A is reflected to element B and reflected again back to the apex of the spherical reflector. Instead of mirror A making a 45° angle to the axis of the light beam 20&#39;, the bisector of the angle 90°-α&#39; formed by the mirror pair may be directed toward the apex of the spherical reflector, if desired. 
     FIGS. 1, 2 and 3 show that a light source S (S 1  and S 2 ) located below the mirror plane directs a radiation (i.e. light) beam 20 to the spherical reflector 11. The source S (S 1  and S 2 ) may be composition tuned Infrared Light Emitting Diodes (IRLED&#39;s) or a laser source of appropriate wavelength. The use of the work light in this specification and claims is intended to include the infrared spectrum. As shown in FIGS. 2 and 3 IR radiation from the source or sources is reflected and refocused by the spherical mirror to fall on mirror section A of mirror pair 1, from mirror section A to B, from mirror section B to the spherical reflector, to mirror section E of mirror pair 2, to mirror section F, from mirror 2 to 3 etc., each time the reflective path including the spherical reflector, until the radiation progresses through mirror pair 12 and reaches the detector D 1 . A spherical mirror R, suspended from mounting 21, intercepts a small amount of radiation from the sources and directs it to detector D 2  to provide a reference so that source intensity changes affect them equally. 
     The function of the mirror pairs is to move the effective location of the source radially so that the beam is reflected across the center of the spherical mirror. In the embodiment disclosed a multiplication of 24r is achieved, where r is the radius of the spherical reflector 11, an 8 cm radius mirror would provide about a 2 meter path length. 
     The light emitting diode sources have wavelengths λ 1  and λ 2  which are coincident and near, but not coincident with the absorption band of the gas to be sensed. They must be bonded to the same header or integrated as close together as possible, so that radiation from both of them will reach a detector of a reasonable size. 
     A block diagram of the sensor electronics is shown in FIG. 5. The IRLED&#39;s have center radiant emission at λ 1  and λ 2  and are alternately switched on by the square wave driver 30. When there is no gas of absorption at λ 1 , in the cell, the detector 31 response for λ 1  and λ 2  is equalized and there is no AC signal at the output of the amplifier 33. Wavelengths λ 1  and λ 2  are chosen close enough together so that dust and humidity changes affect them nearly equally. The presence of a gas absorbing at λ 1  attenuates the radiation when source of λ 1  is on, and does not attenuate λ 2 . Thus an AC signal will appear at the output of the amplifier of amplitude proportional to the concentration of the absorbing gas and be displayed on the indicator. 
     In the construction of the enclosure 10 the sources S 1  and S 2  as well as the detectors D 1  and D 2  may be protected by IR transmissive windows, if it is desirable to protect these elements from the gases flowing through the enclosure. There may be gas inlet and outlet ports (not shown) for the enclosure or the enclosure may allow free flow through it as the application requires.