# US 7,609,375 B2 · Optical cavity for gas sensor > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 7,609,375 B2 - Document ID: US7609375B2 - Status: Granted - Kind: B2 - Application number: 11/922,343 - Filed: 2006-06-16 - Published: 2009-05-28 - Granted: 2009-10-27 - Earliest claimed: 2005-06-16 - Assignees: ELT, Incorporated - Inventors: Jeong-Ik Park - Web page: https://patentgrep.com/patent/US7609375B2 ## CPC - G01N 21/3504: for analysing gases, e.g. multi-gas analysis - G01N 21/031: {Multipass arrangements} - G01N 21/61: Non-dispersive gas analysers {(G01N21/3504 takes precedence)} - G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q) ## Abstract An optical cavity for a Non-Dispersive Infrared gas sensor has invented comprising two oppositely arranged parabolic mirrors having common focus located on the common optical axis of the parabolic mirrors, and a plane mirror arranged along the optical axis between the vertex of each of the parabolic mirrors. The NDIR gas sensor has an extended optical path to increase precision and accuracy in the measurement, and substantially increased ventilation opening size to facilitate in and out of the target gas through the optical cavity thereby decreasing the response time required for measuring the gas concentration. The sensors based on the principal of Non-Dispersive Infrared Detection is used the light-absorbing characteristic of gases to measure the amount of light absorption that occur at the specific wavelength absorbed by a target gas and calculate the target gas concentration. ## Claims ### Claim 1 An optical cavity for gas sensor comprising: a first parabolic mirror having a first focal distance; a second parabolic mirror having a second focal distance that is greater than the first focal distance and the second parabolic mirror being oppositely arranged to the first parabolic mirror; and a plane mirror arranged between one end of the first parabolic mirror and one end of the second parabolic mirror, wherein the two oppositely arranged parabolic mirrors have common focus located on the common optical axis of the parabolic mirrors, the plane mirror is arranged along the optical axis, and the common focus lies on the surface of the plane mirror. ### Claim 2 (depends on 1) The optical cavity according to claim 1 further comprising: a light source located on the side of the second parabolic mirror for radiating lights to the plane mirror arranged along the optical axis; and a light detector located at the connecting point of the first parabolic mirror and the plane mirror for detecting the light that has been reflected within the optical cavity by the first parabolic mirror, the second parabolic mirror and the plane mirror one after another. ### Claim 3 (depends on 2) The optical cavity according to claim 2, wherein the angle of radiation of the lights from the light source is set so that the lights from the lights source are incident into the plane mirror portion between the light detector and the common focus. ### Claim 4 (depends on 3) The optical cavity according to claim 3, wherein the incident point of the light on the plane mirror portion is adjusted between the light detector and the common focus and the length of the optical path within the optical cavity is also adjusted depending on the angle of radiation of the lights from the light source. ### Claim 5 (depends on 4) The optical cavity according to claims 4, further comprising gas ventilating openings provided in a predetermined part of the optical cavity where no reflection of light occurs. ### Claim 6 (depends on 5) The optical cavity according to claim 5, wherein the gas ventilating openings are provided in a predetermined part of the optical cavity between the other end of the first parabolic mirror and the other end of the second parabolic mirror. ### Claim 7 (depends on 5) The optical cavity according to claim 5, wherein the gas ventilating openings are provided in a predetermined part of the optical cavity between the common focus and the one end of the second parabolic mirror. ### Claim 8 An optical cavity for a gas sensor comprising: a top surface; a bottom surface; and sidewalls between the top surface and the bottom surface, wherein the sidewalls comprises a first surface, a second surface, a third surface, and a fourth surface in the clockwise direction, the first surface is formed by a first parabolic mirror having a first focal distance, the third surface is formed by a second parabolic mirror having a second focal distance that is greater than the first focal distance and the second parabolic mirror being oppositely arranged to the first parabolic mirror, the fourth surface is formed by a first plane mirror arranged between the lower end of the first parabolic mirror and the lower end of the second parabolic mirror, and the second surface is formed by a second plane mirror conjoining an ellipsoidal mirror, the second plane mirror being arranged between the upper end of the first parabolic mirror and one end of the ellipsoidal mirror and the ellipsoidal mirror being arranged between one end of the second plane mirror and the upper end of the second parabolic mirror. ### Claim 9 (depends on 8) The optical cavity according to claim 8, wherein the first parabolic mirror and the second parabolic mirror have common focus located on the common optical axis of the parabolic mirrors, the first plane mirror is arranged along the common optical axis, and the common focus is formed on the surface of the first plane mirror. ### Claim 10 (depends on 9) The optical cavity according to claim 9 further comprising: a light source located on the side of the second parabolic mirror for radiating lights to the first plane mirror arranged along the optical axis; and a light detector located at the connecting point of the first parabolic mirror and the first plane mirror for detecting the light that has been reflected within the optical cavity by the first parabolic mirror, the second parabolic mirror and the first plane mirror one after another. ### Claim 11 (depends on 9) An optical cavity according to claim 9, wherein the first parabolic mirror, the second parabolic mirror and the first plane mirror are arranged so that the optical path within the optical cavity can form a convergence system of light, and the light detector is arranged at the light converging point of the convergence system. ### Claim 12 (depends on 11) The optical cavity according to claim 11, wherein one focus of the ellipsoidal mirror is formed near the position of the common focus. ### Claim 13 (depends on 11) The optical cavity according to claim 11, further comprising gas ventilating openings provided in a predetermined part of the optical cavity where no reflection of light occurs. ### Claim 14 (depends on 11) The optical cavity according to claim 11, wherein the gas ventilating openings are provided in the top or bottom surface of the optical cavity.