# US 9,752,931 B2 · Spectrometer with multiple Fabry-Perot filters for gas analysis > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 9,752,931 B2 - Document ID: US9752931B2 - Status: Granted - Kind: B2 - Application number: 14/488,608 - Filed: 2014-09-17 - Published: 2015-04-02 - Granted: 2017-09-05 - Earliest claimed: 2013-10-02 - Assignees: Sick AG - Applicants: Sick AG - Inventors: Torsten Waldmann; Rolf Disch - Web page: https://patentgrep.com/patent/US9752931B2 ## CPC - G01J 3/0205: {Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows} - G01J 3/0235: {using means for replacing an element by another, for replacing a filter or a grating} - G01J 3/12: Generating the spectrum; Monochromators - G01J 3/26: using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters - G01J 3/433: Modulation spectrometry; Derivative spectrometry - G01N 21/3504: for analysing gases, e.g. multi-gas analysis - G01N 33/0062: {concerning the measuring method or the display, e.g. intermittent measurement or digital display} - G01J 2003/1282: {Spectrum tailoring} - G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N) - 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 A spectrometer (10) for gas analysis is provided, the spectrometer comprising a measurement cell (28) having a gas to be investigated, a light source (12) for the transmission of light (14) into the measurement cell (28) on a light path (16), a filter arrangement (22) having a Fabry-Perot filter (24a-c) in the light path (16), in order to set frequency properties of the light (14) by means of a transmission spectrum of the filter arrangement (22), as well as a detector (36, 38) which measures the absorption of the light (14) by the gas (30) in the measurement cell (28). In this connection the filter arrangement (22) has a plurality of Fabry-Perot filters (24a-c) arranged behind one another in the light path (14) and a control unit (44) for the filter arrangement (22) is provided in order to change the transmission spectrum by setting at least one of the Fabry-Perot filters (24a-c). ## Claims ### Claim 1 A spectrometer for gas analysis, the spectrometer comprising: a measurement cell having a gas to be investigated; a light source for the transmission of light into the measurement cell on a light path; a filter arrangement having a plurality of Fabry-Perot filters arranged behind one another in the light path, in order to set frequency properties of the light by means of a transmission spectrum of the filter arrangement; a detector which measures the absorption of the light by the gas in the measurement cell; and a control unit for the filter arrangement for changing the transmission spectrum in accordance with a scanning scheme by setting at least one of the Fabry-Perot filters, the scanning scheme providing a respective transmission spectrum having a narrow band maximum at a scanning frequency, modulating the amplitude of the narrow band maximum at the scanning frequency over a plurality of cycles, and systematically varying the scanning frequency in order to successively evaluate a spectrum at different frequencies with a lock-in-method based on the known modulation. ### Claim 2 (depends on 1) The spectrometer in accordance with claim 1, wherein the filter arrangement has three Fabry-Perot filters. ### Claim 3 (depends on 1) The spectrometer in accordance with claim 1, wherein an association of mirror spacings of the Fabry-Perot filters and of associated transmission spectra are stored in the control unit. ### Claim 4 (depends on 1) The spectrometer in accordance with claim 1, wherein the Fabry-Perot filters have graduated full widths at half maximum. ### Claim 5 (depends on 2) The spectrometer in accordance with claim 2, wherein the three Fabry-Perot filters have graduated full widths at half maximum. ### Claim 6 (depends on 1) The spectrometer in accordance with claim 1, wherein the transmission spectrum has a narrow band maximum at a predefined frequency. ### Claim 7 (depends on 6) The spectrometer in accordance with claim 6, wherein the control unit is configured to vary the transmission spectrum of the filter arrangement cyclically in such a way that the amplitude of the maximum is modulated. ### Claim 8 (depends on 6) The spectrometer in accordance with claim 6, wherein the amplitude is modulated with a predefined function. ### Claim 9 (depends on 7) The spectrometer in accordance with claim 7, wherein the predefined function is one of a delta pulse, a sinusoidal pulse and an exponentially growing and then abruptly falling off pulse. ### Claim 10 (depends on 7) The spectrometer in accordance with claim 7, wherein a signal of the detector is evaluated with a Lock-In-method by means of the known modulation. ### Claim 11 (depends on 1) The spectrometer in accordance with claim 1, further comprising an additional optical band pass filter in the light path, in order to mask out maxima of higher orders of at least one of the Fabry-Perot filters. ### Claim 12 (depends on 11) The spectrometer in accordance with claim 11, wherein the optical band pass filter is provided to mask out maxima of higher orders of the Fabry-Perot filter of the highest free spectral range. ### Claim 13 (depends on 1) The spectrometer in accordance with claim 1, wherein the measurement cell is a photo-acoustic measurement cell. ### Claim 14 (depends on 1) The spectrometer in accordance with claim 1, further comprising a calibration unit that can be introduced into and removed from the light path and that has an interference filter with a known reference frequency or has a gas cuvette having a reference gas. ### Claim 15 (depends on 14) The spectrometer in accordance with claim 14, wherein the calibration unit has a moveable carriage or a filter wheel. ### Claim 16 (depends on 1) A spectrometer in accordance with claim 1, wherein at least one of the Fabry-Perot filters is mounted such that it can be removed from the light path and can be reintroduced into the light path. ### Claim 17 (depends on 2) The spectrometer in accordance with claim 2, wherein: the first Fabry-Perot filter has a low full width half maximum; the second Fabry-Perot filter has a mean full width half maximum; and the third Fabry-Perot filter has a high full width half maximum. ### Claim 18 A method of spectrometric gas analysis, comprising: passing a light source through a filter arrangement having a plurality of Fabry-Perot filters arranged behind one another; setting the frequency properties of the light by changing the transmission spectrum of the filter arrangement in accordance with a scanning scheme; guiding the light into a measurement cell having a gas to be investigated; and measuring the absorption of the light by the gas in the measurement cell, wherein the absorption is measured a plurality of times after a respective adjustment of at least one of the plurality of Fabry-Perot filters and is thereby measured a plurality of times with light of different frequency properties, the scanning scheme providing a respective transmission spectrum having a narrow band maximum at a scanning frequency, modulating the amplitude of the narrow band maximum at the scanning frequency over a plurality of cycles, and systematically varying the scanning frequency in order to successively evaluate a spectrum at different frequencies with a lock-in-method based on the known modulation.