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An optical absorption spectroscopy apparatus comprises a multi-pass optical cell (1) having a first reflector (2) and a second reflector (4, 4′), a first light source (6) for light of a first waveband and a second light source (6′) for light of a second waveband. The cell (1) is constructed and arra

An optical absorption spectroscopy apparatus comprises a multi-pass optical cell (1) having a first reflector (2) and a second reflector (4, 4′), a first light source (6) for light of a first waveband and a second light source (6′) for light of a second waveband. The cell (1) is constructed and arranged such that light entering the cell is reflected one or more times between the first and second reflectors (2, 4, 4′) before exiting the cell. Light of the first waveband enters and exits the cell (1) in a first plane and light of the second waveband enters and exits the cell in a second plane that is different from the first plane.

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1. An optical absorption spectroscopy apparatus comprising a multi-pass optical cell comprising a first reflector and a second reflector, a first light source for light of a first waveband and a second light source for light of a second waveband that is different from the first waveband, the cell be

1. An optical absorption spectroscopy apparatus comprising a multi-pass optical cell comprising a first reflector and a second reflector, a first light source for light of a first waveband and a second light source for light of a second waveband that is different from the first waveband, the cell being constructed and arranged such that light entering the cell is reflected one or more times between the first and second reflectors before exiting the cell, and wherein light of the first waveband comprises a first input beam that enters the cell and a first output beam that exits the cell, wherein the first input beam and the first output beam lie in a first plane, and light of the second waveband comprises a second input beam that enters the cell and a second output beam that exits the cell, wherein the second input beam and the second output beam lie in a second plane that is different from the first plane, and wherein a line normal to the first plane and a line normal to the second plane lie in a common plane that coincides with a main optical axis of the multi-pass cell. 2. An apparatus according to claim 1, including a first entrance window for light of a first waveband, a second entrance window for light of a second waveband, a first exit window for light of the first waveband and a second exit window for light of the second waveband. 3. An apparatus according to claim 2, wherein the first entrance and exit windows lie in the first plane and the second entrance and exit windows lie in the second plane. 4. An apparatus according to claim 2, wherein the first entrance and exit windows are made of a first optical material that is substantially transparent to light of the first waveband, and the second entrance and exit windows are made of a second optical material that is substantially transparent to light of the second waveband. 5. An apparatus according to claim 1, further including a first detector for light of the first waveband and a second detector for light of the second waveband. 6. An apparatus according to claim 1, wherein the multi-pass cell comprises a White cell having a front mirror and first and second back mirrors. 7. An apparatus according to claim 1, further including a measurement cell for containing a sample fluid. 8. An apparatus according to claim 7, further including means for transferring a sample fluid to and from the measurement cell. 9. An apparatus according to claim 1, wherein the apparatus is constructed and arranged for analysing the optical absorption characteristics of a gas. 10. An apparatus according to claim 1, wherein the first waveband is ultraviolet and the second waveband is visible or infrared. 11. An apparatus according to claim 10, wherein the first waveband is ultraviolet and the second waveband is infrared. 12. An apparatus according to claim 1, wherein the first and second reflectors comprise a material that is reflective to light of both the first waveband and the second waveband. 13. An apparatus according to claim 12, wherein the reflective material is aluminium. 14. An apparatus according to claim 1, wherein the first waveband is ultraviolet or visible and the second waveband is infrared. 15. A method of measuring one or more components of a fluid, comprising: a. placing a sample of the fluid in a multi-pass optical cell having a first reflector and a second reflector,b. passing light of a first waveband through the fluid and detecting the transmitted light;c. passing light of a second waveband through the fluid and detecting the transmitted light, wherein the second waveband is different from the first waveband;d. wherein light of the first waveband comprises a first input beam that enters the cell and a first output beam that exits the cell, wherein the first input beam and the first output beam lie in a first plane, and light of the second waveband comprises a second input beam that enters the cell and a second output beam that exits the cell, wherein the second input beam and the second output beam lie in a second plane that is different from the first plane, and wherein a line normal to the first plane and a line normal to the second plane lie in a common plane that coincides with a main optical axis of the multi-pass cell, ande. determining the concentration of one or more components of the fluid from the optical absorption spectra of the detected light. 16. A method according to claim 15, in which light of the first and second wavebands is passed substantially simultaneously through the fluid. 17. A method according to claim 15, including passing the light through the fluid using a White cell. 18. A method according to claim 15, further including containing the fluid within a measurement cell. 19. An optical absorption spectroscopy apparatus comprising a multi-pass optical cell comprising a first reflector and a second reflector, a first light source for light of a first waveband, a first entrance window for light of the first waveband, a first exit window for light of the first waveband, a second light source for light of a second waveband, a second entrance window for light of the second waveband and a second exit window for light of the second waveband, the cell being constructed and arranged such that light entering the cell is reflected one or more times between the first and second reflectors before exiting the cell, and wherein light of the first waveband enters and exits the cell in a first plane, and light of the second waveband enters and exits the cell in a second plane that is different from the first plane, and wherein a line normal to the first plane and a line normal to the second plane lie in a common plane that coincides with a main optical axis of the multi-pass cell.