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A sample cell for a spectrometer, and a spectrometer using such a sample cell. The sample cell may include a housing having a reflective inner surface which is at least a section of a spheroid bounding a cavity. A lens system receives electromagnetic radiation from a source and directs a converging

A sample cell for a spectrometer, and a spectrometer using such a sample cell. The sample cell may include a housing having a reflective inner surface which is at least a section of a spheroid bounding a cavity. A lens system receives electromagnetic radiation from a source and directs a converging beam through a port in the cavity to a focal point inside the cavity, such that the beam undergoes multiple reflections on the inner surface before exiting the cavity. Arrangements for adjusting beam cross-sectional area and angle are optionally provided. Methods which can be performed on such an apparatus and computer program products for performing such methods are further provided.

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1. A spectrometer, comprising: a housing having a reflective inner surface comprising a section of a spheroid bounding a cavity;an illumination unit which directs a converging non-laser light polychromatic light beam, the converging non-laser light polychromatic light beam having a cross-sectional a

1. A spectrometer, comprising: a housing having a reflective inner surface comprising a section of a spheroid bounding a cavity;an illumination unit which directs a converging non-laser light polychromatic light beam, the converging non-laser light polychromatic light beam having a cross-sectional area vertical to a plane of the converging non-laser light polychromatic light beam in the cavity, through a port in the housing to a focal point inside the cavity mid-way between the reflective inner surface and the port in the housing where the light beam enters the cavity, such that the beam undergoes multiple reflections on the inner surface before exiting the cavity, the illumination unit further adapted to vary the vertical cross-sectional area of the beam at the port in the housing where the beam enters the cavity; anda detector to receive light which has exited the cavity. 2. A spectrometer according to claim 1 wherein the section of a spheroid is an annular section of a sphere. 3. A spectrometer according to claim 2, wherein the illumination unit directs the converging light beam to a focal point inside the cavity such that an axis of the beam does not intersect a center of the reflective surface. 4. A spectrometer according to claim 3 wherein the illumination unit varies an angle of the beam with respect to a normal to the reflective inner surface where the beam enters the cavity. 5. A spectrometer according to claim 4, additionally comprising a processor which varies each of the angle of the beam and the vertical cross-sectional area of the beam in response to a signal from the detector. 6. A spectrometer according to claim 4 wherein the processor varies each of the angle of the beam and the vertical cross-sectional area of the beam in response to a signal from the detector which signal represents an intensity of light resulting from interaction of the light beam with a fluid inside the cavity. 7. A spectrometer according to claim 2 wherein the focal point is mid-way between the reflective inner surface and the port in the housing where the light beam enters the cavity. 8. A spectrometer according to claim 1 wherein the illumination unit comprises a polychromatic light source and an achromatic lens system. 9. A spectrometer according to claim 1 wherein the light beam enters and exits the cavity through a same port. 10. A spectrometer according to claim 1 wherein only a portion of the multiply reflected light beam exits the cavity on a first pass around the reflective inner surface. 11. A spectrometer according to claim 10 wherein the light beam exits the cavity through a port which partially reflects the light beam back into the cavity. 12. A spectrometer according to claim 10 wherein the cavity is a resonant cavity. 13. A spectrometer according to claim 1 wherein the inner surface has a curvature in a horizontal direction which is different from the curvature in a vertical direction, so as to at least partially correct any astigmatism in the beam. 14. A spectrometer according to claim 1 wherein the width of the beam at the inner surface is less than the height of the beam at the same location. 15. A spectrometer according to claim 1 configured as a hand-held spectrometer. 16. A sample cell for a spectrometer, comprising: a housing having a reflective inner surface which is at least a section of a spheroid defining a cavity;a lens system to receive light from a source and direct a converging non-laser light polychromatic light beam, the converging non-laser light polychromatic light beam having a cross-sectional area vertical to a plane of the converging non-laser light polychromatic light beam in the cavity, through a port in the cavity to a focal point inside the cavity, such that the beam undergoes multiple reflections on the inner surface before exiting the cavity, the lens system further configured to vary the vertical cross-sectional area of the beam where the beam passes through the port. 17. A sample cell according to claim 16 wherein the lens system varies an angle of the beam with respect to a normal to the reflective inner surface where the light beam enters the cavity. 18. A sample cell according to claim 16 wherein the section of a spheroid is an annular section of a sphere. 19. A sample cell according to claim 16 configured to be disposed within a hand-held spectrometer. 20. A method comprising: directing a converging non-laser light polychromatic light beam through a port in a cavity to a focal point inside the cavity defined by a reflective inner surface comprising a section of a spheroid, the converging non-laser light polychromatic light beam having a cross-sectional area vertical to a plane of the converging non-laser light polychromatic light beam in the cavity, such that the beam undergoes multiple reflections on the inner surface before exiting the cavity;varying the vertical cross-sectional area of the beam where the beam passes through the port; and, receiving at a detector light which has exited the cavity. 21. A method according to claim 20 wherein the converging light beam is directed to a focal point inside a cavity bounded by a reflective inner surface which comprises an annular section of a sphere. 22. A method according to claim 21 additionally comprising varying the angle of the beam with respect to a normal to the reflective inner surface where the light beam enters the cavity. 23. A method according to claim 21 wherein the varying is in response to a signal from a processor. 24. A computer program product carrying a non-transitory computer program which, when loaded into a programmable processor, executes the method of: directing a converging non-laser light polychromatic light beam through a port in a cavity to a focal point inside the cavity bounded by a reflective inner surface comprising a section of a spheroid, the converging non-laser light polychromatic light beam having a cross-sectional area vertical to a plane of the converging non-laser light polychromatic light beam in the cavity, such that the beam undergoes multiple reflections on the inner surface before exiting the cavity;varying the vertical cross-sectional area of the beam where the beam passes through the port; and,receiving a signal from a detector resulting from light which has exited the cavity. 25. A computer program product according to claim 24 wherein the executed method additionally comprises: varying the angle of the beam with respect to a normal to the reflective inner surface where the light beam enters the cavity. 26. A method according to claim 20 further comprising: selecting a hand-held spectrometer to perform the directing, varying and receiving.