An optical mount and spectrometer comprising the optical mount

Abstract

An optical mount comprising a first optical holder 201 for mounting a reflective first optic 214 and an optical selector 202 comprising a second optical holder 229 for a plurality of second optics 216, 216a, 216b. The second holder 229 may be mounted to rotate about a selection axis B-B, wherein at different angular positions about the selection axis B-B the second holder 229 locates a different, selected one of the second optics 216, 216a, 216b in an optical path 215 such that light on an optical input path 213 is delivered via the first optic 2114 and the selected second optic 216 to an optical output path 215. The optical mount may comprise a mechanism for rotating the selected second optic 216 when in the optical path 215 and moving the first optical holder 201 in concert with the rotation of the selected second optic 216 such that light delivered along the optical input path 213 is directed along the optical output path 215 for different angular positions of the selected second optic 216 to the optical output path 215.

Description

[0001]

[0002] AN OPTICAL MOUNT AND SPECTROMETER COMPRISING THE

[0003] OPTICAL MOUNT

[0004] Field of Invention

[0005] This invention concerns an optical mount and spectrometer comprising the optical mount. In particular, but not exclusively, the invention concerns an optical mount arranged for mounting first and second optics to define an optical path, the optical mount comprising a mechanism for moving the first optic in concert with rotation of the second optic such that light delivered along an optical input path is directed along the same output optical path for different angular positions of the second optic.

[0006] Background

[0007] The Raman Effect is a phenomenon in which a sample scatters incident light of a given frequency into a frequency spectrum, which has characteristic peaks caused by interaction of the incident light with the molecules making up the sample. Different molecular species have different characteristic Raman peaks, and so the effect can be used to analyse the molecular species present.

[0008] A known Raman spectroscopy system is described in International patent application WO 2008 / 045497 Al. Referring to Figure 1 of WO 2008 / 045497 Al, light delivered along an input path from a laser source 10 is reflected by an input mirror 14 and by a notch or edge filter 16 (referred to hereinafter as a “Rayleigh filter”), which acts as a dichroic beamsplitter. This directs it along an output optical path into a microscope 18, where it is deflected by a mirror 20 through an objective lens 22 and focused on a sample 24. Raman scattering takes place at the sample, producing Raman-shifted light at different wavenumbers from the incident laser line. The Raman-shifted light is collected by the objective lens 22 and passed back along the optical path via the mirror 20 to the Rayleigh filter 16.

[0009] Whereas the Rayleigh filter 16 reflects light of the laser wavelength, it transmits the Raman-shifted wavenumbers. While doing so, it rejects the much more intense laser line. Further rejection of the laser line takes place in a second, identical filter 26. The Raman-shifted light then passes through a Raman analyser 28, which may comprise a diffraction grating, or filters which accept specific Raman lines of interest. The resulting light is then passed to a detector 30. This may for example comprise a charge-coupled device (CCD), across which a Raman spectrum may be dispersed by a diffraction grating. Or a filter may pass a two-dimensional image of the sample to the CCD, in light of a selected Raman wavenumber.

[0010] The Rayleigh filter 16 is necessarily placed at an angle to the optical path, in order to inject the light from the light source 10 towards the sample 24. However, in order to provide a sharp cut-off between the rejection of the laser line and the acceptance of Raman-scattered light at wavenumbers close to the laser line, this angle should be a low angle of incidence. WO 2008 / 045497 Al discloses angles between 7.5° and 13°.

[0011] Figure 1 of the accompanying drawings schematically illustrates an optical mount 100 for an input mirror 114 and the Rayleigh filter 116 used in Horiba’s LabRAM Raman spectrometer. The optical mount 100 comprises a mechanism arranged to mechanically adjust a position of the input mirror 114 in concert with changes in an angle of the Rayleigh filter 116 such that light delivered along a laser beam input path 113 is directed along the same laser output beam path 115 for different angular positions of the Rayleigh filter 116. Raman backscattered light is directed back along the laser beam output path 115, passing through the Rayleigh filter 116 and along path 117 to an analyser (not shown). Changing the angle of the Rayleigh filter 116 changes the wavenumbers filtered by the Rayleigh filter 116. It may be useful to adjust (tune) the angle of the Rayleigh filter 116 depending on application to either filter wavenumbers further from the laser wavelength to reduce the effect of Rayleigh scattered light on the detected spectrum or to transmit wavenumbers closer to the laser beam wavelength to detect Raman scattered light of these wavenumbers.

[0012] The Rayleigh filter 116 is arranged to rotate about pivot point 107 that is fixed relative to the input and output paths 113, 115. The input mirror 114 is arranged to rotate about pivot point 109, which is movable along a linear slider 121. A motor (not shown) drives linear motion of the input mirror 114 along the slider 121. The Rayleigh filter 116 is connected to the input mirror 114 by a mechanism 111 arranged such that linear movement of the input mirror 114 along the slider 121 causes rotation of the input mirror 114 about pivot point 109 and rotation of the Rayleigh filter 116 about pivot point 107 to direct the laser beam delivered along input path 113 to output path 115 despite the change in angle of the Rayleigh filter 116 to the output path 115.

[0013] A version of the spectrometer comprises a linear selector 131 for changing the Rayleigh filter 116 in the optical path to a different Rayleigh filter 116a, 116b. The linear selector 131 comprises a lead screw 132 that is driven by a motor 133 to move the selector to different positions in which a different Rayleigh filter 116, 116a, 116b is located in the optical path of the laser beam. The linear selector 131 rotates about pivot point 107 creating a large swept area that must be free of other components of the spectrometer. Adding additional Rayleigh filters 116 requires a longer linear selector 131, increasing the space required for the device.

[0014] Furthermore, it is difficult to make a mirror suitable for use as input mirror 114 having a high reflectivity over a broad range of wavelengths.

[0015] Summary of Invention

[0016] According to a first aspect of the invention there is provided an optical mount comprising a first optical holder for mounting a reflective first optic, an optical selector for mounting a plurality of second optics, the optic selector arranged to selectively locate a different one of the second optics into an optical path such that light on an optical input path is delivered via the first optic and a selected second optic to an optical output path, and a mechanism for rotating the selected second optic when in the optical path and moving the first optical holder in concert with the rotation of the selected second optic such that light delivered along the input path is directed along the optical output path for different angular positions of the selected second optic to the optical output path.

[0017] The optical selector may comprise a second optical holder for the plurality of second optics. The second holder may be mounted to rotate about a selection axis, wherein at different angular positions about the selection axis the second holder locates a different, selected one of the second optics into the optical path. The optical holder may comprise a wheel, wherein the selection axis is the wheel axis and the second optics are mounted about the wheel axis. The second optical holder may be arranged to mount four or more second optics, more preferably five or more, even more preferably six or more, yet more preferably seven or more and most preferably eight second optics. The use of a second optical holder that rotates to selectively locate the second optics in the optical path may allow a closer packing of such numbers of second optics compared to the linear optical selector of the prior art.

[0018] The second optical holder may be mounted for rotation to rotate the selected second optic about an incident angle axis perpendicular to an incidence plane of the selected second optic. The incident angle axis may intersect with the selected second optic, and, preferably, an incident surface of the selected second optic. A plane of an incident surface of the selected second optic, and preferably, each of the second optics, may be parallel to the incident angle axis. The incident angle axis may be perpendicular to the wheel axis. The incident angle axis may intersect with the wheel axis. Rotation of the second optical holder about the incident angle axis adjusts an incidence angle of the light (such as laser beam or back-scattered light) on the selected second optic.

[0019] The mount may comprise a base and the optical selector may comprise a support to which the second optical holder is mounted such that the second optical holder can rotate on the support and the support is mounted on the base to rotate relative to the base.

[0020] The optic mount may comprise a motor for rotating the second optical holder. The wheel may be mounted to the motor such that the wheel axis is common with the motor axis, i.e. the wheel is directly driven by the motor. In this way, additional bearings for the wheel are not required.

[0021] The mount may comprise a second optical holder encoder for measuring a rotary position of the second optical holder. The second optical holder encoder may be an absolute encoder.

[0022] The mount may comprise a second optic encoder to measure a rotary position of the selected second optic relative to the optical output path. The second optic encoder may be an absolute encoder. The second optic encoder may be a linear encoder. Use of a linear encoder to measure a rotary position of the selected second optic relative to the optical output path may provide sufficient accuracy because changes in the angle of rotation are small and a linear encoder may be cheaper and less complex to mount than a rotary encoder.

[0023] The optical selector may comprise a detent to mechanically resist rotation of the second optical holder for set positions of the second optical holder, each set position locating a different one of the second optics in the optical path. In this way, the motor may be switched off when the second optical holder is rotated to the required position, reducing heat generated in the device which could affect the positioning of optical components. The detent may comprise notches in the wheel and a catch that is biased towards the wheel such that the catch enters into a corresponding notch when the notch is aligned with the catch. Rotation of the wheel by the motor may push the catch from the notch.

[0024] Each second optic may be kinematically mounted on the second optical holder. The kinematic mounting of each second optic may define a position of the second optic in six degrees of freedom.

[0025] The optical input path may cross the optical output path. The first optical holder may be arranged to mount the first optic such that an angle of incidence of the optical input path to the first optic is less than 45°. The first optical holder may be arranged to mount the first optic such that for all positions of the first optic when mounted on the first optical holder, the angle of incidence is less than 45°. The first optic may be a mirror, such as a dielectric mirror. Dielectric mirrors are readily available that provide a high reflectively across a broad range of wavelength for angles of incidence of the dielectric mirror to an optical input path of less than 45°.

[0026] The plurality of second optics may comprise two or more filters. Each filter may transmit a different range of wavelengths (for the same angle of incidence of incoming light). Each filter may be a notch filter.

[0027] According to a second aspect of the invention there is provided a spectrometer comprising an optical mount according to the first aspect of the invention. The spectrometer may comprise a laser arranged to direct a laser beam along the optical input path and an analyser arranged to receive light transmitted by the selected second optic. The spectrometer may be a Raman spectrometer.

[0028] According to a third aspect of the invention there is provided an optical selector comprising a motor having a motor shaft and a wheel mounted on the motor shaft, the wheel comprises mounting formations for mounting a plurality of optics at circumferential positions about a motor shaft axis.

[0029] In this way, separate bearings are not required for the wheel as the wheel is mounted on the motor shaft having its own bearings, i.e. is a direct-drive mechanism. This reduces the number of components and complexity of the optical selector.

[0030] The optical selector may comprise a base and the wheel may be mounted on the base to rotate about an incident angle axis. The incident angle axis may intersect with a selected optic of the plurality of optics located by the optical selector to be on an optical path of light and, preferably, an incident surface of the selected optic. A plane of the selected optic, and preferably, each of the optics, may be parallel to the incident angle axis. The incident angle axis may be perpendicular to the motor shaft axis. The incident angle axis may intersect with the motor shaft axis. Rotation of the wheel about the incident angle axis adjusts an incidence angle of the light (such as laser beam or back-scattered light) on the selected optic.

[0031] The optical selector may comprise a support to which the wheel is mounted such that the wheel can rotate on the support and the support is mountable on a base to rotate relative to the base.

[0032] According to a fourth aspect of the invention there is provided an optical selector comprising a wheel mounted for rotation, the wheel comprises mounting formations for mounting a plurality of optics at circumferential positions about an axis of rotation and a detent to mechanically resist rotation of the wheel for set positions of the wheel, each set position locating a different one of the optics in an optical path.

[0033] In this way, the motor may be switched off when the wheel is rotated to the required position, reducing heat generated in the device which could affect the positioning of optical components. The detent may comprise notches in the wheel and a catch that is biased towards the wheel such that the catch enters into a corresponding notch when the notch is aligned with the catch. Rotation of the wheel by the motor may push the catch from the notch.

[0034] According to a fifth aspect of the invention there is provided a spectrometer comprising an optical selector according to the third aspect and / or fourth aspect of the invention.

[0035] Description of the Drawings

[0036] FIGURE 1 is a schematic illustration of a prior art optical mount;

[0037] FIGURE 2 is a schematic of a spectrometer according to an embodiment of the invention;

[0038] FIGURE 3 is a perspective view of an optical mount according to an embodiment of the invention from one side;

[0039] FIGURE 4 is a perspective view of the optical mount from the opposite side; and

[0040] FIGURE 5 is a plan view of the optical mount.

[0041] Description of Embodiments

[0042] Referring to Figure 2, light, such as a laser beam, from a laser source 210 is directed along optical input path 213 to an optical device. The optical device comprises an optical mount 200 on which a mirror 214 and a Rayleigh filter 216 are mounted. In this embodiment, the Rayleigh filter 216 is a notch or edge filter, which acts as a dichroic beamsplitter. The mirror 214 and Rayleigh filter 216 reflects the laser beam directed along the optical input path 213 to an optical output path 215. The laser beam directed along the optical output path 215 enters into a microscope 218, wherein the laser beam is deflected by a mirror 220 through an objective lens 222 and focused on a sample 224. Raman scattering takes place at the sample, producing Raman-shifted light at different wavenumbers from the incident laser line. The Raman-shifted light is collected by the objective lens 222 and passed back along the optical path 215 via the mirror 220 to the Rayleigh filter 216.

[0043] Whereas the Rayleigh filter 216 reflects light of the laser wavelength, it transmits the Raman-shifted wavenumbers. While doing so, it rejects the much more intense laser line. The Raman-shifted light then passes through a Raman analyser 228, which may comprise a diffraction grating, or filters which accept specific Raman lines of interest. The resulting light is then passed to a detector 230. This may for example comprise a charge-coupled device (CCD), across which a Raman spectrum may be dispersed by a diffraction grating. Or a filter may pass a two-dimensional image of the sample to the CCD, in light of a selected Raman wavenumber.

[0044] The Rayleigh filter 216 is necessarily placed at an angle to the optical path, in order to inject the laser beam from the light source 210 towards the sample 224. By adjusting an angle of the Rayleigh filter 216 to the output optical path 215, the wavenumbers transmitted or reflected by the Rayleigh filter 216 can be adjusted. However, if the angle of the Rayleigh filter 216 is adjusted alone, for some angles, the laser beam would not a directed along the output optical path 215. Accordingly, mount 200 is arranged to move the mirror 214 in concert with rotation of the Rayleigh filter 216 such that the laser beam delivered along optical input path 213 is directed along optical output path 215 for different angular positions of the Rayleigh filter 216.

[0045] Referring to Figures 3 to 5, the optical mount 200 comprises a first optical holder 201 for mounting the mirror 214, an optical selector 202 for mounting a plurality of second optics 216, 216a, 216b including at least two Rayleigh filters. The optic selector 200 is arranged to selectively locate a different one of the second optics 216, 216a, 216b into an optical path 203 such that the laser beam on the optical input path 213 is delivered via the mirror 214 and the selected second optic 216 to the optical output path 215.

[0046] The mount comprises a mechanism for rotating the selected second optic 216 when in the optical path and moving the mirror 214 via the first optical holder 201 in concert with the rotation of the selected second optic 216 such that light delivered along the input path 213 is directed along the optical output path 215 for different angular positions of the selected second optic 216 to the optical output path 215. The first optical holder 201 is connected to a slider 221 via a pin 209 to be rotatable about axis A-A. The slider 221 is guided for linear motion along guide 223 on a base 205. Movement of the slider 221 along the guide 223 moves the first optical holder 214, and therefore, the axis of rotation A-A in a linear direction. Movement of the slider 221 is driven by motor 208 via a transmission mechanism, in this embodiment a rack 217 and pinion 219. A spring 211 between the slider 221 and the guide 223 takes up backlash in the transmission mechanism.

[0047] The optical selector 202 comprises a second optical holder, in this embodiment, in the form of a wheel 229 mounted to rotate on a support 231. The plurality of second optics 216, 216a, 216b are mounted on the wheel 229, wherein rotation of the wheel 229 to different angular positions about a selection axis B-B selectively locates a different one of the second optics 216, 216, 216b into the optical path 203. The wheel 229 is mounted on a shaft of motor 232 such that rotation of the wheel 229 is driven directly by the motor 232. In one embodiment, a further filter may be attached to a rear of the wheel 229 such that light that passes through the selected filter also passes through the additional filter. Such an additional filter may be used to refine the filtered light delivered to the analyser. There may be a corresponding further optic for each of two or more of the second optics 216, 216, 216b that allows light to pass therethrough.

[0048] The optical selector 202 comprises a detent mechanism that retains the wheel 229 in a position locating the selected second optic in the optical path without the need to supply power to the motor 232. The detent mechanism comprises notches 233 on the wheel 229 and a catch 234 that is biased by a spring, in this embodiment a flat-spring 235, towards the wheel 229 such that when the catch 234 is aligned with a one of the notches, the catch 234 is received in the notch 233 to resist rotation of the wheel 229 about motor axis B-B. The catch 234 is located on a side of the wheel

[0049] 229 rather than at the top of the wheel 229 opposite the base 205. (A side of the wheel is a position to the left or right of the wheel axis with the part of the wheel closest to base being considered the bottom). This may be beneficial in reducing a height of the optical selector 202 from the base 205.

[0050] The wheel 229 is mounted on a support 231 via the motor 232. The support 231 is mounted on base 205 for rotation about an incident angle axis C-C. The incident angle axis is perpendicular to an incidence plane of the selected second optic 216 and intersects with an incident surface of the selected second optic 216. Rotation of the support 231 about the incident angle axis C-C adjusts an incidence angle of the light (laser beam and back-scattered light) on the selected second optic 216. In this way, the wavenumbers reflected and transmitted by the selected second optic 216 can be altered.

[0051] A second optical holder absolute encoder (not shown) is arranged for measuring a rotary position of the support 231, and therefore selected second optic 216, about the incident angle axis C-C. In this embodiment, the second optic encoder is a linear encoder. Use of a linear encoder to measure a rotary position of the support 231 about axis C-C may provide sufficient accuracy because a range of rotation about axis C-C is small, such as less than 10° or less than 5°.

[0052] The wheel 229 is arranged to mount eight second optics (although the drawings only show three 216, 216a and 216b). Each second optic 216, 216a and 216b is mounted on a plate 236. In this embodiment, the plates 236 are triangular shaped such that the plates 236 can be fitted circumferentially around the axis B-B. Each plate is kinematically mounted on the wheel 229. To achieve this, three mounting formations on the plate 236, in this embodiment three slots machined in the plate 236, co-operate with a corresponding set of three mounting formations on the wheel 229, in this embodiment three hemispheres. Fastening members, in this embodiment a pair of bolts, compress springs to bias the mounting formations into engagement. Each hemisphere may be provided by a screw end that protrudes from a hole in the wheel 229. Adjustment of the screws adjusts a position of the hemispheres and therefore, a position of the incident surface of the corresponding second optic 216, 216a and 216b.

[0053] Support 231 further comprises rollers 225, 226, 227. Rigidly connected to the first optical holder 201 is a guide rod 206 that extends between the rollers 225, 226, 227. The rollers 225, 226, 227 are biased against the guide rod 206 by planar spring 237. Movement of the slider 221 moves the axis of rotation A-A in a linear direction, translating the first optical holder 201. The translation of the first optical holder 221 moves the guide rod 206 between the rollers 225, 226, 227. However, as the translation caused by movement of the slider 221 is not parallel to the guide rod 206, to accommodate the translation, the optical holder 201 must also rotate about axis A-A, rotating the guide rod 206. This rotation of the guide rod 206 is followed by the rollers 225, 226, 227, rotating the support 231 about incident angle axis C- C. Accordingly, movement of the slider 221 both translates and rotates the mirror 214 and rotates the selected second optic 216. The mechanism is geometrically arranged such that for different angular positions of the selected second optic 216, the position of the first mirror 214 is adjusted such that laser beam is directed along the output optical path 215. In this embodiment, slider 221 moves in a linear direction that is 45° to the input optical path 213 and 45° to the output optical path 215. A surface normal to the selected second optic 216 intersects with the axis A- A. The guide rod 206 extends along a line parallel to the surface normal of the selected second optic 216. An angle of a surface normal of mirror 214 to the input optical path 213 summed with the angle of the surface normal of the selected second optic 216 to the output optical path 215 is equal to 45°. The mirror 214 can move to positions in which the surface normal of mirror 214 is less than 45° to the input optical beam 214.

[0054] In such an arrangement, the optical input path 213 crosses the optical output path 215. Preferably, the first optic mirror is a dielectric mirror that provide a high reflectively across a broad range of wavelengths for angles of incidence of the optical input path to the dielectric mirror of less than 45°.

[0055] One of the second optics may be a plain mirror, which may be used for calibration.

[0056] In use, a user selects a Rayleigh filter 216, 216a, 216b for use in a particular application. This may be based on the wavelength of the laser light to be used to excite the sample, the sample being analysed and / or the Raman wavenumbers of interest. The user inputs into a controller of the spectrometer, for example via a user interface (not shown), a required Rayleigh filter and the controller controls the motor 232 to locate the required Rayleigh filter into the optical path 203. On locating the required Rayleigh filter 216, 216a, 216b into the optical path 203, power to the motor 232 is switched off. An angle of the Rayleigh filter 216, 216a, 216b to the optical output path 215 is set at an angle preset by the user. For this purpose, the user interface may allow the user to input into the controller the preset angle. The motor 208 is operated by the controller to set the angle of the selected Rayleigh filter 216 based on the preset angle. The user can then change the preset angle for different applications and / or for different sampling requirements. For example, a strong Raman-scattering sample may allow for trading-off Rayleigh blocking to gain a closer edge position, giving access to additional low- wavenumber chemical information.

[0057] Optionally, a tuning process may be carried out using motor 208 to adjust an angle of the selected second optic 216 to filter the required wavenumbers based on a measurement of noise. Again, this may be carried out by the user providing appropriate inputs to the controller of the spectrometer or the apparatus, such as the controller, may be arranged to carry out an automated tuning method based on analysing the Raman data, for example as described in WO2012 / 150434 or WO2014 / 064447.

[0058] It will be understood that modification and alterations may be made to the abovedescribed embodiments without departing from the invention as defined herein. For example, other types of filters could be fitted, such as shortpass or notch filters. A quick release mechanism, such as a mechanism that can be released without tools and / or does not require the releasing of nuts / bolts, may be provided for enabling individual filters 216, 216a, 216b to be changed quickly or the wheel 229 to be exchanged for a different wheel having at least one different filter. To allow changing of the wheel, the wheel and motor axle may comprise kinematic mounting formations to enable the wheel to be kinematically mounted on the motor axis. In this way, mounting of the wheel on the motor axis in a repeatable position may be facilitated.

Claims

CLAIMS1. An optical mount comprising a first optical holder for mounting a reflective first optic, an optical selector comprises a second optical holder for a plurality of second optics, the second holder mounted to rotate about a selection axis, wherein at different angular positions about the selection axis the second holder locates a different, selected one of the second optics in an optical path such that light on an optical input path is delivered via the first optic and the selected second optic to an optical output path, and a mechanism for rotating the selected second optic when in the optical path and moving the first optical holder in concert with the rotation of the selected second optic such that light delivered along the optical input path is directed along the optical output path for different angular positions of the selected second optic to the optical output path.

2. An optical mount according to claim 1 , wherein the optical holder comprises a wheel, wherein the selection axis is the wheel axis and the second optics are mounted about the wheel axis.

3. An optical mount according to claim 1 or claim 2, wherein the second optical holder is arranged to mount four or more second optics.

4. An optical mount according to any one of the preceding claims, wherein the second optical holder is mounted to rotate the selected second optic about an incident angle axis perpendicular to an incidence plane of the selected second optic.

5. An optical mount according to claim 4, wherein the incident angle axis intersects with the selected second optic.

6. An optical mount according to claim 4 or claim 5, wherein a plane of an optical surface of the selected second optic is parallel to the incident angle axis.

7. An optical mount according to claim 6, wherein a plane of an optical surface of each of the second optics is parallel to the incident angle axis.

8. An optical mount according to any one of claims 4 to 7 when dependent through to claim 2, wherein the incident angle axis is perpendicular to the wheel axis.

9. An optical mount according to claim 8, wherein the incident angle axis intersects with the wheel axis.

10. An optical mount according to any one of the preceding claims, comprising a base and the optical selector comprises a support to which the second optical holder is mounted such that the second optical holder can rotate on the support and the support is mounted on the base to rotate relative to the base.

11. An optical mount according to any one of the preceding claims, comprising a motor for rotating the second optical holder.

12. An optical mount according to claim 11 when dependent through to claim 2, wherein the wheel is mounted to the motor such that the wheel axis is common with the motor axis.

13. An optical mount according to any one of the preceding claims, the mount comprises a second optical holder encoder for measuring a rotary position of the second optical holder.

14. An optical mount according to claim 13, wherein the second optical holder encoder is an absolute encoder.

15. An optical mount according to any one of the preceding claims, wherein the mount comprises a second optic encoder to measure a rotary position of the selected second optic relative to the optical output path.

16. An optical mount according to claim 15, the second optic encoder is an absolute encoder.

17. An optical mount according to claim 15 or 16, wherein the second optic encoder is a linear encoder.

18. An optical mount according to any one of the preceding claims, the optical selector comprises a detent to mechanically resist rotation of the second optical holder for set positions of the second optical holder, each set position locating a different one of the second optics in the optical path.

19. An optical mount according to claim 18 when dependent through the claim 2, wherein the detent comprises notches in the wheel and a catch that is biased towards the wheel such that the catch enters into a corresponding notch when the notch is aligned with the catch.

20. An optical mount according to any one of the preceding claims, wherein each second optic is kinematically mounted on the second optical holder.

21. An optical mount according to any one of the preceding claims, wherein the first optical holder is arranged to mount the first optic such that an angle of incidence of the optical input path to the first optic is less than 45°.

22. An optical mount according to any one of the preceding claims, wherein the plurality of second optics comprises two or more filters.

23. A spectrometer comprising an optical mount according to any one of claims 1 to 22.

24. A spectrometer according to claim 23, comprising a laser arranged to direct a laser beam along the optical input path and an analyser arranged to receive light transmitted by the selected second optic.

25. An optical selector comprising a motor having a motor shaft and a wheel mounted on the motor shaft, the wheel comprises mounting formations for mounting a plurality of optics at circumferential positions about a motor shaft axis.

26. An optical selector according to claim 25, comprising a base and the wheel is mounted on the base to rotate about an incident angle axis.

27. An optical selector according to claim 26, wherein the incident angle axis intersects with an incident surface of a selected optic of the plurality of optics located by the optical selector to be on an optical path of light.

28. An optical selector according to claim 26 or claim 27, wherein the incident angle axis is perpendicular to the motor shaft axis.

29. An optical selector according to any one of claims 26 to 28, wherein the incident angle axis intersects with the motor shaft axis.

30. An optical selector according to any one of claims 26 to 29 comprising a support to which the wheel is mounted such that the wheel can rotate on the support and the support is mountable on a base to rotate relative to the base.

31. An optical selector comprising a wheel mounted for rotation, the wheel comprises mounting formations for mounting a plurality of optics at circumferential positions about an axis of rotation and a detent to mechanically resist rotation of the wheel for set positions of the wheel, each set position locating a different one of the optics in an optical path.

32. A spectrometer comprising an optical selector according any one of claims 25 to 31.

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