WO2004070330A1

WO2004070330A1 - Monochromator and spectrophotometer

Info

Publication number: WO2004070330A1
Application number: PCT/AU2004/000145
Authority: WO
Inventors: Jeffrey John Comerford; Alan Geoffrey Wiseman
Original assignee: Varian Australia Pty Ltd
Priority date: 2003-02-10
Filing date: 2004-02-10
Publication date: 2004-08-19
Also published as: AU2003900563A0

Abstract

A monochromator (20) for use in a spectrophotometer, and a spectrophotometer, in which the provision of substantially monochromatic light for absorption or fluorescence measurements is provided by a dispersive means of the monochromator (for example a diffraction grating (34) or prism), or by an alternative wavelength selecting element such as an optical bandpass filter of the spectrophotometer, without altering the geometry of the optical path. This is enabled by the dispersive means (34) of the monochromator (20) being associated with (for example by back-to-back mounting) a non-dispersive means such as a plane mirror (36) whereby the non-dispersive means (36) is positionable in place of the dispersive means (34) by, for example, a rotational drive means (40-38). This allows for light (46) entering the monochromator (20) through entrance slit (22) to be transmitted therethrough to exit slit (32) without dispersion and without altering the geometry of the optical path. The choice of use of different dispersion means of a spectrophotometer without altering the optical path allows for optimation of the spectrophotometer for a wide range of analytical applications.

Description

Monochromator and Spectrophotometer

Technical field

The present invention relates to a monochromator and to a spectrophotometer that contains the monochromator for the analysis and characterisation of samples by measurement of their absorption or fluorescence spectra.

Background of the invention

The following discussion of the background to the invention is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims.

To perform a spectrophotometric measurement of the absorption of light by an analytical sample, a source of substantially monochromatic light of a selected wavelength can be provided for the purpose of illuminating the sample. Such substantially monochromatic light is conveniently obtained by providing a continuum light source such as a xenon arc lamp or flash lamp and providing a monochromator between the continuum light source and the analytical sample. The monochromator can then be adjusted to provide substantially monochromatic light of a desired wavelength. Alternatively, substantially monochromatic light is conveniently obtained by providing an appropriate optical filter, for example a bandpass filter, between the continuum light source and the analytical sample. Monochromators and optical filters have their respective advantages and disadvantages and depending upon circumstances, it is sometimes advantageous to use one of these for wavelength selection in preference to the other.

A means of detecting and measuring the intensity of the substantially monochromatic light after its passage through an analytical sample in an appropriate sample container can also be provided so that the absorption of light by the analytical sample can be measured. To perform a measurement of the fluorescence of a sample, a source of substantially monochromatic light of a first wavelength can be provided for the purpose of illuminating an analytical sample and causing the sample to emit light. The substantially monochromatic light of a first wavelength that illuminates the analytical sample is called the excitation light, and the light emitted by the illuminated analytical sample is called the fluorescently emitted light. Excitation light is conveniently obtained by providing a continuum light source such as a xenon arc lamp or flash lamp and providing a wavelength selective means such as a monochromator or an optical filter, for example a bandpass filter, between the continuum light source and the analytical sample. A means is also provided for isolating fluorescently emitted substantially monochromatic light of a second wavelength and transmitting this light to a light detecting device for detection and measurement. Such means can be for example a monochromator or an optical bandpass or cut-off filter placed between the analytical sample and the light detecting device. Again, the respective advantages and disadvantages of monochromators and optical filters make it desirable to use one or the other of these depending upon circumstances.

Thus, to provide a spectrophotometer that can be optimised for a wide range of specific analytical applications, it is desirable to allow either a monochromator or an optical filter to be used as appropriate in the provision of the substantially monochromatic light of a first wavelength for the illumination of an analytical sample for absorption or fluorescence measurements or for the isolation of a second wavelength of interest from the light emitted by an analytical sample for fluorescence measurements.

However a problem with such proposed optimisation of a spectrophotometer is that the geometry of the optical path through the spectrophotometer may differ between use of a monochromator or use of an optical bandpass or cut-off filter to provide the substantially monochromatic light, for example a likely arrangement is for the monochromator to be bypassed if an optical filter is used to provide the required substantially monochromatic light. This is a problem because the geometry of the optical path is usually optimised, based on the optical characteristics of the monochromator, for some specific purpose such as the efficient and uniform illumination of a sample and/or the efficient collection of light from an illuminated sample.

Summary of the Invention According to a first aspect of the invention there is provided a monochromator for a spectrophotometer, the monochromator having an optical path of predetermined geometry, a dispersive means in the optical path for selecting light of a substantially specific wavelength for the spectrophotometer, the monochromator further including a non-dispersive optical means, wherein the non-dispersive optical means is positionable in place of the dispersive means whereby substantially all light entering the optical path is transmittable therethrough without altering the geometry of the optical path.

The dispersive means may be a diffraction grating or a prism. Preferably the non-dispersive optical means is a plane mirror. Preferably the dispersive means and the non-dispersive optical means are provided by the one element, for example a diffraction grating having a conventional grooved surface for dispersing incident light and having a flat opposite surface which is provided with a reflective coating to thereby provide a mirror surface for non-dispersively reflecting incident light. Such an element may be carried by a rotatable shaft whereby rotation of the shaft can position one or the other of the surfaces in place without altering the geometry of the optical path. Alternatively a diffraction grating as such and a mirror as such may be mounted back to back and likewise carried by a rotatable shaft for positioning a grooved surface of the diffraction grating (that is, a dispersive means) or a reflecting surface of the plane mirror (that is, a non-dispersive optical means) in the optical path without altering the geometry of that path. Thus the dispersive means and the non- dispersive optical means may be mechanically associated whereby the movement of one out of the optical path moves the other into the optical path without altering the geometry of the path. Mechanical association of the two means other than a back to back arrangement are also within the scope of the invention. For example the dispersive means and the non-dispersive optical means may be separate elements provided on a sliding carriage or a rotatable wheel mounted to move the alternate element into the correct position without affecting the optical geometry.

It is not necessary that the dispersive means and the non-dispersive optical means be mechanically interconnected for one to replace the other in the optical path without disturbing the optical beam configuration. Thus each means could be associated with an independent mechanism such as a swing in-out, slide in-out or rotate in-out mechanism, however both mechanisms will need to be mechanically mounted to a common base to provide a common reference point to the other optical components of the optical path. Such independent mechanisms may be computer controlled such that movement of one of the dispersive or non-dispersive means out of position initiates movement of the other into position in the optical path.

Although the non-dispersive optical means is preferably a plane mirror or a plane mirror surface, other non-dispersive optical means, which will generally involve the use of at least one mirror, may be used depending upon what is used as the dispersive means. Whatever constitutes the non-dispersive optical means, it is a requirement that its action be such as to allow a non-dispersed beam to follow the same optical path through the monochromator as would a beam that is dispersed by the dispersive means.

The monochromator may be one of a double monochromator arrangement as a wavelength selective means for a spectrometer. In such a double monochromator arrangement two monochromators are operated in series whereby light transmitted to the second monochromator is first dispersed by the first monochromator, with the advantage that stray light is reduced. Stray light is light that passes through a monochromator system despite the wavelengths of this light being different from the wavelength that the monochromator system is intended to transmit. A known disadvantage of such a double monochromator arrangement is that the efficiency of light transmission is reduced compared to that achieved when only a single monochromator is used. According to a second aspect of the invention there is provided a spectrophotometer having an optical path that includes a monochromator according to the first aspect of the invention, wherein the optical path of the spectrophotometer allows or provides for the use of an alternative wavelength selecting optical element to the dispersive means of the monochromator, wherein either the dispersive means of the monochromator or the alternative wavelength selecting optical element is useable for selecting light of a substantially specific wavelength for the spectrophotometer without altering the geometry of the optical path of the spectrophotometer.

The alternative wavelength selecting optical element is preferably an optical bandpass filter, however it may instead be a cut-off filter, a diffraction grating or a prism.

For a better understanding of the invention and to show how it may be carried into effect, preferred embodiments thereof will now be described, by way of non-limiting example, with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 schematically shows an embodiment of a monochromator arrangement according to the first aspect of the invention set to operate as a monochromator.

Figure 2 schematically shows the monochromator of Figure 1 set to transmit light without dispersion.

Figure 3 schematically shows another embodiment of a monochromator according to the first aspect of the invention set to operate as a monochromator.

Figure 4 shows the monochromator of figure 3 set to transmit light without dispersion.

Figure 5 schematically shows a double monochromator arrangement according to an embodiment of the invention. Figure 6 schematically shows a spectrophotometer according to an embodiment of the invention.

Description of the Preferred Embodiments A monochromator according to an embodiment of the invention is provided with a dispersive element (either a prism or a diffraction grating) that is preferably mounted on a rotatable shaft so that the angle of the dispersive element with respect to light impinging on the dispersive element can be varied to select the wavelength of the light that is transmitted by the monochromator. Furthermore, the monochromator is also provided with a plane mirror (that is, a non- dispersive optical element) mounted on the same shaft as the dispersive element so that by appropriate motion of the shaft the dispersive element can be removed from the light path and the mirror put into the light path at the position formerly occupied by the dispersive element. The mirror can then be appropriately rotated so that substantially all the light entering the monochromator is transmitted, limited only by small losses at each reflective surface. In this configuration the optical geometry through the spectrophotometer is unchanged, apart from the fact that substantially all light entering the monochromator is now transmitted. When it is desired to use an optical filter, the monochromator is configured to transmit substantially all of the incident light as just described and the required filter is placed in position . When it is desired to use the monochromator to select the desired wavelength of light, the monochromator is operated normally. A filter may optionally be used in conjunction with the monochromator to reduce stray light. Alternatively, no filter may be used. Thus, in a spectrophotometer for absorbance measurements according to this embodiment of the invention either a monochromator or an optical bandpass filter can be used to provide the required substantially monochromatic light of selectable wavelength without altering the optical geometry of the spectrophotometer. Such a spectrophotometer can therefore be optimised for specific applications quickly and easily.

In a preferred embodiment of the first aspect of the invention the dispersive means is a diffraction grating and the non-dispersive optical means is a plane mirror which are arranged back-to-back on the shaft of an electric motor. Preferably, the electric motor is a digital stepper motor. Means are provided so that the position of the diffraction grating and mirror in the optical path can be controlled accurately and reproducibly. Such means are preferably appropriate digital electronics for driving the digital stepping motor. Optionally, the shaft can be provided with position detecting or encoding means. The shaft can also be rotated by a motor mechanically coupled to the shaft by any appropriate means such as for example intermeshing gears, or pulleys and belt.

Referring to figures 1 and 2, the schematically illustrated monochromator arrangement 20 is a conventional Czerny-Turner monochromator. It comprises an entrance slit 22 in slit (or aperture) plate 24, a collimating mirror 26, a diffraction grating element 28 (to be described in detail below), a focussing mirror 30 and an exit slit 32 in slit plate 24. Element 28 is a diffraction grating via the provision of a conventional grooved surface 34 for the diffraction of light. A plane mirror 36 is provided on the opposite side of diffraction grating element

28 from a conventional grooved surface 34. Element 28 is mounted on a rotatable shaft 38 of drive means 40, which means is preferably a digital stepping motor but any appropriate rotating means may be used. Each of mirrors 26 and 30 is preferably provided with a mask (42 and 44 respectively) as is known in the art.

When it is desired to use the monochromator 20 as a monochromator, it is set to the state shown in Figure 1. Diffraction grating element 28 has been rotated by rotating means 40 so that its grooved surface 34 is facing mirrors 26 and 30. Polychromatic light 46 entering through entrance slit 22 falls on collimating mirror 26 and is reflected as a substantially parallel beam 48 onto grooved surface 34 of diffraction grating element 28. The angle of incidence of the substantially parallel beam 48 on grooved surface 34 is controlled by rotating drive means 40. Light 50 reflected from grooved surface 34 is dispersed by interference and is reflected from focussing mirror 30 (as indicated by reference 52) to a focus on exit slit 32. Such an arrangement results in substantially monochromatic light 52 passing through exit slit 32. The wavelength of light 52 passing through exit slit 32 is determined by the angle of incidence of the light 48 from collimating mirror 26 on grooved surface 34 of diffraction grating element 28, and is controlled by setting the orientation of diffraction grating element 28 with respect to the incident light 48 by rotating drive means 40.

When it is desired to use filters in a spectrophotometer rather than a monochromator 20 as such, the monochromator is set to the state shown in Figure 2. Diffraction grating element 28 has been rotated by drive means 40 so that plane mirror 36 is facing collimating mirror 26 and focussing mirror 30. Light 46 entering entrance slit 22 is reflected successively by mirrors 26, 36 and 30 and brought to a focus on exit slit 32.

As can be seen from Figures 1 and 2, the path of light through the monochromator 20 arrangement is the same in either case, so there are no changes in the optical geometry of the spectrophotometer in which the monochromator 20 arrangement is used.

It is to be understood that the invention is not limited to the Czerny-Turner monochromator configuration 20 (illustrated in Figure 1 as an example of the invention), but is equally applicable to other monochromator arrangements.

Figures 3 and 4 schematically illustrate a monochromator arrangement 60 that is similar to that of Figures 1 and 2 and in which the same reference numerals have been used to denote corresponding features. The difference is that the diffraction grating element 28 has been replaced by a prism 62 having a mirrored reflecting surface 64 and a non-mirrored normal surface 66.

In Figure 3 the monochromator 60 is shown as set for use as a monochromator wherein prism 62 has been rotated by drive motor 40 so that parallel beam 48 from collimating mirror 26 impinges on its non-mirrored surface 66. Light 48 is refracted into prism 62 at surface 66 and is reflected internally of the prism by "rear" mirrored surface 64 to emerge, with refraction, through surface 66. Thus the emergent light from prism 62 is dispersed by refraction and a selected wavelength thereof, as determined by the angle of incidence of beam 48 on surface 66 of prism 62, which is controlled by rotating drive motor 40, is focussed by focussing mirror 30 on exit sit 32. When it is desired to use filters for wavelength selection rather than a monochromator as such, the monochromator 60 is set to the state shown in

Figure 4. In this state, prism 62 has been rotated by drive means 40 to position its mirrored surface 64 so that it is facing the collimating mirror 26 and focussing mirror 30. Light 46 entering entrance slit 22 is reflected successively by mirrors

26, 64 and 30 and brought to a focus on exit slit 32. The path of light through the monochromator 60 is effectively the same in the Figures 3 and 4 settings, so there is no change in the optical geometry of a spectrophotometer in which the monochromator 60 is used when the monochromator 60 is set in either of the states as shown by Figures 3 and 4. Thus when monochromator 60 is set to the state shown by Figure 4, an optical filter can be used for wavelength selection.

An embodiment of the invention provides a double monochromator arrangement for a spectrometer in which two monochromators are operated in series but the first monochromator can optionally be set to either a first configuration whereby light transmitted to the second monochromator is first dispersed, with the advantage that stray light is thereby reduced, or to a second configuration whereby light is transmitted to the second monochromator with no dispersion. Such a second configuration is advantageous when the light to be measured is of relatively low intensity. A double monochromator instrument according to this embodiment of the invention has the advantage of being conveniently and rapidly settable to either of its two configurations as required.

The same instrument can thereby provide enhanced performance either for a first situation where stray light must be minimised at the expense of light transmission or for a second situation where light transmission must be maximised at the expense of stray light rejection.

In a double monochromator embodiment of the invention two monochromators are so arranged that light entering the second monochromator must first have passed through the first monochromator as is known. The first of the two monochromators is provided with a dispersive element that is preferably mounted on a rotatable shaft so that the angle of the dispersive element with respect to light impinging on the dispersive element can be varied to select the wavelength of the light that is transmitted by the first monochromator to the second monochromator. Furthermore, the first monochromator is also provided with a plane mirror mounted on the same shaft as the dispersive element so that by appropriate motion of the shaft the dispersive element can be removed from the light path and the mirror put into the light path at the position formerly occupied by the dispersing element. The mirror can then be appropriately rotated so that substantially all the light entering the first monochromator is transmitted to the second monochromator, limited only by small losses at each reflective surface. When it is desired to maximise light transmission through the double monochromator at the expense of stray light rejection the first monochromator is configured to transmit substantially all of the incident light as just described. When it is desired to minimise stray light, the first monochromator is operated as known in the art. It is to be understood that the functions of the first and second monochromators just described can be interchanged. The aim of the invention is to have at least one of the monochromators in a double monochromator configurable to transmit light with or without dispersion as may be required. Such a double monochromator can therefore be optimised for specific applications quickly and easily.

Figure 5 schematically illustrates a double monochromator arrangement 70. This arrangement 70 includes effectively two monochromators in series. Thus the monochromator arrangement 70 includes an optical path defined by an entrance slit 72 in plate 74, followed by a masked collimating mirror 76, then a first diffraction grating element 78 which has a grooved diffracting surface 80 and a plane mirrored surface 82 arranged back to back. Element 78 is mounted on shaft 84 and is rotatable by a drive means 86, for example a digital stepping motor. Element 78 in the optical path is followed by a masked focussing mirror 88 which focuses light onto a mirror 90 which is arranged to direct the light through a slit 92 in a slit plate 94 to another mirror 96. The slit 92 is effectively the exit slit of the first monochromator and the entrance slit for the second monochromator in the series monochromator arrangement 70. Mirror 96 is followed by a masked collimating mirror 98, then another diffraction grating element 100, having back to back a grooved diffracting surface 102 and a plane mirror 104, and rotatably movable via a drive means 106. Element 100 in the optical path is followed by a masked focussing mirror 108 for focussing light onto an exit slit 110 in a slit plate 112 of the arrangement 70.

Figure 5 illustrates the monochromator arrangement 70 as set for the transmission of light therethrough without dispersion, wherein the plane mirror surfaces, respectively 82 and 104, of the elements 78 and 100 are in the optical path. In another setting, drive means 106 can be operated to rotate element 100 to position its grooved diffracting surface 102 in place of the plane mirrored surface 104. In this setting, light is transmitting from the first to the second monochromator of the arrangement 70 without dispersion for it to be dispersed in the second monochromator by surface 102 of the element 100 for selection of a particular wavelength, which wavelength of light is focussed by focussing mirror 108 onto exit slit 110. This setting is advantageous when the light to be measured is of relatively low intensity.

In a further setting, both drive means 86 and 106 can be operated to rotate elements 78 and 100 such that both grooved dispersing surfaces 80 and 102 are placed in the optical path. In this further setting light is dispersed by the first monochromator of the arrangement 70 for selection of a substantially specific wavelength of the light to be transmitted to the second monochromator wherein the light is further dispersed for more specific wavelength selection. This further setting has the advantage that stray light is reduced.

A spectrophotometer for absorption measurements according to an embodiment of the second aspect of the invention is provided with a monochromator located between a continuum light source and the analytical sample to provide substantially monochromatic light of a selectable wavelength and a detector for detecting the substantially monochromatic light after it has passed through the sample. Means are provided to place an optical bandpass filter between the continuum light source and the analytical sample in the optical path of the light passing from the continuum source through the monochromator to the analytical sample. For example the optical bandpass filter can be positionable between the monochromator and the analytical sample. Alternatively, the optical bandpass filter can be positionable between the continuum source and the monochromator. The means for placing the optical bandpass filter in the optical path may be a holder for the filter which is located in the optical path and into which the filter is manually insertable. More typically, it may be a carriage, either rotating or sliding, onto which the filter (or a selection of filters) is mountable so that a required filter can be driven into the light path by an actuator which may be under computer control. Such a carriage may be movable to allow an operator access to change one or more of the filters for different applications, or a complete carriage, with pre-mounted. filters, may be removed and replaced to provide a "selection" of filters for a specific application.

For simplicity in the description of the invention, reference is made to the use of a single monochromator or optical filter. It is to be understood that a plurality of optical filters and or monochromators (including double monochromators) can be used if desired where ever the use of a single monochromator or optical filter is described. The principal advantage to be gained by the use of multiple monochromators or optical filters is the reduction of stray light.

A spectrophotometer for fluorescence measurements according to another embodiment of the second aspect of the invention is provided with two monochromators, a first monochromator located between a continuum light source and the analytical sample to provide excitation light, and a second monochromator to isolate fluorescently emitted light having a specific wavelength of interest. The second monochromator is located between the analytical sample and a detector for detecting the fluorescently emitted light. Means are provided to place a first optical filter between the continuum light source and the analytical sample in the optical path. For example the first optical filter can be positionable between the first monochromator and the analytical sample. Alternatively, the optical filter can be positionable between the continuum source and the monochromator. The means for placing the first optical filter in the optical path may be a holder or carriage as previously described. For simplicity in the description of the invention, reference is made to the use of a single monochromator or optical filter. It is to be understood that a plurality of optical filters and or monochromators (including double monochromators) can be used if desired where ever the use of a single monochromator or optical filter is described. The principal advantage to be gained by the use of multiple monochromators or optical filters is the reduction of stray light.

Means are also provided to place a second optical filter between the analytical sample and the detector of fluorescently emitted light in the optical path followed by fluorescently emitted light passing from the analytical sample through the second monochromator to the detector. The means for placing a second optical filter in the optical path may be a holder or carriage as previously described.

Each of the two monochromators is provided with a dispersive means that is preferably mounted on a rotatable shaft so that the angle of the dispersive means with respect to light impinging on the dispersive means can be varied to select the wavelength of the light that is transmitted by the monochromator. Furthermore, each monochromator is also provided with a plane mirror surface on the same shaft as the dispersive means so that by appropriate motion of the shaft the dispersive means can be removed from the light path and the mirror put into the light path at the position formerly occupied by the dispersive means. The mirror can then be appropriately rotated so that substantially all the light entering the monochromator is transmitted, limited only by small losses at each reflective surface. In this configuration the optical geometry through the spectrophotometer is unchanged, but substantially all light entering the monochromator is now transmitted.

When it is desired to use an optical bandpass filter, the required filter is placed in position and the corresponding monochromator is configured to transmit substantially all of the incident light as described in the preceding paragraph. When it is desired to use a monochromator, the monochromator is operated as known in the art. Thus, in a spectrophotometer for fluorescence measurements according to this embodiment of the invention, either a monochromator or an optical bandpass filter can be used for either or both the excitation light or the fluorescently emitted light without altering the optical geometry of the spectrophotometer. Such a spectrophotometer can therefore be optimised for specific applications quickly and easily.

Figure 6 schematically illustrates a spectrophotometer 120 for either absorption or fluorescence measurements. The optical circuit of the spectrophotometer 120 includes a light source 122, for example a xenon arc flash lamp, and source optics including a lens 124 and plane mirror 126 for directing polychromatic light from source 122 into the entrance slit 130 of an excitation monochromator 128. The optical path through monochromator 128 includes collimating mirror 132, diffraction grating element 134 which includes, back-to-back, a grooved diffracting surface and a mirrored surface (such as disclosed in the Figs 1 and 2 embodiment), followed by a focussing mirror 136 to focus light of a selected substantially single wavelength an exit slit 138 of the excitation monochromator 128. If the mirrored surface of diffraction grating element 134 instead of its grooved surface is positioned in the optical path, the light emerging from exit slit 138 will be polychromatic. Light emerging from the exit slit 138 is focussed by a focussing mirror 140 to pass through an excitation polariser 142 to a beam splitter 144. When the excitation monochromator 128 is used to pass polychromatic light therethrough, a filter 146 can be inserted into the optical path before the polariser 142 for wavelength selection. Such a filter 146 is an alternative wavelength selecting optical element to the diffraction grating element 134 of the monochromator 128.

A portion of the monochromatic light that falls on beam splitter 144 is reflected therefrom to a flat mirror 146 and from there to an ellipsoidal mirror 148 that images the monochromatic light into a sample well 150.

For absorbance measurements, sample well 150 will have a transparent base and light emerging therefrom is focussed by optics 152 onto an absorbance detector 154,

For fluorescence (the term "fluorescence" herein is to be read as encompassing "phosphorescence") measurements, fluorescently emitted light from the sample in well 150 follows the same path as the excitation light from beam splitter 144, but in the opposite direction. A portion of the emitted light that reaches the beam splitter 144 passes through the beam splitter 144, then through a polariser 156 and a filter 158 onto an emission focussing mirror 160 which focuses the emission light onto the entrance slit 164 of an emission monochrmator 162. Emission monochromator 162 is similar to excitation monochromator 128 in that it includes diffraction grating element 166 having, back-to-back, a grooved dispersing surface and a mirrored surface (such as disclosed in the Figs 1 and 2 embodiment). When the monochromator 162 is to be used as a monochromator, the grooved dispersing surface of element 166 is positioned in the optical path. When the monochromator 162 is set with the mirrored surface of element 166 positioned in the optical path, wavelength selection for the emission light is provided by insertion of a wavelength selecting filter at an appropriate position into the emission optical path, for example before the beam splitter 144 so as to avoid the emission light mixing with excitation light. The monochromator 162 includes a collimating mirror 168 and focussing mirror 170. Substantially monochromatic emission light (whether isolated by a filter or element 166 of monochromator 162) emerges from an exit slit 172 of the monochromator 162 and falls on a focussing mirror 174 to be brought to a focus on an emission detector 176.

When the spectrophotometer 120 is used for fluorescence emission measurements, the sample well 150 preferably has an opaque base to prevent reflection of light from the absorbance optics 152 and absorbance detector 154 onto the ellipsoidal mirror 148 and thus ultimately into the emission detection circuit 156-158-160-162-174-176 where it would be a potential source of stray light.

As known by persons skilled in the art, a spectrophotometer such as 120 will normally include components in its optical circuit (not shown) for deriving an excitation reference beam for absorption measurements, which is detected by an absorbance reference detector, and for deriving an excitation reference beam for emission measurements, which is detected by an emission reference detector. For absorbance measurements, an electrical signal from absorbance detector 154 is used in conjunction with an electrical signal from an absorbance reference detector (not shown) to generate a measurement of the absorbance of a sample solution in well 150. For fluorescence (or phosphorescence) emission measurements, an electrical signal from emission detector 176 is used in conjunction with an electrical signal from an emission reference detector (not shown) to generate a measurement of the emission of light from a sample solution in well 150.

The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims.

Claims

1. A monochromator for a spectrophotometer, the monochromator having an optical path of predetermined geometry, a dispersive means in the optical path for selecting light of a substantially specific wavelength for the spectrophotometer, the monochromator further including a non-dispersive optical means, wherein the non-dispersive optical means is positionable in place of the dispersive means whereby substantially all light entering the optical path is transmittable therethrough without altering the geometry of the optical path.

2. A monochromator as claimed in claim 1 wherein the dispersive means is a diffraction grafting.

3. A monochromator as claimed in claim 1 wherein the dispersive means is a prism.

4. A monochromator as claimed in any one of claims 1 to 3 wherein the non-dispersive optical means is a plane mirror.

5. A monochromator as claimed in any one of claims 1 to 4 wherein the dispersive means and the non-dispersive optical means are mechanically connected whereby movement of one out of its position in the optical path correspondingly moves the other into that position.

6. A monochromator as claimed in claim 4 wherein the dispersive means is either a diffraction grating or a prism, and the non-dispersive optical means is provided by a reflective coating on a surface of the diffraction grating or prism for that surface to function as a mirror.

7. A monochromator as claimed in claim 5 wherein the dispersive means is a diffraction grafting and the non-dispersive optical means is a mirror which are mounted back-to-back and are carried by a rotatable shaft.

8. A monochromator as claimed in any one of claims 1 to 7, including another monochromator to form a double monochromator arrangement whereby the two monochromators are in series.

9. A monochromator as claimed in claim 8 wherein the first of the two series arranged monochromators is the monochromator as claimed in any one of claims 1 to 7.

10. A monochromator as claimed in claim 8 wherein the second of the two series arranged monochromators is the monochromator as claimed in any one of claims 1 to 7.

11. A spectrophotometer having an optical path that includes a monochromator as claimed in any one of claims 1 to 10, wherein the optical path of the spectrophotometer allows or provides for the use of an alternative wavelength selecting optical element to the dispersive means of the monochromator, wherein either the dispersive means of the monochromator or the alternative wavelength selecting optical element is useable for selecting light of a substantially specific wavelength for the spectrophotometer without altering the geometry of the optical path of the spectrophotometer.

12. A spectrophotometer claimed in claim 11 , wherein the alternative wavelength selecting optical element is an optical bandpass filter.

13. A spectrophotometer as claimed in claim 11 , wherein the alternative wavelength selecting optical element is a prism.

14. A spectrophotometer as claimed in claim 11 , wherein the alternative wavelength selecting optical element is a diffraction grafting.