WO1989000280A1

WO1989000280A1 - Improvements in and relating to spectrophotometers

Info

Publication number: WO1989000280A1
Application number: PCT/GB1988/000526
Authority: WO
Inventors: William Anthony Scott; Robert Neilson Mckenzie
Original assignee: Beckman Riic Limited
Priority date: 1987-07-07
Filing date: 1988-07-05
Publication date: 1989-01-12
Also published as: GB8715949D0

Abstract

A beam of visible or UV light is directed by a sliding mirror (20) onto a diffraction grating (33) through each of a plurality of collimators (29 to 32) in turn. At each turn a photodiode array (35) measures the spectrum of a corresponding portion of the range of frequencies over which the instrument operates. A set of broad-band band-pass filters (16) may eliminate undesired frequencies at each turn.

Description

Improvements in and relating to spectrophotometers

The invention relates to spectrophotometers, that is to say, to instruments for measuring the intensity of radiation of different frequencies present in a spectrum, and especially to diode-array spectrophoto¬ meters in which a beam of radiation is dispersed and caused to fall onto an array of photodiodes each of which is irradiated by a band of frequencies and emits a signal indicative of the intensity of radiation in that band.

Spectrophotometers are known in which a diffraction grating is rotated in order to cause the dispersed beam to sweep over a single photosensitive device, which indicates the intensity of different, angularly separated, parts of the beam in turn. The accuracy with which absolute frequency can be determined using such an instrument is limited by the precision of the mechanism that causes and controls the rotation of the grating. Diode-array spectrophotometers have previously been proposed, but there are practical limits on the number of photodiodes that can be included in the array and arrays with large numbers of photodiodes are expensive, and it has been found that for many purposes a sufficiently high resolution of frequency over a sufficiently large frequency range cannot be achieved with a practical array at a commercially acceptable cost .

It is an object of the invention to provide a spectrophotometer with which the disadvantages of both of those previously proposed forms of spectrophotometer can be avoided at least to a significant extent. The invention provides a spectrophotometer comprising an array of detectors; means for dispersing a beam of radiation and causing it to fall on the array of detectors; and means for selectively directing a beam of radiation to the dispersing means from any of a plurality of directions.

The array of detectors may be an array of photodiodes.

The dispersing means may be such as to cause to fall onto the array of detectors radiation of a respective range of frequencies from each said direction, every frequency within a larger range being included within at least one said respective range. The said respective ranges then advantageously cover the said larger range with little or no overlap. The resolution obtainable from the instrument, for a given array of detectors, is then effectively multiplied by the number of available directions of incidence, and because the directing means is required only to select one of a number of discrete paths, the need for exact control of the moving parts can be greatly reduced.

In practice, some overlap of the ranges is desirable in order to avoid any risk of a gap's being left between two ranges. Also, it has been found to be desirable to keep the total power output of the array within a limited range in order to obtain the best performance from amplifiers and the like processing the output from the array and in order to achieve that it is sometimes desirable to use less than the whole array in some frequency ranges. If that is done then there must be significant overlap in the frequency ranges over the full width of the array to avoid gaps between the parts of those ranges that are actually detected. In special cases the overlap between two adjacent ranges may correspond to up to 50% of the full width of the array, but in general an overlap of from 10% to 20% of the width of the array is preferred.

The directing means may comprise a movable mirror, which may be movable into and out of the path of at least one beam of radiation. The desired beam path may be selected by a mirror that is so rotatable as to change the orientation of its reflecting surface; in that case it is necessary to ensure that the mirror comes into exactly the correct orientation for each beam, but it is not necessary to know the precise orientation of the mirror between those positions. Advantageously, however, the desired beam is selected by means such that a slight error in the positioning of the selecting means does not affect the direction of any beam. The selecting means may then use a movable irror that is translatably mounted and/or is slidable such that the direction of movement of any point on its reflecting surface is tangent to that surface, and is preferably a plane mirror translatable in its own plane. Instead, the selecting means may use a dichroic mirror or other beam-splitter combined with shutters to exclude the unwanted beams.

If a sliding or otherwise translatable mirror is used, it is preferably arranged to be slid by a suitable actuator between discrete positions in each of which every beam that it may be desired to select either falls on a part of the mirror away from any edge thereof or misses the mirror without passing close to any edge thereof. If that is done then no beam will be split or mis-directed if the sliding mirror comes to rest slightly away from its intended position. If shutters are used then they should be so arranged that they will not come to rest incompletely blocking a beam or deflect a beam into another part of the apparatus. Radiation-absorbing shields may be provided to eliminate any radiation that does not form part of a desired beam.

The dispersing means may be a diffraction grating. Advantageously, the arrangement is such that in normal use for a given range of frequencies the diffraction grating causes first-order diffracted light to fall on the array of detectors but diffracted light of a higher order may be caused to fall on the array of detectors instead.

Preferably, the part of the dispersed beam that falls onto the array of detectors leaves the diffraction grating generally perpendicular to the surface of the grating. If that is done, it is found that the angle at which radiation of any given frequency is diffracted is an approximately linear ^•function of the wavelength of the light, minimising the usual tendency for radiation of long wavelengths to be more widely dispersed. The dispersed beam is then divided by an array of detectors of equal widths into approximately equal intervals of wavelength.

The spectrophotometer may comprise collimating means aligned on the dispersing means from each of the said plurality of directions. If the dispersing means is a concave diffraction grating, the collimating means advantageously have foci on the Rowland circle of the grating and the array of detectors then preferably lies along the Rowland circle. Means may be provided for admitting to the directing means only radiation having frequencies within a desired range of frequencies. Advantageously, means are provided for admitting radiation of any of a plurality of ranges of frequencies each corresponding to a respective said direction. Those means may be a set of band pass filters each so selected as to admit substantially only radiation that, if it is directed to the dispersing means from the corresponding said direction, will fall on the array of detectors, radiation that should not fall on the detectors being filtered out. If the dispersing means is a diffraction grating the spectrophotometer may then be so arranged that radiation in two or more different ranges of frequencies from different orders of diffraction of radiation incident on the dispersing means may fall on the array of detectors in normal use, a desired one of those ranges of frequencies being selected by filtering out radiation in the other range or ranges. If that is done, then the number of ranges of frequency that can be detected may be greater than the number of directions from which radiation can be directed to the dispersing means. The spectrophotometer may include a source of electromagnetic radiation, means for guiding radiation from the source to the above-mentioned directing means, and means for so locating a sample that, in use, radiation passes from the source to the directing means through the sample so that the spectrum detected by the array of detectors is a spectrum characteristic of the sample.

The means for locating a sample may comprise means for locating a cell and may also include a cell so located.

The source of electromagnetic radiation may be a source of visible and/or ultra-violet light.

The invention also provides a method of obtaining a spectrum of radiation using a spectrophotometer according to the invention.

The method may comprise successively directing said radiation to the dispersing means from at least two said directions and obtaining spectra of respective ranges of frequencies.

Two forms of spectrophotometer constructed in accordance with the invention will now be described by way of example only with reference to the accompanying drawings, in which:

Pig. 1 is a diagrammatic plan view of the first form of spectrophotometer; and

Pig. 2 is a diagrammatic plan view of part of the second form of spectrophotometer. Referring to Fig. 1 of the drawings, one form of spectrophotometer comprises a radiation source indicated generally by the reference numeral 1, a sample compartment indicated generally by the reference numeral 2, a selector indicated generally by the reference numeral 3, and an analyser indicated generally by the reference numeral 4.

The source 1 comprises a tungsten filament lamp 5 emitting visible light, within a screen 6, and a deuterium discharge lamp 7, emitting ultraviolet light, within a screen 8. The lamps 5 and 7 may be conven¬ tional and are not described in detail. A beam of light from each of the lamps 5 and 7 escapes from the screen 6 or 8 surrounding that lamp through an opening 9 or 10, respectively. The ultraviolet beam is directed towards a mirror 11. The visible beam intersects the ultra-violet beam, and a plane mirror 12, mounted for sliding movement in its own plane under the control of an actuator 13, can be moved into the paths of the beams where they intersect and then, as shown in the drawing, shuts off the ultraviolet beam and reflects the visible beam towards the mirror 11. Each lamp may in addition have a respective shutter to absorb its radiation when it is not being used, in order to avoid unnecessary heating of components in the path of the beams and/or of a sample.

The mirror 11 is concave, and condenses the beam of light through a diaphragm 14 onto a mirror 15. The concave mirror 11 may be a toroidal mirror providing a beam of light that converges as seen in plan view in the drawing but is of constant width, or converges at a different angle, in a plane perpendicular to that of the drawing. The mirror 15 deflects the beam of light into the sample compartment 2, where it comes to a focus. Between the mirror 15 and the sample compartment 2, the beam passes through a selected one of a plurality of filters 16 which, under the control of an actuator 17, transmits light of a selected range of frequencies.

The sample compartment 2 and any means therein for holding a sample to be examined, for example, a cell and means for locating the cell, may be entirely conventional and in the interests of brevity are not described.

The beam of light emerging from the sample compartment 2, diverging from the above-mentioned focus, is made parallel by a concave mirror 18 and then reflected by a plane mirror 19 towards a sliding plane mirror 20 of the selector 3. The mirror 20 slides in its own plane, under the control of an actuator 21, obliquely across the path of the beam from the mirror 19, and will, depending on its position, let the beam pass it or reflect the beam onto a plane mirror 22. The beam reflected by the mirror 22 will then pass the sliding mirror 20 or be reflected onto a further plane mirror 23, depending on the position of the sliding mirror, and in the latter case the beam reflected by the mirror 23 will pass the sliding mirror 20 or be reflected onto a further plane mirror 24, depending on the position of the sliding mirror. The number of mirrors 22 to 24 may be different from that shown; an arrangement with only one such mirror may be useful, and the maximum number may be at least 8 or 10, being limited only by the practical difficulty of fitting the different beam paths and sets of mirrors and lenses together. The beams of light from the mirrors 19, 22, 23, and 24 are spaced far enough apart that the sliding mirror 20 can easily be located with its edge between two beams, so that a slight error in positioning it will not result in a beam's being partly reflected and partly passing the mirror 20.

Each of the mirrors 19, 22, 23, and 24 directs the beam of light onto a respective condensing lens 25, 26, 27, or 28 by which the beam is focused onto a respective entrance slit 29, 30, 31, or 32 of the analyser 4, providing a respective collimated beam that is incident on a fixed holographic diffraction grating 33. The incident light beam is diffracted by the diffraction grating 33, and part of the diffracted

_* light is directed by a plane mirror 34 onto a fixed array 35 of photodiodes, the outputs from which are processed electronically in a processor 36 (not shown in detail). As shown in the drawing, the diffraction grating 33 is a concave grating with the housing of the analyser 4, which defines the slits 29 to 32, describing the Rowland circle of the diffraction grating, and the image of the array 35 in the mirror 34 lying substantially along the Rowland circle, but other means of focussing the diffracted light onto the grating 33 may be used instead.

In operation, the actuator 13 positions the mirror 12 to select the beam from one of the two lamps, the actuator 17 positions the appropriate filter 16 in the beam to select a desired band of frequencies from the light of that lamp, and the actuator 21 positions the sliding mirror 20 to direct the beam onto the grating 33 through an appropriate one of the slits 29 to 32. The arrangement is such that an appropriate portion of -li¬

the diffracted light of a desired order of diffraction from the grating falls on the array 35. There may be equal numbers of filters 16 and slits 29 to 32, with each filter passing a band of frequencies that corresponds to the frequencies diffracted from a respective one of the slits 29 to 32 onto the array 35. Instead or in addition, light may be directed through two different filters and a single one of the slits 29 to 32, causing light of different bands of frequencies and respective different orders of diffraction to fall on the array 35. In any case, the filters 16 are used to prevent diffracted light of more than one order of diffraction from falling on the array at one time. If a spectrum over a wider range of frequencies is required, the process may be repeated for different frequency bands and the results combined subsequently. Because the full operating frequency range of the instrument can be divided into a number of relatively narrow bands of frequencies, each of which can be spread over the entire array 35, each photodiode can be exposed to a correspondingly small frequency range and the resolution of the results obtained is correspon¬ dingly increased compared with an instrument in which the same array had to cope with the entire working range of the apparatus at once. It is also comparatively easy to ensure that the unwanted zero- order and higher-order beams from the diffraction grating within the frequency range that is transmitted by the appropriate filter 16 are reflected away from the array 35 or absorbed by suitably placed collectors. Any desired absorbers and/or shutters for the unwanted beams may also be provided in and around the selector 3.

The only moving parts of the optical system that can appreciably affect the path of any of the light beams are the sliding mirrors 12 and 20. Because they slide in their own planes and their exact position does not matter, so long as they block and unblock the correct beams, it is not necessary to achieve a high degree of precision in the operation of the actuators. The mirrors 12 and 20 must in general not come to rest with a beam of light partly falling on and partly missing either of those mirrors, but the normal resting positions of the mirrors can be selected so as to be a considerable distance from any position in which that might occur. Even the path along which the mirror 20 moves is not important provided that it does not deviate so much from the correct orientation that the reflected beam from the mirror 22, 23, or 24 does not reach the slit 30, 31, or 32, and the length of the mirror 20 makes its orientation very easy to control. Although the mirrors 12 and 20 are described as sliding, they may be provided with rollers or other means to facilitate their movement. Instead of being a sliding mirror, the mirror 12, at least, may be pivotable about an axis perpendicular to its plane.

Referring now to Fig. 2 of the drawings, the second form of spectrophotometer includes a radiation source (not shown) which may be the same as the radiation source 1 shown in Fig. 1. The beam of light from the source 1 comes to a focus, as described above, within a sample compartment 2 which may be conventional. ° The diverging beam of light emerging from the sample compartment 2 falls on a three-element achromatic lens 41 which causes it to converge. The converging beam from the lens 41 falls on a neutral- density beam splitter 42. The reflected beam from the beam splitter 42 falls on a dichroic mirror 43 by which blue light is reflected onto a path parallel to the transmitted beam from the beam splitter 42 (which serves as a beam of ultraviolet light) , while red light is transmitted. The transmitted red beam from the dichroic mirror 43 falls on a plane mirror 44 which reflects it into a path parallel to the other two beams. The three parallel beams from the beam splitter 42 and the mirrors 43 and 44 fall on slits 45 to 47, respectively, carried on a common mount 48. The mirrors and slits are so positioned that each beam is focussed by the lens 41 on its respective slit. The slits 45 to 47 are associated with shutters (not shown) controlled by actuators (not shown) which may be of conventional design, and which, together with the slit mount 48, exclude from the analyser all light emerging from the sample compartment 2 except for a selected beam. The three beams from the slits 45 to 47 fall on plane mirrors 49 to 51, respectively, that direct each beam onto a concave holographic grating 52 from which the first order refracted light in a selected frequency range from each beam is focussed onto a fixed array .53 of photodiodes the outputs from which are processed electronically in a processor 54. The centre of the array 53 is on the central axis of the grating 52.

The array of photodiodes may be a Hamamatsu Linear Image Sensor Type 52301-512Q having 512 photodiodes, and the effective useful wavelength ranges for the three beams may be 190-370 n (UV beam) , 370-575 nm

(blue) , and 575-800 nm (red) , so that each photodiode is exposed to a range of wavelengths of no more than 0.5 nm from each beam. It will be appreciated that the actual ranges may extend slightly beyond those specified in order to provide some overlap.

As an example of suitable dimensions for the system, the optical path lengths from the light sources to the toroidal mirror 11 may be about 18.75 cm, from the toroidal mirror to the sample focus about 24.25 cm, and from the sample focus to the centre of the achromatic lens 41 about 10.5 cm. The path length along each beam from the achromatic lens 41 to the respective slit 45, 46, or 47 may be about 10 cm, from each slit to the centre of the grating 52 my be about 27 cm, and from the centre of the grating 52 to the centre of the array 53 may be about 6.5 cm. As will be seen from Fig. 2, the three beams are so arranged that the path lengths from the beam splitter 42 to the grating 52 are substantially equal. It will be appreciated that the exact paths for perfect focussing of the three beams by the lens 41 onto the slits 45 to 47 and for perfect focussing by the grating 52 from all three slits onto the array 53 must be found by calculation or experiment depending on the strengths of the various optical elements, and that the three beams may need to have slightly different path lengths before and/or after the slits 45 to 47. In particular, the correct optical path length from a slit to the grating will usually depend on the angle of incidence of the beam from that slit onto the grating.

As with the first form of spectrophotometer, the number of slits and respective beams may be made more or less than three and/or diffracted light of more than one order from a single beam may be used, although it will be appreciated that the use of successive neutral- density beam-splitters would result in a corresponding reduction in the intensity of the light falling on the array 53. The second form of spectrophotometer has, however, the advantage that there are no moving parts at all, except for shutters, in the optical paths downstream of the sample compartment 2. The operation of the second form of spectrophoto¬ meter is very similar to that of the first form. A beam of light of suitable spectral composition is produced by the radiation source, and the shutter associated with a selected one of the slits 45 to 47 is opened while the shutters associated with the other slits are closed, so that a single beam of light falls onthe grating 52 from a selected direction, and diffracted light of a desired order of diffraction of a corres- ponding range of frequencies falls on the photo-diodes in the array 53.

Claims

hat we claim is : -

1. A spectrophotometer comprising an array of detectors; means for dispersing a beam of radiation and causing the dispersed beam to fall on the array of detectors; and means for selectively directing a beam of radiation to the dispersing means from any of a plurality of directions.

2. A spectrophotometer as claimed in claim 1, wherein the array of detectors is an array of photodiodes.

3. A spectrophotometer as claimed in claim 1 or claim 2, wherein the dispersing means is such as to cause to fall onto the array of detectors radiation of a respective range of frequencies from each said direction, every frequency within a larger range being included within at least one said respective range.

4. A spectrophotometer as claimed in claim 3, wherein the said respective ranges cover the said larger range with little or no overlap.

5. A spectrophotometer as claimed in any one of claims 1 to 4, wherein the dispersing means is a diffraction grating.

6. A spectrophotometer as claimed in claim 5, wherein the arrangement is such that, for a given range of frequencies, the diffraction grating causes only first-order diffracted light to fall on the array of detectors.

7. A spectrophotometer as claimed in claim 5 or claim 6, wherein the part of the dispersed beam that falls onto the array of detectors leaves the diffraction grating in a direction generally perpen¬ dicular to the surface of the diffraction grating.

8. A spectrophotometer as claimed in any one of claims 1 to 7, which comprises collimating means aligned on the said dispersing means from each of the said plurality of directions.

9. A spectrophotometer as claimed in any one of claims 1 to 8, wherein the directing means comprises a movable mirror, advantageously, a plane mirror which is preferably mounted for translation in its own plane and is especially preferably slidably mounted.

10. A spectrophotometer as claimed in claim 9, wherein the movable mirror is movable into and out of the path of at least one beam of radiation, preferably such that the direction of movement of any point on its reflecting surface is parallel to a tangent to that surface at that point.

11. A spectrophotometer as claimed in any one of claims 1 to 8, wherein the directing means comprises at least one dichroic mirror or other beam-splitter.

12. A spectrophotometer as claimed in any one of claims 1 to 11, which comprises means for admitting to the directing means only radiation having frequencies within a desired range of frequencies, preferably, for admitting radiation of any of a plurality of ranges of frequencies each corresponding to a respective said direction.

13. A spectrophotometer as claimed in any one of claims 1 to 12, which comprises a source of radiation, preferably, a source of visible and/or ultraviolet light, means for guiding a beam of radiation from the source to the selectively directing means, and means for so locating a sample that, in use, radiation passes from the source to the selectively directing means through the sample.

14. A method of obtaining a spectrum of radiation using a spectrophotometer as claimed in any one of claims 1 to 13.

15. A method as claimed in claim 14, which comprises successively directing said radiation to the dispersing means from at least two said directions and obtaining spectra of respective ranges of frequencies.