GB2208430A

GB2208430A - Monochromators

Info

Publication number: GB2208430A
Application number: GB8717542A
Authority: GB
Inventors: Aleksandrs Termanis
Original assignee: Hilger Analytical Ltd
Current assignee: Hilger Analytical Ltd
Priority date: 1987-07-24
Filing date: 1987-07-24
Publication date: 1989-03-30
Also published as: GB8717542D0

Abstract

A scanning monochromator has a linear array of optical sensors 26, a secondary light source, and an optical path 27 from the light source to the sensors via either the diffraction grating 12 of the monochromator or a further grating 20 fixedly mounted with respect thereto. As the selected wavelength of the spectrum of the main emission source is altered, the diffracted light from the second light source scans across the optical sensor array 26 and thereby changes the output, so that an accurate indication of the current position of the monochromator grating 12 can be obtained. Light from the main and secondary sources passes through a slit 8, is collimated by a mirror 10, is reflected by the gratings 12 and 20 and is focussed by a mirror 16 onto an extra slit 14 and a deflecting prism 24, respectively. <IMAGE>

Description

MONOCHROMATORS This invention relates to monochromators.

In many applications employing a scanning monochromator it is important to be able to control the monochromator in order to select precisely and reproducibly a specific wavelength position on a spectrum. This is particularly desired in emission spectrophotometry, and especially in scanning ICP AES (inductively coupled plasma-atomic emission spectroscopy) systems.

One method commonly used for positioning relies on so-called "peak searching", ln whion a predetermined spectral window is scanned until a selected peak is located. This involves a number of disadvantages: (a) If the peak is weaker than a nearby peak, the peak search method can result in the system latching on to the wrong peak.

(b) If the selected peak is missing (and it should be noted that the monochromator may be employed to determine whether a peak is present), similar consequences to those outlined in (a) could arise.

(c) The scanning of the spectral window to search for a particular peak results in a delay in reaching the selected spectrum position.

Another method used in monochromators in which a diffraction grating is rotated to alter the selected spectrum position involves a high quality angle encoder to indicate the precise position of the grating.

However, other parts of the optical system are not monitored and therefore displacements caused by, e.g., temperature changes affect the accuracy of the system.

According to one aspect of the present invention there is provided a monochromator in which radiation is directed along a path through the monoch-omc or to an array of sensors, the path being such that as the selected spectrum point of the monochromator is altered the radiation scans across the sensor array and thereby changes the output of the array. The output can therefore be used to indicate the selected spec'rum position of the monochromator.

The radiation is preferably diffracted, either by the main diffraction grating of the monochromator or by a different diffraction grating mounted in a predetermined position with respect to the main diffraction grating, and the diffracted light is then directed to the sensor array such that at least one of the diffraction orders is incident on the array. The advantage of this is that it increases the angular range over which the sensor array is effective to indicate the selected spectrum point without requiring a long sensor array, because as the spectrum of the main emission source of the monochromator is scanned, successive diffraction orders scan across the sensor array.

Preferably, the position of the monochromator can be controlled by electro-mechanical means, such as an angle encoder or a simple servo-motor, to within a certain angular range which is sufficient to ensure that a given diffraction order is incident on the diode array, and the fine control of the monochromator position can then be achieved in response to the output of the array. For any given spectrum point, the output of the array may not be unique, because there may be another spectrum position where a different diffraction order is incident at substantially the same position on the array. Thus the array output may be used simply as an indication of the precise position of the selected spectrum point within a range determined by other means.

There may be an automatic control system for altering the selected scanning point until the output of the sensor array indicates that a predetermined point has been reached.

The monochromator may have control means responsive to the position within the array of a sensor producing the maximum output for determining the selected spectrum point. However, in a preferred embodiment of the invention, greater accuracy of control is achieved by making the control means responsive to the relative outputs of at least two, and preferably three, adjacent sensors in the array.

An arrangement embodying the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic view showing the main optical components in a scanning monochromator in accordance with the present invention; Fig. 2 shows the optical paths of a main beam and a position-indicating beam adjacent the exit slit of the monochromator; and Fig. 3 is a graph illustrating exemplary outputs of some of the sensors in an array in the monochromator of Fig. 1.

Fig. 1 illustrates the main optical components of a Czerny-Turner type of plane grating monochromator 2. The main axis of the instrument is indicated at 4, and the path of light from a main emission source is indicated at 6. The light diverges from an entrance slit 8, and is collimated by a collimating mirror 10. The collimated light is dispersed by a diffraction grating 12, and subsequently focussed at an exit slit 14 by a focussing mirror 16.

The diffraction grating 12 can be rotated about a substantially vertical (as shown in Fig. 1) axis 18, as indicated by the arrow A. This causes the dispersed image to scan across the exit slit 14, and thus alters the wavelength of the light passing through the exit slit 14. Thus, by turning the diffraction grating 12 to a selected angular position, a predetermined wavelength point on the emission spectrum of the main emission source can be selected.

The arrangement so far described is standard. In addition, the monochromator 2 is provided with a positlon-indicating means which will be described below.

A secondary diffraction grating 20 is mounted in a predetermined position with respect to the main diffraction grating 12, and is preferably fixed thereto. The monochromator has a position-indicating radiation source (not shown), which may for example be a low-power laser, such as a He-Ne laser. Light from the laser passes through the entrance slit 8 and along a path 22. The light fans out in a horizontal (as seen in Fig. 1) direction. The light is collimated by the mirror 10, dispersed by the diffraction grating 20, and reflected by the focussing mirror 16. The laser beam from the mirror 16 is directed onto a reflector 24 located adjacent the exit slit 14, and is then reflected onto an array 26 of sensors.As seen in Fig. 2, the exit slit 14 is disposed in a plane defined by the line A'-A" and the normal to the drawing, and the array is disposed in a plane defined by the line B'-B" and the normal to the drawing. The array is a linear array with the sensors arranged successively along the direction which is normal to the drawing.

It will be appreciated that as the main diffraction grating 12 is rotated, the secondary grating 20 is also rotated and thus the image which is focussed onto the sensor array by the focussing mirror 16 is scanned along the length of the array. This image is formed of successive diffraction orders produced by the grating 20.

The spatial frequency of the grating 20 is chosen such that at least one diffraction order is incident on the array. If the main grating 12 is intended for use with ultraviolet light, it is likely that the grating 20 will be substantially coarser than the grating 12. For example, the grating 12 may have approximately 2400 to 3600 grooves per millimetre, and the grating 20 approximately 20 grooves per millimetre. If, however, the grating 12 is intended for analysis of spectra in the infrared region, the spatrial frequency of the gratings 12 and 20 may be substantially the same. It will be appreciated therefore that in such circumstances the grating 20 may not be necessary, and the grating 12 can be employed for diffraction both of the main beam and the radiation from the position-indicating source.

Fig. 3 shows a typical intensity distribution across some of the sensors of the array. In the graph of Fig. 3, the ordinate represents the digitised output voltages of the sensors (in units which are linearly related to voltage such that 4096 units corresponds to approximately 6 volts), and the abscissa represents the sensor number.

The sensor with the maximum output is sensor n, and the output voltages fall with distance from this sensor.

It will be appreciated that, as the grating 12 is rotated to shift the wavelength of the light passing through exit slit 14, the light incident on the sensor array will shift, so tat the sensor producing the maximum output will also change. It would be possible to fine-tune the monochromator in response simply to the number n of the sensor producing the maximum output, so that a wavelength could be selected by adjusting the monochromator so that the appropriate diffraction order is incident on the sensor array, and then shifting the diffraction order along the array until the appropriate sensor n generates the maximum output.

However, in the preferred embodiment, an additional level of control is used whereby small adjustments to the angular position of the gratings 12 and 20 are made until the relative voltages of at least a pair of, and preferably three for greater positional reproducibility, adjacent sensors correspond to predetermined ratios.

This produces a finer and more accurate control of the monochromator. Preferably, the -voltages of the sensors immediately adjacent the sensor generating the maximum output are utilised. By considering the relative voltages, which can be achieved by normalising the values to the maximum output value, variations in the intensity of the radiation beam have substantially no effect on the system.

Calibration of the apparatus can be achieved by tuning the monochromator to a predetermined wavelength musing a known emission source, and recording the pattern of output voltages of a group of sensors of the array, and the position within the array at which this pattern occurs. Subsequent location of tis wavelength point can be achieved by shifting the gratings 12 and 20 until a substantially identical output is generated by the sensors.

A scanning monochromator in accordance with the invention can control the spectrum position with great accuracy. Assuming that the focal length of the monochromator is 1 metre, it is possible to control the angular position of the gratings 12 and 20 to within an accuracy of 2 to 3 arc seconds. This could be achieved in a conventional manner using a stepper motor in an open loop mode, i.e. just counting the steps to determine position. Assuming that the sensors are positioned near the exit slit with a spacing of 13 ym, it is possible then to fine-control the positioning of the gratings to select any arbitrary wavelength with an accuracy of 0.005 nm just by taking into account the position of the sensor producing the maximum output. This will enable most unknown spectral lines to be identified with a reasonable degree of certainty.By additionally taking into account the relative outputs of adjacent sensors, it is possible to achieve a positioning accuracy of 0.2 arc seconds, corresponding to a wavelength error of plus or minus 7.5 x 10-4 nm at 300 nm (using a 2400 grooves/mm grating).

The sensors are preferably formed from photodiodes, preferably arranged as a charge-coupled device array.

Alternatively, common diode arrays can be used. The array may include for example 2048 diodes.

The gratings may be rotated by a stepper motor (not shown). This may be arranged so that each arc second of rotation requires, e.g., about 7 steps. In the preferred embodiment, the gratings are rotated at a substantially constant speed until the positioning is controlled in response to the output of the sensor array, and the fine control of the positioning then occurs as the stepper motor decelerates.

The above embodiment is particularly advantageous because the radiation beam used to indicate the selected position on the emission spectrum follows substantially the same optical path as the beam from the main emission source, including optical elements 10 and 16, which in this case are mirrors but which could be lenses. This means that slight shifts in the positions of these elements are taken into account.

Various alternatives to the arrangement described above are possible.

The exit slit and the sensor array could be movable during scanning, instead of the diffraction gratings.

The main dispersing means may be a prism instead of the grating 12. Means such as a shutter may be provided for selectively blocking off the grating 20 to prevent light from the main emission source from reaching the grating, as reflections from the grating 20 would detrimentally affect the output from the exit slit. The grating 20 could be replaced by some other optical means for redirecting radiation along a pat to the sensor array.

It would, for example, be possible to use a reflector.

However, unless special provisions were to be made to produce multiple images (e.g. a multifacetted device such as a polygon), this would mean that there would be only a single reflected image so that the optical sensor array would have to be differently positioned, or much longer, if it were to be effective over a wide angular range.

It is possible also to envisage arrangements in which the light from the main emission source is directed to the optical sensor array for generating a signal indicative of position, thereby avoiding the need for a separate radiation source. In this case, if the light is to be diffracted to produce different diffraction orders which can successively scan across the sensor array, means are preferably provided for allowing only predetermined wavelengths known to be present in the emission spectrum to reach the array.

It would also be possible to have the sensor array fixed with respect to the main dispersing means of the monochromator, and arrange for the incident radiation beam to scan across the sensors as the monochromator is adjusted.

Although the term light is occasionally used herein, it will be appreciated that tis term is used in a broad sense to cover any radiation within the electromagnetic spectrum.

The invention has been described primarily in connection with monochromators for spectrophotometry of emission spectra, but it is clearly applicable also to absorption spectrophotometry, or indeed anywhere that precise control of monochromators is requIred.

Claims

CLAIMS:

1. A scanning monochromator in which a spectrum can be scanned to select a particular point thereon, and in which such scanning causes a radiation beam to traverse an array of radiation sensors so that the output of the array can be used in determining whether said particular point has been selected.

2. A monochromator as claimed in claim 1, which has dispersing means for dispersing radiation from an emission source to form said spectrum, and an exit slit, the position of the exit slit with respect to the dispersed radiation determining the selected point on the spectrum, wherein the array of sensors is mounted in a position which is fixed with respect to the exit slit during scanning of the monochromator.

3. A monochromator as claimed in claim 2, wherein the dispersing means or a member which is fixedly positioned with respect thereto during scanning is arranged to direct the radiation beam along a path to the sensor array.

4. A monochromator as claimed in claim 3, including means for diffracting the radiation beam which traverses the sensor array.

5. A monochromator as claimed in claim 4, wherein the diffracting means is formed by the dispersing means.

6. A monochromator as claimed in claim 4, wherein the diffracting means is formed by said member which is fixed with respect to the dispersing means during scanning.

7. A monochromator as claimed in claim 6, wherein the dispersing means is a first diffraction grating and the member forms a second diffraction grating which is coarser than the first diffraction grating.

8. A monochromator as claimed in any one of claims 2 to 7, wherein the dispersing means is rotatable for selection of a spectrum point.

9. A monochromator as claimed in any preceding claim, further including a radiation source for generating the radiation beam.

10. A monochromator as claimed in claim 9, wherein the source is operable to emit radiation having one or more discrete, narrow wavebands.

11. A monochromator as claimed in claim 10, wherein the radiation source is a laser.

12. A monochromator as claimed in any preceding claim, wherein the sensors are charge-coupled devices.

13. A monochromator as claimed in any preceding claim, including means responsive to the position within the array of a sensor producing a maximum output for generating a signal indicative of the selected spectrum point.

14. A monochromator as claimed in any preceding claim, including means responsive to the relative outputs of at least two adjacent sensors within the array for generating a signal indicative of the selected spectrum point.

15. A monochromator as claimed in claim 14, wherein the means responsive to the relative outputs of at least two adjacent sensors is responsive to the output of a sensor generating a maximum output and to the outputs of the sensors on each side thereof.

16. A monochromator as claimed in any preceding claim, including means for altering the selected spectrum point until the output of the sensor array indicates that a predetermined point has been reached.

17. A monochromator substantially as herein described with reference to the accompanying drawings.