GB2269230A

GB2269230A - Measuring light wavelength.

Info

Publication number: GB2269230A
Application number: GB9216307A
Authority: GB
Inventors: Stephen Vivian Charles Ramm
Original assignee: SINAR AGRITEC Ltd
Current assignee: SINAR AGRITEC Ltd
Priority date: 1992-07-31
Filing date: 1992-07-31
Publication date: 1994-02-02
Also published as: GB9216307D0

Abstract

A device for developing a signal related to the wavelength of substantially monochromatic light comprises means to direct the light through a filter 12 onto a sensor 7, the filter having a known (e.g. linear) wavelength transmission characteristic. The output of the sensor is supplied to calculating means 25 associated with a memory 26 which contains data about the characteristic. The calculating means calculates a signal related to the wavelength of the incident light. The device is particularly suitable for use with a monochromator, and may provide a signal to a motor 31 for adjusting a monochromator element (grating or prism). A reference signal representative of light intensity is provided by a further sensor 6. A calibration signal is provided by a further filter 13 having known absorption peaks, with a sensor 8. Temperature compensation is provided by a thermistor 17. <IMAGE>

Description

DESCRIPTION OF INVENTION "IMPROVEMENTS IN OR RELATING TO A DEVICE FOR DEVELOPING A SIGNAL RELATED TO THE WAVELENGTH OF INCIDENT LIGHT" THE PRESENT INVENTION relates to a device for a developing a signal related to the wavelength of incident light and, in one embodiment, relates to a device adapted to develop an output signal indicative of the wavelength of incident light.

Monochromators comprise a source of light adapted to produce light of a substantially single wavelength, the wavelength being controllably adjustable. Thus, a monochromator may be employed, for example, when measuring a continuous absorption spectrum of a sample for analytical purposes. Monochromators also have other uses.

Various types of monochromator exist including monochromators which employ different types of active optical element, such as a diffraction grating of some sort, a prism or similar dispersive element, or filters or etalons. The monochromators utilise a multi-frequency light source, such as an incandescent bulb.

All of these monochromators achieve a change in wavelength of light exiting the monochromator by moving the optical element relative to the light path (or by moving the light path relative to the optical element). For example, a plane reflection grating may be turned, an interference filter may be tilted, or an interference filter of variable wavelength may be moved across the light path.

In present-day monochromators the wavelength at the exit slot or exit port of the monochromator is predicted from the relative position of the optical element using some form of prediction equation.

The existing monochromators have proved to be less than totally accurate with regard to the prediction of the wavelength present at the exit slot or port, especially when there are changes in the monochromator optics or when the position measuring device used to measure the relative position of the optical elements and the light beam develops inaccuracies which causes the prediction of the wavelength at the exit slot to be no longer valid.

Inaccuracies may arise if the components are moved, due to shock, vibration, temperature change or other environmental changes. Also, wear in mechanical parts, such as bearings, may render the position measuring device inaccurate.

A further factor that can lead to errors is the change in the optical system that will probably take place when a component is replaced due to failure, and in particular when a light source, such as an electric lamp, is changed. In a typical monochromator which employs a tungsten-halogen bulb as a light source, the bulb may fail at relatively frequent intervals during use of the monochromator, requiring replacement of the bulb, usually by an unskilled person. It may prove to be undesirably expensive to achieve the necessary mechanical accuracy in replacing the bulb.

In some cases the actual movement of the optical element may be such that a measurement of its position cannot be made reliably to the required resolution necessary to provide an accurate prediction of wavelength at the exit slot or, alternatively, the cost of so doing may be prohibitive.

The present invention seeks to provide a device adapted to develop a signal related to the wavelength of incident light, thus effectively enabling the measurement of the wavelength of light exiting a monochromator independently of the monochromator optics. In one apparatus embodying features of the invention, the wavelength of light exiting a monochromator is measured, and a signal is created which is directly indicative of the wavelength of the light. Thus the signal which is utilised as indicating the wavelength of the light is not derived from any measurements made of the relative position of the monochromator optics.

According to this invention there is provided a device for developing a signal related to the wavelength of substantially monochromatic incident light comprising means to direct the light through a filter on to a sensor adapted to detect the light, the filter having a predetermined output characteristic which is related to the wavelength of the incident light, the output of the sensor being supplied to calculating means associated with a memory which contains data about the said characteristic, the calculating means being adapted to calculate a signal related to the wavelength of the incident light. The signal may be developed in real time, that is to say sufficiently quickly that it is suitable for measuring changing wavelengths.

Preferably the calculating means is adapted to determine the wavelength of the incident light and to provide an output representative of the wavelength of the incident light.

Conveniently the said filter is an absorption filter with a predetermined relationship between the proportion of light falling on the filter which passes through the filter and wavelength.

Advantageously the relationship between the proportion of light which is transmitted by the filter and wavelength is substantially linear.

Preferably the filter is a red glass filter.

Advantageously the filter is such that 50% of the incident light is transmitted through the filter when light of substantially 1,000nm is incident upon the filter.

In a modified embodiment the filter comprises an interference filter, the sensor being adapted to count fringes as the wavelength of incident light changes.

Preferably the sensor comprises two detector means adapted to detect fringes "in quadrature".

Conveniently means are provided to sense the temperature of at least the filter, the calculating means being adapted to correct for changes in the characteristic of the filter or the combination of the filter and the sensor which are related to temperature.

Advantageously a thermistor is mounted in thermal contact with the filter, the thermistor forming part of a resistive array, a reference voltage being applied to the resistive array and a further voltage being derived from the resistive array which is indicative of the temperature of the thermistor.

Preferably a further sensor is provided which senses the intensity of incident light, the output from the sensor associated with the filter being compared with the output of the sensor which senses the intensity of the incident light.

Advantageously the output of the sensor associated with the filter and the output of the sensor to which senses incident light are both supplied to means which produce an output signal representative of the ratio between the two signals.

Conveniently the output of the sensors is supplied to a ratio-metric analogue-to-digital converter the output of which is passed to a microprocessor which constitutes the said calculating means.

Advantageously a further sensor is provided, adapted to receive incident light which is passed through a further filter, the further filter having predetermined absorption peaks at predetermined optical wavelengths, means being provided to determine when the incident light is at a wavelength equal to the wavelength of one of said peaks, means being provided to correct a value for wavelength calculated from the first filter.

In one embodiment the output of the calculating means simulates the output of an incremental shaft encoder.

The device may be in combination with a monochromator, the said calculating means being adapted to control the movement of an active optical element in the monochromator.

In one embodiment the output of the calculating means is passed to a digital-to-analogue converter having two outputs which are each connected, through an operational amplifier, to a motor which drives the active optical element of the monochromator.

The device may be provided with means to sample a signal from an external detector, and to pass the signal to the calculating means.

The or each sensor or filter may be associated with thermostatically controlled means to maintain a substantially constant temperature.

The invention also provides a method of measuring the wavelength of substantially monochromatic incident light comprising the steps of directing the light through a filter on to a sensor adapted to detect the light, the filter having a predetermined output characteristic which is related to the wavelength of the incident light, the output of the sensor being supplied to calculating means associated with a memory which contains data about the said characteristic, the calculating means being adapted to calculate a signal related to the wavelength of the incident light.

In one embodiment the calculating means calculate the wavelength of incident light and the rate of change of wavelength of incident light and produce a signal indicative of the wavelength of incident light at the end of the calculation generating the said signal.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which, FIGURE 1 shows the exit slot of a monochromator showing a supplementary exit, FIGURE 2 is a top plan view of the exit slot shown in Figure 1 illustrating a sensor arrangement associated with the supplementary exit, FIGURE 3 is a schemmatic block diagram illustrating the sensor arrangement of Figure 2 and associated electrical circuitry, including a motor adapted to drive an operative part of a monochromator, and FIGURE 4 is a plan view of a modified embodiment of the invention.

Referring now to Figures 1 to 3 of the accompanying drawings a monochromator has an exit slot and is adapted to generate light, which at any instant is of a substantially constant wavelength, which exits through the exit slot.

The wavelength of the light is adjustable and the monochromator may operate to provide light with a wavelength which changes over a period of time. Thus, the light at the exit slot may "scan" across a predetermined wavelength range.

Figure 1 illustrates a plate 1 which defines the exit slot 2 of the monochromator. The plate 1 also defines, however, a supplementary exit slot 3.

Figure 2 is a top plan view of the plate 1, and it is to be noted that, aligned with the exit slot 3 is a mirror 4 which serves to direct light passing through the supplementary exit slot 3 to a sensor array 5. The mirror is a reflecting plate which is coated with a diffusely reflecting material.

The sensor array 5 is illustrated in greater detail in Figure 3. The sensor array 5 comprises three lightsensitive detectors 6, 7 and 8. Each detector is a silicon PIN diode, and each detector is associated with a respective filter 9,10,11, adapted to block out light of a wavelength shorter than 800nm so as to block out second order diffracted light from a grating present within the monochromator.

The sensor 6 is a reference sensor and is located so that light reflected from the mirror 4 passes directly through the associated filter 9 to the sensor 6.

The sensor 7 is associated with a filter 12 which is so located that light from the mirror 4 passes through the filter 12 before passing through the filter 10 and impinging on the sensor 7. The filter 12 is a coloured glass filter which has a monotonic and increasing transmission with increasing wavelength in the range of 800nm to 1,100nm. Thus the proportion of light falling on the filter which passes through the filter increases with increasing wavelength. The relationship between the proportion of light which is transmitted and wavelength is substantially linear. The filter may be of a type known as "RG 1,000" which is a red glass filter in which the middle of the transmission range (that is to say the part of the range where 50% of the incident light is transmitted through the filter) is located at 1,000nm.A typical filter of this type is temperature-sensitive, and the curve, which is substantially linear, which relates intensity of transmitted light to wavelength may shift by 0.38 nm per OC.

The third sensor 8 is associated with a filter 13 which is so located that light from the mirror 4 passes through the filter 13 before passing through the filter 11 and impinging on the sensor 8. The filter 13 is a filter, termed a didymium glass filter, which has pronounced absorption peaks at predetermined points in the optical spectrum. In this example the absorption peaks are located at substantially 807nm and 879nm, with a further less pronounced peak being located at approximately 1,068nm. It is a property of absorption filters of this type that the three absorption peaks are invariant in position, (in other words the peaks always occur at the same wavelength), and do not vary with the temperature of the filter, and.also do not vary with the age of the filter. The absorption peaks are thus at precisely predetermined optical wavelengths.

The output from each of the photo-detectors 6,7,8 is in the form of a current that is converted into an appropriate reference voltage using high impedance low noise operational amplifier arrangements. Thus, detector 6 is associated with an operational amplifier arrangement 14, and which produces an output signal in the form of a voltage Va Similarly the detector 7 is associated with an operational amplifier arrangement 15 which produces an output signal in the form of a voltage Vb.

Finally, detector 8 is associated with an operational amplifier arrangement 16 which produces an output signal in the form of a voltage Vc A thermistor 17 is provided which is mounted in a unitary housing which contains the various detectors 6,7,8 and the filters 9 to 13 discussed above. The various components may be mounted on a single substrate of thermally conducting material, such as a metal substrate.

The thermistor 17 is thus subjected to the same temperature as the temperature of the detectors and the filters. The thermistor 17 is associated with other resistors which form a resistive network 18 associated with a further operational amplifier 19, part of the resistive network being supplied with a reference voltage Vr The output of the operational amplifier 19 is a voltage Vt which is a voltage which has a value related to the resistance of the thermistor 17 and which is thus related to the temperature of the sensors and the filters. One particular resistive network 18 is illustrated schematically, but other resistive arrangements may be utilised.

The output of the operational amplifier 16, carrying the voltage Va, is connected to one input of a multiplexer 20, another input of which, 21, receives the reference voltage Vr as supplied to the operational amplifier 19. The output of the multiplexer 20 is connected to a sample and hold circuit 22, the output of which is connected to the reference input of a 16 bit twochannel ratio-metric analogue-to-digital converter 23.

The output, voltage Vb, from the operational amplifier 15 is connected to a further input of the analogue-to-digital converter 23.

The voltage Vc from the operational amplifier 16, the voltage Vt from the operational amplifier 19 and a further voltage Vx are all connected to inputs of a multiplexer 24. The output of the multiplexer 24, which is at any instant a selected one of the inputs, is connected to another input of the analogue-to-digital converter 23.

The signal supplied by the sample and hold circuit 22 is either the voltage Va, which is proportional to the intensity of incident light, or the reference voltage Vr The analogue-to-digital converter 23 is adapted to provide a digital output representative of the ratio of (or a comparison between) the signal supplied by the sample and hold circuit 22 and either the signal from the operational amplifier arrangement 15 or the signal from the multiplexer 24.

The analogue-to-digital converter 23 is connected to a microprocessor 25 which also controls the operation of the components described above.

The microprocessor 25 is connected to a Read-Only Memory 26, which may be constituted by an EPROM and a second Read/Write Memory 27. An output of the microprocessor is connected to a digital-to-analogue converter 28 having two outputs each connected to a respective operational amplifier 29,30, the outputs of the amplifiers being connected to drive a motor 31. The motor 31 is mechanically connected to an optical component, such as diffraction grating, present within the monochromator, to drive the optical component to cause the wavelength of light exiting through the exit slot 2 to be varied.

Data is stored in the Read-Only-Memory 26. This data comprises the operating programme for the microprocessor and also information concerning the transmission characteristic of the filter 12, and also temperature coefficients which enable the microprocessor to calculate the transmissivity of the filter 12 at any temperature and at any specific wavelength. Since the sensors may also be temperature sensitive, data relating to the temperature coefficients related to the output of the sensors may also be stored in this memory. If the detectors 6,7,8 are temperature responsive, appropriate data may be present in the memory 26.

The Read/Write Memory 27 is utilised during operation of the microprocessor to store data that is needed as the microprocessor performs the necessary calculations.

The microprocessor 25 may be adapted to provide an output to a digital-to-analogue converter 28, which supplies signals, through operation of amplifiers 29 and 30, to a motor 31 which moves an optical element, such as a grating or a prism to change the wavelength of light from the monochromator. Motor speed may thus be controlled by the microprocessor to produce the required rate of change of wavelength.

In a first operational state the temperature of the detectors and the filter is determined. The multiplexer 20 is switched so that the reference voltage Vr present on input 21 is passed to the sample/hold circuit 22 and is thus subsequently passed to the analogue-to-digital converter 23. The multiplexer 24 is also switched so that the voltage Vt present on the output of the operational amplifier 19 is fed to the analogue-to-digital converter 23.The microprocessor 25 thus receives a signal, from the analogue-to-digital convertor 23, which represents the ratio between reference voltage Vr and temperature dependent voltage Vt, which is itself a function of Vr Thus the ratio is independent of the instantaneous value of Vrt and is consequently dependent solely on the resistive values present in the resistive network 18 and the temperature of the thermistor 17. The microprocessor 25 can thus perform the appropriate calculations to determine the temperature of the thermistor 17 and consequently the temperature of the sensors and the filters. It is to be appreciated that any variation in reference voltage Vr is effectively cancelled out.

Once the temperature of the detectors and the filters have been determined by the process outlined above, an appropriate value is stored in the Read/Write Memory 27 for future use by the microprocessor. Thus the microprocessor is able to select, from the Read Only Memory 26, details of the transmission characteristic of the filter 12 at the appropriate temperature, and details of the performance of the sensors at that temperature.

In a second operational state, under the control of the microprocessor 25 a signal Vb from the sensor 7, which is associated with the RG 1,000 filter 12 is supplied to the analogue-to-digital converter 23 and, simultaneously, the signal Vat from sensor 6 is passed, by the multiplexer 20, to the sample/hold circuit 22 and thus to the analogue-to-digital converter 23. The output of the analogue-to-digital converter 23 is then the ratio between the signal Va and the signal Vb. This ratio is not Va dependent upon the intensity of the incident light. The ratio is dependent principally upon the absorption of the filter 12, but may be influenced by slight differences in temperature related characteristics of the sensors.Thus the microprocessor, since it has available to it information concerning the temperature of the filter 12 and the sensors, and the optical characteristics of the filter 12 at that temperature, together with details of the performance of each of the sensors at that temperature, can calculate or otherwise determine the wavelength of the incident light.

From time-to-time, at regular intervals (especially if the wavelength of the incident light is changing) the multiplexer 24 causes the signal Vc from the sensor 8 to be applied to the analogue-to-digital converter 23, and the analogue-to-digital converter 23 generates an output which is representative of the ratio of the signal V a and the signal Vc Again, it is to be noted that the signals both represent incident light of the same intensity, and so the absolute value of the intensity of the light will not alter the ratio. The only factor that effectively alters the ratio is the transmission characteristic of the filter 13 at the wavelength of the incident light. The microprocessor responds when a "peak" of absorption of the filter 13 is detected.This enables a "calibration" to be effected, since it is known that the various absorption "peaks" are at precisely predetermined wavelengths, and information concerning these wavelengths is stored in the Read Only Memory 26. Thus, if there has been any error that has arisen due to the re-calculation of wavelength from the signal Vb, then when the wavelength passes one of the "peaks" of the filter 13, the appropriate correction can be made.

The multiplexer 24 is provided with an input for an external source Vx This external source may be a detector of the same or of a similar type as the detectors 6, 7 and 8. The sensor may be illuminated by the same incident light, although the light may be attenuated. This input channel is not essential but must be considered to be an optional feature. The microprocessor may be operated so that it takes alternate samples of the voltages Vx and Vb, storing the results in the Read/Write Memory 27. The resulting data may be processed as required or transmitted to other processing means by an appropriate interface.

At periodic intervals the device may enter a recalibration mode, the microprocessor then drives the grating or the prism through the entire wavelength range, taking alternate samples of the signals Vb and Vc As described above the signal Vc is examined to find the various "peaks" of absorption, and a value of Vb is noted at these points. These values are compared with the stored values and an appropriate correction is applied to the stored coefficients in order to compensate for any changes such as electronic drift in the measuring circuitry. By this method long-term accuracy of the wavelength sensing system is assured.

The microprocessor 25 may be provided with a further output 32. This output may provide data concerning the wavelength of light at any instant, and may provide an output signal which emulates an incremental shaft encoder, thus producing pulses corresponding to the angular increments of a grating in an ideal monochromator.

It is to be appreciated that the device, as described, effectively measures the wavelength present at the supplementary exit slot 3, and, inevitably, this wavelength is not precisely the same as the wavelength of light passing through the principal exit slot 2. However, due to the nature of the optics of the monochromator, there is always a readily determinable relationship between the wavelength of the light at supplementary slot 3 and the wavelength of the light at the principal exit slot 2. An appropriate correcting programme is applied to the microprocessor.

It is to be noted that in a modified embodiment of the invention the light which is supplied to the sensor array is taken from the principal exit slot, thus obviating the need providing a supplementary exit slot. This can be achieved by using a beam splitter or a reflector that receives light from part of the exit slot,- although an optical fibre arrangement can be utilised to direct light from the exit slot to an appropriate sensor array. In such an embodiment any errors due to changes in the linear dispersion at the exit slot are obviated.

In a further modified embodiment of the invention the signal V a from the detector 6 which is not associated with any filter is supplied to one of the input channels of the analogue-to-digital converter, rather than the reference channel, and the signal Vb is applied, optionally via means of a multiplexer, to the other input channel. A fixed reference voltage may then be applied to the analogue-to-digital converter. Channels 1 and 2 are then sampled simultaneously with the aid of an additional sample and hold amplifier, and the appropriate ratio can then be calculated by the microprocessor.

It is to be appreciated that the need for the thermistor 17 and the associated circuitry can be obviated if the temperature of the detectors and filter is controlled, for example by providing the detectors and filters with a thermostatically controlled housing.

It is to be appreciated that the description above relates to filters specifically selected for use in one part of the optical spectrum. If a device is to be used in other parts of the spectrum, filters and detectors having different properties will be required.

It is to be noted that when a monochromator is operated, by moving, for example, a grating within the monochromator at a substantially constant speed, the wavelength of the light at the output is changing continually. It does take a finite time for the apparatus described above to calculate the wavelength, and the microprocessor may thus include a programme which determines not only the wavelength at an instant, but also the rate of change of wavelength, thus enabling the microprocessor to provide an output signal which is truly representative of the wavelength at the instant that the output signal is generated.

Whilst reference has been made, in the foregoing description, to the use of absorption filters, it would equally be possible to utilise interference filters. When an interference filter is utilised with a light source of continually changing wavelength, interference fringes are detected. In order to determine the wavelength at any instant, it is necessary to count the number of fringes which pass a detector. It is envisaged that in order to facilitate these procedures it may be best to utilise two sensors adapted to measure signals in "quadrature". This is a known technique which'enables accurate measurement of repeating signals, such as fringes and, indeed, using this technique it is possible to determine the direction of movement of the fringes, thus avoiding any errors which may arise if the direction of change of wavelength of the light reverses.

Whilst the invention has thus far been described with reference to an arrangement in which the wavelength of light is measured, it is to be appreciated that an arrangement of the type described above may be incorporated in an apparatus where the intensity of light passing through a sample is measured. Figure 4 illustrates part of such an apparatus. Incident light 33, from a monochromator, is passed through a sample chamber 34 to a sensor 35 and is also passed to a sensor array 36, which may include a plurality of sensors of the general type described with reference to Figure 3. Signals from sensors 35 and 36 are passed to appropriate processing circuity 37 which can incorporate multiplexers, sample and hold circuits, and an analogue-to-digital converter 23 and a microprocessor 25. A signal representative of the wavelength of the light is determined within the microprocessor 25 and is used to perform further calculations concerning the optical characteristics of the sample 34, to provide an output signal which is related to the wavelength of the incident light.

Claims

CLAIMS:

1. A device for developing a signal related to the wavelength of substantially monochromatic incident light comprising means to direct the light through a filter on to a sensor adapted to detect the light, the filter having a predetermined output characteristic which is related to the wavelength of the incident light, the output of the sensor being supplied to calculating means associated with a memory which contains data about the said characteristic, the calculating means being adapted to calculate a signal related to the wavelength of the incident light.

2. A device according to Claim 1 wherein the calculating means is adapted to determine the wavelength of the incident light and to provide an output representative of the wavelength of the incident light.

3. A device according to Claim 1 or Claim 2 wherein the said filter is an absorption filter with a predetermined relationship between the proportion of light falling on the filter which passes through the filter and wavelength.

4. A device according to Claim 3 wherein the relationship between the proportion of light which is transmitted by the filter and wavelength is substantially linear.

5. A device according to Claim 3 or 4 wherein the filter is a red glass filter.

6.- A device according to Claim 5 wherein the filter is such that 50% of the incident light is transmitted through the filter when light of substantially 1,000nm is incident upon the filter.

7. A device according to Claim 1 or Claim 2 wherein the filter comprises an interference filter, the sensor being adapted to count fringes as the wavelength of incident light changes.

8. A device according to Claim 7 wherein the sensor comprises two detector means adapted to detect fringes "in quadrature".

9. A device according to any one of the preceding Claims wherein means are provided to sense the temperature of at least the filter, the calculating means being adapted to correct for changes in the characteristic of the filter or the combination of the filter and the sensor which are related to temperature.

10. A device according to Claim 9 wherein a thermistor is mounted in thermal contact with the filter, the thermistor forming part of a resistive array, a reference voltage being applied to the resistive array and a further voltage being derived from the resistive array which is indicative of the temperature of the thermistor.

11. A device according to any one of Claims 1 to 6 or Claim 9 or 10 wherein a further sensor is provided which senses the intensity of incident light, the output from the sensor associated with the filter being compared with the output of the sensor which senses the intensity of the incident light.

12. A device according to Claim 11 wherein the output of the sensor associated with the filter and the output of the sensor to which senses incident light are both supplied to means which produce an output signal representative of the ratio between the two signals.

13. A device according to Claim 12 wherein the output of the sensors is supplied to a ratio-metric analogue-todigital converter the output of which is passed to a microprocessor which constitutes the said calculating means.

14. A device according to any one of the preceding Claims wherein a further sensor is provided, adapted to receive incident light which is passed through a further filter, the further filter having predetermined absorption peaks at predetermined optical wavelengths, means being provided to determine when the incident light is at a wavelength equal to the wavelength of one of said peaks, means being provided to correct a value for wavelength calculated from the first filter.

15. A device according to any one of the preceding Claims wherein the output of the calculating means simulates the output of an incremental shaft encoder.

16. A device according to any one of the preceding Claims in combination with a monochromator, the said calculating means being adapted to control the movement of an active optical element in the monochromator.

17. A device according to Claim 16 wherein the output of the calculating means is passed to a digital-to-analogue converter having two outputs which are each connected, through an operational amplifier, to a motor which drives the active optical element of the monochromator.

18. A device according to any one of the preceding Claims provided with means to sample a signal from an external detector, and to pass the signal to the calculating means.

19. A device according to any one of the preceding Claims wherein the calculating means calculate the wavelength of incident light and the rate of change of wavelength of incident light and produce a signal indicative of the wavelength of incident light at the end of the calculation generating the said signal.

20. A device according to any one of the preceding Claims wherein the said sensor and filter are associated with thermostatically controlled means to maintain a substantially constant temperature.

21. A method of measuring the wavelength of substantially monochromatic incident light comprising the steps of directing the light through a filter on to a sensor adapted to detect the light, the filter having a predetermined output characteristic which is related to the wavelength of the incident light, the output of the sensor being supplied to calculating means associated with a memory which contains data about the said characteristic, the calculating means being adapted to calculate a signal related to the wavelength of the incident light.

22. A device for generating a signal related to the wavelength of incident light substantially as herein described with reference to and as shown in the accompanying drawings.

23. Any novel feature or combination of features disclosed herein.