GB2162308A

GB2162308A - Wavelength detection

Info

Publication number: GB2162308A
Application number: GB8511740A
Authority: GB
Inventors: B E Jones; S J Collier; G Murtaza
Original assignee: Sira Ltd
Current assignee: Sira Ltd
Priority date: 1984-06-08
Filing date: 1985-05-09
Publication date: 1986-01-29
Also published as: GB8414671D0; GB8511740D0

Abstract

A method and apparatus for determining the wavelength of a radiant signal and method and apparatus for determining a measurand comprising providing a sensor 12 whose wavelength transmission or production characteristics are a function of the value of the measurand, the wavelength of the radiant signal being determined by providing two detector means 21,22, one of which is wavelength dependent by a first factor and the other of which is wavelength dependent by a different factor, and the outputs of the two detector means being compared (28). <IMAGE>

Description

SPECIFICATION Wavelength detection The present invention relates to a method and apparatus for determining the wavelength of a radiant signal or the change of wavelength.

Although described primarily with respect to visible wavelengths the principle of the invention is applicable to ultra-violet, infra-red or other wavelengths.

Recent research has been concerned with the investigation of wavelength-dependent optical sensors ('colour' sensors) which have fibre-optic links to, for example, a control point in a process plant. Such sensor systems have potential advantages in that they use multimode-fibre technology and non-coherent light sources, and exclude the need for referencing which is required for amplitude (intensity) dependent optical sensors to overcome the effect of attenuation within the system.

Several workers have noted the potential use of wavelength dependent effects in optical sensors. However, the usefulness of such devices is limited by the need to perform some form of spectral analysis at the output end of the fibre and this results in an increase in cost and complexity over equivalent amplitude-dependent sensors.

The present invention provides a method for determining the wavelength of a radiant signal comprising dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a second detector which is wavelength dependent by a different factor and comparing (eg by providing a ratio) the output of the two detectors.

The present invention also provides a method of determining change of wavelength of a radiant signal comprising dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a detector which is wavelength dependent by a different factor and comparing the output of the two detectors, and detecting changes in the comparison.

The present invention also provides a method for determining a measurand comprising providing a sensor whose wavelength transmission or production characteristics are a function of the value of the measurand, and determining the wavelength of a radiant signal of the sensor by dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a second detector which is wavelength dependent by a different factor and comparing the output of the two detectors.

The present invention also provides a method for determining a plurality of measurands comprising providing, for each measurand, a sensor whose wavelength transmission or production characteristic is exclusive and is a function of the value of the measurand, and determining the wavelengths of the radiant signals from the sensors in a plurality of detector assembly means, each detector assembly means being sensitive to wavelength of radiant signal from a respective one sensor only, and in each detector assembly means dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a second detector which is wavelength dependent by a different factor and comparing the output of the two detectors.

The present invention also provides apparatus for determining the wavelength of a radiant signal comprising two detectors, a first detector which is wavelength dependent by a first factor and a second detector which is wavelength dependent by a different factor, means for dividing said radiant signal into two parts, and for passing one part to the first detector and the second part to the second detector, and means for comparing the outputs of the two detectors.

The present invention also provides apparatus for determining the change of wavelength of a radiant signal comprising two detectors, a first detector being wavelength dependent by a first factor and a second detector being wavelength dependent by a different factor, means for dividing said radiant signal into two parts and for passing one part to the first detector and the second part to the second detector, means for comparing the outputs of the two detectors and for detecting changes in the comparison.

The present invention also provides apparatus for determining a measurand comprising a sensor whose wavelength transmission or production characteristics are a function of the value of the measurand and two detectors, a first detector which is wavelength dependent by a first factor and a second detector which is wavelength dependent by a different factor, means being provided to receive a radiant signal from said sensor and to divide said radiant signal into two parts, and passing the first part of the first detector and the second part to the second detector whereby the wavelength of the radiant signal may be determined by comparing the outputs of the two detectors.

The present invention also provides apparatus for determining a plurality of measurands comprising, for each measurand, a sensor whose wavelength transmission or production characteristic is exclusive and is a function of the value of the measurand, detector assembly means for each sensor sensitive to the wavelength of radiant signal from the respective one sensor only, each detector assembly means comprising two detectors, one detector being wavelength dependent by a first factor and the second detector being wavelength dependent by a second factor, and means for dividing said radiant signal into two parts and passing one part of the first detector and passing the second part of the second detector whereby the wavelength of the radiant signal from the respective factor may be determined by comparing the output of the two detectors.

Preferably the two parts of the radiant signal should be of approximately equal intensity.

The two detectors may comprise identical detector elements, one or both of which are mounted behind a wavelength dependent filter and the radiant signal is passed to one or both of the detector elements through a respective wavelength dependent filter, or the detector elements may in themselves by wavelength dependent.

In a preferred arrangement the first detector is wavelength dependent to a factor of one, in other words, the detector is not wavelength dependent.

It will be understood that using the method of the invention or the apparatus of the invention attenuation or variation of attenuation in the system producing the radiant signal will have an equal effect on both detectors and will therefore not affect the result. This makes the system of particular practical significance.

In a preferred arrangempnt the radiant signal may be provided by means of a sensor whose wavelength transmission or production characteristics are a function of some factor to be measured, for example, displacement, pressure or temperature.

Preferred arrangements of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of apparatus for carrying out the method of the invention, Figure 2 is a more detailed representation of the apparatus of Fig. 1, Figure 3 shows the transmission characteristics of a typical filter for use with a detector, Figure 4 shows a first sensor comprising a prism, Figure 5 shows a second type of sensor comprising a reflective diffraction grating, Figures 6 and 7 are graphs of the output characteristics of the sensors of Figs. 4 and 5 respectively, Figure 8 illustrates in diagrammatic form a third form of sensor comprising two prisms, Figure 9 shows an alternative form of the arrangement of Fig. 8, Figure 10 shows a fourth type of sensor in the form of a lens, Figure 11 shows a detail of part of Fig. 1 0, Figure 12 shows an alternative form of the sensor shown in Fig. 10, Figure 13 shows a detail of a part of a preferred form of detector showing the filter arrangement, Figures 14, 15 and 16 show in logic diagram form, three different ways of analysing the output of the detector, Figure 1 7 shows diagrammatically a variety of different means for providing a multiplex arrangement of the apparatus of the invention, and, Figure 18 shows a diagrammatic form of one form of multiplexing apparatus of the invention.

The elements of the apparatus of the invention are shown in Fig. 1. In principle the apparatus comprises a source of light 10, a fibre-optic link 11 passing light from the source 10 to a sensor 1 2 the output of which is varied by changes in the measurand 1 3 which may be, for example, temperature, displacement, rotation, pressure, the output from the sensor 1 2 being passed by a fibre-optic link 1 4 (which may be the same fibre as link 11) to a detector 1 6. It is necessary that all of the parts of the apparatus 10, 11, 1 2, 14 and 1 6 are wavelength compatible so that they have an optimum performance over the desired wavelength range.

In general the source 10 will be a broad band white light source. The sensor will be a device whose wavelength transmission characteristics are a characteristic of the measurand 1 3. Examples of devices which may be used as sensors 1 2 include diffraction gratings, prisms, Fresnel zone plates, colour glasses, semi-conductor filters, interference filters and Christiansen filters.

Fig. 2 illustrates the apparatus of Fig. 1 in more detail. The white light source 10 may comprise a tungstem halogen lamp 1 7 receiving power from a transformer 1 8. As is clear from Fig. 2 the detector 1 6 includes a twinelement photodiode 19, each component 21, 22 of the photodiode 1 9 passing an output signal via respective amplifiers 23, 24 and via scale adjustment potentiometers 26, 27 to a meter 28 which compares the two signals and produces a digital output. The amplifiers 23, 24 are driven by a power supply 29.

As can be seen from Fig. 2 in front of component 21 of photodiode 1 9 there is provided a filter 31. The filter 31 is wavelength dependent so that the amount of light transmitted through to the component 21 of photodiode 1 9 will be dependent upon the wavelength of the incident radiation.

Fig. 3 shows the transmission/wavelength characteristic of the filter 31. It will be seen therefore that for an input radiation signal of a wavelength L, the amount of radiation transmitted is very much less than for the wavelength L2 even though the input radiation signal is of the same amplitude.

However there is no filter in front of the component 22 of photodiode 1 9 and so a comparison of the output signals of the two components 21, 22 of photodiode 1 9 will produce a signal which is dependent upon wavelength. Any variation in attenuation in the fibre-optic links 11, 14 of their connectors or in the light source 10 will apply equally to both signals and will therefore not affect the results.

Two types of sensors 1 2 are illustrated in Figs. 4 and 5. In Fig. 4 the sensing element comprises a suitable prism 32, eg of sapphire which receives radiation from the fibre-optic link 11 via a rod lens 33 the prism 32 breaking up the input beam of white light into a continuous spectrum of different wavelengths and displacement along the line of arrow 34 of the input end to the fibre optic link 1 4 will cause a change in 5 the wavelength of the light transmitted to the link 14.

In this way displacement in the direction of the arrow 34 can be changed into changes of optical wavelength.

In Fig. 5 the input radiation from the fibreoptic link 11 is collimated by a lens 36 into a parallel beam 37 which is reflected from a reflective diffraction grating 38 back to the collimating lens 36 and hence to the fibreoptic link 14. Rotation of the grating 38 about the axis 39 will change the wavelength of light reflected from the grating and transmitted to the optic link 14 and hence the apparatus of Fig. 5 provides a sensor for measuring rotational displacement.

The output characteristics of the sensors of Figs. 4 and 5 are shown in Figs. 6 and 7 respectively; in Fig. 7 the detection system output in mV is plotted on the vertical axis and the angle of displacement of the grating 38 is plotted on the horizontal axis and in Fig.

6 the wavelength in nanometers of the output is plotted on the vertical axis and the pressure in inches of water is plotted on the horizontal axis, the pressure being converted to a linear displacement along the line of arrows 34 in Fig. 4.

Use of the invention or preferred aspects of the invention provide a method and apparatus in which the output signal is largely independent of intensity and spectral changes in the light source and in the fibres and in the connectors. The dynamic range is limited to the characterics of the filter but there is a wide range of filters which may be used to provide a variety of characteristics. No further form of reference signal is required. By using an integrated twim element photodiode 19, no auxiliary optics are required at the detector 1 6 and hence the overall cost of the system is low. Since the characteristics of the detector 1 6 are defined at the detector 1 6 they will not be affected by environmental changes at the sensor 1 2 except those changes to be measured.

Fig. 8 shows an arrangement of sensor 12 in the form of two prisms arranged oppositely with respect to one another. The prisms are chosen so as to provide no deviation between the incident beam 51 from fibre optic link 11 and the emergent beam 52 to fibre optic link 14 but that there should be dispersion only.

Such an arrangement requires two prisms 53, 54 of different angle A1, A2 respectively and the prisms being of different material, commonly prism 53 being of calcium fluoride (Ca F2) and prism 54 being of sapphire (A1203).

Clearly for a particular wavelength of the incident beam and for the refractive indices of the material chosen for the prisms 53, 54 a particular combination of angles A1, A2 will provide a dispersed emergent beam 52 parallel to the incident beam 51.

Other factors which should be taken into account in choosing the relevant materials are the changes of refractive index with temperature, the environmental stability of the materials, their low solubility and lack of birefringence. If materials with high and low dispersions are paired one may tend to cancel the effective low temperature dependence of the refractive index.

This is important for good temperature stability of systems designed to measure physical quantities other than temperature. However, the temperature dependence of refractive index in these materials can be put to full use in the design of a temperature sensor having the same design and working principle but different pair of materials for the two prisms.

Materials from which the prisms may sensibly be chosen are zinc sulphide (ZnS), sapphire (A12O3), barium, calcium, lithium and sodium fluoride (BaF2, Cay2, LiF and NaF).

An advantage of the arrangement shown in Fig. 8 is that the emergent beam may be reflected from a plane mirror as shown in Fig.

9 and thereby pass back through the two prisms to disperse the beam even more and reflects the emergent beam back to the same optic fibre. Such an arrangement is shown in Fig. 9 and the plane mirror 56 may be rotated by the measurand which means that the wavelength of the radiation emerging from the prism 53 back to the fibre optic link 11/14 will be dependent upon the rotation of the plane mirror. Alternatively, in place of rotation of the mirror 56 by the measurand there may be rotation of either of the prisms 53, 54 about the optical axis to vary the wavelength of the emergent beam. In this way the effec-tive value of the angle of the prism is varied so far as the radiation beam passing through the prism is concerned.

Such an arrangement of sensor utilising two prisms and plane reflecting mirror provides excellent mechanical and thermal stability and the output remains independent of the axial displacement of the components which might be caused, for example, by changes of temperature. Rotational movement of the mirror 56 about the axis 57 or one of the prisms 53, 54 about the optic axis may easily be caused by a lever arrangement, the length of the lever being used to vary the degree of movement caused by the measurand to cause a particular wavelength change. The wavelength range over which the sensor operates may also be easily adjusted by rotation of the mirror or one of the prisms.

Fig. 10 illustrates an alternative arrangement of sensor utilising a lens. The sensor makes use of the chromatic dispersion displayed by a lens when uncorrected for chromatic aberration. In this case white light from the optical fibre 11/14 is collimated by the collimator 62 and passed to a lens doublet 63. The lens doublet 63 is designed to minimise all aberrations except the chromatic aberration. A plane mirror 64 is mounted to be movable along th optical axis as illustrated by the arrows 65. The plane mirror 64 is mounted at a distance f(LO) from the lens 63 so as to be in the image plane for a particular wavelength LO. The rays of wavelength L0 are reflected back to the lens 63.The remainder of the spectrum, that is the parts of the wavelength greater than or smaller than L0 are filtered out by a spatial filter 66 so that in an ideal case only light of wavelength L0 will be reflected back to the optical fibre 62. The plane mirror 64 is movable along the optical axis of the lens 63 and this movement selects the wavelength which is reflected through the system back to the optical fibre 11 /1 4.

Fig. 1 2 illustrates an alternative form of lens type sensor in which the beam is not collimated but is brought to an initial focus at point 67 before passing to the lens 63 and mirror 64 and the spatial filter 66 can comprise a pinhole.

Various different types of detector will now be described. As shown in Fig. 1 3 the detector in the preferred arrangement may comprise a collimating lens 71 for collimating the radiation received from the optic fibre 14, the collimated beam 72 passing to the sensor itself, the detector being in the form of a wafer 73 of silicon on which two detector elements 74, 75 are mounted, one half of the wafer 73 being covered by a filter 76 so that one of the elements 75 is covered by the filter 76. The detectors themselves may comprise a Centronic (R.T.M.) LD2-OB which includes two detectors which give a current output dependent upon the intensity of light falling on them.The filter 76 may comprise a glass filter and may include either an interference filter may be of doped glass and a doped glass manufactured by Schott under the number UG3 has been found to be particularly suitable as it has an approximately linear characteristic, that is a graph of the transmission against frequency is approximately linear.

We prefer to use glass filters because these are less sensitive to temperature, vibrations and weathering than gelatine filters.

Figs. 14 to 1 6 show three schematics of arrangements in which the output signals from detectors 74, 75 may be treated. In Fig.

14 the outputs from detectors 74, 75 are amplified by separate current amplifiers 81, 82, the outputs of amplifiers 81, 82 are divided in divider 83, the output of divider 83 is amplified by amplifiier 84 and the signal output from 84 is adjusted in a signal processor 85 which effectively adjusts the zero level and the scaling. The output from 85 is passed to an indicator and/or a computer. In an alternative arrangement, shown in Fig. 15, the outputs from detectors 74, 75 are passed direct to the divider 83 and the output from divider 83 is amplified in amplifier 84 and adjusted in signal processopr 85. Once again the output from signal processor 85 may be passed to a display or computer.

In the arrangement of Fig. 1 6 the signals from the detector 74, 75 are passed to a single divider/amplifier unit 86 and the output from that may pass either to a signal processing unit 85 or alternatively directly to a display or computer.

Several alternative arrangements of a different embodiment of the apparatus are illustrated in Fig. 1 7. Thus far the apparatus has been described with respect to a single sensor and a single detector. In practical environments, however, it is frequently required to have a plurality of sensors and the present arrangement of the invention facilitates the use of a plurality of sensors.

Referring to Fig. 1 7 it will be seen that in principle a single light source 110 is provided which passes light along an optical fibre 111 to a plurality of sensors 112, 113, 114 (although more sensors can be provided). The sensors return their wavelength dependent signals to the optical fibre 111 and there is provided either a separate detector 1 22, 123, 124 for each sensor 112, 113, 114 or alternatively a single detector 1 30.

Such an arrangement of apparatus is referred to as a multiplex arrangement.

In detail, each sensor 112, 113, 114 is set so as to operate in a unique wavelength band so that variations of the measurand for a particular sensor will cause the sensor to return to the fiber 111 a signal which is within that wavelength band. Clearly in a practical arrangement there is a separation between adjacent wavelength bands used by the sensors.

The detectors 122, 123, 1 24 are arranged to detect only those wavelengths in the band of their associated sensors. In this way although a single fibre 111 may be used, the apparatus is able to distinguish between the signals provided by each of the sensors 11 2, 113, 114.

In an alternative arrangement of detector 130, a single detector is used but there is provided a multiple filter arrangement 1 35 which includes filters 132, 133, 1 34 which are selectively moved over one of the detector elements 136, the filters 132, 133, 1 34 passing radiation which corresponds to the wavelength bands of respective sensors 112, 113, 114.

In practical terms, the total possible sensitive wavelength range of the detectors is between, say, 500 and 1000 nm, and therefore if there are provided three sensors then sensor 11 2 can operate in the range of 500-600 nm, sensor 11 3 in the range 650 to 750 nm and sensor 114 in the range 800 to 900 nm. Thus each sensor has a 100 nm wavelength band with a 50 nm wavelength band between each sensor.

There are considerable advantages in this arrangement. A single optical fibre may be passed between each of the sensors which renders installation simpler. Either a single detector is used of if a multiplicity of detectors is used they are identical although the mirror lens or prism is set differently so as to operate in a different wavelength band. A continuous surveillance is provided. Most importantly, the operation is independent of the length of the optical fibre where in prior arrangements the intensity is measured, the length of the optic fibre will vary the intensity and hence create problems with regard to measurement. This is particularly problematical in a practical environment where the optical fibre may be broken and have to be rejoined.In arrangements using the intensity of the light, recalibration has to be provided whereas in the present arrangement joins and breaks in the optical fibre which may affect the overall intensity of the light will not affect the wavelength and thus recalibration will not be necessary. Also with the present multiplexing arrangement failure of one sensor does not affect the others.

Fig. 1 8 illustrates an alternative arrangement of Fig. 1 7. In this arrangement, instead of a single light source there is provided a series of LEDs (light emitting diodes) 151, 152, 153, 1 54. Each LED provides a light output of optical fibre 111 of a different wavelength band distinct from the wavelength band of the other LEDs.There are associated sensors 161, 162, 163, 164 which are sensitive to the wavelength of respective LEDs 151, 152, 153, 154 and detectors 171 and 172, 173, 1 74 which are also sensitive to the wavelength bands of LEDs 151, 152, 153, 1 54. There is provided a solid state or mechanical synchronised switching arrangement 180 incorporating a switch 181 which causes detector 1 71 and LED 1 51 to be selected or 1 72 and LED 1 52 or detector 1 73 and LED 153, or detector 1 74 and LED 1 54. The output of the detectors 1 71 to 174 is displayed on display 183.

The apparatus of Fig. 18 thus operates as follows. One detector/LED pair is selected (for example detector 171/LED 151), and as light from LED 151 passed along optical fibre 111 to the sensor 1 61 where the wavelength of the reflected signal is within the wavelength band of detector 1 71 and is passed back along optical fibre 111 to detector 1 71. This detector 1 71 gives an output to display 1 83.

The switch arrangement 1 80 then switches the switch 1 81 so as to select the next detector/LED pair, for example detector 1 72/LED 1 52 and the operation is repeated.

In this way an individual sensor may be selected and displayed on display 1 83.

The invention is not restricted to the details of the foregoing example. For example in the arrangement of Fig. 2 a second different filter may be placed in front of the component 22 of photodiode 1 9 in which case the response of the detector 1 6 would then be determined by the ratio of the transmission of the respective filters at a particular wavelength. In place of the filters which automatically reduce the intensity of radiation passing to the photodiode 19, two different photodetectors with different response characteristics may be used.

It is also possible to increase the accuracy or to extend the spectral range of the apparatus by processing the responses from more than two photodetectors. This does however increase the difficulties of signal analysis.

Although the detector 1 6 has been described with respect to sensors 12, the detector can be used by itself in, for example, spectrometers as a convenient means of calibrating the wavelength and giving a readout where more conventional methods are not suitable. The detector 1 6 may also be used by itself in other places where colour changes or wavelength shifts are to be detected.

Claims

1. A method for determining the wavelength of a radiant signal comprising dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a second detector which is wavelength dependent by a different factor and comparing the outputs of the two detectors.

2. A method of determining change of wavelength of a radiant signal comprising dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a detector which is wavelength dependent by a different factor and comparing the outputs of the two detectors, and detecting changes in the comparison.

3. A method for determining a measurand comprising providing a sensor whose wavelength transmission or production characteristics are a function of the value of the measu rand, and determining the wavelength of a radiant signal of the sensor by dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a second detector which is wavelength dependent by a different factor and comparing the output of the two detectors.

4. A method for determining a plurality of measurands comprising providing, for each measurand, a sensor whose wavelength transmission or production characteristic is exclusive and is a function of the value of the measurand, and determining the wavelengths of the radiant signals from the sensors in a plurality of detector assembly means, each detector assembly means being sensitive to wavelength of radiant signal from a respective one sensor only, and in each detector assembly means dividing said radiant signal into two parts, passing one part to a first detector which is wavelength dependent by a first factor and passing the other part to a second detector which is wavelength dependent by a different factor and comparing the output of the two detectors.

5. A method as claimed in claims 1 to 4 in which the first detector is wavelength dependent by a factor of 1.

6. A method as claimed in any of claims 1 to 5 in which the two detectors are of the same type, the radiant signal being passed to one or both of the detectors through a wavelength dependent filter.

7. A method as claimed in any of claims 1 to 6 in which the two detectors are wavelength dependent.

8. A method as claimed in any of claim 1 to 5 in which the two parts of the radiant signal are of approximately equal intensity.

9. A method as claimed in any of claims 1 to 6 in which the radiant signal is provided by means of a sensor whose wavelength transmission or production characteristics are a function of a measurand.

10. A method as claimed in any of claims 1 to 9 in which the comparison of the outputs of the two detectors comprises providing a ratio of their outputs.

11. A method as claimed in claim 1, or 2, or 3, or 4, substantially as hereinbefore described.

1 2. Apparatus for determing the wavelength of a radiant signal comprising two detectors, a first detector which is wavelength dependent by a first factor and a second detector which is wavelength dependent by a different factor, means for dividing said radiant signal into two parts, and for passing one part to the first detector and the second part to the second detector, and means for comparing the outputs of the two detectors.

1 3. Apparatus for determining the change of wavelength of a radiant signal comprising two detectors, a first detector being wavelength dependent by a first factor and a second detector being wavelength dependent by a different factor, means for dividing said radiant signal into two parts and for passing one part to the first detector and the second part to the second detector, means for comparing the outputs of the two detectors and for detecting changes in the comparison.

14. Apparatus for determining a measurand comprising a sensor whose wavelength transmission or production characteristics are a function of the value of the measurand and two detectors, a first detector which is wavelength dependent by a first factor and a second detector which is wavelength dependent by a different factor, means being provided to receive a radiant signal from said sensor and to divide said radiant signal into two parts, and passing the first part to the first detector and the second part to the second detector whereby the wavelength of the radiant signal may be determined by comparing the outputs of the two detectors.

1 5. Apparatus for determining a plurality of measurands comprising, for each measurand, a sensor whose wavelength transmission or production characteristic is exclusive and is a function of the value of the measurand, detector assembly means for each sensor sensitive to the wavelength of radiant signal from the respective one sensor only, each detector assembly means comprising two detectors, one detector being wavelength dependent by a first factor and the second detector being wavelength dependent by a second factor, and means for dividing said radiant signal into two parts and passing one part to the first detector and passing the second part to the second detector whereby the wavelength of the radiant signal from the respective factor may be determined by comparing the output of the two detectors.

1 6. Apparatus as claimed in any of claims 1 2 to 1 5 in which the first detector is wavelength dependent by a factor of 1.

1 7. Apparatus as claimed in any of claims 1 2 to 1 6 in which the two detectors are of the same type, and include different wavelength dependent filters through which the radiant signal part is passed to the relevant detector.

1 8. Apparatus as claimed in claim 1 7 and claim 1 5 in which separate detector assembly means are provided for each sensor.

1 9. Apparatus as claimed in claim 1 7 and claim 1 5 in which the detector assembly means comprises a single pair of detectors with a plurality of pairs of wavelength dependent filters movable to cooperate with the detectors so that selection of a pair of filters allows the detectors to determine the wavelength of radiation from a particular sensor.

20. Apparatus as claimed in any of claims 1 2 to 1 9 in which the means for comparing the outputs of the two detectors provides a ratio of said outputs.

21. Apparatus as claimed in claim 15 or 1 9 in which the sensor is substantially as described with reference to Fig. 4.

22. Apparatus as claimed in claim 1 5 or 1 9 in which the sensor is substantially as described with reference to Fig. 5.

23. Apparatus as claimed in claim 1 5 or 1 9 in which the sensor is substantially as described with reference to Figs. 8 and 9.

24. Apparatus as claimed in claim 1 5 or 1 9 in which the sensor is substantially as described with reference to Figs. 10, 11 and 12.

25. Apparatus as claimed in any of claims 1 2 to 1 6 in which the two detectors are wavelength dependent.

26. Apparatus as claimed in any of claims 1 2 to 25 in which the two parts of the radiant signal are of approximately equal intensity.

27. Apparatus as claimed in any of claims 1 2 to 25 in which the output signals of the two detectors are processed in apparatus as described with reference to Fig. 1 4.

28. Apparatus as claimed in any of claims 1 2 to 25 in which the output signals of the two detectors are processed in apparatus as described with reference to Fig. 1 5.

29. Apparatus as claimed in any of claims 1 2 to 25 in which the output signals of the two detectors are processed in apparatus as described with reference to Fig. 1 5.

30. Apparatus as claimed in claim 12, or 13, or 14, or 14, substantially as hereinbefore described with reference to tha accompanying drawings.