GB1605425A - Improvements in or relating to spectrophotometric intruments. - Google Patents

Description

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(54) IMPROVEMENTS IN OR RELATING TO SPECTROPHOTOMETRIC INSTRUMENTS

(71) [, the Secretary of State for Defence, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement : This invention relates to spectral and position resolution devices, and more particularly to apparatus for positionally locating objects or regions of space exhibiting a particular geometry and characterised by a spectral signature.

In the field of spectroscopy it is possible to identify materials or objects by the radiation they absorb, or by the radiation they emit when excited. The radiation exhibits a spectrum characteristic of the material, and this may be detected using spectrophotometric equipment. In the optical field, objects may be positionally located, using for example a telescope, by emitted or reflected light. Similarly, radar techniques employ what might be termed a microwave searchlight to reflect signals from a target. Modern optical and radio telescopes can be capable of extremely high positional resolution and energy sensitivity. Moreover, spectrophotometric techniques have been so refined that they are able to provide very high energy sensitivity and wavelength resolution.

Conventional optical telescopes and spectrophotometers are, however, incapable of distinguishing between an object and its background if the object and the background exhibit the same optical reflection, absorption or emission characteristics. Applicant is totally unaware of any conventional technique capable of detecting a source of radiation in the presence of a much more intense spectrally similar background, or capable of

analysing mixtures of chemical compounds spectrophotometrically if the compounds exhibit the same z : l characteristic spectra.

According to the present invention, a location device for the detection of target sources of electromagnetic energy includes a telescope, a spectrum analyser adapted to select at least one wavelength interval from the output of the said telescope, a detector capable of detecting the electromagnetic energy in the said wavelength

interval, modulation means operable both to scan spatially and to scan spectrally the flow of electromagnetic energy to the said detector, together with electronic signal processing means to process the output of the said detector and to isolate electronic signals possessing a waveform corresponding to the combination of the characteristics of the modulation means, whereby, in operation, the location device is preferentially sensitive to target sources exhibiting selected spectral and spatial properties of size and shape as seen through the telescope.

In a preferred embodiment of the invention, the location device is operable in at least one of the ultra-violet, visible and infra-red regions of the electromagnetic spectrum, the spectrum analyser being a wavelengthdispersing monochromator, and the modulation means including a spatial filter and a spectral scanner each sited in the path of electromagnetic energy when flowing from the telescope to the detector.

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The spatial scanning is done by means of a spatial filter which preferably, but not necessarily, comprises a rotatable reticle sited at a focal plane of electromagnetic energy when flowing between the telescope and the detector. The spatial filter alternatively comprises a mask formed from the Fourier transform of a reticle, the Fourier transformed reticle being rotatable and sited in the path of electromagnetic energy when flowing from the telescope to the detector.

A location device of the invention alternatively comprises an optical telescope, a monochromator adapted to select at least one wavelength interval from the output ofthe telescope, the monochromator being also adapted to disperse the said wavelength interval spatially over an output area greater than that which would be occupied by the image of a target source, modulation means comprising a combined spectral scanner and spatial filter

having a balanced field and a spatial period which is an integral sub-multiple of the dispersed width of the said wavelength interval, and detector means capable of detecting the electromagnetic energy in the said wavelength interval, whereby, in operation of the location device, electromagnetic energy in the said wavelength interval reach ing the detector from the said target source is modulated in a manner corresponding to the combined characteristics of the spectral scanner and the spatial filter, whereas electromagnetic energy from sources which do not exhibit spectral and spatial properties which correspond to the combined characteristics of the spectral scanner and the spatial filter is not so modulated.

Conveniently, the telescope is of the reflecting Cassegrain focus type. A preferred type of modulation means

is a light chopper in the form either of a picket fence reticle or of a peripherally castellated disc.

ZD In order that the invention may be more fully understood, one embodiment thereof will now be described, by way of example only, with reference to the drawings accompanying the provisional specification.

Figure I illustrates a location device of the invention Figure 2 illustrates a light chopper in the form of a picket fence reticle

Figure 3 illustrates a light chopper in the form of a peripherally castellated disc. m Referring to Fig 1, the location device comprises a telescope 1, an intermediate lens 2 and a monochromator 3, together with a light chopper 4 acting as a combined spectral scanner and spatial filter, a photomultiplier 5 and a combined amplifier/phase sensitive detector 6. The telescope I is of the Cassegrain type in which light is focused by a concave mirror 7 in combination with a convex mirror 8. Light baffles 9 are fitted to the telescope I to restrictthe light falling on the mirror7 to that parallel to its axis. The monochromator 3 employs the Czerny-

Turner configuration ofa collimating mirror 10, a reflection diffraction grating 11 and a telescope mirror 12, 0 z : l z : l together with adjustable entrance and exit slits 13 and 14 respectively. The mirrors 7 and 8 of the telescope focus parallel light rays 15 to a point 16. The intermediate lens 2 focuses light rays 17 diverging from the point 16 through the entrance slit 13 to a point 18. The point 18 is at the focus of the monochromatortelescope mirror 10, and therefore light rays 19 diverging from the point 18 become parallel rays 20 after reflection in the mirror 10. The f-number ofthe mirror 10 sets the acceptance solid angle ofthe monochromator 3, and the telescope 1 is sufficiently large to match this. The rays 20 are diffracted and therefore spectrally dispersed by the

diffraction grating 11, and are focused by the telescope mirror 12 to a point 22 through the exit slit 14, in the 0 tD plane of the I ight chopper 4. The monochromator 3 acts as a band-pass filter whose pass-band is spread across the width of the exit slit 14. The chopper 4 when in operation selectively modulates the energy from the designated target, but does not modulate either background sources of energy or those other sources which do not exhibit the designated spectral and spatial properties. The modulated energy thereafter falls on the photomultiplier 5. The photomultiplier 5 produces an AC output signal varying at the frequency of the light intensity modulation produced by the chopper 4. The amplifier/phase-sensitive detector 6 is switched in synchronism with the modulation frequency, and therefore preferentially detects signals modulated at that frequency. This is of considerable benefit in increasing signal to noise ratio.

Figures 2 and 3 illustrate alternative types of tight chopper 4 which are suitable for use with the location device shown in Fig 1. When the location device is being used, it is quite probable that the field of view of the telescope

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I will include part of the horizon. If such is the case, it is important that the chopper 4 should interrupt the light output from the monochromator in such a way that the differing light intensities above and below the horizon are either not modulated, or that any such modulation is minimised. Figure 2 shows a chopper 4 in the form of a picket-fence reticle operable horizontally in the direction of the arrow 23. The chopper 4 is composed of opaque bars 24 attached to support members 25 and defining between them spaces 26. The bars 24 and spaces 26 are of equal width giving a mark-space ratio of unity. In use the bars 24 move horizontally past the exit slit 14 of the monochromator 3 which has been adjusted to be equal in area to the area of a bar 24 plus that of a space 26. This matching of the chopper areas to that of the exit slit 14 gives a so-called balanced field chopper which exposes a constant area of the exit slit 14 irrespective of the position of the bars 24 with respect to the slit 14.

An alternative form of chopper 4 is shown in Fig 3 and comprises a disc 31 which is circumferentially castellated to form chopper blades 32 defined by equi-spaced radii 33 of the disc 31. The disc 31, which may also be termed a reticle, is axially offset from the exit s) it ! 4 of the monochromator 3. The disc 31 has a radius which is large compared to the height of the exit slit 14, and the disc axis is sufficiently offset from the exit slit 14 such that vertical modulation of the exposed area the exit slit 14 by the chopper blades 32 is small compared to the

corresponding horizontal modulation. The disc-type chopper 4 of Fig 3 is more convenient to use than the picket-fence type chopper of Fig 2, and is therefore preferred, provided that (as is usually the case) the resulting small vertical modulation of the exit slit 14 can be tolerated. The blades 32 and their spacing achieve the same balanced field effect as that achieved by the arrangement of Fig 2. With reference now to the operation of the location device, suppose that the field of view of the telescope is filled by an extended source of light. Let that source of light contain a component having a characteristic spectrum, such as for example the sodium D line doublet at 589 and 589.6nm. Suppose further that the field of view of the telescope 1 also includes a target light source emitting sodium D light and subtending at the monochromator a solid angle smaller in extent that the acceptance solid angle of the monchromator 3. The monochromator 3 is wavelength tuned and the entrance slit 13 is adjusted such that, at the exit slit 14, the spectral lines in the sodium D doublet fill the exit slit 14 with sodium light which originated at the extended source. The target light source however gives rise to an image which does not fill the exit slit 14, since the target source is smaller than the area projected by the acceptance solid angle of the monochromator 3. As the chopper 4

moves across the exit slit 14 it scans the light field at the slit 14 and therefore scans spectrally as well as spatially. Since the chopper 4 is of the balanced field type, the light from the extended source falling on the photomultiplier 5 is not modulated. The photomultiplier 5 therefore receives a modulated light beam only if the exit slit 13 is non-uniformly illuminated, ie when the target source is present in the field of view of the telescope. The combined amplifier and phase-sensitive detector 6 responds only to electrical signals modulated at the chopping frequency, ie AC signals at the correct frequency. When the photomultiplier 5 receives a constant light intensity, corresponding to an extended source of light fillingthe monochromator acceptance angle, the phasesensitive detector 6 gives a zero output. The present invention therefore provides a means of locating a target light source having particular spatial characteristics of shape and size as seen through the telescope in the presence of a spectrally interfering extended source, this result being achieved by combining spectral and spatial scanning. The use of a monochromator and spectral scanner removes sources of light at other wavelengths as well as sources of wide waveband energy, and spatial filtering restricts sensitivity to target sources of designated spatial characteristics.

The foregoing example of the invention is a specific embodiment of general principles which are not necessarily obvious at first sight. The general principles ofthe invention comprise the two processes of spatially scanning the radiation from a scene in order to identify targets having particular spatial properties, together with scanning the dispersed spectrum of the scene to isolate particular spectral characteristics. Only those targets which have spectral and spatial properties corresponding to the characteristics ofthe spectral scanner and spatial filter give rise to radiation reaching the detector which is modulated byboth the spectral scanner and the spatial filter.

In the embodiment of the invention hereinbefore described, as has been mentioned, the chopper 4 simultaneously performs the functions of spectral and spatial scanning, and it therefore modulates only that

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radiation in a particular waveband which originated at a target possessing a geometry appropriate to the spatial filter characteristics. However, this combination of functions is by no means essential, and the operations of spatial and spectral scanning may be performed sequentially at different points of an optical system; in this case, the following considerations apply with respect to the positioning ofthe spectral scanner and spatial filter within the optical system.

It is usual for a conventional spatial filter reticle to operate on a real image of a scene, in order that it might be superimposed optically on the scene to be spatially scanned in so far as the detector is concerned, and so that signals modulated in a manner appropriate to target extent may be obtained. Accordingly, a spatial filter in the form of a reticle should be employed at the focal points of the optical system concerned, in Fig I these are the points 16 or 18, or at the exit slits 14. However, it is quite possible to obtain spatial scanning at other

points in the optical path between a telescope and detector. The images are imaginary at points other than foci, being the Fourier transforms of real images; in consequence, a Fourier transformed spatial filter must be employed at non-focal points, such as a hologram of a conventional reticle. In the art of spatial filter design, it is known that there exists a number of target geometries for which different varieties of filter can be made specific, each type of filter being adapted to select preferentially a different target geometry. It is accordingly a major virtue of the invention that sensitivity to a particular target geometry can be achieved.

The spectral scanner is necessarily sited to scan the dispersed spectral output of the monochromator, and therefore, in the case of a conventional planar reticle, the output focal plane ofthe monochromator is coincident with the plane of the reticle. If the spatial filter operates to modulate appropriate target emission at a frequency f and the spectral scanner at a second frequency f,, then the amplitude X of the resultant waveform at the detector is of the form :- X=A, sin2T) : f, t. sin 27if, t (I) Where t is the time in seconds and Ao is a constant ; the waveform described mathematically in equation I will be familiar to those versed in the art of radio receivers; it is the equation of an amplitude-modulated carrier wave, and the principles of isolating and processing such signals are well known.

As previously mentioned the example of the invention hereinbefore described employs a combined spectral scanner and spatial filter for reasons of simplicity. In addition, at optical wavelengths, the mechanical characteristic of choppers and the time constant limitations of certain detectors are such that it can be difficult in practice to satisfy the condition that the two frequencies f1 and fof equation should differ considerably, as required by considerations of carrier wave modulation theory. Consequently, it is particularly convenient to employ a combined spectral scanner and spatial filter.

The foregoing modulation and detection techniques are appropriate for targets which are stationary with respect to the field of view of the telescope of a location device of the invention. However, targets which are

moving across this field of view give rise to signals modulated at frequencies other than that which corresponds to a similar but stationary target. This conclusion can be reached by merely considering the modulation of the energy emitted by a target moving instantaneously either with or counter to the rotation of the chopper 4 in the monochromator output focal plane in the optical system of Fig I. The frequency shift effect associated with moving targets can be circumvented by the use of, for example, electronic delay techniques, phase locked loops and signal processing associated with Doppler shifted signals which are well known, and accordingly it is to be understood that the present invention is by no means restricted to the location of stationary targets.

It will be apparent to those skilled in the field of optics that the principles embodied in the foregoing specific description are applicable to all wavelengths from the near ultra-violet, through the visible, to the far infra-red regions ofthe electromagnetic spectrum, provided that suitable transmissive, reflective, or diffracting elements are employed appropriate to the particular wavelength. The technique is also capable of further refinement. For example, it is possible to employ a multiple exit slit or correlation mask arrangement, as used in correlation

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spectrometry, with one slit per group of components. In the case of the sodium D doublet, this would require two exit s ! its at the output focal plane of the monochromator. The monochromator would then be finely tuned, or correlated, to that particular radiation. Similarly, the monchromator could be tuned to other more complex multiplet spectra.

The optical system hereinbefore described is suitable for monitoring radiation produced by reflection or when excited atomic states decay. The necessary excitation may be inherent as a result of the high temperature of the atoms concerned, or alternatively external excitation may be required. This may be effected by, for example a laser beam operating on an output wavelength suitable for absorption or reflection by the atomic or molecular species concerned. Furthermore, molecular species may be detected by the Raman, or

frequency-shifted, scattering they produce. Monitoring naturally emitted radiation may be termed a"passive" mode of operation, while monitoring reflected radiation or externally excited emission may be termed an "active"mode of operation.

A monochromator with an appropriate detector can be employed to monitor far infra-red wavelengths. At longer wavelengths the radar regions are approached.

The invention is concerned with combining spectral scanning and spatial filtering to provide ameans of locating targets exhibiting particular spectral and spatial characteristics, and variations thereof within the scope of the invention will be apparent to those skilled in the appropriate arts.

What I claim is: 1. A location device for the detection of target sources of electromagnetic energy including a telescope, a spectrum analyser adapted to select at least one wavelength interval from the output of the said telescope, a detector capable of detecting the electromagnetic energy in the said wavelength interval, modulation means operable both to scan spatially and to scan spectrally the flow of electromagnetic energy to the said detector, together with electronic signal processing means to process the output of the said detector and to isolate electronic signals possessing a waveform corresponding to the combination of the characteristics of the modulation means, whereby, in operation, the location device is preferentially sensitive to target sources exhibiting selected spectral and spatial properties of size and shape as seen through the telescope.

Claims (1)

1. 2. A location device according to claim I wherein the spectrum analyser is operable to select at least one wavelength interval from the combined ultra-violet visible and infra-red regions of the electromagnetic spectrum.

   3. A location device according to claim 2 wherein the spectrum analyser comprises a wavelength-dispersing monochromator, the modulation means comprising spatial filter means and spectral scanner means sited in the path of electromagnetic energy when flowing from the telescope to the detector.

   4. A location device according to claim 3 wherein the spatial filter means is either a rotatable reticle or a rotatable mask equivalent to the Fourier transform of a reticle.

   5. A location device according to any preceding claim comprising an optical telescope, a monochromator adapted to select at least one wavelength interval from the output of the telescope, the monchromator being also adapted to disperse the said at least one wavelength interval spatially over an output area greater than that which would be occupied by the image of a target source, modulation means comprising a combined spectral scanner and spatial filter having a balanced field and a spatial period which is an integral sub-multiple of the dispersed width of the said wavelength interval, and detector means capable of detecting the electromagnetic energy in the said wavelength interval, whereby, in operation of the location device, electromagnetic energy in the said wavelength interval reaching the detector from the said target source is modulated in a manner corresponding to the combined characteristics of the spatial filter and the spectral scanner, whereas

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   electromagnetic energy from sources which do not exhibit spectral properties which correspond to the combined characteristics of the spectral scanner and the spatial filter is not so modulated.

   6. A location device according to claim 5 wherein the telescope is of the reflecting Cassegrain focus type.

   7. A location device according to anyone of claims 3 to 6 wherein the monochromator comprises a multiple exit slit arrangement.

   8. A location device according to any one of claims 3 to 7 including means to stimulate targets to emit electromagnetic radiation in a wavelength interval appropriate for the spectrum analyser.

   9. A location device accordingto claim 8 including a laser adapted to stimulate emission of electromagnetic radiation from targets.

   10. A location device according to any one of claims 3 to 9 wherein the monochromator is operable to disperse

   sodium D radiation at wavelengths of589nm and 589. 6nm for subsequent detection.

   ]]. A location device according to any one of claims 3 to 10 wherein the monochromator includes a diffraction grating.

   12. A location device according to any one of claims 3 to 11 including a phase-sensitive detector.

   13. A location device substantially as herein described with reference to Figures 1 and 2 or Figures I and 3 of the drawings accompanying the provisional specification.