This invention relates to an improved optical sensor which is capable of analysing a light signal to obtain information related to the intensity and position of spectral features, and to a method of performing such analysis. Such information is indicative of the properties of the optical source and/or the optical path through which the light has passed.

It is well known that multiplex or Fourier Transform spectrometers can be realised using interferometers where two paths exist for the passage of the input light. (J Strong, J Opt. Soc. A er. 47-354, P B Fellgett (1951) Thesis, University of Cambridge). Furthermore such interferometers can be of the amplitude division or wavefront division type (R B Blackman and J W Turkey (1958) The Measurement of Power Spectra, Dover Publications, New York). It is also well known that a particular family of amplitude division interferometers where the rays follow nearly the same path is the polarisation interferometer (M Francon and S Mallick, Polarisation Interferometers, iley-Interscience 1971) . The variation in difference in optical path length for the two rays can be

introduced by moving an optical element in time (time dispersing instruments) or with a variation in optical path length across the aperture (spatially dispersing instruments). We are taught by T Okamoto et. al. (T Okamoto, S Kawata, S Minami, App. Opt. 23, 2,269-273) that combining a spatially dispersing instrument with a photo-diode array can realise a Fourier Transform spectrometer with no moving parts. We learn further from Japanese patent application JP-A-No. 59,105,508 that such a spatially dispersing interferometer can be of the polarisation type.

It is also well known that images can be analysed for particular characteristics by examination in the Fourier domain (Optics ■ Hech and Zajac, pub Addison Wesley, 2nd edition 1987). We learn from Young et. al. (R C D Young, C R Chatwin, B F Scott Optical Engineering, 32, p 2608-2615) that such image processing techniques are very fast and of value in the acquisition and identification of military targets and the like. We further learn from US patent 5,172,000 that such image processing techniques can also have commercial use in the inspection of semiconductor wafers.

It is an object of this invention to form a simple cost effective mechanism for the analysis of optical signals by combining the output of a spatially dispersive Fourier Transform spectrometer with image processing techniques in the Fourier domain to form a compact, rugged analyser instrument.

The present invention, from one aspect, provides an optical sensor comprising a spectrometer arranged to receive light from a light source and to generate therefrom a spectral image, and image analysis means

for deriving data from the spectral image; characterised in that the spectrometer is of the spatially dispersive Fourier Transform type producing a spectral image which is dispersed in the Fourier domain, and the image analysis means comprises spatial filter means having at least one mask derived from the Fourier Transform of a spectral feature of interest.

From another aspect, the invention provides a method of spectral analysis of light, comprising forming a spatially dispersed Fourier Transform spectrum image of said light, and filtering said image by spatial filter means having spatially varying optical properties related to the Fourier Transform of at least one spectral feature of interest.

Particularly preferred features of the apparatus and method of the present invention will be apparent from the following description and claims.

.Embodiments of the invention will now be described, by way of example, with reference to the drawings, in which:

Fig. 1 is a schematic flow diagram of the invention; Fig. 2 is a schematic view of a tilted mirror Sagnac interferometer; Fig. 3 is a schematic view of a polarisation interferometer; Fig. 4 is a schematic view of a tilted mirror Michelson interferometer; Fig. 5 is a typical wavelength absorption spectrum of an aromatic chemical such as benzene; Fig. 6 is the intensity representation of the interferogram resulting from the wavelength

spectrum shown in Fig. 5 after processing by a spatially dispersing Fourier Transform spectrometer; Fig. 7 is a grey scale representation of the intensity spectrum shown in Fig. 6; Fig. 8 is a sectional strip formed by enlarging part of the grey scale representation shown in Fig. 7; Fig. 9 is a combination formed of the sectional strip of Fig. 8 and its negative; Fig. 10 is an overall schematic view of a preferred embodiment; and Fig.11 is a schematic view of the use of the preferred embodiment to examine the presence of an analyte in a free path region.

Fig. 1 shows the overall flow diagram of a preferred embodiment. The optical signal to be analysed enters an optical relay and conditioning element 1 which may for example collimate the beam and optimise its shape to match the aperture of the remainder of the optical system. The optical signal is then passed to a dispersing Fourier Transform spectrometer 2 which may be of any convenient type providing that it forms a spatially dispersed image of the Fourier components of the spectrum of the input optical system. The output of the spatially dispersing Fourier Transform spectrometer is then passed to another relay and conditioning element 3 and then enters an optical image recognition device 4. Electrical signals 5 will then pass from the optical image recognition device 4 to further data-processing or actuation devices 6. The electrical signals may encode the level of spectral match of the input optical signal to specific spectral patterns or they may be simple logic levels.

Figs. 2 to 4 illustrate examples of Fourier Transform spectrometers which may be used.

Referring now in particular to Fig. 2. Light from a light source 21 is converted into a parallel beam by a condenser lens and collimating lens system shown schematically at 23, 24 and is then incident on a beam splitter 24. The clockwise propagating beam then encounters mirror 25, mirror 26, beamsplitter 24, imaging lens 27 and image plane 28. The anti-clockwise propagating beam by contrast encounters beamsplitter 24, mirror 26, mirror 25, beamsplitter 24, imaging lens 27 and image plane 28. Tilting movement of mirror 26 as indicated by dashed line 29 introduces a variation in optical path difference with movement in the image plane from the centre 30 to either side 31, 32.

Referring now in particular to Fig. 3. Light from a light source 41 is converted into a parallel beam by condenser lens 42 and collimating lens 43 and is then incident on a linear polariser 44 which has its axis at 45°. The light is then incident on a three component polarising prism 45 of the Wollaston type. Such prisms are produced from optically active materials such as Calcite, Magnesium Fluoride or Quartz. Optically active materials have the property that they exhibit different index of refractions for different polarisations. As a consequence of this property a ray striking an interface with this material at an angle of incidence that is not at 90° to the tangent to the surface at the point of intersection will be refracted by an angle which depends on the polarisation of the light and the alignment of the crystal structure of the material. At the first interface 46 the light is incident at right angles and passes straight through. At the second interface 47 the light resolves into its

two constituent polarisations 48, 49 and at the third interface 50 the two rays are refracted to form a converging system which process continues at the final interface 51. The rays are then resolved again into the same polarisation plane by a second polariser 52 to produce a fringe pattern at the image plane 53. At the central ray position 54 the difference in optical path between the mutually orthogonal polarisations is zero. This optical path difference increases for aperture positions 55, 56 away from the central position 54.

Referring now in particular to Fig. 4. Light from a light source 61 is converted into a parallel beam by condenser lens 62 and collimating lens 63 and is ^■ then incident on a quartz prism pair 64, the angles 65, 66 of this prism pair being varied from exact right angles. The ray then is incident on a partially reflecting surface 67 with half the ray proceeding into the air-gap 68 and half reflected towards the fully reflecting surface 69. This first ray then passes through the beamsplitter 67 and the air-gap 68 to the second prism and then to an imaging lens 70. The alternative ray path is through the beamsplitter 67 and the air-gap 68 to the fully reflecting surface 71, back to the air-gap 68 and the beamsplitter 67 and then to the imaging lens 70. The optical path difference increases for zones away from the central fringe 72.

Referring now to Fig. 5, this shows the wavelength absorption spectrum of benzene, an aromatic material that is often desirable to measure the presence of. In Fig. 5 the vertical scale indicates relative intensity and the horizontal scale shows wavelength in nanometres.

Figure 6 shows the interferogram that results after an

optical input signal from broadband light from, for example, a xenon arc lamp, is passed through a region containing benzene. Such an interferogram may be recorded by placing a linear photodiode array at the image plane of a spatially dispersing Fourier Transform spectrometer with the axis of the linear array at 90° to the linear fringes. In Fig. 6 the vertical scale is relative intensity and the horizontal scale is photo- diode number assuming a 1024 element photo-diode linear array.

Fig. 7 shows a grey scale image of the linear fringes shown in Fig. 6. The interference fringes produced by the spectrometers of interest are linear for all practical purposes, provided the effective mirror separation is small. Such an image could be recorded by placing a photographic plate in the image plane of a spatially dispersing Fourier Transform spectrometer. For clarity the images only show the central portion of the fringe pattern.

Fig. 8 shows a sectional strip formed by enlarging the central area of the image of Fig. 7.

Fig. 9 shows two sectional strips combined, one being the positive image and the other being its reversal or negative in the conventional photographic sense of 'positive' and 'negative'.

Referring now to Fig. 10, this shows the operation of the preferred embodiment of the optical image recognition device referred to as element 4 in Fig. 1. The spatially dispersed fringe pattern 81 is superimposed on grey scale transmissive strips 82, 83 which are the Fourier components of the spectral feature that it is desirable to detect the presence and

level of. The light that passes through these strips 84, 85 then passes to an optical detector 86, 87 which spatially integrates the signal by use of, for example, a collection lens 88. The two electrical signals 89. 90 then pass to an electrical circuit 91 which compares the two signals and thus deduces the proportion of optical signal that matches the particular spectral profile that has been selected to examine.

Referring now to Fig. 11, this shows the use of the preferred embodiment to examine the presence of a particular material in a path through air containing the particular material, for example benzene.

Light from a light source, conveniently a Xenon Arc Lamp 101 enters an optical relay and conditioning element 102 which for example may collimate the beam. The light beam then passes through a region 103 which contains the material to be detected. Further relay and conditioning of the signal by means 104 then may be required before the light enters the spatially dispersing Fourier Transform spectrometer, shown here as of the Modified Michelson type 105. The output image which consists of fringes encoding the Fourier components of the absorption spectrum of the material present in region 103 then passes to the optical image recognition device 106 where it is spatially filtered by one or a multiplicity of strip spatial filters 112. The light passing through these filters is then collected by for example a cylindrical lens 107 and passed to one or more detectors 108. The signals from these detectors then pass to signal conditioning electronics 109 which compares them by sum, difference and ratios and then outputs the results to a monitoring or display system 110.

In addition to the analysis of known light passed through a substance of interest, the invention may be used to examine characteristics of the light source itself. In this case, the light source could be the sun or another astral body, a luminous gas or plasma, or light produced by Raman type emissions stimulated by laser.

The spatial filters 112 referred to above may be made by any convenient method. By using traditional photographic techniques spatial filters may be made that have a continuous grey scale representation of the intensity of Fourier component at that particular position. Alternatively under particular conditions, for example where identification of the presence of material by examination of its characteristic spectral signature is required in the presence of many other materials, it may be desirable to use a film structure of the 'Photolith' type which has a very steep gamma response curve. If traditional photographic substrates do not have adequate transmission characteristics then it may be desirable to use photographic representation using chrome on quartz substrates defined by etching processes and the like. Alternatively, a selectively reflective mask may be used as a filter, rather than a transmissive filter.

It may not be possible or desirable to form the spatial filters by direct exposure photographic processes. Under these circumstances it may be useful to generate the required Fourier Transforms using a computer and directly writing the spatial filters from computer generated information. Such an approach would be of particular value where extraneous information needs to be suppressed. It is often useful to apply greater 'weighting' or 'confidence' on signals at the centre of

a Fourier Transform pattern than those at the extremes. Such an approach is referred to as apodisation and all the normally used apodisation algorithms may be conveniently and readily applied by building them in to the computer generated spatial filters so that they may be applied directly to the optical measurement.

It will be seen from the foregoing that the present invention provides an apparatus for measuring the amount of optical signal within an overall optical signal that conforms to particular spectral characteristics. The apparatus may be used for detecting and measuring changes within a light emitting source, or for detecting and measuring change produced by chemical species through which broad band optical signal transmits.

The invention may be used to ascertain the amount of light that is emitted conforming to particular spectral characteristics from a chemical species following excitation by electric or optical means.

In summary the apparatus according to the present invention is characterised in that the light to be analysed is introduced into a spatially dispersive Fourier Transform spectrometer. The resulting image which consists of interference fringes is then relayed into an analyser which superimposes the image onto a single or a plurality of spatial filters. The light passing through the spatial filter(s) is then detected. Such detection can be spatially discriminatory or can integrate over space. By comparison of the signal passing through the various filters the level of match to particular spectral characteristics can be ascertained and the properties of the light source inferred.

The invention may also be used to analyse the colour of a light source, or the colour produced by reflection from or transmission through a substrate.

Obviously many modifications and variations of the present invention are possible and therefore it is to be understood that the present invention is not limited to the above described preferred embodiment but may be otherwise embodied within the scope of the following claims.