WO1995024621A1

WO1995024621A1 - Coherence imaging system

Info

Publication number: WO1995024621A1
Application number: PCT/GB1994/000454
Authority: WO
Inventors: Philip Sutton; Herbert Alan French; Andrew Shaw; Duleep Wickramasinghe
Original assignee: The Secretary Of State For Defence
Priority date: 1994-03-09
Filing date: 1994-03-09
Publication date: 1995-09-14

Abstract

The invention concerns an optical detection system for coherence mapping of a field of view wherein the received light is separated into a plurality of pixels and interference means process, the light from each pixel as a function of path difference, to provide parameters characteristic of the coherence function for each pixel. A coherence image may be displayed. The interference means may comprise a Michelson interferometer with one stepped mirror, or an array of interference modules, such as Fabry Perot elements.

Description

Coherence Imaging System

The invention relates to optical detection systems employing processing of received radiation in the optical domain prior to electronic detection and in particular to systems for coherence imaging.

The coherent state of light received by a detector from an object (whether illuminated or self luminous) provides a broad description of various mechanisms associated with the light: its generation, its scattering from objects and its transmission through the medium to the detector. The state of coherence can be determined by performing various types of temporal and spatial correlations on the received optical field.

The classical description of light as a sinusoidal waveform is applicable only in an idealised limiting case. Light from practical sources show fluctuations in intensity and phase due to various mechanisms in the sources, in the transmission medium, and on the detector. Hence a random function description, with appropriate statistical characteristics, is more suitable for optical radiation.

To describe such a random function completely, correlations of all orders are necessary. However, for certain classes of stationary random functions such as Gaussian, limiting the description to lower order correlations, the first and second, is sufficient. The first order correlation is the average value, while the second order measures the intensity as well as correlation of the disturbance between two space/time positions. Observing the second order correlation at a position between two different times gives the degree of temporal coherence, while observing this correlation at different points in space at a given time gives the degree of spatial coherence.

Whether light is emitted from a self luminous source or scattered from other objects, it carries spectral information about the source or scatterer. An instrument capable of forming an image of the different physical and chemical properties of emitting and scattering media, provides much more information than a simple picture of the spatial distribution of the corresponding objects.

The object of the invention is to provide means for coherence imaging by measurement of the temporal coherence, such that the state of coherence of . light from an object field is mapped to form an image displaying the coherence characteristics of objects at their respective positions in the image.

The invention provides an optical detection system for coherence mapping of a field of view comprising: means to receive light from the field of view and to provide as an output light from a plurality of picture elements or pixels within the field of view; interference means to process light from each pixel so as to divide the light into two beams and recombine them with a range of path differences therebetween; light detection means to detect the interference patterns produced by the recombined light beams; computing means to process the signals from the detection means for each pixel as a function of path difference and to provide parameters characteristic of the coherence function for each pixel; and display means connected to the computing means to receive the coherence parameters and display them as a coherence image of the field of view.

In one form the light receiving means may include a scanning arrangement to sequentially provide light from pixels within the field of view. In this form the interference means may be a single Michelson interferometer.

The interferometer may have two mirrors in the respective light paths of the two divided beams and in one arrangement one of the mirrors may be stepped so as to provide a range of path lengths. By using a stepped mirror problems associated with large data volume and consequent large computational effort for coherence image formation are alleviated. In an alternative arrangement the range of path lengths may be provided by providing for cyclical movement of one mirror.

At least one spectral filter may be included in the light path before the interference means for spectral pre-processing of the received light for target/background contrast improvement.

In a further form of the invention the light receiving means includes a micro-lens array to provide a plurality of separate input pixel light beams for optical processing by the interference means. The interference means may comprise an array of interference modules with each interference module corresponding to a different pixel in the optical field of view. The interference modules may be Fabry-Perot or Fresnel bi-prism elements. In these arrangements a detector array is used. Since the Fabry-Perot and Fresnel bi-prism interference elements provide a sheared interference pattern for each pixel in the field of view the detector array has N x L detectors where N is the number of pixels and L is the number of detectors required to provide the interference fringe/coherence information. The advantage of a shearing method is that an instantaneous interference pattern is produced whose visibility profile can easily be measured or photographed.

Advantageously the detection means will monitor the phase of the signal in the interference patterns. By monitoring the phase of the interference signal the computing means can provide as a display parameter the path difference at which a discrete phase change occurs. In an arrangement employing amplitude measurements from the detector the output signals from the detector are low pass filtered so as to provide a signal representing the envelope of the coherence function. In this arrangement a display parameter based on the amplitude and the width of the envelope can be calculated.

The computing means may include signal processing techniques to enhance the detection and display of the coherence image.

The invention will now be described by way of example only with reference to the accompanying Drawings of which:

Figure 1 shows a schematic representation of a detection system receiving light from a field of view; Figure 2 illustrates a Michelson interferometer based coherence imaging sensor with a scanning mirror;

Figure 3 is a graphical representation of the coherence profile measured by the Figure 2 sensor;

Figure 4 is a modification of the Figure 2 arrangement using a stepped mirror;

Figure 5 illustrates a schematic representation of the invention employing parallel processing;

Figure 6 illustrates a part of a micro-lens array used in the Figure 5 arrangement; and

Figures 7 and 8 respectively show Fabry-Perot and Fresnel bi-prism module elements used in the modular interference array of Figure 5«

In a conventional imaging system 10 the intensity distribution from points such as 11 and 12 of the object field 13 is mapped into the image field. Further information about objects in the field of view, however, can be obtained if information about the temporal coherence of light from the two points 11 and 12, for example, can be determined.

To measure the degree of first order temporal coherence of light entering the receiver 10 an amplitude interferometer is used. This measures the quality of interference due to superposition of a radiation field with a delayed version of itself. By varying the relative delay between the two components, a coherence profile is obtained. With a suitable optical arrangement at the input, the coherence profile of light from a small element of the object field can be obtained. At the origin, where the path difference is zero, the non-normalised coherence function is simply the intensity and thus the conventional intensity detection system can be considered as a special case of the coherence measuring system. Generally, however, the measured coherence function is normalised with respect to the zero path difference value to give the complex degree of coherence.

If light from a field of view as shown in Figure 1 is expressible as a constant amplitude sinusoidal temporal wave, then the disturbance at the two points 11 and 12 from which light emanates is related with a linear phase dependent on the separation. In this case the resultant correlation function seen by the light receiver 10 will also be another sinusoidal function. If however, the phase or amplitude is fluctuating in time, the correlation will be reduced. For example, with a disturbance whose phase fluctuation increases with time, the correlation function decreases compared to that near the origin. The functional form of the correlation function is thus an indication of the underlying variation of temporal phase and amplitude fluctuations. This in turn makes it possible to ascertain information on the various processes involved in the generation and transmission of light, via the observed statistics.

The degree of first order temporal coherence (second order correlation) is related to the spectral profile of the radiation via the Fourier transform. Narrow frequency band radiation will display a relatively slowly varying coherence profile. Measurement of the degree of first order temporal coherence is equivalent to observing the complete spectral profile. Due to the Fourier relation between the coherence and spectral pictures, small features on one will be magnified in the other. Hence, fine spectral features, which may be beyond the resolving capability of a spectral instrument, will be resolvable through temporal coherence measurements.

A coherence function displaying a non-monotonic decrease would indicate the existence of processes (either in the generator or scatterers) with certain characteristic time scales. If these time scales are long compared to typical line width broadening mechanisms, such as due to macroscopic motion of the emitters or scatterers, the resulting frequency shifts would be buried within the broad emission. However, measurements of temporal coherence will bring out the long temporal effects. This shows that the degree of coherence is a more general measurement than simple intensity.

The temporal coherence image according to the present invention can be constructed using the arrangement shown in Figure 2. Light 20 from a field of view is scanned by means of a scanning mirror arrangement 21 through a telescope 22. The received light is incident on a Michelson interferometer 23 coupled to a computer 24 which controls the scanner and also the data acquisition by a detector 25 responsive to light transmitted through the interferometer. Light 20 from a small element of the field of view of the instrument is reflected by the scanning mirror arrangement 21 to a beam splitter 26 in the interferometer 23- One beam of light 27 reflected from a fixed mirror 28 is then combined with a second beam 29 reflected from an oscillating mirror 210 by the beam splitter 26 to produce an output beam 211 focused by lens 212 to the detector 25. The oscillating mirror 210 is attached to a piezoelectric crystal under the control of a signal from the computer 24 so as to produce an oscillating temporal delay in the in the beam 29 compared with the beam 27.

The electrical output from the detector 25 is a sinusoidal like oscillation 30 representative of the interference profile with an overall decaying envelope 3 as shown in Figure 3- Filtering this signal using a low pass (or band stop) filter then provides a signal representative of the envelope function 3 . which is recorded by the computer 24. The position of the scanning mirror 210 of the interferometer 23 (effectively measuring the path difference between the two recombined beams 27 and 29) is also monitored by the computer 24. The recorded signal representing the envelope signal as a function of the interferometer path difference (31) is the temporal coherence profile.

The scanning arrangement 21 is of conventional design using two mirrors rotεtable about mutually perpendicular axes to provide 2-dimensional x,y scanning of the field of view by the telescope. This enables a scanned picture of the temporal coherence profile of the object field to be formed.

Once the temporal coherence profile image has been recorded, it is then desirable to be able to display this in a readily comprehensible picture form. This is relatively difficult since the coherence profile corresponding to each image point in the field of view needs to be reduced to one or two numbers to enable display as a pixel.

If the profile could be approximated to a function such as a Gaussian or Lorentzian, then two parameters corresponding to the amplitude and width could be used to control the colour and/or brightness of the pixel. Using a parameter corresponding to the width, would show an image associated with the coherence length of the various emitters. Other possibilities are to use a measure of the area under a normalised coherence profile and measures of the spread of the visibility values and gradient of the coherence profile over selected regions of path difference. For profiles normalised to intensity, long coherence sources will result in larger areas under the corresponding curve than short coherence sources. Measuring the spread of the visibility values (eg. variance with a weighting function proportional to the path difference) indicates whether the coherence profile decays slowly or is sharply peaked at the zero path position, corresponding to long and short coherence sources respectively.

Comparing the statistics of the visibility data with the statistics of calibration targets may enable similar/dissimilar sources to picked out on the image. The gradient can be as a measure of the sharpness of the profile, over narrow regions of path difference known a priori.

A coherence imaging instrument in the current form shown in Figure 2 cannot be expected to form wide field images of non-stationary objects in real time due to the complete profile being recorded and processed for each pixel. However, it can be envisaged that the interferometer system and the detector electronics can be specially configured to measure certain characteristics of the coherence profile, to overcome the problems of large data volume and the computational effort required to form an image. One such approach is the arrangement shown in Figure 4. The Figure 4 arrangement provides an interferometer with fixed path differences by use of a stepped mirror 40 in place of a scanning mirror. Light from an optical imaging system 4l is separated by a beam splitter 42 into two beams 43 and 44 which after reflection respectively from a plane mirror 4 and the stepped mirror 40 are recombined by the beam splitter 42 to provide an output beam. A linear array of detectors 46. - 46₈ (as shown) are processed to provide measures of the fringe visibility of the spatial fringes, indicated by reference numeral 47. corresponding to the path differences introduced by the stepped mirror 40, in order to determine the fringe visibility profile 48. As in the Figure 2 arrangement the imaging system 4l scans a field of views to provide coherence image information. The processors 49- - 49₈ connected to the respective detectors 46. - 46₈ provide the means for extracting the fringe visibility.

The Michelson Interferometer arrangements described in Figures 2 and 4 ignore phase information in the measured signal. For example it can be shown that a change of phase occurs in the fringe patterns corresponding to a certain path difference and observation of this path difference can provide a sensitive determination of particular objects in a field of view. The choice of coherence feature to be used will depend on the nature of the object and its background.

The coherence imager thus far described operates in the same manner as an imaging Fourier Transform (FT) spectrometer. The instrument can be expected to perform satisfactorily over spectral regions where a FT spectrometer is used; the main advantage of a FT spectrometer over a conventional spectrometer being in the more efficient utilisation of the input light. This is described by the well known Felgett's and Jacquinot's advantages. A coherence imager, however, may not need to operate over the full spectral range of an imaging spectrometer and . is could lead to cost/complexity savings in use of optics that do not need chromatic correction and also in use of detectors of less uniform/wide spectral response.

Figure 5 is a schematic drawing of a further coherence imager consisting, for simplicity, of 7 separate modules 0 - 56. Not shown are transfer optics which might be needed between the optical modules nor electronic interfaces between the electronic modules.

Light from a field of view is collected by the input optics module 50. This module could be of any design, making use of reflecting or refracting components or both with the final design (as with all parts of the system) depending on technical requirements, cost, size and other limitations.

Light from the input optics module 50 is then transmitted through a lens module 51- This comprises a regular array of individual micro-lenses 60 as is shown in greater detail in Figure 6. This array may be necessary where the subsequent optical modules, the spectral pre-processing module 52 and the interference module 53 may (depending upon application constraints) have certain light entry-angle limitations. The lens module 51 is shown close to the image plane and generates n x m separate "collimated" light channels (where n and m are the numbers of micro-lenses 60 respectively along the X and Y axes) which propagate through the imager and thus each channel corresponds to an image pixel. This contrasts with the arrangements of Figures 2 and 4 where the optical components correspond to just one of these pixel channels.

The spectral pre-processing module 52, when required, can comprise any one or a combination of components such as interference filters, tunable acoustooptic or Fabry-Perot filters, coloured glass. Spectral pre-processing is included broadly for two reasons; a) to limit the optical signal to a spectral band of interest thus reducing the total light falling on the detectors and hence reducing the photon and shot noise levels; and/or b) to enhance some particular feature.

After spectral pre-processing the light is incident on the interference module 53. This module is the key component in the imager and it is designed to extract the defined coherence parameter. The module 53 can consist of a single large interferometer as previously described or, for example, it could comprise an n x m array of Fabry-Perot etalons, each comprising two partially reflecting mirrors 70 and 71 set at an angle A as illustrated in Figure 7• In this particular case, the mirror separation d would be chosen to introduce the desired path difference.

Alternatively, the n x m array could comprise a series of Fresnel bi-prisms 80 each designed to have a path delay achieved for example using a simple piece of glass 81 of thickness d' as shown in Figure 8.

A potentially advantageous feature of using an array of small interferometers in the interference module 53 is that the pixels do not have to have the same path difference on each interference component. Thus it would be possible to arrange for a selected area of the array, a "foveal patch", to look at some more subtle coherence feature whilst the surrounding elements are optimised for initial detection or detection of some secondary importance object.

In the Figure 5 arrangement as described, each element of the interference module 53 will generate a sheared interference pattern due to the angling of the etalon mirrors or the Fresnel bi-prism roof angle. Hence to read each pixel will require more than one detector element in the detector array module 54. Thus the detector array must comprise k x m detector elements where the number k = L x n, L being the number of detectors required to process the interference fringe/coherence information from one coherence image pixel and N = k x n being the total number of pixels.

After detection the signals from the detector elements are processed by an electronic processing module 55- The electronic processing would consist of hardware and software capable of extracting the desired coherence feature: absolute visibility, visibility ratio, position of phase change etc. This processing could be performed in parallel (processing all pixels at once) or in series (sequentially processing pixels). The result of the processing would then be passed to a display module 56 which could incorporate any combination of display techniques such as pseudo-colour, simple grey-scale, internally generated symbology.

As an alternative to the interference elements described, the individual elements of the interference module could comprise non-lateral shearing elements. For example, they could be electronically modulated to scan over a small distance (compared with path difference d) to generate a time-modulated waveform. In this case, only n x m detector elements would be needed with the coherence information being extracted from the individual time waveforms.

Another variation would be to combine the parallel (array) structure shown in Figure 5 with a single pixel concept, similar to the data gathering Michelson arrangement of Figure 4 to realise a serial/parallel scan system similar to current Thermal Imaging Common Modules. This approach moves the emphasis of complexity away from producing large component arrays towards the design of mirror scanners etc.

Coherence imaging as described above is concerned with presenting certain features of light from particular objects in such a way as to achieve enhanced contrast. Imagers responsive only to intensity of illumination do not exploit all the key features of the illumination. It is known that spectral processing and polarisation processing prior to signal detection can both enhance the contrast of certain objects. In its most general sense coherence imaging can take advantage of the benefits of all these processes.

Application areas for the invention can be addressed via two approaches: the physical effect leading to coherence difference and the industrial activity where that effect could be important. Possible physical effects: a) chemical/gas detection where atomic and molecular perturbations produce high gradients and characteristic structure in the spectral domain b) biological activity where various responses and activity cycles yield small changes in the interaction with EM radiation c) solid state physics were energy band-gap engineering can produce characteristic emission and absorption spectra d) microscopy and the general study of microscopic phenomena were scattering of EM radiation of comparable wavelengths lead to spectral changes e) "small structure" (compared to EM wavelength) physics where thin films or multi-layer (based on amplitude or phase gradients) effects cause interference which again affects spectrum

Industrial activity applications:-

a) Earth resource monitoring:

(1) crop activity

(2) geological changes

(3) build-up/decay of critical atmospheric gases and particles

(4) meteorology

(5) ocean life (eg algae) activity

(6) land, sea and air pollution b) Astronomy c) Process control including:

(1) surface finishes

(2) crack and fault detection (3) chemical impurities d) Safety monitoring:

(1) aging effects (decay etc)

(2) heating, radiation, chemical induced changes

Although the arrangements of the invention have been described in relation to measurements in the visible part of the electro-magnetic (EM) spectrum the principles are completely applicable across the EM spectrum and indeed to any wave-based phenomena where suitable components are available (the higher order coherence processing is not even restricted to just wave phenomena) .

Claims

1. An optical detection system for coherence mapping of a field of view comprising: means to receive light from the field of view; interference means to divide the received light into two beams and recombine them with a path difference therebetween; characterised in that there is provided: a) means to separate the received light into a plurality of picture elements or pixels within the field of view and the interference means processes the light from each pixel so as to divide the light into two beams and recombine them with a range of path differences therebetween; b) computing means to process the signals from the detection means for each pixel as a function of path difference and to provide parameters characteristic of the coherence function for each pixel; and c) display means connected to the computing means to receive the coherence parameters and display them as a coherence image of the field of view.

2. An optical detection system as claimed in claim 1 characterised in that the light receiving means includes a scanning arrangement to sequentially provide light from pixels within the field of view.

3. An optical detection system as claimed in claim 2 characterised in that the interference means is a single Michelson interferometer.

4. An optical detection system as claimed in claim 3 characterised in that the interferometer has two mirrors in the respective light paths of the two divided beams and one of the mirrors is stepped so as to provide a range of path lengths.

5- An optical detection system as claimed in claim 3 characterised in that the interferometer has two mirrors in the respective light paths of the two divided beams and there is provided means for cyclical movement of one mirror to thereby provide the range of path lengths.

6. An optical detection system as claimed in any one preceding claim characterised in that at least one spectral filter is included in the light path before the interference means for spectral pre-processing of the received light for target/background contrast improvement.

7. An optical detection system as claimed in claim 1 characterised in that the light receiving means includes a micro-lens array to provide a plurality of separate input pixel light beams for optical processing by the interference means.

8. An optical detection system as claimed in claim 7 characterised in that the interference means comprises an array of interference modules with each interference module corresponding to a different pixel in the optical field of view.

9- An optical detection system as claimed in claims 7 or 8 characterised in that the light detection means is a detector array.

10. An optical detection system as claimed in any one of claims 7 to 9 characterised in that the interference modules are Fabry-Perot elements.

11. An optical detection system as claimed in any one of claims 7 to 9 characterised in that the interference modules are Fresnel bi-prism elements.

12. An optical detection system as claimed in claim 10 or 11 characterised in that the detector array has N x L detectors where N is the number of pixels and L is the number of detectors required to provide the interference fringe/coherence information for each pixel.

13- An optical detection system as claimed in any one preceding claim characterised in that the detection means is arranged to monitor the phase of the signal in the interference patterns.

14. An optical detection system as claimed in claim 13 characterised in that the computing means provides as a display parameter the path difference at which a discrete phase change occurs.

15- An optical detection system as claimed in any one preceding claim characterised in that output signals from the detection means representing amplitude measurements of the interference patterns are low pass filtered so as to provide a signal representing the envelope of the coherence function.

16. An optical detection system as claimed in claim 15 characterised in that the computing means provides a display parameter based on the amplitude and the width of the envelope. 17- An optical detection system as claimed in any one preceding claim characterised in that The computing means may include signal processing techniques to enhance the detection and display of the coherence image.