GB2113832A - Electromagnetic radiation scanning of a surface - Google Patents

Abstract

A method of scanning a surface with electromagnetic radiation from a source thereof, wherein a detector is employed preferentially to detect a reflection of the incident radiation from the source and/or emission from the surface being scanned as a result of the incident radiation, the scanning is effected by reference to a time cycle so that any given point in an image formed by the reflected and/or emitted radiation can be referred to some instant in the cycle, a signal is produced representative of the occurrence of the detection of the image point and the time thereof in the cycle, and the signal recorded. The source may be a laser and the detector a camera. The surface contour of a tunnel or the shape and dimensions of an article may be determined on drawings or characters identified.

Description

SPECIFICATION Method and apparatus for electromagnetic radiation scanning of a surface The present invention relates to a method and apparatus for scanning a surface with an electromagnetic radiation.

It is known to scan a surface with such radiation for determining some characteristic of the surface; a detector, for instance, a camera, being used to detect the radiation reflected by the surface to permit the determination to be made.

The scanning may be to determine the contour of the surface, e.g. the contour of a tunnel surface in a diametrical plane thereof, or the profile or the shape and dimensions where the surface is that of an article or object; or to identify a drawing or sign or character, whether printed or not, appearing thereon.

It is an object of the present invention to provide a method and apparatus for scanning a surface by electromagnetic radiation, which facilitates the handling and/or utilisation of data provided by the scan.

As intimated above, a camera may be used as the detector. In this specification, unless otherwise expressly stated or unless the context otherwise requires, the word "camera" is used to mean any device capable of forming an image of an object by electromagnetic radiation reflected therefrom or emitted thereby, and of recording the image and/or of producing signals which can be used for producing a replica of the image. It thus includes a photographic camera, whether a movie camera or one taking individual exposures, and also a video camera such as a T.V. camera or a CCD camera.

A particular ccnvenient form of electromagnetic radiation, at least in many applications, is that of a laser beam and the present invention will be described hereinafter with reference to the use of electromagnetic radiation in that form. However, it will be understood that this is done purely in the interests of convenience of description and that the present invention generally is not limited to using a laser beam as the electromagnetic radiation.

In the present invention, the scanning apparatus is arranged so that the detector detects preferentially a reflection of the incident radiation from source and/or emission from the surface being scanned as a result of the incident radiation rather than any other radiation; and the scanning is effected by reference to a time cycle so that any given point in an image formed by the reflected and/or emitted radiation can be referred to some instant in the cycle, a signal is produced representative of the occurrence of the detection of the image point and the time thereof in the cycle, and the signal recorded. It is preferred to establish the time cycle as a scan sequence comprising a predetermined number of scan positions. The scan signals then represent the count from start of the sequence to the position at which any given signal is produced.The scan sequence may take the form of a frame comprising a plurality of lines each of a given number of scan positions and each scanned in turn; the scan starting at some fixed position preferably at the top or bottom of the frame at an end of the respective line. This is particularly convenient for scanning a page of written or printed characters or a drawing of an article to be produced by a machine or even the article to permit a drawing thereof to be produced.

In using a scan sequence, a video camera is a particularly convenient form of detector with the normal frame and line sync pulses being used to establish timing data. For instance, each frame pulse can be used to identify the particular scan sequence being made while the line pulse is used to start a counter and any detector signal produced in the sequence used to operate the counter to produce an output representing the position in the line at which the detector signal occurred. However, a detector such as a single diode could be used together with steered beam and/or steered movement of the thing being scanned; the steering mechanism producing timing signals in an analogous manner to the video camera.The information provided by the scan may be held in a memory arranged to correspond to the scan sequence so as automatically to address a signal originating with the detector to the relevant part of the memory. In the case where a scanning frame is employed, the memory will be in the form of a corresponding matrix providing a memory element for each scan position. For instance, in a scanning e.g. a page of characters, the memory may be arranged as, say, a 7 x 7 matrix to build up an image of a letter; the signals originating with the detector being stored in the memory in terms of X and Y co-ordinates.

Such a matrix would, of course, be used for each possible character position on the page.

A computer may be used to recognise the stored data and to read out the memory in a suitable code. The code will depend upon the purpose the data are to be used for. It may be a machine code such as the ASC II code e.g. to operate a printer or machine tool. In the case where the data relates to a page of written or printed words, the code conversion may be one producing a translation of the language of the original text e.g. into a foreign language or into braille or even into sound i.e. the spoken word.

The data however may relate to the drawing of an article to be produced by a machine. With the data being converted to a machine code for operating the machine, or it could relate to the article with the data being converted to a code to operate a drawing machine or some kind of image reproducer.

As an alternative, the signals from the detector may be stored into a permanent store such as a magnetic tape or drum, film, video tape or floppy disc, either directly or after hardware conversion to a suitable code. In the alternative, the signals originally with the detector may first be digitised so that each digital signal provides the information as to the time in the cycle the detector produced the signal.

In the case where the detector must also record the intensity of the reflected beam, for instance in scanning the surface of a tunnel, the output signals to be recorded must additionally provide this intensity information. This may be done by digitising the analogue signals produced by the detector and storing them in a memory such that the positional information recorded as described above in a permanent store can then be used to control read out from the memory to permit a representation of the original image to be reproduced.

Scanning may be effected by the use of plural laser beams (which may be derived from a single beam from source by means of a beam splitter) rotated at a speed to produce the effect of a 'wall of light'. In the case of scanning a page of printed or written material the wall is then tracked vertically down the page so that each line of characters and each intervening space is scanned in succession. Two sets of laser beams may be used each inclined at an angle between vertical and horizontal to describe the surface of a cone with the two cones being set base to base and with the periphery of each base meeting at the surface to be scanned. More than two sets of laser beams could be used in like manner. Use may also be made of superimposition and of colour per se as will be explained below.Further diverging or converging radiation can be used by introducing beam expanders and/or other lens combinations, e.g. cylindrical lenses; fibro optics, scanning mirrors, axions, prisms, obstructions, gratings, holograms and/or accoustic-optical deflector.

To achieve optimum resolution by the detector of the reflected light, the special and polarisation properties, and intensity distribution of the incident beams or beam and the fluorescent properties of the surface may be made use of. In the particular case of a laser beam, for instance, the emission from source extends over a known narrow frequency band. By the use of a spectral narrow band filter before the detector, the detector will only "see" light of that frequency band. Again, the intensity distribution of the incident beam on the surface is of gaussion form with the peak intensity rising from points radially of the beam axis to a maximum at the axis of the beam. By using a threshold device in the detector, the detector can be rendered incapable of detecting or recording intensities less than the maximum by more than a predetermined amount.

Thus, in using the video camera, the line by line video signal may be sent to a threshold device to record the video signal only if it exceeds the threshold; the threshold detector being arranged then to stop or pulse the counter to indicate the instant of operation of the device. This in itself assists in reducing the amount of information needed to represent the reflected beam and also renders the detector less susceptible to extraneous radiation. The degree of reflection by a given surface of the laser beam is also dependent on the angle of polarisation of the incident beam in relation to the incident surface; and by choice of the angle of polarisation of the laser beam and by using a suitable polarisation filter before the detector, the intensity of the reflected beam can be increased.The surface itself may also modify the reflection due to its fluorescent properties and the detector can be arranged to recognise these properties in terms of the intensity, polarisation and spectral properties of the fluorescence. It is conceivable that, in some instances, it might be sufficient to detect solely the fluorescence but, in instances where this is not so, setting up the detector to recognise fluorescence characteristics can assist in increasing the resolution of the system and in reducing the amount of data to be handled.

In using a detector such as a video camera, the above techniques lead to a considerable increase in the signal to noise ratio of the detector output; and the high resolution that may be achieved by these techniques in turn facilitates associating intensity information, if any, with timing or positional information and thereby the need to hold only intensity information in the memory employed.

The present invention, as intimated above, may readily be used to scan, for instance, an engineering drawing of an article and to produce therefrom information needed to programme one or more machine tools producing the article.

Likewise, it may be used to control a machine tool carrying out any machining operation of an article from written or drawn information. Conversely, it may be used to establish the shape and dimensions of an article and to programme a drawing machine to produce a drawing of the article whether on the same, a reduced or enlarged scale.

In scanning for contour of a tunnel or like structure, then by use of a formula to express the mass of the material forming the contour, the centre of gravity of the contour scanned can be determined as a single point representing the centre of gravity of the contour. This is analagous to reproducing the contour on a sheet, cutting out the contour, rotating each of two or more equi angularly spaced points on the contour to lie uppermost with the contour lying in a vertical plane and drawing on the sheet for each point in turn when so positioned a straight vertical line in said plane and noting the point at which the lines intersect, that point being the centre of gravity. By carrying out the analogy of this operation in a computer using said formula expressing the mass of the material forming the contour, then, in respect of any one scan of a contour, only the information representing that point need be stored to permit a comparison with a future determination of that contour.

In using a camera as a detector, since its field of view is invariably wider than the width of the scan in any direction parallel with the surface, it is feasible to detect more than one scan whether effected simultaneously or not. The simultaneous or sequential scans may be produced by beam splitting and/or by using more than one laser source. Further, background lighting can be used and the background recorded by a camera so as to provide a record thereof should a fault be located.

Sequential scanning may be used to obtain a plurality of readings at a given location for the purpose of averaging random variations in any one scan to obtain a more accurate indication of the surface properties.

If, in carrying out a scan, a reference orientation or axis is needed, a separately set up laser may be used in some fixed relationship with the detector to provide a reference input thereto or it may be, in some instances, feasible to use part of the beam of the laser being used to effect the scan.

In a further embodiment, which may be used independently of the above described invention.

use may be made of colour in effecting a determination so that the colour per se provides information about a contour. Thus, in scanning the surface of a structure such as a tunnel, a first scan to be made could be effected using light of one colour and a second scan at the same location carried out using a light of a different colour. The scans are then reproduced on a colour video display as lines. If the contour has not changed in the interim between the two scans and, assuming the scans are displayed in the same orientation, the two lines will coincide and produce a third colour. To the extent that they do not coincide, this indicates that a deformation of the contour has taken place and the lines will separate or partly overlap in the display to give corresponding colour changes thereon.If the two scans were effected in a diametrical plane, their orientation on the display will automatically be the same. If not, adjustment could be made in the display to bring this about. Rather than use different colour sources, the different colours may be created in the video camera (if a colour one) or in the display unit by use of the appropriate colour channel.

Further, it will be appreciated that a plurality of scans may be carried out each in respect of a different colour. The scan on each occasion may be carried out with a single beam or by use of two light "cones" base to base. In the latter instance, the beams can be arranged so that variations in the radial distance of the surface will appear as a partial or complete line separation. If the two cones are of different colours, then again colour changes will appear on the display as line separation takes place.In using two or more scans whether effected simultaneously or not, and the plurality of scans in toto is taken as representing the surface then it follows that any deformation therein or mark thereon will produce a greater change on the colour and/or intensity and/or polarisation and/or fluorescence of the reflected and/or omitted light than would be the case if the surface is left to be characterised by a single scan.

Similarly in carrying out one scan with beams that are intended to intersect at the surface. Further, the coherent properties of the light may be used to assist place resolution, and to maximise the signal to noise ratio in the case where the scan is represented by electrical signals. Thus, the mixing of two coherent wave fronts will lead to a beat frequency or phase signal and the signal can be detected to give a greater intensity resolution with the given intensity distribution of the beams themselves i.e. the greater the intensity produced by the beams the greater will be the magnitude of the beat signal.

This further embodiment may be used to detect any radiation or indeed oscillation, whether electromagnetic or acoustic, the detected oscillation being converted to an electrical signal representative thereof which is then displayed on a display screen along with the trace of a previous detection of the oscillation. The display screen may be a video screen, an oscilloscope, VDU screen of a computer, a film screen, a film projection screen, slides, holograms or fibre optics, and can be activated with electromagnetic radiation or other energy, e.g. an electron beam on a phosphor of a T.V. screen. It is also possible to use electrical energy directly, e.g. on electro illuminescent screen. In this instance, the two traces need not be in different colours.For instance, the arrangement could be one in which, provided the two traces stay within a predetermined tolerance as to the separation between them, the comparison is regarded as being acceptable; but, if the two traces diverge beyond the tolerance, a signal is automatically provided to indicate this.

The traces may in any event be scanned by a camera or diode and the information processed as described above. This permits a determination to be made of the extent of the divergence at any instant in the scan cycle and therefore at the instant when said signal indicating a divergence beyond the set tolerance is produced as the case may be. In this connection, a camera or scanning diode in combination with a computer can be used as a transducer. For instance, a microphone will display a voice as a trace on an oscilloscope and a camera with related hardware may then be used to digitise the signal so that a computer with related hardware can be used to manipulate the data, e.g. instruct a robot, or store the data on a peripheral. On the other hand, the data reduction can be applied as described above where the display screen can be filmed or video taped for data management.

Claims (49)

1. A method of scanning a surface with electromagnetic radiation from a source thereof, wherein a detector is employed preferentially to detect a reflection of the incident radiation from the source and/or emission from the surface being scanned as a result of the incident radiation, the scanning is effected by reference to a time cycle so that any given point in an image formed by the reflected and/or emitted radiation can be referred to some instant in the cycle, a signal is produced representative of the occurence of the detection of the image point and the time thereof in the cycle, and the signal recorded.

2. A method according to claim 1, wherein the time cycle is established as a scan sequence comprising a predetermined number of scan positions whereby the scan signals then represent the count from start of the sequence to the position at which any given signal is produced.

3. A method according to claim 2, wherein the scan sequence takes the form of a frame comprising a plurality of lines each being of a given number of scan positions and each being scanned in turn; and the scan is started at some fixed position in the frame.

4. A method according to claim 3, wherein the scan is started at the top or bottom of the frame at an end of the respective line.

5. A method according to claim 2, 3 or 4, wherein a video camera is used as the detector with the normal frame and line sync pulses being used to establish timing data.

6. A method according to claim 5, wherein each frame pulse is used to identify the particular scan sequence being made while the line pulse is used to start a counter and any detector signal produced in the sequence is used to operate the counter to produce an output representing the position in the line at which the detector signal occured.

7. A method according to claim 2, 3 or4, wherein the detector is a single diode; and a steering mechanism is used to steer the incident radiation and/or to steer movement of the surface being scanned; the steering mechanism producing timing signals in an analogous manner to a video camera.

8. A method according to any of claims 1 to 7, wherein information provided by the scan is held in a memory arranged to correspond to the time cycle so as automatically to address a signal originating with the detector to the relevant part of the memory.

9. A method as claimed in claim 8 as dependent on claim 3, wherein the memory is in the form of a corresponding matrix providing a memory element for each scan position.

10. A method according to claim 9, wherein the memory is arranged as a matrix to build up an image of a letter; the signals originating with the detector being stored in the memory in terms of X and Y co-ordinates.

11. A method according to claim 10 for scanning a page of characters, wherein said matrix is employed in plurality, with one matrix being used for each possible character position on the page.

12. A method according to any of the preceding claims, wherein a computer is used to recognise stored data and to read out the memory in a suitable code.

13. A method according to claim 12, wherein the code is a machine code to operate a printer or machine tool.

14. A method according to claim 13, wherein the code is the ASC II.

15. A method according to claim 12, 13 or 14.

wherein, in the case where the data relate to a page of written or printed words, the code conversion is one producing a translation of the original text into another language or into braille or into sound.

1 6. A method according to claim 13 or 14, wherein the data relate to the drawing of an article to be produced by a machine tool and the data are converted to a machine code for operating the machine tool to produce the article.

17. A method according to claim 13 or 14, wherein the data relate to an article and the data are converted to a code to operate a drawing machine or some kind of image reproducer.

1 8. A method according to any of claims 1 to 11, wherein the signals from the detector are stored in a permanent store either directly or after hardware conversion to a suitable code, with the signals originating with the detector first being digitised so that each digital signal provides the information as to the time in the cycle the detector produced the signal.

1 9. A method according to any one of the preceding claims wherein, in the case where the detector is also to record the intensity of the reflected radiation; the output signals from the detector are digitised and stored in a memory; with the timing information being recorded in a permanent store and being used to control read out from the memory to permit a representation of the original image to be reproduced.

20. A method according to any of the preceding claims, wherein a scanning is effected by the use of a laser beam.

21. A method according to claim 20, wherein a plurality of laser beams are employed derived from a single beam from source by means of a beam splitter or from separate sources, and rotated at a speed to produce the effect of a 'wall flight'.

22. A method according to claim 21, wherein, in the case of scanning a page of printed or written material, the 'wall of light' is tracked vertically down the page so that each line of characters and each intervening space is scanned in succession.

23. A method according to claim 21 or 22, wherein two laser beams are used each angled to describe the surface of a cone with the two cones being set base to base and with the periphery of each base meeting at the surface to be scanned.

24. A method according to claim 24, wherein further laser beams are used in the manner defined in claim 23.

25. A method according to any of the preceding claims, wherein diverging or converging radiation is used.

26. A method according to claim 25, wherein said diverging or converging radiation is produced by beam expanders and/or other optical combinations.

27. A method according to claim 26, wherein cylindrical lenses, fibro optics, scanning mirrors, axions, prisms, obstructions, gratings, holograms and/or accoustic-optical deflector constitute or constitutes said other optical combination.

28. A method according to any of the preceding claim, wherein the detector is such as to be able to detect spectral and/or polarisation properties and/or intensity distribution of the incident beams or beam and/or the fluorescent properties of the surface.

29. A method according to any of the preceding claims 1 to 21 , wherein, in scanning for contour of a tunnel or like structure, a formula is used to express the mass of the material forming the contour to determine the centre of gravity of the contour scanned as a single point and the parameter representing the centre of gravity stored.

30. A method according to any of the preceding claim, wherein a camera is used as the detector simultaneously or sequentially to effect a plurality of scans of different locations.

31. A method according to claim 30, wherein the incident radiation is produced by beam splitting and/or by using more than one radiation source.

32. A method according to claim 30 or 31, wherein background lighting is used and the background recorded by a camera so as to provide a record thereof to permit a fault to be located.

33. A method according to any of the preceding claims, wherein a plurality of scans are effected at a given location for the purpose of averaging random variations in any one scan to obtain a more accurate indication of the properties being scanned.

34. A method of determining a contour by scanning, wherein use is made of colour in effecting a determination so that the colourperse provides information about a contour.

35. A method according to claim 34, wherein, in scanning the surface of a structure such as a tunnel, a first scan is made using light of one colour and a second scan is made subsequentially at the same location using a light of a different colour, and the scans are then reproduced on a colour video display as lines, whereby if the contour has not changed in the interim between the two scans and, assuming the scans are displayed in the same orientation, the two lines will coincide and produce a third colour.

36. A modification of the method of claim 35, wherein, rather that use different colour sources, the different colours are created in a colour video camera or in a colour display unit by use of the appropriate colour channel.

37. A method of determining a contour by scanning, wherein the or each scan is carried by use of two light "cones" base to base, and the beams are arranged so that variations in the radial distance of the surface will appear as a partial or complete line separation.

38. A method according to claim 37, wherein the two cones are of different colours, whereby colour change will be produced on the display of the scans as lines if line separation takes place.

39. A method according to any of the claims 34 to 38, wherein, in using two or more scans of any one location, whether effected simultaneously or not, the plurality of scans in toto is taken as representing the surface whereby any deformation therein or mark thereof produces a greater change on the surface property being detected than would be the case if the surface were left to be characterised by a single scan.

40. A method of scanning a surface, wherein coherent properties of light are used to assist place resolution, and to maximise the signal to noise ratio in the case where the scan is represented by electrical signals.

41. A method according to claim 40, wherein two coherent wave fronts of respective beams of oscillating energy are used to produce a beat frequency or phase signal and the signal is detected to increase the intensity resolution with the given intensity distribution of either the beams themselves.

42. A method according to claim 41, wherein the detected oscillation is converted to an electrical signal representative thereof which is then displayed on a display screen.

43. A method according to claim 42, wherein the display screen is a video screen, an oscilloscope, VDU screen of a computer, a film screen, a film projection screen, slides, holograms or fibre optics.

44. A method according to claim 43, wherein the display screen is activated with electromagnetic radiation or other energy.

45. A method according to claim 44, wherein the screen is an electro-illuminescent screen and is activated with electrical energy.

46. A method according to claim 42, 43, 44 or 45, wherein if any two traces on the screen of a given location stay within a predetermined tolerance as to the separation between them, the comparison is regarded as being acceptable; but, if the two traces diverge beyond the tolerance, a signal is automatically provided to indicate this.

47. A method according to any of claims 42 to 46, wherein traces on the screen are scanned by a camera or diode and the information stored; said camera or diode constituting said detector in the method claimed in any of preceding claims 1 to 33.

48. A method of scanning a surface substantially as hereinbefore described.

49. Apparatus for effecting the method of any of the preceding claims, substantially as hereinbefore described.