SCANNING SYSTEM

Abstract: A light source directs beam of light photons to impinge on a first scanner to effect scanning of photons in a first general scan direction "line scan"; a second scanner subsequently effects scanning of said photons in a second general scan direction "frame scan" substantially at right angles to said first general scan direction thereby enabling an image to be perceived. The light from the source is typically modulated to transmit image information. The scanners are preferably synchronised with modulation of the image producing light beam by means detecting synchronisation light beams reflecting from the respective scanners, the synchronisation light beams being distinct from the image producing beam.

Description: The present invention relates to a scanning system and apparatus for use in a scanning system, particularly, but not exclusively, such a scanning system and apparatus for use in image projection. According to a first aspect, the present invention provides a scanning system comprising: first scanner means; a light source arranged to direct a beam of photons to impinge on the first scanner means to effect scanning of photons in a first general scan direction; second scanner means arranged to effect scanning of said photons in a second general scan direction substantially at right angles to said first general scan direction thereby enabling an image to be perceived. It is preferred that the first scanner means is arranged to effect a linear photon sweep or trace "line scan" across the second scanner means, the second scanner means scanning the line trace to effect a two dimensional "frame scan" image. The light beam directed toward the first scanner means is preferably modulated. It is preferred that synchronisation means is provided for the system arranged to enable modulated light sequences scanned from at least one of the scanner means to be accurately repeated, the synchronisation means comprising: i) a synchronisation beam distinct from the modulated image forming beam, the synchronisation beam being directed to impinge on at least one (preferably both) of the scanner means; and, ii) sensor means arranged to sense the synchronisation beam reflecting from the scanner means. Desirably, the synchronisation beam and the sensor means of the synchronisation means are configured such the sensor means senses the reflected beam proximate the beginning of the scan sweep of the reflected synchronisation beam. The synchronisation means advantageously comprises separate synchronisation beams (each distinct from the modulated image forming beam) , each synchronisation beam being directed toward a separate one of the first and second scanner means. Separate respective sensors are preferably provided, each arranged to detect respective synchronisation beams reflected from respective scanner means. The system preferably also provides a reference enabling a phase relationship between the scan rate of the first and second scanner means to be maintained. It is preferred that one or both of the first and second scanner means comprises reflector means having one or more light reflecting surfaces arranged to effect scanning of the light photons. Advantageously the reflector means comprise rotatable mirrors preferably a first one comprising the first scanner means and a second one comprising the second scanner means. Desirably the mirrors comprising the respective scanner means are arranged to be driven by respective motors. In one embodiment, the first scanner means may comprise a rotatable ultifaceted polygonal mirror, the second scanner means preferably comprising a driven rotatable element having a reflective surface extending about a rotational axis for reflecting the light beam, the reflective surface having a generally continuously curved surface profile about the rotational axis except for at least one localised angular step or discontinuity. In this embodiment it is preferred that the mirrors are rotated at differing respective speeds, the first rotatable scanner mirror effecting the "line scan" being rotated at a substantially greater speed that the rotational speed of the second rotatable scanner element effecting the "frame scan". The use of a rotatable reflector element having a reflective surface as defined having relatively few (preferably only one or two) localised steps or discontinuities provides significant technical advantages when used as a "frame" scanner downstream of a rotatable polygonal mirror having a relative multiplicity of facets used as a "line" scanner. This is because, for example, the low number of scans per revolution of the "frame" scanner relative to the "line" scanner enhances the resolution of the image. Furthermore, the "frame" scanner may be run at relatively high rotational speeds at which rotational stability is greater. According to a second aspect, the invention provides scanner means for scanning a light beam, the scanner means comprising a driven rotatable element having a reflective surface extending about a rotational axis for reflecting the light beam, the reflective surface having a generally continuously curved surface profile about the rotational axis except for at least one localised angular step or discontinuity, the curved surface profile and localised angular step or discontinuity being configured such that for the scanner rotating at a constant rotational speed with the light beam impinging upon the reflective surface, the beam is scanned to move across a flat screen at substantially a constant speed. An advantage of such a linear scan is that the intensity and resolution of particular pixels making up the projected image should be independent of their respective locations in the image because the photon density would be uniform over the total extent of the image for an unmodulated scanning beam. The generally continuously curved profile of the mirror surface is preferably not of a continuous degree of curvature; rather the degree of curvature varies about the axis. It is preferred that the locus of the mirror surface about the rotational axis is asymmetrical about at least one axis of symmetry extending transversely through the rotational axis. Desirably, the locus of the mirror surface about the rotational axis is symmetrical about two or less (preferably only one) axes of symmetry extending transversely through the rotational axis. Where only a single localised step or discontinuity is provided the scanner means effects a single scan of light photons per revolution of the driven rotatable element. (Alternatively two steps or discontinuities may be provided relatively spaced at 180° intervals about the rotational axis; in this embodiment two scans of light photons are effected per revolution.) The localised step or discontinuity serves to reset the frame scan to its start position and initiate the "screen refresh" of the perceived image. Desirably, the driven rotatable element is balanced to reduce vibration and out of balance forces when in operation. The invention will now be further described in a specific embodiments, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a scanning system according to the invention for use in projecting an image; Figure 2 is a schematic block diagram of the system of Figure 1; Figure 3 is a schematic representation similar to Figure 1 showing an alternative scanning system according to the invention; Figure 4 is a schematic perspective and plan view of a first embodiment of scanner means according to the invention; Figure 5 is an explanatory diagram showing the linear scan produced by the scanner means according to the invention; and, Figure 6 is a schematic representation of an alternative embodiment of scanner means according to the invention. Referring to the drawings, a projection system comprises a modulated laser light source 1 arranged to direct generated photons to a first scanner 10 (line scanner) , to be reflected toward a second scanner 20 (frame scanner) subsequently to produce an image 2 on a screen 3. The first scanner comprises a multi-facetted polygonal mirror 11 driven by an a.c. synchronous motor 12 to rotate at very high speed (typically, for example 80,000 r.p.m.). The first scanner 10 scans photons to produce a "line scan" (which ultimately represents the horizontal scan on the formed image) each facet producing one scan line. The "line scanned" photons reflected from the first scanner 10 impinge on second scanner 20 which again comprises a rotating curved mirror 21 (driven by motor 22) having its axis orientated perpendicularly to the rotational axis of the first mirror 11. Each successive scan line impinging on the curved facet of the second mirror 21 is reflected onto the screen, the progressive rotation of the facet causing successive scan lines to be displaced vertically on the screen 3 such that the reflective surface of the second mirror 21 provides a "frame scan" producing a completed screen of information or "image". In order to ensure that a stable image is perceived on the screen, it is also important that as the motor rotates bringing the next successive mirror facet of scanner 10 into alignment with the photon light beam la the sequence of modulation is synhronised at exactly the same point on the new facet as the preceding facet. By repeating the modulated light pattern over the course of each facet of the second mirror 21 a stable two dimensional image is produced. In order to achieve the required synchronisation of light modulation and scanner rotation to enable successive line and frame scans to be accurate imagewise a synchronisation system comprising a synchronisation laser light source 4 and photodetectors 5,6 are provided. Laser light source 4 produces respective beams, each directed to impinge upon a respective scanner mirror 11,21. Photodetectors 5,6 are positioned so as to detect a respective reflected synchronisation beam at a point in time corresponding to the initiation of a line scan and frame scan (of the modulated image producing beam) respectively. With conventional methods it has been proposed to use photodetectors to detect the scanning modulated image producing beam itself in order to synchronise modulation for successive scans. The presently described technique is preferable because when synchronising modulation and mirror rotation by detecting the scanning modulated image producing beam itself, there is an inherent (although small) time lag because the scanning image producing beam is modulated only when the beam detection information is received by the modulator. By using the synchronisation beams produced at a beam source 4 distinct from modulated image producing source 1, the scanned image producing beam may be continuously modulated whilst detecting the beginning of the following line or frame to be scanned using photodetectors 5,6. The modulation pattern that controls the photon generator 1 is generated by a processor and control unit 7. The principle of operation is similar to conventional TV/monitor technology where a serial modulation pattern is produced, the speed and timing being governed by the scan speed and the synchronisation respectively. Control unit 7 therefore receives inputs from the photodetectors 5,6 to determine the synchronisation of the beam modulation and the rotation of mirrors 11 , 21 . In order to maintain a stable image of the correct aspect ratio with the required number of scan lines per frame, the rotational speed of both first and second mirrors 11,21 has to be exactly pre-determined or controlled. A.C. hysteresis motors may be used, their speed being determined by the frequency of the a.c. supplied to them. Both frequencies are derived from one high frequency master clock (divided down by the appropriate amount) thus once the mirrors 11,21 are revolving at the required speed, their exact ratio and relative phase is maintained constant to avoid the image "tumbling" or wandering on the screen (such as occurs for example with loss of synchronisation on a faulty television picture) . If the scanning system is used with apparatus which permits the resolution of the image to change (for example the image resolution on a computer display screen changing from 1024 x 768 pixels to 640 x 480 pixels) the change of resolution can be detected and the speed of the motors 12,22 adjusted to compensate thereby enabling the image size to remain constant. Speed of rotation of the mirrors 11,21 is also controlled by the processor controller 7. Conventional systems may use a multi-faceted rotational mirror to produce the "frame scan" in the vertical direction to produce the image on the screen. A problem with this is that motors typically used for rotating optical mirror assemblies achieve stable rotational velocities between 1,000 rpm and 120,000 rpm. This produces a vertical "frame scan" frequency which is unnecessarily high for present purposes; in order to solve this problem an alternative rotational scanner has been devised as shown in Figures 4 and 5 which can be used with stable high rotational speed motors. If the number of facets of a conventional polygonal multifaceted mirror were reduced (to say four of five) the scan angle of each facet would be too great to project an image for practical purposes without the need for expensive optical arrangements. As shown in Figures 4 and 5, the scanner 20 comprises rotatable element 21 rotatably mounted on a motor assembly 22, and provided with a curved peripheral reflective surface 23 extending about its rotational axis. The locus of the curved reflective surface about the axis is not symmetrical about more than one axis of symmetry and is generally substantially continuously curved except for one major step (or discontinuity) 24 which acts to reset the scan to its original position as the step rotates through the light beam. Although the reflective surface is generally continuously curved about the rotational axis, the degree of curvature varies about the axis. The important feature about the curved reflective surface extending about the axis is that it is configured to provide at least a close approximation to a linear scan in which, for a constant rotational speed of element 21 the reflected beam moves at a substantially constant linear speed on a flat screen. As shown in figure 5, for an exact linear scan the incremental time δt for the beam to travel a uniform incremental distance δy on the screen is constant. The locus of a curved mirror surface about the rotational axis providing a close approximation to a linear scan can be expressed mathematically using the following expressions: and Where A is the scan angle and R is the radius of the inscribed circle. It is possible to envisage the curved mirror surfaces for producing one or more frame scans per revolution being arrived at by incremental modification or trial and error and suitable machining which perform to produce a linear scan or a close approximation thereto. The scanner 40 (shown in figure 6) is an alternative to the scanner 20 of Figure 4 and comprises a rotatable element 41 mounted on a driving motor assembly 42. The curved peripheral reflective mirror surface 43 is provided with two discontinuities or steps 44,45 spaced at 180° about the rotational axis. The locus of surface 43 about the rotational axis therefore has two axes of symmetry passing transversely through the rotational axes (perpendicular to one another) . the surface 43 ensures two scans are effected per revolution about the rotational axis. The locus of the curved mirror surface about the rotational axis for rotatable element 42 can be expressed mathematically using the following expressions: and /w= Where A is the scan angle and R is the radius of the inscribed circle. Returning to the single discontinuity mirror 21 shown in figures 1,3 and 4, due to the asymmetrical shape of the mirror, dynamic balancing about the rotational axis is required to reduce out of balance forces and vibration. Furthermore, the angle of divergence of the reflected light beam varies, and needs to be compensated for. As shown in Figure 3, a scan lens 15 may be provided in the path of light reflected from the first mirror 11 to ensure the reflected light is adapted to be focused on a flat (planar) surface (not a curved focal surface as would be the case for unadapted reflected light) . In order to compensate for the divergence of the light reflected from mirror 21 an arrangement of aspherical lenses 9 is introduced in the path of the scanned light. The modulated light source 1 comprises respective red, green and blue lights. A scanning system according to the invention is suitable for use in a projection system, particularly a virtual reality display system such as a direct retinal projection arrangement or head mounted display. Alternatively, the system may be used in "head up display" applications in which an image is projected onto a transparent screen enabling the user to simultaneously see through the screen and observe an image projected onto the screen. As a further alternative, the scanning system would be suitable for industrial marking applications, in which visible laser light would be replaced with higher energy sources such as a high power carbon dioxide laser source. The system may further more be adapted to act as a camera system, such as a video camera system. In this application, the light source is replaced with photosensors/photodetectors. It is believed that such a system is both novel and inventive per se.

Claims: Claims : 1. A scanning system comprising: first scanner means; a light source arranged to direct a beam of photons to impinge on the first scanner means to effect scanning of photons in a first general scan direction; and, second scanner means arranged to effect scanning of said photons in a second general scan direction substantially at right angles to said first general scan direction thereby enabling an image to be perceived. 2. A scanning system according to claim 1 wherein the light beam directed toward the first scanner means is modulated and wherein the system further comprises synchronisation means arranged to enable modulated light sequences scanned from at least one of the scanner means to be accurately repeated, the synchronisation means comprising: i) a synchronisation beam distinct from the modulated image forming beam, the synchronisation beam being directed to impinge on at least one of the scanner means; and, ii) sensor means arranged to sense the synchronisation beam reflecting from the scanner means. 3. A scanning system according to claim 2, wherein the synchronisation beam and the sensor means are configured such the sensor means senses the reflected beam proximate the beginning or end of the scan sweep of the reflected synchronisation beam. 4. A scanning system according to claim 3, wherein the synchronisation beam and the sensor means are configured such the sensor means senses the reflected beam proximate the beginning of the scan sweep of the reflected synchronisation beam. 5. A scanning system according to any preceding claim, wherein the first scanner means is arranged to effect a linear "line scan" photon trace across the second scanner means, the second scanner means scanning the "line scan" trace to effect a two dimensional "frame scan". 6. A scanning system according to any of claims 2 to 5, wherein the synchronisation means enables phase relationship between the scan rate of the first and second scanner means to be monitored and/or controlled. 7. A scanning system according to any of claims 2 to 6, further comprising processor means arranged to process information derived from the sensor means. 8. A scanning system according to claim 7, further comprising control means acting in response to information processed by the processor means enabling the relative scan rate of the first and second scanner means, and/or the modulation of the scanned light to be manipulated. A scanning system according to claim 8, wherein the processor and control means comprises a microprocessor. 10. A scanning system according to any of claims 2 to 9, wherein the synchronisation means comprises separate synchronisation beams distinct from the modulated image forming beam, each synchronisation beam being directed toward a separate one of the first and second scanner means. 11. A scanning system according to claim 10, wherein the synchronisation means comprises separate sensors each arranged to detect respective synchronisation beams reflected from respective scanner means. 12. A scanning system according to any preceding claim, wherein either or both of the first and second scanner means comprises reflector means having one or more light reflecting facets arranged to effect scanning of the light photons. 13. A scanning system according to claim 12, wherein the reflector means comprises a rotatable mirror reflector. 14. A scanning system according to claim 13, wherein the reflector means comprise rotatable mirror reflectors, a first mirror reflector comprising the first scanner means and a second mirror reflector comprising the second scanner means, the mirror reflectors being driven by respective motors. 15. A scanning system according to claim 14, wherein the rotational speeds of the mirror reflectors are controllable relative to one another to achieve a desired phase relationship. 16. A scanning system according to any of claims 12 to 15, wherein at least one of the respective mirror reflectors comprising the first and second scanner means comprises a rotatable multi-faceted polygonal mirror. 17. A scanning system according to claim 16, wherein the mirrors are rotated at differing respective speeds, the mirror effecting the "line scan" being rotated at a substantially greater speed than the rotational speed of the mirror effecting the "frame scan". 18. A scanning system according to any preceding claim, wherein the first scanner means effects a line scan and the second scanner means effects a frame scan, the second scanner means comprising a driven rotatable element having a reflective surface extending about a rotational axis for reflecting the light beam, the reflective surface having a generally continuously curved surface profile about the rotational axis except for at least one localised angular step or discontinuity. 19. A scanning system according to claim 18, wherein the curved surface profile and localised angular step or discontinuity of the reflective surface are configured such that, for the scanner rotating at a constant rotational speed with the light beam impinging upon the reflective surface, the beam is scanned to move across a flat screen at substantially a constant speed. 20. Scanner means for scanning a light beam, the scanner means comprising a driven rotatable element having a reflective surface extending about a rotational axis for reflecting the light beam, the reflective surface having a generally continuously curved surface profile about the rotational axis except for at least one localised angular step or discontinuity, the curved surface profile and localised angular step or discontinuity being configured such that for the scanner rotating at a constant rotational speed with the light beam impinging upon the reflective surface, the beam is scanned to move across a flat screen at substantially a constant speed. 21. Scanner means according to claim 20, wherein the reflective surface of the driven rotatable element is arranged to reflect the impinging light beam substantially perpendicularly to the rotational axis of the driven element. 22. Scanner means according to claim 20 or claim 21, wherein the continuously curved profile of the mirror surface is not of a continuous degree of curvature, the degree of curvature of the mirror surface varying about the rotational axis of the rotatable element. 23. Scanner means according to any of claims 20 to 22, wherein the locus of the mirror surface about the rotational axis of the rotatable element is asymmetrical about at least one axis of symmetry extending transversely through the rotational axis. 24. Scanner means according to any of claims 20 to 23, wherein the locus of the mirror surface about the rotational axis is symmetrical about two or fewer axes of symmetry extending transversely through the rotational axis. 25. Scanner means according to any of claims 20 to 24, wherein a single localised step or discontinuity only is provided on the generally continuously curved mirror surface of the rotatable element, the scanner means effecting a single scan of the light beam per revolution of the driven rotatable element. 25 . Scanner means according to any of claims 20 to 24 , wherein the locus of the mirror surface about the rotational axis is def ined at least closely approximately by : /(*) = and Where A is the scan angle and R is the radius of the inscribed circle or and Where A is the scan angle and R is the radius of the inscribed circle. 26. Scanner means according to any of claims 20 to 25, wherein two steps or discontinuities are provided on the generally continuously curved mirror surface of the rotatable element, the two steps or discontinuities being relatively spaced at a 180° interval about the rotational axis of the rotatable element. 27. Scanner means according to any of claims 20 to 26, wherein the shape of the driven rotatable element is asymmetrical about it's rotational axis balanced to ameliorate vibration and out of balance forces when in operation. 28. A scanning system according to any of claims 1 to 19, comprising scanner means according to any of claims 20 to 27. 29. Projection apparatus comprising a scanning system according to any of claims 1 to 19 and/or scanner means according to any of claims 20 to 27. 30. Retinal display apparatus for projecting an image directly onto a users retina, the retinal display apparatus comprising a scanning system according to any of claims 1 to 19 and/or scanner means according to any of claims 20 to 27.

Inventors: CAPEY MARTIN, JONES GARETH, MAILLAUD FABRICE, CAPEY, MARTIN, JONES, GARETH, MAILLAUD, FABRICE
Applicants: CAPEY MARTIN, JONES GARETH, MAILLAUD FABRICE, CAPEY, MARTIN, JONES, GARETH, MAILLAUD, FABRICE
Application year: 1996
Main classification: G02B26
IPC codes: G02B5, G02B26