FOCUS-CORRECTED CONVERGENT BEAM SCANNER

Abstract: A focus-corrected convergent beam scanner includes a lens arranged to receive light from a scene and to form an image of the scene in an image surface. A mirror annulus is arranged behind the lens to reflect light from the scene which is passed through the lens. A detector is arranged to receive light from the scene which has been reflected by the mirror annulus. The mirror annulus is rotatable about an axis of rotation in order to scan the image of the scene across the detector. The tilt angle of the reflecting surface of the mirror annulus varies as a function of the position of the reflecting surface along the circumferential axis is arranged at a varying distance from the lens such that the image of the scene is always in focus onto the detector.

Description: The invention relates to an image pickup device comprising: lens means arranged to receive light from a scene and to form an image of the scene in an image plane, said lens means having an optical axis; movable reflective means being arranged behind the lens means to reflect light from the scene which is passed through the lens means, thereby scanning the image in one direction; detector means arranged to receive light from the scene which have been reflected by the reflective means. An embodiment of a known image pickup device is shown in Figure 1. In this image pickup device, light rays 8 from a distant object 10 are converged by a double convex lens 12. The converging light rays are reflected by a scan mirror 14 to produce an image 16 of the object 10 on a detector array 18. The detector array 18 is, for example, a linear array extending perpendicular to the plane of the drawing. In order to scan the image 16 across the detector array 18, the scan mirror 14 is pivoted around axis 20. Because the scan mirror 14 is reflecting converging rays 8 (as opposed to the parallel rays between the object 10 and the lens 12), this image pickup device is sometimes referred to as a convergent beam scanner. A number of problems arise when one attempts to scan the image 16 across the detector array 18 in a television-compatible manner. The scan frequency for television-compatible scanning is, for example, 60 cycles per second. Each scan cycle consists of an active portion and an inactive or flyback portion. In the active portion, the pivot or tilt angle of the mirror 14 varies linearly with time. An electrical signal representing the image is produced at the output of the detector array. During the inactive or flyback portion of the scan cycle, the mirror 14 is brought back to its initial position in order to ready it for the next active scan. During flyback, no image signal is produced. The flyback of mirror 14 must be achieved in 8% of the 1/60 second scan cycle (0.00133 seconds). In order to accomplish such a fast flyback, the size of mirror 14 must be reduced. However, as the size of mirror 14 is reduced, the mirror must be moved closer to the focal point 22 of lens 12. By moving the mirror closer to the focal point 22, the focusing error due to scanning a convergent beam is increased. As a result, a blurred image is scanned across the detector array. The pickup device shown in Figure 1 also suffers from the problem of vibration. The vibration arises from repeatedly changing the direction of rotation of the scan mirror 14. It is an object of the invention to provide an image pickup device having a convergent beam scanner which avoids defocusing error. It is another object of the invention to provide a convergent beam scanner having a high scan efficiency (i.e. a fast flyback). It is a further object of the invention to provide an image pickup device with reduced vibration. The image pickup device according to the invention is characterized in that the movable reflective means is a twisted reflective annulus rotatable about an axis of rotation perpendicular to the mean plane of the annulus; said reflective annulus having a circumferential axis which intersects the said optical axis through the entire rotation of the reflective annulus; the reflecting surface of the annulus is arranged at a tilt angle about the circumferential axis, the tilt angle of the reflecting surface varying as a function of the position of the reflecting surface along the circumferential axis; and the circumferential axis is arranged on the said optical axis at a distance from the lens means, the distance varying as a function of the tilt angle of the reflecting surface such that the image is always focussed onto the detector means. The tilt angle is the angle between the reflecting surface of the mirror and the optical axis of the lens means at the intersection of the two. In general the distance of the circumferential axis from the lens means can be expressed in a rather complicated analytical form. It is remarked that EP-A-0.051.894 discloses an imaging apparatus comprising movable reflective means for scanning the image over a detector means whereby the reflective surface of the reflective means is arranged at a tilt angle varying as a function of the position of the reflecting surface along the circumference of the reflective means. In this device however the reflective means consists of a number of discrete mirrors which are assembled in a truncated cone and which are arranged at angles of substantially 45 o with respect to the rotation axis of the cone. Upon rotation of the cone the succession of discrete mirrors performs the scanning of the image in one, first, direction. The purpose of the varying tilting angle is to scan the image in a second direction, perpendicular to the first direction. In the image pickup device of the present invention, the detector means is preferably located in a plane. Preferably, the detector means is located in a plane perpendicular to the optical axis of the lens means, and the distance from the detector to the optical axis equals the focal distance of the objective lens minus the distance between the objective lens and said detector plane. It is also preferred, according to the invention, that the distance, x, of the circumferential axis from the detector plane is a function of the tilt angle, , as given by the equation where Y d is the distance of the detector means from the optical axis of the lens means (which is also equal to the distance of the detector plane from the focal point of the lens means). This expression is a valid approximation of the above mentioned analytical form, when the total field width is such that the focal surface can be approximated by a plane. The field width is the angular width of the scene to be detected. The lens means may be, for example, an ordinary spherical objective lens or an anamorphic lens. Figure 1 is a schematic representation of a known image pickup device. Figure 2 is a partly schematic, partly perspective view of an embodiment of an image pickup device according to the invention. Figure 3 is a schematic view of a portion of the image pickup device according to the invention with the mirror annulus at a first rotational position. Figure 4 is a schematic view of a portion of the image pickup device according to the invention with the mirror annulus at a second rotational position. Figure 5 is a perspective view of another embodiment of the image pickup device according to the invention. Figure 2 shows an embodiment of an image pickup device according to the invention. The image pickup device includes a lens means 24, a mirror annulus 26, a detector means 28, and a means 30 for rotating the mirror annulus. The lens means 24 is arranged to receive light rays 32 from a distant scene (that is, a scene located well beyond the focal distance of the lens means 24). From these light rays, lens means 24 forms an image of the scene in an image surface. The lens means 24 has an optical axis 34. The lens means 24 may be one or more optical components for forming a real image of a scene. For example, lens means 24 may be a conventional objective lens. Preferably, lens means 24 is an anamorphic lens. (Figure 5). An anamorphic lens is a lens having different focal lengths in different planes, for example focal length A in the horizontal plane and focal length B in the vertical plane. The anamorphic lens 24 forms the ray bundle onto a narrow line on the mirror annulus 26 in order to minimize the difference in the tilt angle from one side of the ray bundle to the other. When lens 24 is anamorphic, a second anamorphic lens 25 is provided between the mirror 26 and the detector means 28, in order to reconstruct the image of the scene on the detector means. As shown in Figure 2, the mirror annulus 26 is arranged behind the lens means 24. Mirror annulys 26 is rotatable about an axis of rotation 36 so that the light rays 32 pass sequentially across the entire reflecting surface 38 of the mirror annulys 26 as the mirror annulus is rotated. Light rays 32 from the scene which have passed through the lens means 24 are reflected by reflecting surface 38 onto the detector means 28. The detector means 28 comprises, for example, a linear aaray 40 of individual detector elements. The array 40 is oriented so that its projection onto optical axis 34 is perpendicular to optical axis 34. The detector elements in array 40 may be any conventional light detectors. In order to scan the light rays 32 across the detector means 28, the reflecting surface 38 of the mirror annulus 26 is arranged at a tilt angle about the circumferential axis 42 of the mirror annulus 26. As shown in Figure 2, the tilt angle of the reflecting surface 38 varies as a function of the position of the reflecting surface 38 along the circumferential axis 42. (A gap is shown between the ends of annulus 26 to clearly show the changing tilt angle. In practice, however, this gap would not be present.) Now, the means 30 for rotating the mirror annulus 26 (for example, an electric motor) causes the mirror annulus 26 to spin on its axis of rotation 36. The mirror annulus 26 and the axis of rotation 36 are arranged such that the circumferential axis 42 always intersects the optical axis 34 through the entire rotation of the mirror annulus 26. As a result, each segment of the reflecting surface 38 which presents itself at the optical axis 34 appears at a different tilt angle. Rotation of the mirror annulus 26 thus approximates the rocking motion of the scan mirror 14 in the prior art arrangement shown in Figure 1. In order to scan a sharp image across the detector means 28, the detector means 28 must be arranged at the focal surface of the lens means 24. Figures 3 and 4 show how this is achieved in one embodiment of the present invention. Figure 3 schematically shows a cross-section through a part of the image pickup device of Figure 2. The mirror annulus is at a first rotational position such that the mirror segment 44 of the reflecting surface 38 subtends a tilt angle , around the circumferential axis 42. The tilt angle, , is measured with respect to the optical axis 34, and in Figure 3 it is between 45 o and 90 o . The detector means 28, in this embodiment of the invention, is arranged in a plane 46 which is perpendicular to the optical axis 34. The perpendicular distance between the detector means 28 and the optical axis 34 is Y d . In the absence of the mirror annulus 26, the lens means 24 would form an image of the scene at image surface 48. The distance between the image surface 48 and the detector plane 46 measured along the optical axis is selected to be equal to the distance, Y d , between the optical axis 34 and the light-sensitive surface of the detector means 28. As the mirror annulus 26 is rotated, the tilt angle , is varied. As a result, the actual image surface 50 is scanned across the detector means 28. In order to maintain the image in focus at the surface of the detector means 28, the distance, x, between the circumferential axis 42 and the detector plane 46 must be varied according a complicated form which expresses x as a function of Y d ,  and the focal distance of the lens. When the total field width is limited such that the focal surface 48 can be approximated by a plane, the expression relating x, Y d ,  and the focal distance can be approximated by the following formula: Thus, when the tilt angle is 45 o , the distance x is 0. The circumferential axis 42 then intersects both the optical axis 34 and the detector plane 46. In Figure 4, the image pickjp, device according to the invention is schematically illustrated with the tilt angle , between 22.5 o and 45 o. The distance, x, between the circumferential axis 42 and the detector plane 46 is again given by the equation When the tilt angle is 45 o , the distance x is again equal to 0. If a field width is chosen such that the focal surface deviates significantly from a plane a numerical computer may be used to determine the distance x for any given tilt angle . The limits on the tilt angle are determined by considerations such as field width, angular sensitivity of the detector means and construction of the embodiment. In practice, the tilt angle need only range from 30 o to 60 o . Mirror annulus 26 may be manufactured, for example, by starting with a thick aluminium ring. The ring can be machined roughly into the desired shape of annulus 26, after which the precise shape can be produced on a numerically-controlled cutter. If the annulus is to be used to reflect infrared in the 3 to 5 or 8 to 12 micron regions, small imperfections in its surface will not degrade the image quality. If shorter wavelengths are to be used, the aluminum surface can be hand polished after it is cut. While the mirror annulus produced in the manner described above can be used in a scanner, preferably it is used as a master to produce other mirrors. For example, a negative mold can be produced from the master. Then further mirror annulli can be produced from the negative mold. Preferably, each aluminum mirror annulus is provided with a conventional overcoat to prevent oxidation and to maintain a highly reflective surface. Figures 2, 3 and 4 show only one embodiment of the present invention. For example, it is not necessary that the detector means 28 be arranged in a plane which is perpendicular to the optical axis 34. The detector plane 46 could be arranged at any angle with respect to the optical axis 34, and then the equations for the location of the circumferential axis 42 can be derived accordingly. In the drawings the mirror annulus is represented with the reflecting surface substantial perpendicular to the axis of rotation. Of course, depending on the particular embodiment of the invention the reflecting surface may be substantial parallel to the rotation axis, or have any other angle convenient to the embodiment. In another variation of the invention, the mirror annulus produces two complete scans of the scene for each complete rotation. Each half of the mirror annulus produces a complete scan. However, the two halves are in slightly nonparallel planes in order to provide interlace.

Claims: An image pickup device comprising: lens means (24) arranged to receive light from a scene and to form an image of the scene in an image plane, said lens means having an optical axis (34); movable reflective means being arranged behind the lens means to reflect light from the scene which is passed through the lens means, thereby scanning the image in one direction; detector means (28) arranged to receive light from the scene which have been reflected by the reflective means; characterized in that: the movable reflective means is a twisted reflective annulus (26) rotatable about an axis of rotation (36) perpendicular to the mean plane of the annulus; said reflective annulus having a circumferential axis (42) which intersects the said optical axis (34) through the entire rotation of the reflective annulus; the reflecting surface (38) of the annulus is arranged at a tilt angle () about the circumferential axis, the tilt angle of the reflecting surface varying as a function of the position of the reflecting surface along the circumferential axis (42); and the circumferential axis (42) is arranged on the said optical axis (34) at a distance from the lens means (24), the distance varying as a function of the tilt angle of the reflecting surface such that the image is always focussed onto the detector means (28). An image pickup device as claimed in Claim 1, characterized in that: the detector means (28) is located in a plane perpendicular to the optical axis (34); and the distance (Y d ) from the detector means (28) to the optical axis (34) equals the focal distance of the lens means minus the distance between lens means and said plane. An image pickup device as claimed in Claim 2, characterized in that the total field width is no more than about 30 o and that the distance x, of the circumferential axis (42) from the detector plane is a function of the tilt angle, , as given by the equation where Y d is the perpendicular distance of the detector means (28) from the optical axis (34). An image pickup device as claimed in Claim 3, characterized in that the lens means (24) is a conventional objective lens. An image pickup device as claimed in Claim 3, characterized in that the lens means (24) is an anamorphic lens. An image pickup device as claimed in Claim 4 or 5, characterized in that the device further comprises means (30) for rotating the twisted reflective annulus on the axis of rotation (36).

Inventors: BASTIAN, ROBERT, DIEPEVEEN, NEAL
Applicants: MAGNAVOX GOVERNMENT AND INDUSTRIAL ELECTRONICS COMPANY
Application year: 1985
Main classification: H04N3
IPC codes: H04N3, H04N5, G03B42, G02B26