HIGH EFFICIENCY OPTICAL SCANNER

Abstract: An improved optical system having an electro-mechanical scanner (44) for increasing the duty cycle of the scanner while maintaining a constant optical aperture throughout the scan. Two alternating optical paths (A,B) are provided between the scanner (44) and a transducer (59) by use of an optical multiplexer (54) synchronized with the scanner rotation (Fig. 3a).

Description: The present invention relates to an optical system for scanning a collimated beam according to the preamble of claim 1. Opto-mechanical scanning systems are used in high performance applications to scan an optical collimated beam either to view a scene or illuminate a photo-sensitive material. Typical applications include infrared (IR) line scanners, forward looking infrared scanners (FLIS), and laser printers. In one widely used type of opto-mechanical scanner, reflecting facets of a rotating polygon move through a collimated beam of light or IR energy. Figure 1 of the drawings shows a simplified schematic drawing of this type of scanning system. The teachings of this invention readily apply to this type of scanner. As will be appreciated by those skilled in the art, practical optical and meachanical limitations restrict the size of a scanning element relative to the size of the collimated energy beam. Since the system can effectively scan the beam only so long as the beam lies wholly on the scanning surface, as the boundary between adjacent reflective surface moves through the collimated beam, there is a portion of the scanning cycle during which the beam is being retraced. Typical systems have a relatively low scanning duty cycle, in a range of 50 to 80 percent. In certain infrared information gathering applications, such as IR line scanners in low flying reconnaissance aircraft, it is desirable to provide practically 180 degree horizon-to-horizon coverage. However, with a need to block all energy from one horizon before scanning the other horizon, and a need to use scanning elements of a reasonable size, the scanning coverage in these systems is limited to less than 180 degrees because retrace between horizons occupies the entire interval in which the collimated beam passes through a boundary between adjacent facets of the scanning element. A laser printer is another device which employs a mechanical scanner and in which it is highly desirable to have the duty cycle as close to 100% as practical in order to maximize throughput and minimize or eliminate the need for buffer storage. Figures 2, 3a and 3b show optical systems which serve to increase the duty cycle of the scanner and which are closely related to prior art systems as for instance known from US-A-4 124 269 and FR-A-1 552 349. Departing from those known optical systems, it is the object of the present invention to further improve the duty cycle and to indicate a system which can be implemented easier. This object is achieved according to the characterizing features of claim 1. Further advantageous embodiments may be taken from the dependent claims. The invention is described in greater detail with reference to the following drawings, in which: Figure 1 is a simplified schematic diagram of a scanner of a type commonly used in the prior art and to which the teachings of this invention apply; Figure 2 is a simplified schematic diagram of an improved scanning system; Figure 3a is an optical diagram of a further embodiment of an infrared line scanner; Figure 3b is a plan view of a mechanical optical multiplexer used in the embodiment of Figure 3a; and Figures 4a and 4b are an optical diagram of an infrared line scanner according to the present invention. Referring now to Figure 1 of the drawings, a scanner widely used in the prior art comprises a constantly rotating polygon 10 with four reflective surfaces 12 and edges or boundaries 22. An objective lens 14 forms an optical path between sensor 16 and the optical deflector or scanner 10. The lens 14 focuses a collimated beam 18 of optical energy from the reflective faces 12 of the scanner on sensor 16 which has an effective aperture that is small relative to the size of the beam. In practical scanning systems, the size of the beam necessarily covers a substantial portion of the face of a scanning element; e.g., in the order of between 20% and sometimes as much as 50 percent. The time it takes a boundary 22 between two successive scanning elements 12 to pass through the beam 18 determines the retrace interval of the scan. The duty cycle of the scanner is the ratio of the time the beam is completely hitting the scanning faces during one revolution of the total time for one complete revolution of the scanner. Referring now to Figure 2, it shows an improvement in the context of a constantly rotating square scanning element 10 and a sensor 16, the same as those illustrated in Figure 1. There are two optical paths labeled A and B between the scanning element and the transducer. One optical path includes an objective lens 24, an optical multiplexer 26, and an elliptical focussing mirror 28. The other path is similar. It includes an objective lens 32, the same multiplexer 26 and an elliptical mirror 34. The optical multiplexer 26 is a high speed optical switch that alternatively blocks radiant energy along one path and transmits it along the other and directs the image to the sensor element 16 alternatively from one of the two objective lenses 24 or 32. The multiplexer 26 switching is synchronized with the rotation of the scanner 10. The lenses 24 and 32 focus the beams at the multiplexer. With the energy of the beams thus concentrated, they can be rapidly switched by a mechanical chopper 26 rotating synchronously with the scanner 10, thus providing a high duty cycle by reducing the retrace time to the time required for each edge of the chopper to traverse the concentrated beam at the image of sensor 16 on chopper 26. In this embodiment, collimating lenses 24 and 32 preferably are displaced from one another in the scanning direction by nominally one-half of the length between boundaries 22 of the scanning elements 12. The multiplexer switches beam paths when the boundaries 22 pass through beams A and B. The beam which is completely hitting face 12 of scanner 10 is selected by multiplexer 26. A mechanical chopper edge rotating synchronously with the scanner 10 can cross the focussed transducer image in less than 1 percent of the total scan period, providing a duty cycle on the order of 99 percent since one beam path will always totally hit on a scanning face. In the preferred embodiment, a mechanical chopper is used. However, it will be appreciated that any suitable fast acting multiplexer can be used in the practice. The mechanical chopper and scanner are preferably arrayed in sectors on rotating members whose rotation can be synchronized by various techniques known in the art. For example, the rotating members can be mechanically geared to one another, either directly or by a differential gear if variable phasing is desired as in an IR scanner to compensate for aircraft roll. The rotating member carrying the optical multiplexer elements can be controlled by a phase-lock-loop with optical pickups on the scanning member, preferably provided by the scanning elements themselves for accuracy. The optical multiplexer elements and the scanning elements can also be arrayed on the same rotating member in concentric rings, with the optical path between elements folded back with the prisms, mirrors or other suitable optical relay elements. Figures 3a and 3b show an infrared scanner which uses a three-sided mirror 44 to scan 180° between horizons left and right. The optical multiplexer or scanner 44 is an equilateral triangle and it rotates at a constant angular velocity about its axis 46. A pair of eliptical mirrors 48 and 50 are disposed to project respectively a beam along two optical paths, one from the left side and one from the right side of the rotating mirror, to a common primary focal plane 52. An optical multiplexer 54, shown in plan in Figure 3b, comprises a thin disk with alternating reflective and transmissive sectors 57 and 55, respectively, around its periphery. The reflective and transmissive paths pass through the common focal plane 52. The disk is driven by a servo motor 56 whose rotational speed is phase-locked to the rotational speed of the scanning mirror 44. An elliptical relay mirror 58, common to both beam paths, refocuses the beam on an IR detector or detector array 59. Switching between beam paths occurs when a boundary between the reflective and transmissive sectors of the optical multiplexer pass through the image of the detector 59 focused on the multiplexer. Since the detector image is very small at this point and the beam energy is concentrated, switching can occur in less than 1 percent of the time required for a 180 degree scan. The multiplexer rotation is synchronized with the scanner rotation so that switching occurs every 90 degrees, at the horizons and at the nadir. A light emitting diode 62, a lens 64, lens 66 and a detector 67 optically sense the scanning mirror position and provide a series of reference pulses to the controlling servo motor 56 in a phase-lock loop. Electronic control of the optical multiplexer permits adjustment of the phase of the switching to compensate for motion of the scanning system itself. Thus, signals from an auto-pilot or an inertial navigation system can be used to compensate for roll of an aircraft carrying the infrared imaging system. Similarly, a differential gear between the shafts of the optical multiplexer and the scanning mirror could be used to provide mechanical phase adjustment of the switching. In the specific embodiment shown in Figure 3a, reflective lenses are used to provide wide band infrared operation. Alternatively the required optical configuration could be achieved with refractive elements and mirrors. Now referring to Figures 4a and 4b, elements similar to or the same as those of the embodiment of Figure 3a are, for ease of explanation, marked with the same element numbers. The embodiment of Figures 4a and 4b provides an improvement in the scanning efficiency over that of the prior art as well as the embodiment of Figure 3a. Essentially, the three-sided scanner 44 is replaced with a flat, circular scanning mirror 110 which is held by bearings 112 and 114. The scanning mirror 110 rotates about axis of rotation 118 through centers of mirror 110 and elliptical mirrors 48 and 50. The surfaces of mirror 110 are parallel to each other and their planes are inclined about 45 degrees from the axis of rotation. The hollow-shaft drive motor 120 drives the scanning mirror 110. The hollow-shaft components include the drive motor 120, reference encoder 122 (coupled to synchronize motor 56) and bearings 112, so as to allow a circular bundle of rays reflected off mirror 110 to pass to elliptical mirrors 48 and 50 without loss. The optical chopper blades 124 (as shown in Figure 4b) switch the rays received by optical multiplexer 54' from mirror 48 to those from mirror 50, and vice versa, when mirror 110 reaches a position ninety degrees from the position shown in Figure 4a. The switching time is extremely fast due to the very small size of the detector-array image at the optical multiplexer's plane 52. Switching times in the order of 0.4 percent of the scanning period are easily obtained in typical applications, thus providing typical scanning efficiencies of 99.6 percent.

Claims: An optical system for scanning a collimated beam, comprising: a) moving means (120); b) an optical deflection means (110) having a plurality of scanning surfaces, said optical deflection means being coupled to said moving means for rotation about an axis (118); c) optical sensor means (59); d) means (50, 58) arranged in a first optical path between said deflection means and said sensor means so as to focus a first beam at said sensor means; e) means (48, 58) arranged in at least one second optical path between said deflection means and said sensor means so as to focus a second beam at said sensor means; and f) an optical multiplexer (54'), said multiplexer being disposed in both said first and second optical paths and operating synchronously with the motion of said deflection means to alternatively block one said path and allow transmission along the other said path, said optical system being characterized in that said deflection means (110) includes two flat parallel surfaces and said deflection means is inclined from its axis of rotation (118) by a predetermined angle of preferably 45 degrees. The system according to claim 1, characterized in that said optical deflection means (110) is circular in shape as viewed along the axis of rotation (118). The system according to claim 1, characterized in that said optical deflection means (110) and said optical multiplexer (54') are mechanically coupled together.

Inventors: WESTELL, WILLIAM E.
Applicants: HONEYWELL REGELSYSTEME GMBH
Application year: 1988
Main classification: G02B26
IPC codes: G02B26