Collimating lens having doublet element between positive-power elements

Abstract: A collimating lens (10) is constructed of a lens assembly having a first lens set (12, 14) at a first end of the assembly and a second lens set (20) at a second end of the assembly with a doublet (16, 18) disposed between the first lens set and the second lens set. The first lens set includes a plurality of lens elements (12, 14) each of which has positive optical power. The second lens set has a single lens element (26) of positive optical power and a concave surface (40) facing subject matter to be viewed by the collimating lens. The doublet lens element includes a lens element (16) constructed of flint glass and having a negative optical power plus a lens element (18) constructed of crown glass and having a positive optical power. In the doublet lens element, the negative power lens element has two concave surfaces, the positive-power lens element has two convex surfaces, and the two lens elements are cemented together to provide for a broad-band chromatic correction. The second end of the optical assembly of the lens is characterized by a telecentric entrance pupil for the viewing of a radiant subject. An external aperture stop (50) is provided in front of the first end of the optical assembly at a distance of at least one focal length from the first lens element of the collimating lens. This provides for more efficient collection of light and a wider field of view.

Description: BACKGROUND OF THE INVENTION This invention relates to a collimating lens with an external aperture stop and, more particularly, to a collimating lens having a doublet element of both crown and flint glasses disposed between front and back positive-power lens elements for increased field of view and reduced chromatic aberrations. Collimating lenses are used in numerous situations, one situation of interest being in an optical apparatus for scanning a display, such as a display formed of an array of light-emitting diodes (LED's), wherein an image of a subject on the display is transmitted via a scanning mirror, or drum, to an objective lens for a viewing of the subject by a person. Existing collimating lenses perform the foregoing function, but at a cost. One aspect of the cost is the fact that the light collection ability is less than that which would be desired. A further aspect of the cost is the fact that the overall physical size of the collimating lens is larger than that which would be desired. Another disadvantage in existing collimating lenses in such scanning apparatus is the sensitivity of the lens characteristics to fabrication and alignment errors attendant the construction of such a visual display system. Also, prior lenses of the foregoing type have had limited spectral bandwidth such that, upon a change in the light emission spectrum of the display, a further design of the collimating lens has been required. As an example of sensitivity to fabrication, it is noted that a typical construction of collimating lens of the prior art has employed a field lens element which is closely positioned to the display being viewed. The close proximity to the display is attained by building the concave field lens as part of a composite structure including the display. Thus, it becomes necessary during fabrication of the display system to align the field lens element with the other elements of the collimating lens. It is difficult to maintain optical integrity of a lens, or lens assembly, wherein elements of the lens are mounted mechanically in different parts of the system, rather than as portions of a unitary optical assembly. SUMMARY OF THE INVENTION According to the present invention there is provided a lens assembly as specified in the claims hereinafter. The lens assembly, constructed in accordance with the invention with an external aperture stop, includes a collimating lens comprising a doublet element of glasses of different indices of refraction along with a front positive lens element and a back positive lens element. This arrangement of the lens elements eliminates the need for a field lens, thereby allowing the collimating lens to be spaced apart a sufficient distance from a display being viewed with the aid of the lens, so as to permit manufacture of the display completely separate from the structure of the collimating lens. The use of a doublet element which comprises a concave lens element cemented to a convex lens element, provides for increased chromatic correction of the lens, thereby allowing use of the lens over a wider spectral band. The configuration of the collimating lens also provides for increased efficiency of light collection as well as an enlarged viewing angle. Additionally, the collimating lens has a more compact configuration than lenses of the prior art which have been employed for scanning a displayed subject. BRIEF DESCRIPTION OF THE DRAWING The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing wherein: Fig. 1 is a diagrammatic side view of the collimating lens of the invention showing rays of light propagating through the lens; and Fig. 2 is a simplified stylized view of an optical system for scanning subject matter on a display for viewing by a person, the optical system including the collimating lens of the invention. DETAILED DESCRIPTION With reference to Fig. 1, a collimating lens 10 of the invention comprises an assembly of lens elements including a first lens element 12, a second lens element 14, a third lens element 16, a fourth lens element 18, and a fifth lens element 20. The third lens element 16 and the fourth lens element 18 are cemented together along an interfacing surface 22 to provide a doublet lens element 24. The first, the second, the fourth and the fifth lens elements 12, 14, 18, and 20 have positive optical power, and are fabricated of crown glass. The third lens element 16 has negative optical power, and is fabricated of flint glass. In a preferred embodiment of the invention, the first, the second, and the fifth lens elements 12, 14, and 20 have the same indices of refraction, namely, 1.59. The fourth lens element 18 has an index of refraction of 1.51. The third lens element 16 has an index of refraction 1.71, this being significantly higher than the indices of refraction of the other lens elements. The following dimensions are employed in constructing the preferred embodiment of the collimating lens 10. The first lens element 12 has a left surface 26 and a right surface 28, as viewed in Fig. 1, the surfaces 26 and 28 being spherical and having radii of curvature, respectively, of 2.2676 cm (0.89276 inches) and 7.4952 cm (2.95087 inches). The diameter of the first lens element 12 is 1.4478 cm (0.57 inches). The second lens element 14 has a left surface 30 and a right surface 32, the surfaces 30 and 32 being spherical surfaces and having radii of curvature, respectively, of 1.2145 cm (0.47813 inches) and 28.4983 cm (11.21984 inches). The center of curvature of the surface 26 is to the right of the lens element 12, and the center of curvature of the surface 28 is to the left of the lens 12. In the lens element 14, the center of curvature of the surface 30 is to the right of the surface 30, and the center of curvature of the surface 32 is to the right of the surface 32. The doublet lens element 24 has a left surface 34, the aforementioned interfacing surface 22, and a right surface 36. The surfaces 34, 22, and 36 are spherical surfaces having radii of curvature, respectively, of 2.6118 cm (1.02827 inches), 0.8175 cm (0.32185 inches), and 1.2790 cm (0.50353 inches). The center of curvature of the surface 34 is to the left of the surface 34, the center of curvature of the surface 22 is to the right of the surface 22, and the center of curvature of the surface 36 is to the left of the surface 36. The fifth lens element 20 has a left surface 38 and a right surface 40, the surfaces 38 and 40 being spherical and having radii of curvature, respectively, of 0.7570 cm (0.29803 inches) and 1.3691 cm (0.53900 inches). The center of curvature of the surface 38 is to the right of the surface 38, and the center of curvature of the surface 40 is to the right of the surface 40. As described above, all of the optical surfaces of the lens elements of the collimating lens 10 are spherical; the spherical construction is employed to facilitate manufacture of the lens 10. The following thicknesses and spacings, as measured along a central optic axis of the lens 10, are employed in constructing the preferred embodiment of the lens 10. The first lens element 12 has a thickness of 0.2540 cm (0.100000 inches). The spacing between the first lens element 12 and the second lens element 14 is 0.01270 cm (0.005000 inches). The thickness of the second lens element 14 is 0.4961 cm (0.195313 inches). The third lens element 16 abuts against the second lens element 14 at an annular optical flat 42 provided on the left side of the third lens element 16, the optical flat 42 located outside of the active region of the lens element 16. The spacing along the central axis of the lens 10 between the surfaces 32 and 34 of the lens elements 14 and 16 is 0.06781 cm (0.026696 inches). The thickness of the third lens element 16, as measured along the central axis of the lens 10 is 0.2442 cm (0.096145 inches). Also, as measured along the central optic axis, the thickness of the fourth lens element 18 is 0.6443 cm (0.253660 inches). The distance between the surfaces 36 and 38 of the lens elements 18 and 20 is 0.0987 cm (0.038876 inches). The thickness of the fifth lens element 20 is 0.254 cm (0.100000 inches). The collimating lens 10 is employed, by way of example, with a display 44 comprising an array of light-emitting diodes (LED's) 46 supported by a substrate 48. The center of the array of LED's 46 is located at a distance of 0.2078 cm (0.081825 inches) from the right surface 40 of the fifth lens element 20, as measured along the central axis of the collimating lens 10. The focal length of the lens 10 is 1.27 cm (0.500000 inches), this being the minimum distance between the left surface 26 of the first lens element 12 and an optical stop 50. Also shown in Fig. 1 are numerous rays 52 of light which emanate from the LED's 46 and propagate through the lens 10 towards the stop 50. By tracing the rays 52, it is observed that the rays 52, emanating from any one of the LED's 46, are parallel to each other in the region of the stop 50. Fig. 2 shows, by way of example, an optical system 54 which advantageously employs the collimating lens 10 in accordance with the practice of the invention. The system 54 comprises an infrared detector array 56 which views radiation 58 emanating from a subject 60. Electric signals are produced by detectors of the array 56 in response to incident rays of the radiation 58, the electrical signals being multiplexed and sampled by a sampling unit 62. The samples produced by the sampling unit 62 are in digital format and represent data of pixels of the subject 60. The system 54 further comprises a signal processor 64 and the aforementioned display 44. The signal processor 64 receives the signal samples from the sampling unit 62, and applies customary signal-processing technology, such as filtering and storing of the signal samples, to output image data of the subject 60 for presentation upon the display 44. A person, represented by a human eye 66, views the display 44 by means of the collimating lens 10, a rotating mirror assembly 68 and a relay lens 70 which are positioned serially along an optical path between the display 44 and the eye 66. The relay lens 70 is located between the mirror assembly 68 and the eye 66. The collimating lens 10 is located between the mirror assembly 68 and the display 44. The mirror assembly 68 is rotated about an axis 72 by a motor 74. The mirror assembly 68 comprises a series of mirrors 76 located about a periphery of the assembly 68 and arranged symmetrically about the rotational axis 72. While one of the mirrors 76 may be oriented parallel to the rotational axis 72, the other ones of the mirrors 76 are angled slightly, with different angles of inclination, relative to the rotational axis 72 so as to reflect a beam 78 of light from the lens 10 at different angles relative to the rotational axis 72. Thereby, one of the mirrors 76 directs an upper portion of the image presented in the display 44 to the eye 66, another of the mirrors 76 directs a lower portion of the image on the display 44 to the eye 66, and the remaining ones of the mirrors 76 direct intermediate regions, between the foregoing upper and lower regions of the image, to the eye 66. Thus, assuming that an image portion is swept by the mirror 76 from the person's left to the person's right, as viewed by the eye 66, successive ones of the mirrors 76 provide a scanning from top to bottom of the image on the display 44 to the eye 66. As shown in Fig. 1, the rays 52 from a plurality of the LED's 46 cross in the plane of the stop 50. If the distance between the display 44 and the lens 10 were increased such that the rays propagating from the LED's 46 to the lens 10 were substantially parallel, then the foregoing crossover point of the rays 52 would move closer to the lens 10, to appear at a distance substantially equal to the focal length of the lens 10. In the situation depicted in Fig. 1. wherein the display 44 is sufficiently close to the fifth lens element 20 to permit collection of rays of radiation from the LED's 46 over a wide angle of emergence of the rays, the aforementioned crossing over of the rays 52 occurs at a distance substantially greater than the focal length of the lens 10. It is shown, by a tracing of the rays 52, that the width of a cone of light emitted by an LED 46 in the center of the display 44, and captured by the lens 10, is limited by the stop 50 which is located at the external aperture of the lens 10. As shown in Fig. 2, the aperture stop 50 is formed by each of the mirrors 76 as each mirror 76 comes into position for reflecting the beam 78 from the lens 10 to the lens 70. The physical size of each mirror 76 determines the physical dimensions of the stop 50. Thus, a larger mirror 76 permits the collection of a larger cone of rays emanating from an LED 46 for transmission to the eye 66. A smaller mirror 76 would decrease the conic angle of the cone of rays emanating from an LED 46 to be transmitted to the eye 66. The numerical ratio of the focal length of the lens 10 to the nominal value of a diameter of the aperture stop 50 is referred to as the focal ratio and determines the amount of radiation emitted by the display 44 which is intercepted and collected by the lens 10 for transmission to the eye 66. In the case of the preferred embodiment of the lens 10, the focal ratio has a value of 1.4 enabling the lens 10 to gather radiation emitted from an LED 46 lying in a cone having a vertex angle of approximately 40 degrees. Thereby, the collimating lens 10 of the invention provides for increased capacity for gathering light from the source of light, namely the array of the LED's 46, while maintaining each of the mirrors 76 at a relatively small size. This aids in attaining a compact packaging for the major optical components of the system 54, namely the collimating lens 10 plus the mirror assembly 68. In addition, the collimating lens 10 provides for high resolution of the image presented on the display 44, and provides for increased spectral bandwidth. As noted above, the use of spherical surfaces on all of the lens elements facilitates the manufacturing process. The construction of the collimating lens 10 provides for a telecentric pupil for maximizing reception of radiant energy emitted by the array of LED's 46. The collimating lens 10 projects a viewing angle of 18 degrees, this being 9 degrees to a side of the central optic axis of the lens assembly. For viewing a square subject, the angles to the centers of the sides of the subject from the central axis are 6.4 degrees, and 9 degrees out to the corners of the square subject. With the foregoing construction, the lens 10 operates over a spectral band of 5200 - 6800 angstroms. The construction of the doublet lens element 24 improves the quality of an image produced by the collimating lens 10 by reducing chromatic aberrations which may be present in collimating lenses. The doublet lens element 24 furthermore corrects for chromatic dispersion which may be introduced by the other lens elements 12, 14, and 20. The curvature of the right surface 40 of the fifth lens element 20 may be altered, if desired, to accommodate a different spacing between the lens 10 and the display 44 from that disclosed above. A longer radius of curvature is provided for an increased spacing, and a shorter radius of curvature is provided for a decreased spacing, between the lens 10 and the display 44. In view of the foregoing construction, there is provided a collimating lens, in accordance with the invention, which has improved light-collection ability, improved chromatic correction, is operable over a wider field of view, is operable over a bandwidth sufficiently large to encompass displays radiating in different colors over a relatively large portion of the spectrum, provides for a lower weight and physical size, and simplifies manufacture by avoidance of the need for a field lens element secured to a display being viewed. It is to be understood that the above described embodiment of the invention is illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiment disclosed herein, but is to be limited only as defined by the appended claims.

Claims: A collimating lens assembly having a first end and a second end opposite said first end, said lens assembly comprising: a first group including first and second positive-power singlets having a first index of refraction, said first group being located at said first end of said assembly; a third positive power singlet located at said second end of said assembly, said third singlet also having said first index of refraction; a doublet disposed between said first group and said third singlet, said doublet including a positive-power lens having an index of refraction less than said first index, and a negative-power lens having an index of refraction greater than said first index, wherein said assembly is operative to form a telecentric pupil along said optical axis external to said second end of said assembly. The lens assembly of claim 1, wherein said first singlet is spaced apart from said second singlet, and wherein said negative-power lens has an optical flat that is abutted against said second singlet. The lens assembly of claim 2 wherein all surfaces of said singlets and said doublet are spherical. A system which includes the lens assembly of claim 1, comprising the lens assembly (10), an optical display (44), and scanning means (68); wherein the second end of said lens assembly is in optical communication with the display, and said scanning means is in optical communication with the first end of said lens assembly for scanning said display. The system of claim 4 wherein each lens in said first group is spaced apart from said display. The system of claim 4 wherein said assembly is operative to form a telecentric pupil along an optical axis of said assembly, said pupil being external to said first end of said assembly. The system of claim 4 wherein said scanning means includes a plurality of mirrors (76), said mirrors having a shape and size that defines an aperture stop for said assembly. The system of claim 7 wherein said second end of said assembly is located from said aperture stop a distance greater than one focal length of said assembly. The system of claim 8 wherein said singlets have a first index of refraction, and wherein said doublet includes a positive-power lens element and a negative-power lens element, said positive-power lens element of said doublet having an index of refraction less than said first index, said negative-power lens element of said doublet having an index of refraction greater than said first index. The system of claim 9, wherein said negative-power lens element has an optical flat (42), and wherein said first group includes a singlet (14) that is abutted against said optical flat of said negative-power lens element.

Inventors: SPENCER HARVEY M, SPENCER, HARVEY M.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1993
Main classification: G02B13
IPC codes: B41J2, G02B25, G02B13, G02B26, G02B27