HIGH APERTURE LENS SYSTEM AND PRINTER USING THE LENS SYSTEM

Abstract: A high aperture finite conjugate lens system has a high numerical aperture on both object and image sides of the lens system. The lens system comprises an aperture stop located approximately as the center of the system and a plurality of lens elements centered on an optical axis and arranged into two lens groups of positive power to provide the high numerical apertures on both the object and the image sides of the lens system. The two lens groups are located on each side of the aperture stop and each lens group comprises an outer-most lens element and an inner-most lens element. The inner lens elements are located in close proximity to the aperture stop and each of the inner-most lens elements have a convex surface facing towards the aperture stop. It is also an object of the invention to provide a printer, for example, a laser thermal printer, using such a lens system.

Description: High aperture lens system and printer using the lens system 5 TECHNICAL FIELD This invention relates generally to finite conjugate lens systems and more particularly to a well corrected finite conjugate micro relay lens system having high numerical apertures on both object and image sides. It also relates to a 10 printer using such a lens. BACKGROUND ART Commercially available high aperture systems such as microscope objectives and Petzval-type lenses are generally 15 designed for an object location which is far from the lens system when compared to the location of the image. Typical magnification provided by a microscope objective is 40:1 and while the f-number of the lens system is low on the image side, it is high on the object side. For a laser thermal printer, a 20 micro relay lens system requires a very low f-number and thus extremely high numerical apertures on both object and image side of the lens system. No such lenses are known to exist. DISCLOSURE OF THE INVENTION 25 The object of this invention is to provide a high speed, well corrected, finite conjugate imaging lens system, with unusually high numerical apertures and correspondingly very low f-numbers on both image and object side of the lens system. 30 This object is accomplished by a lens system comprising an aperture stop located approximately at the center of the system and a plurality of lens elements centered on an optical axis and arranged into two lens groups of positive power to provide the high numerical apertures on both the 35 object and the image sides of the lens system. The two lens groups are located on each side of the aperture stop and each lens group comprises an outer-most lens element and an inner¬ most lens element. The inner lens elements are located in close proximity to the aperture stop and each of the inner-most lens elements have a convex surface facing towards the aperture stop. It is also an object of the invention to provide a printer, for example, a laser thermal printer, using such a lens system. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a profile drawing of an optical system of a first embodiment; Fig. 2 is a schematic of a laser diode thermal printing system; Fig. 3 is a schematic of a multi-fiber array printing head; Fig. 4 is a profile drawing of an optical system of a second embodiment; Fig. 5 is a profile drawing of an optical system of a third embodiment; Fig. 6 is a profile drawing of an optical system of a fourth and fifth embodiment; and Fig. 7 is a profile drawing of an optical system of a sixth embodiment. BEST MODE OF CARRYING OUT THE INVENTION The lens system of the present invention has a general application to systems requiring high numerical aperture on both object and image side while working at finite conjugates. An example of one such application is the use of the lens system in a new thermal printer system, an example of which is described below. In the graphic arts industry, digital scanning printers are widely used for making color separation negatives. After the negatives are made, a color proof image is created, and is approved or rejected. This process may be repeated one or more times before approval is obtained from the customer. An alternate procedure, which uses the lens systems described herein, is to make the color proof image directly, bypassing the making of color separation negatives. Negatives may be made only after the color proof has been approved. This alternate procedure is less costly and is less time consuming than is the procedure that is presently in wide usage. The images that are presently created by digital scanning printers require from 1000 to 3000 dots per inch. The image sizes typically range from A4 (8"xl0") to A2 (18"x24"). The size of the image and the large amount of digital image data requires a laser scanning system which is quite expensive, using precision optical components and high bandwidth electronics. The system utilizing the lenses of our present invention is simpler, less expensive and produces results faster. Referring to Figures 2 and 3, the printer system utilizing the lens systems of our present invention comprises a rotating drum 10, a printhead 14 and a leadscrew 12. A receiver material 16 is wrapped around the drum 10. A sheet of thermal sensitive media-containing colored dye is placed over the receiver material. The printhead 14 comprises a relay imaging lens system 15, and a linear array 20 of optical fiber ends, which are secured in position by a silicon wafer 13 (see Fig. 3) which is etched to have a series of adjacent V-grooves 17. The fibers 19 are made to lie in these grooves 17 in such a way that the ends of the fibers lie in a common plane. The center-to-center spacing between the fibers is typically 0.15 mm. There may be up to 20 such fibers in the array, so that the length of the array may be almost 3 mm. The fiber arrays are further described in U.S. Patents 4,389,655 and 4,911 ,526. The energy that is emitted by this array of fiber ends is picked up by the relay lens 15 and is focused on the thermal sensitive media 16 on the drum 10, forming an array of very small heated spots. The thermal sensitive media 16 releases the dye, which is then transferred to the paper underneath. The printhead is carried along on a rail 18 by the leadscrew 12 in synchronization with the drum rotation. As the drum rotates, the energy coming out of each fiber end varies in accordance with the signal which is sent to the laser diode which feeds that particular fiber. If there are 20 such fibers, there are 20 parallel scan lines created simultaneously on the media, as the drum 10 rotates. The characteristics of the relay lens systems of the illustrated embodiments are as follows: 1. They are color corrected to perform well at wavelengths between 790 and 830 nanometers. The glasses that are used must be highly transparent at these wavelengths. 2. They are designed to work at finite conjugates, at a fixed magnification, generally between 1/3 and 1/2, so that the distance between the object and the image (called the total track length) is on the order of about 2 to 3 inches. 3. They have high numerical apertures, on both object and image sides, such as are associated with microscope objectives. The numerical aperture on the object side of the lens should be large enough to capture a large fraction of the energy leaving the fiber ends. Multimode fibers typically have numerical apertures of 0.3 to 0.4, and single mode fibers will typically have numerical apertures of 0.1 to 0.2. The relay lenses described herein have object-side numerical apertures of 0.2 to 0.253, which is large enough to capture most of the energy emitted by the fibers. The numerical apertures on the object and image sides are related to the magnification MAG of the lens by the following equation: MAG = (numerical aperture at object)/(numerical aperture at image) For example, for embodiment #1 , MAG = (0.25)/(0.5) = 0.5. 4. The image quality, or sharpness of the imagery, does not have to be as good as is normally associated with microscope objectives. The spots that are formed on the image plane are approximately about 0.02 to 0.04 mm in diameter, whereas in a microscope objective having a numerical aperture of say, 0.25, the visible light diameter of the Airy disc would be about 0.003 mm. Thus, these lenses do not have to be "diffraction limited". Also, the useful depth of focus of these lenses is several times larger than that suggested by the Rayleigh criterion. Another consideration is that these lenses can be fabricated with looser tolerances than those associated with normal microscope objectives. The first illustrative embodiments for the lens systems are described in the following table: Embodiment number 1 Number of Elements Object Diagonal Image Diagonal Magnification Object Numerical Aper. Image Numerical Aper. ANSI f-number of lens Total Track Length Embodiment number Number of Elements Object Diagonal Image Diagonal Magnification Object Numerical Aper. Image Numerical Aper. ANSI f-number of lens Total Track Length *not including the beamsplitter prism used for autofocusing. In illustrative embodiments 3, 4 and 5, the rear element L may move +/- 0.1 mm with respect to the rest of the lens. The focused image position also moves, in such a way that the distance from the rear surface of the rear element stays nearly constant, changing less than 0.01 mm. This feature in these embodiments is quite novel in a lens of this high aperture. In illustrative embodiments 4 and 5, a beamsplitter prism B is added on the front or object side of the lens, permitting the use of a servo driven focusing rear lens element. In this system (not shown in the drawings), an infrared beam at wavelengths between 900 and 1,000 nanometers is introduced into the beamsplitter prism from the end of a fiber located near the multi-fiber array. The beam goes through the relay lens and is focused on the media. The beam has no heating effect on the media. After reflection at the media, the beam retraces itself backwards through the lens, and is reflected by the beamsplitting interface within the beamsplitter prism. This beam is then analyzed for focus. An electrical signal is generated, and used to drive the rear lens element into a new position so as to cause the heating beams to focus properly on the media. The lens system 100 of the first embodiment of the present invention is illustrated in Fig. 1. It was designed for use in a laser diode thermal printing system (Fig. 2) using a multi-fiber array printing head (Fig. 3) coupled to high power laser diodes. The printer system uses a drum 10 for the line direction scan movement and a stepping motor or a linear translation stage for the page direction scan movement. A custom designed lens was necessary to meet the requirements for the numerical aperture and the field of view requirements. The numerical aperture of the beam on the object side of the lens system is 0.25, and the numerical aperture of the beam on the image side of the lens system is 0.5. The ANSI f-number of the lens system is 0.6667. The field angle is 3.08 degrees. Relative illumination due to cosine effects exceeds 1.0 at the edge of field of view. The distance from the object to image is 50.8 mm (2 inches). The lens system 100 is used in the rear infrared IR (630 to 810 nm) to relay an object having a diameter of 2.4 mm to an image of diameter 1.2 mm giving a magnification of 1/2. The lens system 100 is used to form an image of a source consisting of a linear array of fibers, each transmitting energy from remotely located infrared laser diodes. The combination of very high numerical aperture and sharp imagery over an extended field of view dictated the need for a new lens of a very unusual construction. The lens system 100 comprises a plurality of lens elements and an aperture stop located approximately at the center of the lens system. Following from object to image, the lens elements are: a biconvex positive singlet LI, a meniscus negative singlet L2 with a concave surface S3 facing an object and a convex surface S4 towards an aperture stop ST, a plano- convex positive singlet L3 located near the aperture stop with its convex surface S6 facing the aperture stop, a meniscus doublet comprised of a bi-convex positive element L4 cemented to a bi-concave negative lens element L5, a bi¬ convex positive singlet L6 and a positive meniscus lens element L7. The numerical data for the optical system is as follows: The performance of the system is excellent. Sagital and tangential MTFs exceed .5 at 50 cycles/mm over the entire field of view, in a fixed flat focal plane and over the wavelength region from 800 to 820 nm. The lens system also has a low distortion. The focusing function is performed by moving an entire lens system. The second embodiment of the present invention is illustrated in Fig. 4. The optical system 200 of the second embodiment is illustrated in Fig. 4 and used to relay an object of a diameter 3.6 mm to an image of diameter 1.2 mm giving a magnification of 1/3. The numerical aperture of the optical system at the object side is 0.2. The numerical aperture of the optical system at the image side is 0.6. The ANSI f-number is 0.625. This lens system may be used as a 1/3 X relay lens in a laser thermal printer described in Figs. 2 and 3. The lens system 200 comprises a plurality of lens elements arranged into two lens groups on either side of the aperture stop ST. The aperture stop ST is again located approximately at the center of the lens system 200. However, the shapes of the lens elements are quite different from those of the first embodiment. Following from the object side to the image side the lens elements are: a positive meniscus lens element LI with its convex surface oriented towards an aperture stop; a biconvex lens element L2; a meniscus doublet comprised of a meniscus negative lens element L3 and a meniscus positive lens element L4 followed by an aperture stop; a cemented doublet following the aperture stop comprised of a positive lens element L5 and a negative lens element L6 cemented together; and a bi-convex singlet lens element L7 and a convex- piano singlet lens element L8. Again, as in a preceeding embodiment, the surfaces facing the aperture stop of the components adjacent to the aperture stop are convex, thus forming a air space having a shape of a negative lens element at the aperture stop and the meniscus components of the two optical groups on either side of the aperture stop having their concave surfaces facing away from the aperture stop. The numerical data for the optical system of the second emobdiment is as follows: Table 2 On the image side of the lens system, there is a 1 mm thick protective window, which can be easily cleaned. The focusing function is performed by moving an entire lens system. The third embodiment is illustrated in Fig. 5. The lens system 300 of the third embodiment is designed for the wavelength range of 750 to 850 nm with the principal wavelength of 800 nm. The focusing function is performed by the movement of the rear-most lens element L8. The track length (i.e. the distance from the object to the image), nominally 2 inches or 50.8 mm will thus change by ±0.1 mm. Unlike the first two embodiments, this lens system does not use a protective window at the rear. The object diameter is 2.4 mm, while the image diameter is 0.8 mm, thus the optical system 300 has a magnification of 1/3. The numerical aperture at the object side is 0.2 and the numerical aperture at the image side is 0.6. The lens system 300 is comprised of 6 components in the following order from object to image: A meniscus singlet lens element LI, a bi-convex singlet lens element L2, a doublet comprised of a bi-concave lens element L3 cemented to a bi-convex lens element L4, a doublet comprised of a convex-piano element L5 cemented to a plano-concave element L6, a bi-convex singlet L7 and a convex-piano singlet L8. The aperture stop ST is located between the third and the fourth components. The unusual feature of this lens system is that the sixth component can be varied in position, with respect to the preceeding components, by as much as +/- 0.1 mm, while the location of the image surface will stay nearly fixed with respect to the rear piano surface of the sixth component. The nominal lens is slightly undercorrected (-.015 mm) for lateral spherical abeπation. Sagittal and tangential longitudinal astigmatic field are flat to less than .01 mm. Axial color is coπected to bring beams having wavelengths of 750 nm and 850 nm to a common focus postion. Lateral color is coπected to less than .002 mm. Lateral tangential coma is coπected to less than .005 mm. Lateral distortion is corrected to less than .0001 mm. This data all applies at the image, i.e., on the short conjugate side of the lens. The object consists of a linear array of 15 circular spots, each 0.1 mm in diameter, with center to center spacing of 0.15 mm. At 1/3 magnification, the images of the spots are .0333 mm in diameter. This is relatively coarse image structure. By comparing the size of the lateral abeπations to the size of the spots, it is clear that the lens is able to satisfactorily relay the images of these spots. Because of the high numerical aperture of the beam at the image, the depth of focus according to the Rayleigh criterion is about +/- .00111 mm, but because of the relatively coarse nature of the image structure, a more realistic tolerance for the depth of focus is about +/- 0.01 mm, about 9 times larger than the Rayleigh depth. The numerical data for the lens system is as follows: Table 3 The fourth and the fifth illustrative embodiments are similar to each other and are illustrated in Fig. 6. They are designed to have a track length of 70.7 m and 58.5 mm respectively. The object side numerical aperture is 0.253. The image side numerical aperture is 0.55. The ANSI f-number is 0.62 and magnification is 0.46 for both lens system designs. Both lens systems are designed for 750-850 nm range and the lens comprises 6 lens components which are: a first, positive meniscus lens element LI; a bi-convex cemented doublet component comprised of a meniscus negative lens element L2 cemented to a bi-convex positive lens element L3; a meniscus cemented doublet component comprising a concave-piano negative lens element L4 cemented to a plano-convex lens element L5, having a convex surface facing the aperture stop AS; a meniscus cemented doublet component comprising a bi-convex lens element L6 cemented to a bi¬ concave lens element L7; a positive bi-convex lens element L8; and a convex-piano lens element L9. The numerical parameters for the lens systems are given in tables 4 and 5 respectively. Table 4 The lens system 600 of the sixth embodiment of the present invention is illustrated in Fig. 7. The numerical aperture of the beam on the object side of the lens system is 0.15, and the numerical aperture of the beam on the image side of the lens system is 0.3. The ANSI f-number of the lens system is 1.11. The semi field angle is 2.54 degrees. The distance from the object to the image is 50.8 mm. The lens system 600 is used in the near infrared (760 to 860 nm) to relay an object having a diameter of 3.0 mm to an image of diameter 1.5 mm giving a magnification of 1/2. The lens system 600 comprises six lens elements organized in two lens groups, I and II. An aperture stop ST is located approximately at the center of the lens system between the two lens groups I and II. Following from object to image, the lens elements are: a biconvex positive singlet LI, a meniscus negative singlet L2 with a concave surface S3 facing an object and a convex surface S4 towards an aperture stop ST, a biconvex positive singlet L3 located near the aperture stop, a bi-convex positive lens element L4, a meniscus negative L5, and a biconvex positive singlet L6. The numerical data for the optical system is as follows: An unusual feature of this sixth embodiment is the use of two lens elements in one lens group that are identical but reversed in direction from two lens elements included in the second lens group; i.e. lens elements LI and L6 are identical and lens elements L3 and L4 are identical in their structure. The term identical means that these have the same radii of curvature, the same thickness and are made of the same type of material, although their aperture sizes are different depending on the position that they occupy within the lens system. This feature permits for lower manufacturing costs. For the purposes of this application, a protection plate P, shown in Fig. 7, is not considered to be a lens element. Although in the prefeπed embodiment the lens element's surfaces are spherical, in a modification, aspherical surfaces can be used. The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims: CLAIMS: 1. A finite conjugate lens system having high numerical apertures on both object and image sides comprising an aperture stop located approximately at the center of said high aperture lens system and a plurality of lens elements centered on an optical axis and aπanged into two lens groups of positive power to provide said high numerical apertures on both object and image sides of said lens system, said lens groups located on each side of said aperture stop, said lens groups each comprising an outer-most lens element and an inner-most lens element, said inner-most lens elements each located in close proximity to said aperture stop, and having a convex surface facing towards said aperture stop. 2. A finite conjugate lens system according to claim 1, wherein said groups of lens elements each have an element with a concave surface facing away from said aperture stop. 3. A finite conjugate lens system according to claim 2, wherein said element with a concave surface is a second element from the aperture stop. 4. A finite conjugate lens system according to claim 1, wherein every positive lens element in said system has a convex surface facing the aperture stop. 5. A finite conjugate lens system according to claim 1, wherein said two lens groups are a front lens group and a rear lens group, said front lens group located in front of said aperture stop, said rear lens group located behind said aperture stop, said rear lens group comprising three lens components. 6. A finite conjugate lens system according to claim 5, wherein said three lens components comprise four lens elements. 7. A finite conjugate lens system according to claim 6, wherein one of said lens elements comprising said rear lens group is negative. 8. A finite conjugate lens system according to claim 1, wherein said two lens groups are a front lens group and a rear lens group, said front lens group located in front of said aperture stop, said rear lens group located behind said aperture stop, said front lens group comprising three lens components. 9. A finite conjugate lens system according to claim 5, wherein said front lens group comprises three lens components. 10. A finite conjugate lens system according to claim 9, wherein one of said three componenets of said front lens group includes one negative lens element. 11. A finite conjugate lens system having high numerical apertures on both object and image sides comprising an aperture stop located between two lens groups of three lens components each, each lens group being of positive power to provide said high numerical apertures on both object and image sides of said lens system, wherein each of said lens groups comprises in order a positive, negative and a positive lens component. 12. A finite conjugate lens system according to claim 11, wherein said two lens groups are a front lens group and a rear lens group, said front lens group located in front of said aperture stop, said rear lens group located behind said aperture stop, said front lens group and said rear group each comprising three lens components and two pairs of lens components in said lens system are identical. 13. A finite conjugate lens system according to claim 11 satisfying the following design parameters: 14. A finite conjugate lens system according to claim 1 having numerical data substantially as set in the following table: 15. A finite conjugate lens system according to claim 1 having numerical data substantially as set in the following table: 16. A finite conjugate lens system according to claim 1 having numerical data substantially as set in the following table: 10 15 20 _!_) 30 5 0 17. A finite conjugate lens system according to claim 1 having numerical data substantially as set in the following table: 18. A finite conjugate lens system according to claim 1 having numerical data substantially as set in the following table: 19. A printer means for generating a plurality of modulated coherent light beams and means for projecting said light beams onto a surface, characterized in that said light projection means includes a finite conjugate lens system having high numerical apertures on both object and image sides comprising an aperture stop located approximately at the center of said high aperture lens system and a plurality of lens elements centered on an optical axis and aπanged into two lens groups of positive power to provide said high numerical apertures on both object and image sides of said lens system.

Inventors: BAEK SEUNG HO, DEJAGER DONALD, BAEK, SEUNG, HO, DEJAGER, DONALD
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1992
Main classification: G02B13
IPC codes: B41J2, G02B13