IMAGE FORMING APPARATUS USING HIGH NUMERICAL APERTURE LENS AND OPTICAL FIBER

Abstract: An image forming apparatus includes a plurality of laser diodes (111) which provide a source of writing radiation. The output of each laser diode is fed by an optical fiber (60) to a printhead where a lens (56) having a high numerical aperture images the fiber endings onto a sensitive medium. The apparatus also has a calibration mode in which the output of each fiber is individually projected onto a detector (210). The response of the detector is used to balance the driving current for the laser sources to provide a balanced output for the printhead. The numerical aperture of the fibers is picked to be not substantially greater than the acceptance numerical aperture of the lens, at least during calibration, thereby substantially reducing noise which adversely affects such calibration.

Description: IMAGE FORMING APPARATUS USING HIGH NUMERICAL APERTURE LENS AND OPTICAL FIBER TECHNICAL FIELD This invention relates to an image forming apparatus in which radiation emitted from one or more optical fibers is imaged on a medium sensitive to that radiation by a high aperture lens. Although not limited thereto, it is particularly usable in a thermal printer using light emitting diodes to provide thermal energy to selectively transfer dye to a receiver to form an image. BACKGROUND ART In commonly-assigned U.S. Patent Application Serial Nos. 07/451,655 and 07/451,656 both filed December 18, 1989, a thermal printer is disclosed which may be adapted for use as a direct digital color proofer with halftone capabilities. This printer forms an image on a thermal print medium in which a donor element transfers a dye to a receiver element in response to a sufficient amount of thermal energy. It includes a plurality of diode lasers which can be individually modulated to supply energy to selected areas of the medium in accordance with an information signal. The printer includes a printhead which includes one end of a fiber optic array having a plurality of optical fibers coupled to the diode lasers. A thermal print medium is supported on a rotatable drum, and the printhead with the fiber optic array is movable relative to the drum. Dye is transferred by sublimation to the receiver element as the radiation, transferred from the diode lasers to the donor element by the optical fibers is converted to thermal energy in the donor element. Commonly-assigned WIPO application US92/01933, filed March 11, 1992, describes a high quality version of the color proofer from the earlier applications which is capable of consistently and accurately writing pixels at a rate of 1800 dots per inch and higher to generate halftone proofs having a resolution of 150 lines per inch and above. Each dot or mini-pixel is held to a density tolerance of better than 0.1 density unit from that prescribed in order to avoid visible differences between the original and the proof. The writing beams are presented to the apparatus in the printhead by optical fibers from their laser diodes. The ends of the optical fibers are imaged on the donor medium by a high numerical aperture lens. SUMMARY OF THE INVENTION In designing a calibration system for the apparatus disclosed, a detector is positioned outside of the moving drum surface. The printhead is movable to a position in optical alignment with the detector. Each laser diode is energized separately and the detector senses the illumination from the laser diode and feeds that information to an appropriate logic and control unit. The logic and control unit then uses that information to adjust the drive current to each laser diode. With this system the printhead can be periodically calibrated. Most important for highest image quality the output from the respective fiber endings can be accurately balanced. In coupling the laser diodes to the optical fibers a number of schemes were tried to obtain maximum intensity at the print medium. For example, a 10-stripe gain guided laser diode was coupled to an optical fiber using a tapered fiber pigtail design. Such laser diodes have a large numerical aperture by nature of the 10-stripe design. The fibers are of the step index type. To obtain the largest amount of radiation from the diode, fibers having an numerical aperture of 0.40 were used. A lens was used that had an extremely high acceptance numerical aperture of approximately 0.25. With this scheme the printhead was found to have a 5% variability in output power as measured during calibration. The power variabilities are believed to be due to a sensitivity to fiber movement, diode current and temperature. This variability in output power had greater effect on the ultimate image if it occurred during calibration than if it occurred during printing, because variability in calibration affects the balance of the printhead. It is an object of the invention to reduce the variability of output power in such a printhead, especially during the calibration mode. This and other objects are accomplished by choosing an optical fiber having a numerical aperture not substantially greater than the numerical aperture of the lens, at least during the calibration mode of the printer. This is effective to reduce the variability because the light exiting the fibers is distributed into various modes or paths. As the fiber is moved the amount of light in each of the various modes changes. This is due to the changes in the path the light takes as the fiber is bent in different directions. As the light travels through the fiber and reflects back from the interface between its core and the cladding it may change into different modes of transmission which are also capable of being transmitted through the fiber. If the numerical aperture of the fiber is not substantially greater than the acceptance numerical aperture of the lens, virtually all of the output of the fiber will pass through the lens providing consistency of intensity at the detector and the medium. This contrasts with the first mentioned approach in which the numerical aperture of the fiber is substantially higher than that of the lens. In this latter instance, much of the radiation which successfully gets through the fiber is not passed by the lens. Since some of that radiation had changed modes due to movement of the fiber or other outside causes, the actual intensity at the detector is more variable. According to a preferred embodiment, a 5-stripe laser diode is used with a fiber having a 0.29 numerical aperture. This is coupled with the 0.25 numerical aperture lens. Partially because of the low numerical aperture of the 5-stripe diode, this system is also more efficient than the 10-stripe diode and 0.40 numerical aperture fiber as well as being less variable in intensity. In the preferred form of the apparatus, both writing and calibration is done at comparable numerical apertures for the fiber and lens. However, in an alternative embodiment, the lens includes a stop or other means for varying its aperture. Calibration is done with the lens at its maximum numeric aperture and the fiber at a numerical aperture not substantially more than it. However, writing is done with the lens aperture reduced. This will add optical noise to the print as well as some variability in intensity. However, the optical noise is random and can be used in writing to overcome image and printer artifacts. SPECIFIC DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of the imaging apparatus of the present invention, partially cut-away to reveal hidden portions thereof. Fig. 2 is a sectional view of the writing head and lens assembly taken along line 2-2 of Fig. 1. Fig. 3 is an end view of the print head assembly. Fig. 4 is a plan view of an optical fiber supporting substrate. Fig. 5 is a perspective schematic illustrating the relationship of the drum, calibration detector, and a portion of the printhead. Fig. 6 is a side schematic illustrating the relative numerical apertures of the lens and the optical fibers. BEST MODE OF CARRYING OUT THE INVENTION Referring now to Fig. 1, there is shown a thermal printer 10 comprising a drum member 12 mounted for rotation about an axis 15 in frame member 14. The drum member 12 is adapted to support a thermal print medium, not shown, of a type in which a dye is transferred by sublimation from a donor element to a receiver element as a result of heating the dye in the donor. The donor element and the receiver element are superposed in relatively intimate contact and are held onto the peripheral surface of the drum member by means such as by vacuum applied to the superposed elements from the drum interior. A thermal print medium for use with the printer 10 can be, for example, the medium disclosed in U.S. Patent No. 4,772,582, which includes a donor sheet having a material which strongly absorbs at the wavelength of the exposing light source. When the donor element is irradiated, this absorbing material converts light energy to thermal energy and transfers the heat to the dye in the immediate vicinity, thereby heating the dye to its sublimation temperature for transfer to the receiver element. The absorbing material may be present in a layer beneath the dye, or it may be admixed with the dye and is strongly absorptive to light having wavelengths in the range of 800nm - 880nm. An example of a preferred embodiment of a receiver element that can be used with the present invention is disclosed in co-pending, commonly assigned EPO Application No. 91118514.8, filed October 30, 1991. The receiver element disclosed therein incorporates a reflective layer which improves the efficiency of the dye transfer to the receiver element. The light source is movable with respect to the drum member and is arranged to direct a beam of actinic light to the donor element. Preferably the light source comprises a plurality of laser diodes which can be individually modulated by electronic signals which are representative of the shape and color of the original image, so that each dye is heated to cause volatilization only in those areas in which its presence is required on the receiver to reconstruct the color of the original object. In the preferred embodiment, the laser diodes are mounted remotely from the drum member 12 (as shown schematically in Fig. 5), on the stationary portion of the frame 14 (Fig. 1), and each direct the light produced thereby to the input end of a respective optical fiber which extends to and transfers the light to a movable writing head 20 adjacent the drum member. Each of the laser diodes is selected to produce a first beam of light having wavelengths in the range of 800nm - 880nm, and preferably predominantly at a wavelength of 830nm. The writing head 20 is moveably supported adjacent drum member 12 and is mounted on a moving translator member 21 which, in turn, is supported for slidable movement on bars 22 and 24. The bars 22 and 24 are sufficiently rigid that they do not sag between the mounting points at their ends and are arranged as exactly parallel with the axis of the drum member as possible. The upper bar 22 is arranged to locate the writing head precisely on the axis of the drum with the axis of the writing head perpendicular to the drum axis. The upper bar 22 locates the translator in the vertical and the horizontal directions with respect to the axis of the drum member. The lower bar 24 locates the translator member only with respect to rotation of the translator about the bar 22 so that there is no over-constraint of the translator which might cause it to bind, chatter, or otherwise impart undesirable vibration to the writing head during the generation of an image. The translator member 16 is driven by means of a motor (not shown) which rotates a lead screw 26 parallel to bars 22 and 24 to move the writing head parallel with the axis of the drum member. The coupling (not shown) which connects the translator member to the lead screw is carefully chosen so that the only force imparted to the translator by the lead screw is parallel to the drum axis. The writing head 20 is removably mounted on the translator member 16 so that it automatically adopts the preferred orientation with respect to the drum axis noted above. The writing head is selectively locatable with respect to the translator, and thus with respect to the drum surface and axis, with respect to its distance from the drum surface, and with respect to its angular position about its own axis. Accordingly, a pair of adjustable locating means are provided to accurately locate the writing head with respect to these two axes on the translator member 16. Only one of the adjustable locating means, a micrometer adjustment screw 25, is illustrated. A torsion and compression spring 27 is provided to load the writing head against these locating means. The end of the writing head 20 adjacent the drum member 12 is provided with a pair of photosensors 29 aimed at the surface of the drum member. The photosensors may each include an infrared source or they may rely on an outside source of light energy. The photosensors are disposed on diametrically opposite sides of the optical axis of the writing head in a fixed relationship thereto. A cross section of the writing head 20 is illustrated in Fig. 2 and comprises a generally cylindrical barrel portion 50 having a flange 52 at the drum end thereof. The interior of the barrel portion is arranged to accept a stationary lens barrel 54 at the writing end, containing a stationary lens 56. Only the rearmost elements of lens 56 are shown. However, it is a high aperture lens having a number of elements located in barrel 54. A printhead assembly 58 is selectively oriented within and at the opposite end of the barrel from the writing end. The printhead assembly comprises a tubular member selectively oriented within barrel portion 50 and contains a linear array of optical fibers which includes a fiber-supporting wafer 34 having a plurality of optical fibers 60 mounted thereon. The optical fibers have a writing end 36 facing the drum member 12 at the opposite end of the barrel. The optical fibers 60 extend from the end of the printhead assembly and out of the writing head barrel through a protective sheath 64 to the diode lasers (Fig.5). A cup-shaped closure member 66 is arranged to mate with the flange 52 of the writing head barrel 50 and forms a housing for a focusing drive means. The end of the closure member adjacent drum member 12 is provided with an axially disposed opening which is bridged by a pair of sheet flexure members, 68 and 70, mounted at the outer periphery thereof by annular plate means 72 and 74 to the closure member 66. The central portions of the sheet flexure members are mounted to a movable rigid cylindrical lens housing 76 which contains moveable lens 80. Lens 80 is movable for focusing by a focus detection system which is not germane to this invention and will not be described in detail. The fiber optic array (see Figs. 2 and 3) comprises a plurality of fibers 60 which are each connected to a respective, remotely mounted diode laser (Fig. 5). The diode lasers can be individually modulated to selectively project light from the writing end 36 of the optical fibers through the lens assembly, consisting of stationary lens 56 and movable lens 80, onto the thermal print medium carried by the drum member 12. The fiber optic array can be of the type shown in Figure 3 and comprises optical fibers 60 which are supported on the substrate 34. The array may be of the type shown in co-pending, commonly assigned U.S. Application Serial No. 07/451,656, filed December 18, 1989. Each of the optical fibers includes a jacket, a cladding, and a core, as is well known in the art. As disclosed in the copending application, the fibers extend from the laser diodes to the array and are mounted in sets of grooves 100 (Fig. 4) which are formed in the substrate 34 so that the fibers at the writing end 36 are disposed substantially parallel and adjacent to each other in very close proximity, with the ends disposed in a common plane perpendicular to ' the fiber axes. In a preferred embodiment of the array, twenty writing fibers 60 are employed. As illustrated in Fig. 3, the substrate 34 is disposed in the tubular member of the printhead assembly 58. The tubular member is provided with a keyway 59 which mates with a corresponding key (not shown) on the inner surface of barrel portion 50 so that the orientation of the linear array 60 is at a preselected angle R with respect to the drum axis 15. The orientation of the keyway 59 in the outer surface of the printhead assembly 58, the corresponding key on the interior of the barrel portion 50, and the photosensors 29 disposed on diametrically opposite sides of the writing head axis, all correspond so that when the two photosensors 29 are exactly parallel with the axis 15 of drum member 12, the writing angle of the linear array 60 is that which has been preselected for the present apparatus. The determination of this relationship is relatively simply achieved with the present construction inasmuch as a visible line 61 is provided on the drum surface which is carefully fabricated to be parallel with the drum axis. Accordingly, when the photosensors 29 both detect line 61 simultaneously, the writing head has the proper angular orientation to provide the desired angle of the linear array with respect to the drum axis. Adjustment of the angular positioning of the writing head is equally easy to obtain. Hold down clamps 102, which lock the writing head 20 on the translator member 16, are loosened, and the micrometer adjustment screw 25 is adjusted against a stop on the translator member to rotate the head member against the force of the torsion spring 27, or to permit the torsion spring to rotate the writing head in the opposite direction, should that be necessary. When the photosensors 29 both simultaneously detect line 61, which may be accomplished when the drum is either moving or stationary, with or without the writing element disposed thereon, the desired angle R between the linear array and the drum axis is achieved. With this construction it is possible to replace the writing head in the field with a new writing head without requiring elaborate setup or alignment, since the predetermined relationship has already been established between the photosensors 29 and the linear array when the writing head is assembled. The focus detection system utilizes a second array of optical fibers 62 mounted on the opposite surface of the substrate 34 with respect to the writing array 60. The foregoing is a general description of printer 10. Figs. 5 and 6 best illustrate the invention. Referring to Fig. 5, drum or drum member 12 is shown with a portion of printhead 20 aligned with a donor and receiver combination 200. Printhead 20 is supplied with illumination from laser diodes 111 which are connected by fibers 60 to the printhead. The printhead moves slowly in a direction parallel to the axis of drum 12 while the drum is rotated rapidly to helically scan the drum as described with respect to Figs. 1 and 2. The output of the laser diodes and the fibers varies substantially over time. It is necessary that they be regularly recalibrated. This is accomplished by moving printhead 20 to the left as seen in Fig. 5 until it is aligned with a silicon detector 210 which is sensitive to the writing illumination. With the printhead so aligned with detector 210, each of laser diodes 111 is sequentially turned on and its output at the detector 210 is measured. This measurement is fed back through the logic and control of the apparatus and is used to adjust the driving current for each laser diode. This calibration allows the outputs of the different fibers to be accurately balanced and is important for highest image quality. As described above, a printhead constructed with illumination from 10-stripe laser diodes emitting at approximately 830nm and fed into optical fibers having numerical aperture of 0.40, exhibited an excessive amount of noise in the calibration process. This was determined to be due to the use of a lens which had a lower numerical aperture than the optical fibers. For example, using a lens having a numerical aperture of 0.25 a relatively intense writing beam was produced out of a fiber having a numerical aperture of 0.40. However, it was difficult to balance the driving currents for the laser diodes to achieve uniformity between the projected spots. To solve this problem, a step index fiber with a numerical aperture of 0.29 was used in combination with a 5-stripe diode. Because of efficiency between the lower numerical aperture fiber and the 5-stripe diode this combination was more efficient than the earlier printhead. However, more importantly, the noise was substantially reduced which made it much easier to balance the system during calibration. We believe this result is due to the fact that with the numerical aperture of the fiber not being substantially above that of the lens virtually all of the radiation emitting from the fiber is passed on by the lens to the detector 210. This is illustrated in Fig. 6 in which the acceptance angle 216 of the lens 56 is shown to correspond with the emission angle 206 of the fiber 60. If the illumination from fiber 60 were at a greater angle than shown at 206 a portion of the illumination outside that angle would not get through lens 56 to detector 210. Since turns, temperature and other variables in the fiber cause some illumination to change modes and exit at different angles than it began (up to the maximum emission angle 206), a more accurate calibration can be effected if all of the illumination is used. For example, using a 0.40 NA fiber with a 0.25 NA lens, radiation entering the fiber at a low angle, due to errors in transmission, may exit at a larger angle. If a particular bend in the fiber causes this to happen to a meaningful portion of the radiation, the angular distribution of the fiber output can vary randomly. The lower NA lens essentially filters out the large angle radiation causing a variance in calibration caused by the bend in the fiber. If all the radiation is passed by the lens, then such variation is reduced or eliminated. This aspect is much more important for calibration than it is for writing since calibration determines the basic balance of the printhead. In writing the increased noise is not as harmful to the image and, in fact, may be useful in a system prone to visible image artifacts. The noise, being random, will mask the image artifact. This suggests an alternative embodiment in which an adjustable aperture 230 can be used to reduce the numerical aperture of lens 56 during writing, while the entire aperture of the lens is used during calibration. This embodiment is useful in a system in which a major problem is produced by image artifacts. It would not ordinarily be preferred since it involves reducing the intensity of the image creating radiation during writing. It would not use the full capability of the lens during writing. 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 as described hereinabove and as defined in the appended claims.

Claims: We Claim: 1. An image forming apparatus comprising: at least one laser source (11) of writing radiation, an optical fiber (60) optically coupled to said laser source, and having a fiber ending from which writing radiation is emitted, a high aperture lens (56) for projecting radiation emitted from said fiber onto a member sensitive to the writing radiation, said lens having a high acceptance numerical aperture, characterized in that the optical fiber (60) has a numerical aperture not substantially greater than the acceptance numerical aperture of the lens (56). 2. An image forming apparatus according to claim 1 which apparatus is operable in at least two modes, including 1) a writing mode in which images are formed in response to a projection of writing illumination on said sensitive member by said lens, and 2) a calibration mode in which said lens projects said writing radiation onto a detector, which detector provides a signal for control of the apparatus. 3. An image forming apparatus according to claim 2 further including means for adjusting the numerical aperture of the lens to provide a substantially lower numerical aperture for the lens and the numerical aperture of the fiber during the writing mode. 4. Apparatus according to claim 2 wherein the optical fiber has a numerical aperture not substantially greater than the acceptance of the numerical aperture of the lens in the writing mode. 5. An image forming apparatus comprising: a plurality of laser sources of writing radiation, a separate optical fiber optically coupled to each of said laser sources and, each optical fiber having a fiber ending from which writing radiation from its source is emitted, a high aperture lens for projecting an image of said fiber endings onto a member sensitive to the writing radiation, said lens having a high acceptance numerical aperture, said apparatus being operable in modes including 1) a writing mode in which images are formed in response to projection of images of said fiber endings by said lens onto said sensitive member, and 2) a calibration mode in which said lens projects writing radiation from each of said fiber endings onto a detector, which detector provides a signal to control the output of each of said laser sources, characterized in that the optical fibers each have a numerical aperture not substantially greater than the acceptance numerical aperture of the lens in at least the calibration mode.

Inventors: GENTZKE JOHN DARRELL, SANGER KURT MICHAEL, GENTZKE, JOHN, DARRELL, SANGER, KURT, MICHAEL
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1992
Main classification: G06K15
IPC codes: B41J2, G02B6, G06K15