Abstract:
A method for setting focus of a multichannel printhead for an imaging apparatus ( 10 ) comprises moving the printhead ( 500 ) to a premove position. A first set of patches ( 82 ) is printed wherein a focus position of the printhead is varied in a first random pattern from one patch to another. A second set of patches ( 83 ) is printed wherein the focus position of the printhead is varied in a second random pattern from one patch to another. A density for each of the patches is measured in the first series of patches. A density for each of the patches is measured in the second series of patches. An optimum focus position is calculated for the multichannel printhead based on a polynomial curve ( 72 ) of density and focus position for each of the patches.

Description:
FIELD OF THE INVENTION  
         [0001]    The presented invention relates to a method for focusing a laser thermal printhead and more specifically imaging a randomized focus pattern and measuring the resulting density to calculate a focus position.  
         BACKGROUND OF THE INVENTION  
         [0002]    Color proofing is a process used by the printing industry to simulate proofs generated on a press run. By using color proofing the printing industry saves time and money simulating how the press will look before the costly press run is performed. The advantage of a color proof is that it is a representation of an ideal press run. The color proof should reflect exactly what the printing industry would like to see coming off a press. The press is continually adjusted to match the color of the proof. The color proof therefore needs to be precise color, reproducible from proofer to proofer, and pre-press shop to pre-press shop. Proofs that exhibit color variation are deemed unacceptable.  
           [0003]    One commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,428,371, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying thermal energy to the dye donor material, to transfer dye to the thermal print media, thereby forming an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, and vacuum imaging drum), and thermal print media and dye donor material exit transports.  
           [0004]    The operation of the image processing apparatus comprises feeding a sheet of thermal media from the media roll to the vacuum drum, partially wrapped around the drum, cut to length, then wrapped fully around the drum. A length of dye donor from a roll form is similarly transported to the drum, partially wrapped around the drum, cut to a desired length, then fully wrapped around the vacuum drum. The dye donor material is wrapped around the vacuum imaging drum, such that it is superposed in registration with the thermal print media. The translation drive, part of the scanning subsystem, traverses the printhead and translation stage member axially along the vacuum imaging drum in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.  
           [0005]    The printhead includes a plurality of laser diodes which are coupled to the printhead by fiber optic cables which can be individually modulated to supply energy to selected areas of the donor in accordance with an information signal. The printhead includes a plurality of optical fibers coupled to the laser diodes at one end and at the other end to a fiber optic array within the printhead. The printhead moves relative to the longitudinal axis of the vacuum imaging drum and dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead to the dye donor material, is converted to thermal energy in the dye donor material.  
           [0006]    Color variation is typically a result of variation of the individual color density used to define to desired color. There are many factors that influence the density variation of a color in a proof. One factor that influences proof density uniformity is the focus position of a printhead. Focus is defined by the distance between the lens and the imaging plane along the optical axis. There exists an optimum distance between the imaging plane and the end of the lens for maximum density transfer. As the focus position varies from the optimum position density will also vary. The relationship between focus position and density is often parabolic in nature. As the actual focus position moves away from the theoretical best focus position the density will vary exponentially.  
           [0007]    Several factors contribute to focus related errors. Some existing color proofs, for example U.S. Pat. No. 5,428,371, use a translation system to move the printhead across the imaging plane. Without perfect alignment of translation components an optical axis distance error will be introduced. Thermal media used on color proofing systems have some level of thickness variability. Thickness variation is typically highest in the same direction as the proofer translation. Choices of hardware to position the printhead in the optical axis direction influence the ability to place the printhead to a desired position. Environmental effects, specifically changes in temperature, can introduce optical axis distance changes and instability of a focus position.  
           [0008]    Some early digital proofers, such as U.S. Pat. No. 5,268,708 utilized an auto focus system to detect changes in optical axis distance due to drum and rod alignment and media thickness changes. This system used a reflected laser to measure distance changes. Disadvantages of this system include the presence of unwanted noise which did not allow the system to work properly. The system was also damaged frequently and was costly to replace. This lead many service engineers to simply disconnect the auto-focus device, essentially leaving the system as a fixed-focus system.  
           [0009]    One color proofing system uses a fixed focus design. The system is designed to allow acceptable print uniformity at a optimum focus position. The best focus patch is determined by selection of patches for a course adjustment proof and then a fine adjustment proof. The course and fine focus proofs are written in a similar manner. A single linear ramp of focus positions is imaged across the translation direction. The best patch is defined as the patch with maximum density. This patch selection method leads to several disadvantages.  
           [0010]    Using a method of printing patches to determine the best focus position introduces errors due to print noise as well as errors due to selecting a single patch, rather than interpreting between patches. The best focus position is essentially limited in accuracy to the focus step size of the fine adjustment proof. Print noise comes from various sources. One error as previously started is changes in focus position. Another error is coating quality of thermal media, both receivers and donors. Coating non-uniformities typically are seen in the same direction as the translation system of the proofer. Errors arrive from optical noise and fiber optic blooming as a result of moving fiber optic cables during the printing process as well as variability within lasers and laser power supplies. A densitometer or spectrophotometer used to measure patches introduces random density errors. Without using statistical methods to find the optimum focus position results will exhibit unacceptable amounts of error.  
         SUMMARY OF THE INVENTION  
         [0011]    Briefly summarized, according to one aspect of the present invention, a method for setting focus of a multichannel printhead for an imaging apparatus comprises moving the printhead to a premove position. A first set of patches is printed wherein a focus position of the printhead is varied in a first random pattern from one patch to another. A second set of patches is printed wherein the focus position of the printhead is varied in a second random pattern from one patch to another. A density for each of the patches is measured in the first series of patches. A density for each of the patches is measured in the second series of patches. An optimum focus position is calculated for the multichannel printhead based on a polynomial curve of density and focus position for each of the patches.  
           [0012]    According to one embodiment of a series of test patches covering the range of mechanical motion of the printhead focus device and arranged in a linear ramp are first printed as a coarse adjustment. The patch corresponding to the maximum density is chosen as the best coarse focus position. Next, a series of randomly focused patches are imaged in order to locate the optimum focus position. The randomized nature of the data points, the distance the points cover, and the number of points are used in order to gain high accuracy with the presence of many noise sources. The fine adjustment proof is designed to be scanned in allowing for polynomial interpolation between patches for precise location of the optimum focus position.  
           [0013]    An advantage of the present invention is increased accuracy in locating the best average focus position of a printhead.  
           [0014]    An advantage of the present invention is that the best focus position is not limited to a patch choice. Interpolation allows for a focus position to be determined between patches.  
           [0015]    The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a side view and vertical cross-section of an image processing apparatus of the present invention.  
         [0017]    [0017]FIG. 2 is a perspective view of the lathe bed scanning subsystem of the present invention viewed from the rear of the image processing apparatus.  
         [0018]    [0018]FIG. 3 shows a perspective view of the printhead and support components.  
         [0019]    [0019]FIG. 4 shows a representation of a fine focus adjustment proof.  
         [0020]    [0020]FIG. 5 shows an example of densities resulting from a fine focus proof 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0022]    Referring to FIG. 1, there is illustrated an image processing apparatus according to the present invention having an image processor housing  12  which provides a protective cover. A movable, hinged image processor door  14  is attached to the front portion of image processor housing  12  permitting access to a media carousel  100 . A roll of donor roll material  34  is connected to a media carousel  100  in a lower portion of image processor housing  12 . Up to seven rolls of roll media can be used. One roll of media  33  is thermal print media used to transfer the donor material  34  onto. This thermal print media  33  is passed to vacuum imaging drum  300  and is ultimately cut into donor sheet material (not shown). In this regard, a media drive mechanism  110  is attached to the thermal print media  33 , and includes three media drive rollers  112  through which the thermal print media  33  of interest is metered upwardly into a media knife assembly  120 . After thermal print media  33  reaches drum load roller  113 , media drive rollers  112  cease driving the donor roll material  34  and two media knife blades  122  positioned at the bottom portion of media knife assembly  120  cut the thermal print media  33  into thermal print media (not shown). Drum load roller  113  presses the cut thermal print media against the vacuum imaging drum  300  while the vacuum imaging drum  300  slowly rotates the cut media (not shown) around vacuum imaging drum  300 .  
         [0023]    Each remaining roll of donor material  34  is a different color, typically black, yellow, magenta and cyan. These donor roll materials  34  are passed to vacuum imaging drum  300  and are ultimately cut into donor sheet materials (not shown) for forming the medium from which colorant imbedded therein are passed to thermal print media resting thereon, which process is described in detail below. In this regard, a media drive mechanism  110  is attached to each roll media of donor roll material  34 , and includes three media drive rollers  112  through which the donor roll material  34  of interest is metered upwardly into a media knife assembly  120 . After donor roll material  34  reaches drum load roller  113 , media drive rollers  112  cease driving the donor roll material  34  and two media knife blades  122  positioned at the bottom portion of media knife assembly  120  cut the donor roll material  34  into donor sheet materials (not shown). Drum load roller  113  presses the cut media against the vacuum imaging drum  300  while the vacuum imaging drum  300  slowly rotates the cut media (not shown) around vacuum imaging drum  300 . The donor sheet material now rests atop the thermal print media (not shown) with a narrow space between the two created by microbeads embedded in the surface of the thermal print media.  
         [0024]    A laser assembly  400  includes a quantity of laser diodes  402  in its interior. Laser diodes  402  are connected via fiber optic cables  404  to a distribution block  406  and ultimately to printhead  500 . Printhead  500  directs thermal energy received from laser diodes  402  causing the donor sheet material to pass the desired colorant across the gap and onto the thermal print media (not shown).  
         [0025]    In operation, vacuum imaging drum  300  rotates at a constant velocity. Printhead  500  begins at one end of the thermal print media and traverses the length of the thermal print media, transferring dye for a particular donor sheet material resting on the thermal print media. After printhead  500  has completed the transfer process, for the donor sheet material resting on the thermal print media the donor sheet material is then removed from the vacuum imaging drum  300  and transferred out of image processor housing  12  via a skive or donor ejection chute  16 . The donor sheet material eventually comes to rest in a donor waste bin  18  for removal by the user. The process is then repeated for the other desired rolls of donor roll media  34 .  
         [0026]    After the color from all desired sheets of the donor materials have been transferred and the donor sheet materials have been removed from vacuum imaging drum  300 , the thermal print media is removed from vacuum imaging drum  300  and transported via a transport mechanism  180  to a exit tray (not shown). Thermal print media is then laminated to a paper stock using a Kodak 800XL laminator.  
         [0027]    Referring to FIG. 2, show a perspective view of lathe bed scanning subsystem  200  of image processing apparatus  10 , including vacuum imaging drum  300 , printhead  500 , and lead screw  250  assembled in lathe bed scanning frame  202 . Printhead  500  is attached to a lead screw  250  via a lead screw drive nut  254  and a drive coupling (not shown) for moving the printhead along the longitudinal axis of vacuum imaging drum  300  for transferring the dye to create the intended image onto the thermal print media  33 .  
         [0028]    Vacuum imaging drum  300  is mounted for rotation about an axis X in lathe bed scanning frame  202 . Printhead  500  is movable with respect to vacuum imaging drum  300 , and is arranged to direct a beam of light to the donor sheet material. The beam of light from printhead  500  for each laser diode  402  (not shown in FIG. 2) is modulated individually by modulated electronic signals from image processing apparatus  10 , which are representative of the shape and color of the original image, so that the color on the donor sheet material is heated to cause volatilization only in those areas in which its presence is required on the thermal print media  33  to reconstruct the shape and color of the original image.  
         [0029]    Translation  
         [0030]    Printhead  500  is mounted on a movable translation stage member  220  which, in turn, is supported for low friction slidable movement on translation bearing rods  206  and  208 . Translation bearing rods  206  and  208  are sufficiently rigid so as not to sag or distort as is possible between their mounting points and are arranged to be as parallel as possible with axis X of vacuum imaging drum  300  with the axis of printhead  500  perpendicular to the axis X of vacuum imaging drum  300 . Front translation bearing rod  208  locates translation stage member  220  in the vertical and the horizontal directions with respect to axis X of vacuum imaging drum  300 . Rear translation bearing rod  206  locates translation stage member  220  only with respect to rotation of translation stage member  220  about front translation bearing rod  208  so that there is no over-constraint condition of translation stage member  220  which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to printhead  500  during the generation of an intended image.  
         [0031]    Writing  
         [0032]    Printhead  500  travels in a path along vacuum imaging drum  300 , while being moved at a speed synchronous with the vacuum imaging drum  300  rotation and proportional to the width of a writing swath (not shown). The pattern that printhead  500  transfers to the thermal print media  33  along vacuum imaging drum  300  is a helix.  
         [0033]    Focus Motor  
         [0034]    Referring to FIG. 3, a focus motor  51  mounted against a mounting block provides linear movement needed to focus the printhead  500 . In the preferred embodiment, focus stepper motor acts as a linear actuator, effecting linear movement by incrementing its shaft forward or backwards with each motor step. The preferred embodiment uses a model no. 110702 (417-11-19-13 0.46A) linear actuator with a 22 k gear box manufactured by Lin Engineering. This device provides 0.07215 microns linear movement per motor step.  
         [0035]    An important characteristic of the stepper motor is the relationship of the driving phase currents that cooperate to rotate the motor. The phase state relationship of currents that drive the stepper motor which energize different windings in the stator at each successive phase state, gives the rotor shaft only a finite number of possible angular positions. It is this capability that allows the stepper motor to run “open-loop,” that is, without encoder feedback, to achieve a precisely known angular position when driven using a precise number of steps. Once a fixed reference home position is established, the stepper motor provides a precise rotation, which can then be mechanically translated to provide precise linear movement, using techniques well-known in the art.  
         [0036]    The focus motor is controlled in increments of four steps to provide repeatable positioning. The resolution is therefore reduced to 0.2886 um per four motor steps.  
         [0037]    Movement of the Printhead  
         [0038]    The printhead  500  is mounted in place on the translation stage m ember  220  and is moved forwards and backwards by action of the focus stepper motor  51 . The focus stepper motor  51  connects to a micrometer  53  which is connected to a mounting block  58 . The model of the micrometer is 350-712-30 manufactured by Mitotoyo Corporation. The micrometer attaches to a printhead clamp  55  by a magnetic couple  59 . The printhead is held in place using magnetic force and v shaped blocks. Teflon pads are attached to the top of the magnets providing low friction necessary for forwards and backwards motion. Motor torque of the focus stepper motor  51  is large enough to overcome the magnetic force provided by the magnets mounted in the V grooves.  
         [0039]    Printhead Focus Home  
         [0040]    Two types of flags with associated light emitting sensors are used for determining focus home. A linear flag  54  is attached to the printhead mounting bracket. A rotary flag  52  is attached to the stepper motor. Sensors are positioned to detect each flag. The linear focus home sensor and the rotary focus home sensor are standard optical sensors type 1A05HR, manufactured by sharp Electronics Corporation. Focus home sensors have a conventional emitter-receiver leg configuration well-known in the art. The mounted position of the focus home linear sensor allows for clear passage of the focus home linear flag  54  between the legs of the sensor. The mounted position of the focus home rotary sensor allows for clear passage of the focus home rotary flag  52  between the legs of the sensor.  
         [0041]    First installation and use of the printhead requires the linear flag  54  and rotary flag  52  positions to be set up. To do this the micrometer  53  is set to a mechanical zero position and the readout is set to 0 mm. The printhead  500  is then moved such that the micrometer  53  readout is 6.750 mm. The best method to move the printhead  500  is by rotating the coupling magnet  59  attached to the micrometer  53 . The linear flag  54  is then positioned such that the light emitted from the linear flag sensor is just blocked. The rotary flag  52  is then adjusted such that the flag notch is in the center of the rotary home flag sensor.  
         [0042]    The homing procedure requires the printhead  500  to travel away from the vacuum drum until the linear flag  54  interrupts emitted light from a focus home linear sensor. The printhead  500  continues in traveling until the rotary flag  52  blocks light emitted from the focus home rotary flag sensor. The printhead  500  then travels towards the vacuum drum until the focus home rotary sensor becomes unblocked. All positions requests will move the printhead  500  towards the vacuum drum. The focus drive system is always driven in one direction to remove effects of backlash, or system slack.  
         [0043]    A pre-move operation of the focus system is performed which consists of homing the printhead  500 , moving out to the best focus position if available, or a preset position, re-homing the printhead  500 , moving back out to the best focus position if available, or a preset position, and finally re-homing the printhead  500 . This operation allows the printhead  500  to break free of any time induced static friction.  
         [0044]    Focus Test Proofs  
         [0045]    The process for location an optimum focus position requires two test proofs to be written. The first proof is a coarse adjustment proof, while the second test proof is a fine adjustment proof  80  seen in FIG. 4.  
         [0046]    Coarse Focus Proof  
         [0047]    Obtaining a coarse focus position accomplished by imaging a set of 25 solid patterns, each at a predetermined focus motor  51  step count and starting position in reference to the focus home position. The course focus step count is setup such that the distance traveled is maximized between mechanical hard stops for the desired number of patches. The exact sequence of events is to execute a pre-move, load thermal print media  33  and dye donor roll material  34 , position the printhead  500  in front of the thermal receivers on the vacuum imaging drum  300 , position the printhead  500  focus to home, turn the focus motor  51  off to avoid building up of heat, image a solid test patch at predetermined exposure, move the translation system over to provide a small gap between patches, focus home the printhead  500 , move the printhead focus to 464 steps away from home, image the solid test pattern. This process is repeated such that a series of 25 patches is written. This will create a 25 patch sequence with focus changes of 33.5 um per patch. The user will select the darkest patch from the series of coarse focus patches. A software interface stores the best course focus value and will center the fine focus adjustment proof  80  around the best course focus position. See FIG. 4.  
         [0048]    Fine Focus Proof  
         [0049]    Obtaining a final focus position involves imaging a fine focus adjustment proof  80 . This proof uses at least one row of density patches  82  written with different focus positions and at most 5 rows of density patches written with different of focus positions. Each row  81  consists of 40 patches. There are a maximum of 200 fine focus patches.  
         [0050]    Focus position for each patch  82  are pseudo-randomized in the translation direction for each row  81 . The pseudo-randomized pattern was developed using a randomize function on a equal step ramp of focus position requests. The randomized focus positions are then used to create a pseudo-randomized pattern, essentially a static random pattern.  
         [0051]    The range of distance the fine focus patches cover is centered around the best coarse focus position. Focus distances will not exceed two coarse focus patch distances or 67 um. Each focus position is imaged resulting in density. An important feature of the fine focus proof is that there are no duplicate focus position requests located in the same y-axis direction (drum travel direction).  
         [0052]    The fine focus proof is designed to be densitometered using a scanning spectrophotometer Model number DTP-41 manufactured by X-Rite. The relatively small distance range of focus positions on the fine focus proof causes the patches  82  to appear to be similar density, and with the randomized nature of the proof, a user will not be able to manually select a patch  82 . In addition, measuring many patches by hand is time consuming and can lead to user induced error. The X-Rite DTP41 scanning spectrophotmeter requires a minimum level of patch  82  to patch  82  contrast in order to recognize and process the requested pattern. 14 mm patches  82  containing density produced by a desired focus position can range from 0.5 density to 1.7 density. Levels beyond 0.5 to 1.7 are rejected. As stated there is little patch  82  to patch  82  density variation. Imaging 0.7 mm patches  83  between desired patches  82  at a high exposure allows for the necessary contrast required for DTP41 measurement by exceeding the exposure level for maximum density output.  
         [0053]    A printhead  500  pre-move is issued before writing the fine focus proof  80  to eliminate any time induced static friction. For each fine focus patch  82  the printhead  500  is homed then sent to the requested position. After the printhead  500  is positioned the focus motor  51  is turned off to reduce heat buildup.  
         [0054]    Density Processing  
         [0055]    Densities are read in using a software density interface. The number of rows  81  imaged on the fine focus proof  80  will correlate to the number of rows  81  to be scanned. The fine focus proof  80  is laminated onto a paper stock using the Kodak XL-800 laminator, then cut to allow clearance for the scanning spectrophotometer and fed through the scanning densitometered. An arrow is located on the fine focus proof  80  indicating the direction to feed the fine focus proof  80  into the scanning spectrophotometer. A save and a close button is located at the bottom of the density entry window. Pressing save then close allows for processing of entered density and calculation of the best focus position.  
         [0056]    To avoid unpredictable results, focus data points below 0.5 density and above 1.7 density are eliminated. Also a check for outlier data is performed by fitting a 2-degree polynomial to the data and removing points more than three standard deviations away from the polynomial curve fit.  
         [0057]    As shown in FIG. 5, to calculate the best focus position, polynomials  72  are formed for every focus position request and resulting density data  71  set that encompasses 60 microns of focus (z-axis) distance. The best focus position for each curve is calculated by locating the focus position at the peak of the 2-degree polynomial  72 . For each 60 um data set the distance from the calculated best focal position to the center position of the 60 um data set is calculated. The data set that has a minimized distance between the best calculated focus position and the center of the 60 um data set is used to determine the final best focus position  73 . Polynomial  72  is essentially centered around the best focus position. The final calculated focus position is the closest position to the calculated best focus position that is divisible by four. This quantization is necessary due to the quad step nature of the focus stepper motor  51 . The best focus position is stored in the computer registry.  
         [0058]    A check that the calculated focus position  73  is performed to guarantee the calculated focus position  73  exists in the range of distance that the fine focus proof  80  covers. The final step is to position the printhead  500  to the calculated focus position.  
         [0059]    The randomization of patches, the number of patches, the range of distance for the patches, and the offset for each row of patches are all necessary to calculate an accurate focus position given different noises in the proofing system. Noises present in the proofing system include thermal media coating variability, lamination variability, spectrophotometer measurement variability, environmental effects on thermal media, positional repeatability of the focus system, distance changes between the printhead  500  and the vacuum imaging drum  300  in the printhead translation direction of the proofing system, optical noise, and distance changes between the printhead  500  and the full circumference of the vacuum imaging drum  300 .  
         [0060]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.  
       Parts List  
       [0061]    [0061] 10  Image processing apparatus  
         [0062]    [0062] 12  Image processor housing  
         [0063]    [0063] 14  Image processor door  
         [0064]    [0064] 16  Donor ejection chute  
         [0065]    [0065] 18  Donor waste bin  
         [0066]    [0066] 33  Thermal print media  
         [0067]    [0067] 34  Donor roll material  
         [0068]    [0068] 51  Focus stepper motor  
         [0069]    [0069] 52  Rotary flag  
         [0070]    [0070] 53  Micrometer  
         [0071]    [0071] 54  Linear flag  
         [0072]    [0072] 55  Printhead clamp  
         [0073]    [0073] 58  Mounting block  
         [0074]    [0074] 59  Magnetic couple  
         [0075]    [0075] 71  Resulting density data  
         [0076]    [0076] 72  Polynomial  
         [0077]    [0077] 73  Focus position  
         [0078]    [0078] 80  Fine focus adjustment proof  
         [0079]    [0079] 81  Row  
         [0080]    [0080] 82  Patch  
         [0081]    [0081] 83  Patch  
         [0082]    [0082] 100  Media carousel  
         [0083]    [0083] 110  Media drive mechanism  
         [0084]    [0084] 112  Media drive roller  
         [0085]    [0085] 113  Drum load roller  
         [0086]    [0086] 120  Media knife assembly  
         [0087]    [0087] 122  Media knife blades  
         [0088]    [0088] 180  Transport mechanism  
         [0089]    [0089] 300  Vacuum imaging drum  
         [0090]    [0090] 400  Laser assembly  
         [0091]    [0091] 402  Laser diodes  
         [0092]    [0092] 404  Fiber optic cables  
         [0093]    [0093] 406  Distribution block  
         [0094]    [0094] 500  Printhead