Patent Publication Number: US-2022219448-A1

Title: Drop characteristic measurement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of copending U.S. patent application Ser. No. 16/719,666, filed Dec. 18, 2019, which claims benefit of U.S. Provisional Patent Application Ser. No. 62/783,767 filed Dec. 21, 2018, and U.S. Provisional Patent Application Ser. No. 62/810,481 filed Feb. 26, 2019, each of which is incorporated herein by reference. 
    
    
     FIELD 
     Embodiments of the present invention generally relate to inkjet printers. Specifically, methods and apparatus for monitoring and control of print materials during deposition processes are disclosed. 
     BACKGROUND 
     Inkjet printing is common, both in office and home printers and in industrial scale printers used for fabricating displays, printing large scale written materials, adding material to manufactured articles such as PCB&#39;s, and constructing biological articles such as tissues. Most commercial and industrial inkjet printers, and some consumer printers, use piezoelectric dispensers to apply print material to a substrate. A piezoelectric material is arranged adjacent to a print material reservoir. Applying a voltage to the piezoelectric material causes it to deform in a way that applies a compressive force to the print material reservoir, which is constructed in turn to eject print material when the compressive force is applied. 
     Some inkjet printing applications rely on extreme precision in positioning of print nozzles, quantity and type of print material ejected, and velocity and trajectory of droplet ejection. When nozzles fail to eject print material on demand, with the correct volume, velocity, and trajectory, printing faults result and time and money must be spent correcting the faults. Optical systems are routinely used to monitor droplet size and flight from print nozzles to substrates. Such systems typically rely on illuminating droplets of print material to determine droplet characteristics. The droplets are typically very small, for example 10-15 μm in diameter, and sharp focus of the images captured is helpful in ascertaining droplet characteristics with precision. It is most useful, in addition, to capture the images while the droplets are close to the ejection nozzle. These considerations can constrain the geometry of illumination apparatus. There is need in the art for flexible droplet illumination hardware. 
     SUMMARY 
     Embodiments described herein provide a droplet measurement apparatus, comprising a light source having a collimating optical system; an imaging device disposed along an optical path of the collimating optical system; and a droplet illumination zone in the optical path of the collimating optical system, the droplet illumination zone having a varying droplet illumination location, wherein the light source, the imaging device, or both are adjustable to place a focal plane of the imaging device at the droplet illumination location. 
     Other embodiments described herein provide a printing system, comprising a substrate support; and a print assembly operatively coupled to the substrate support, the print assembly comprising a print support; a dispenser assembly movably coupled to the print support; and a droplet measurement apparatus coupled to the print support, the droplet measurement apparatus comprising a housing; a light source disposed in the housing, the light source comprising a collimating optical system; a droplet illumination zone having a varying droplet illumination location; and an imaging device disposed in the housing to receive radiation from the droplet illumination zone, the light source, droplet illumination zone, and imaging device defining a focal plane that is adjustable to the droplet illumination location. 
     Other embodiments described herein provide a method, comprising positioning a dispenser of an inkjet printer in proximity to an illumination zone of a droplet measurement apparatus; emitting a beam of collimated light from a light source through the illumination zone; aligning a nozzle of the dispenser with an optical path of the beam of collimated light; emitting a droplet from the nozzle into the illumination zone; illuminating the droplet at an illumination location using the beam of collimated light to form a radiation signature of the droplet; receiving the radiation signature at an imaging device; and adjusting a focal plane of the imaging device to the illumination location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments. 
         FIG. 1  is a top isometric view of a printing system according to one embodiment. 
         FIG. 2A  is a cross-sectional view of a droplet measurement apparatus according to another embodiment. 
         FIG. 2B  is a close-up cross-sectional view of the droplet illumination zone of the droplet measurement apparatus of  FIG. 2A . 
         FIG. 3  is a cross-sectional view of a droplet measurement apparatus according to another embodiment. 
         FIG. 4  is a flow diagram summarizing a method according to another embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     A printing system is described herein that has a droplet measurement apparatus with flexible geometry for capturing droplets at different focal planes for various print ejector designs. 
       FIG. 1  is an isometric top view of a printing system  100  according to one embodiment. The printing system  100  has a substrate support  102  mounted on a base  104 . The base  104  comprises one or more solid massive objects that provide a stable foundation for the printing system  100 . In some cases, the base  104  is one or more granite blocks. Using a solid massive object as the base minimizes unwanted vibration or other movement of the printing system  100 . 
     The substrate support  102  includes a working portion  106 , a first staging portion  108 , and a second staging portion  110 . The working portion  106  is disposed between the first staging portion  108  and the second staging portion  110 . The working portion  106  is supported directly on the base  104 , while each of the staging portions  108  and  110  are supported by base extensions  112  attached to the base  104  and extending laterally from the base  104 . The base extensions  112  may be made of any structurally strong material, such as steel. The stability of the base  104  minimizes uncontrolled motion of the substrate and/or other printer components at the location where material is dispensed onto the substrate. 
     The substrate support  102 , in particular the working portion  106 , is a table that supports a substrate in a printing position in the printing system  100 . The substrate support  102  has a supporting surface  103  that provides a low-friction or frictionless support to allow precise movement and positioning of a substrate for printing. The table is rectangular, with a long dimension in a first direction  114  and a short dimension in a second direction  116  perpendicular to the first direction  114 . During printing, the substrate is moved in the first direction  114 . The short dimension is similar to a maximum dimension of a substrate in the second direction  116 , which is a cross-scan direction. The long dimension may be up to about 10 m, while the short dimension is typically 2-3 m. A third direction  118  is perpendicular to both the first direction  114  and the second direction  116 . 
     The printing system  100  has a print assembly  120  juxtaposed with the working portion  106 . The print assembly  120  includes a dispenser support  122  and a dispenser assembly  124 . The dispenser support  122  comprises two stands  126 , one on either side of the working portion  106  and aligned along the cross-scan direction. The stands  126  rise from the base  104 , and may be attached or integrally formed with the base  104 . The stands  126  support a rail  117  that extends across the working portion  106  in the second direction  116  from one stand  126  to the other stand  126 . Multiple stands may be used on each side of the working portion  106  to support the rail  117 , and multiple rails  117  may be used to support devices that scan across the working portion  106 , such as imaging devices and drying devices. 
     The dispenser assembly  124  is coupled to the rail  117  by a carriage  128 , which includes an actuator that moves the carriage  128  along the rail  117  to position the dispenser assembly  124  at a desired location in the second direction  116 . During a print job, the substrate moves by the dispenser assembly  124  in the first direction  114 , sometimes called the scanning direction, while the dispenser assembly  124  is positioned in the second direction  116 , sometimes called the cross-scan direction, by operation of the carriage  128  to deposit material in a desired location on the substrate. A dispenser housing  130  is coupled to the carriage  128 . One or more dispensers (not shown) may be disposed in the dispenser housing  130  to dispense print material toward the working portion  106 . The dispensers dispense print material toward the substrate as the substrate moves by the dispenser assembly  124 . Each dispenser typically has a plurality of ejection nozzles (not shown) at an ejection surface of the dispenser facing the working portion  106 . 
     A print job may include depositing droplets of print material on a substrate in an extremely precise manner. Droplets having dimension of 10-15 μm are deposited at a target location on the substrate. The target location may have dimension of 15-20 μm. The droplets are deposited by ejecting droplets having the requisite dimension from the ejection nozzles at a time, velocity, and trajectory toward a droplet deposition location, which is a predetermined location above the substrate support where the target location of the substrate will be positioned when the droplet arrives at the droplet deposition location. The size, ejection time, velocity, and trajectory of the droplets is determined to place the droplets at the droplet deposition location when the movement of the substrate brings the target location to the droplet deposition location. The extreme precision of such processing requires excellent control of droplet size, ejection time, velocity, and trajectory from the ejection nozzles. 
     In order to achieve such control of droplet properties, the output of the ejection nozzles is measured. The printing system  100  includes a droplet measurement apparatus  132  located near one of the stands  126 . The droplet measurement apparatus  132  is an optical system that detects the interaction of specifically configured electromagnetic radiation with droplets ejected from the dispensers in the dispenser housing  130  to determine the ejection characteristics of the dispenser—droplet size, velocity, and trajectory—as a function of impulse input to the dispenser and characteristics of the print material. The droplet measurement apparatus  132  can be attached to the stand  126 , or as shown here may be supported on the base  104  near the stand  126 . The droplet measurement apparatus  132  is supported beside the working portion  106  of the substrate support  102  to allow the dispenser assembly  124  to access the droplet measurement apparatus  132 . The dispenser assembly  124  moves along the rail  117  to the stand  126 , or vicinity thereof, to engage with the droplet measurement apparatus  132 . Typically, a controller controls positioning of the dispenser assembly  124  based on predetermined location data for the droplet measurement apparatus  132  stored in, or accessible to, the controller. An optional actuated platform  134  is shown here if the ability to raise and lower the droplet measurement apparatus  132  is desired. Such ability may be useful to move the droplet measurement apparatus  132  away from the dispenser housing  130 , or other equipment, when not in use. The actuated platform  134  may also be useful to precisely position the droplet measurement apparatus  132  with respect to the dispenser housing  130  for best results in recording droplet characteristics. 
       FIG. 2A  is a cross-sectional view of a droplet measurement apparatus  200  according to one embodiment. The droplet measurement apparatus  200  may be used as the droplet measurement apparatus  132  in the printing system  100 . The section plane of  FIG. 2A  is perpendicular to the first direction  114  of  FIG. 1 . A frame  202  holds a light source  206 , a droplet illumination zone  208 , and an imaging device  210  in optical engagement. The light source  206  is a collimated light source with a light source  207  and a collimating optical system  209  optically coupled to the light emitter. The light source  206  emits collimated light toward the droplet illumination zone  208 . The dispenser housing  130 , with dispensers  216 , are shown installed in a dispenser tray  218  in position to engage the droplet measurement apparatus  200 . A droplet is ejected from an ejection nozzle (not shown) at an ejection surface  217 , which may be a surface of an ejection plate, of a dispenser  216  toward the droplet illumination zone  208  and into receptacle  214 . 
     The collimated light is steered by a first steering optic  222  and a second steering optic  224  through a droplet measurement zone  212  of the droplet illumination zone  208  to interact with the droplet passing through the droplet illumination zone  208 . An optional alignment optic  220  aligns the collimated light with the first steering optic  222 . Each of the first steering optic  222  and the second steering optic  224  are prisms in this case, but other optical devices, or combinations, can be used as steering optics. The alignment optic  220  is also a prism in this case. The steering optics allow the light source  207  and the imaging device  210  to have optical axes that are not parallel, reducing the footprint of the droplet measurement device  200 . The steering optics optically couple the light source  207  and the imaging device  210  along an optical path that proceeds through the droplet illumination zone  208 . 
     Following interaction with the droplet, the light is steered to the imaging device  210 . The imaging device  210  captures an image of the light from the interaction with the droplet to record data about the droplet. The imaging device  210  may be a camera, CCD array, photodiode array, or other imaging apparatus. The imaging device is supported in operating position by a holder  226 . The holder  226  is coupled to a linear positioner  228 , which in turn is coupled to a mount  230  inside the frame  202 . 
       FIG. 2B  is a close-up view of the droplet illumination zone  208  to illustrate operation of the droplet measurement apparatus  200 . The light source  206 , droplet illumination zone  208 , and imaging device  210  are positioned along an optical path  250  of the collimated light. The optical path  250  is not straight in this case to allow the light source  206  and imaging device  210  to be contained in a small footprint. The ejection nozzles of the dispensers  216  eject droplets along paths that intersect with the optical path  250  at droplet illumination locations. Here, four droplet illumination locations  252 A,  252 B,  252 C, and  252 D are shown corresponding to four ejection nozzles in the ejection surface  217  of the dispenser  216 . The droplet illumination locations may correspond to the positions of ejection nozzles in the ejection surface  217 . So, if the ejection surface  217  has five ejection nozzles arranged to eject droplets into the optical path, there will be five droplet illumination locations. Generally, the droplet illumination locations will be defined by the intersection of a flight path from each ejection nozzle with the optical path  250 . At each droplet illumination location  252 A-D, a droplet may be illuminated by the collimated light resulting in illumination signature propagated along the optical path  250  to the imaging device  210 . The light is collimated to a change in spot size of less than about 1% per meter. Here, the optical path  250  is about 0.25 meters long. 
     In order to image the droplet illumination and recover usable data on droplet size, velocity, and trajectory, the imaging device  210  focuses the light received from the droplet illumination zone  208  along the optical path  250 . Focusing the light locates a focal plane of the imagining device  210  at the droplet illumination location  252 A,  252 B,  252 C, or  252 D, depending on where the droplet is to be ejected. Thus, the droplet measurement device  200  has an adjustable focal plane. Because the optics of the imaging device  210  have high resolution and/or optical power, the focal depth or working range of the imaging device  210  is small. For that reason, the imaging device  210  is movable with respect to the frame  202 . In alternative embodiments, adjusting the focal plane of the imaging device can include adjusting a focus component of the imaging device, such as a lens, mirror, or prism. 
     Referring again to  FIG. 2A , the linear positioner  228  is actuated to extend or retract, moving the imaging device  210  toward or away from the illumination zone  208 . Moving the imaging device  210  changes the length of the optical path  250 , along with the focal limits of the imaging device  210  such that the working range of the imaging device  210  includes the droplet illumination location  252 A,  252 B,  252 C, or  252 D, of interest. Thus, if a droplet is to be ejected from a nozzle aligned with the droplet illumination zone  252 A, the linear positioner  228  is actuated to position the imaging device  210  such that the working range of the imaging device includes the droplet illumination location  252 A, and likewise with the locations  252 B,  252 C, and  252 D. 
     The prisms  222  and  224  are positioned on either side of the droplet illumination zone  208 , with the receptacle  214  between them. Thus, the prisms  222  and  224  are located either side of the receptacle  214 . The prisms  222  and  224  protrude upward above the upper extent of the receptacle  214  in order to direct light from the light source  206  along the optical path  250  through the droplet illumination zone  208 . Here, the prisms  222  and  224  are triangular, each having one right-angle edge, all three faces being rectangular. The prisms  222  and  224  are oriented such that a face  227  adjacent to the right angle edge faces the droplet illumination zone  208 . In this orientation, an edge  229  adjacent to the face  227  extends toward the dispenser housing  130  beyond the optical path  250 . The upward extension of the prisms  222  and  224  locates the optical path  250  in the illumination zone at a distance from the receptacle  214  that allows the dispenser housing  130  to be positioned with the optical path  250  near a flexure plate  260  of the dispenser housing  130  such that the droplet measurement apparatus  200  captures an image of the droplet before characteristics of the droplet change as the droplet travels through the atmosphere. 
     The flexure plate  260  has a plurality of channels  262 . In this view, the channels  262  extend into the plane of  FIGS. 2B  (and  2 A). The channels  262  are generally located around the dispensers  216  and spaced apart a distance D that is generally similar to a distance E between the two prisms  222  and  224 . Specifically the distance E is a distance between top extremities of the prisms  222  and  224 . The channels  262  allow the dispenser housing  130 , properly positioned to register the prism  222  and  224  upward protrusions with the channels  262 , to approach the optical path  250  more closely by providing additional space to accommodate the upward protrusions of the prisms  222  and  224 . The channels  262  are generally located between the dispensers  216  such that a portion of any dispenser  216  can be positioned between the prisms  222  and  224 , by positioning the upward protrusions of the prisms  222  and  224  in the channels  262  on either side of the desired dispenser  216 . In this way, the flexure plate  260  can be positioned as close as possible to the optical path  250  for best imaging results. 
     Additionally, the optical path  250  is configured to bring the optical path as close to the dispensers  216  as possible in the droplet illumination zone  208 . Thus, the optical path  250  is brought as close as possible to the face  227  that faces across the droplet illumination zone  208  so that the light path through the prisms  222  and  224  is near the top of the upward projections of the prisms. Such an arrangement provides closer engagement of the ejection surface of the dispensers  216  to the optical path  250 . 
       FIG. 3  is a cross-sectional view of a droplet measurement apparatus  300  according to another embodiment. The droplet measurement apparatus  300  is similar in many respects to the droplet measurement apparatus  200  of  FIGS. 2A and 2B , and the same elements are labeled with the same reference numerals. The chief difference between the droplet measurement apparatus  300  and the droplet measurement apparatus  200  is that the light source  206  and the imaging device  210  are aligned. The receptacle  214  is disposed in a support  302  that extends outward to support the light source  206  and the imaging device  210  in a transverse position relative to the droplet illumination zone  208 . The light source  206  is positioned on one side of the droplet illumination zone  208 , and the imaging device  210  is positioned opposite from the light source  206  with the droplet illumination zone  208  between the light source  206  and the imaging device  210 . The optical axes of the light source  206  and the imaging device  210  are aligned here along an axis  310  that extends from the light source  206 , through the droplet illumination zone  208 , directly to the imaging device  210 . 
     In this case, the imaging device  210  is coupled to a linear positioner  304  that is supported on the support  302 . The linear positioner  304  moves the imaging device  210  closer to or further from the droplet illumination zone  208  in a transverse direction, as shown by the arrow  308 , to move the focal plane of the imaging device to coincide with one of the droplet illumination locations  252  where a droplet is to be measured. 
     Positioning the light source  206  and the imaging device  210  in line can present challenges in getting a high resolution image of some droplet locations. In  FIG. 3 , a print assembly  312  has three dispensers, a first dispenser  320 , a second dispenser  322 , and a third dispenser  324 , coupled into a frame  314 , which may be part of a dispenser housing like the housing  130  of  FIG. 1 . The three dispensers  320 ,  322 , and  324  are aligned along the optical axis  310  with a spacing set by operational specifications of the printing system. The dispensers are shown positioned between the light source  206  and the imaging device  210  in order to get an image of droplets  306  immediately upon exiting the dispenser before any characteristics of the droplet change appreciably. Positioning the dispenser between the light source  206  and the imaging device  210  limits how close the imaging device can be positioned to some droplet illumination locations, for example those furthest from the imaging device  210  and closest to the light source  206 . In some cases, the desired image resolution and working range may be difficult to harmonize merely by moving the imaging device  210 . 
     To augment the working range of the imaging device  210 , a tunable lens  330  may be coupled to the imaging device  210 . A tunable lens can extend the working range over which the imaging device  210  can deliver high resolution images, so the imaging device  210  can capture high resolution images of the closest droplet illumination location of the first dispenser  320  and the furthest droplet illumination location of the third dispenser  324 . 
     As shown in  FIG. 3 , in order to position the third dispenser  324  over the receptacle  214  to deliver a droplet, the dispenser assembly must be moved to the right by a distance A which is larger than a clearance B between the dispenser assembly  312  and the imaging device  210 . The imaging device  210  must therefore be moved to the right by actuating the linear positioner  304 , increasing the distance between the imaging device  210  and the droplet illumination zone  208 . If the imaging device  210 , without augmentation by a tunable lens, does not have sufficient working range to deliver high resolution images at the increased distance, the tunable lens  330  can be used to increase the working range of the imaging device  210 . In this way, suitable images can be captured of droplets ejected from all three of the dispensers  320 ,  322 , and  324 . It should be noted that in any of the embodiments described herein, a tunable lens such as the tunable lens  330  may be used along with, or instead of, the linear positioner  304  or  228 . 
     The droplet measurement apparatus  300  of  FIG. 3  has, in some respects, a simpler configuration than the droplet measurement apparatus  200  of  FIGS. 2A and 2B . The droplet measurement apparatus  200 , however, engages the dispensers  216  with the optical path  250  with minimal insertion of the dispensers  216  into the imaging apparatus. In the apparatus  300 , the entire dispenser housing  130  is inserted between the light source  206  and the imaging device  210 , while with the droplet measurement apparatus  200 , only the end of one dispenser  216  is inserted between the upper tips of the prisms  222  and  224 , with less vertical movement needed. The most useful design can be tailored to the individual application. 
       FIG. 4  is a flow diagram summarizing a method  400  according to another embodiment. The method  400  is a method of obtaining a measurement of a characteristic of a droplet ejected from a print material dispenser of a printing system such as an inkjet printer. The method  400  can be practiced using the apparatus described herein. At  402 , the print material dispenser is positioned adjacent to a droplet measurement apparatus of the printing system. If more than one dispenser is used, the dispensers may be housed in a dispenser assembly. The droplet measurement apparatus is typically located outside of a substrate processing area of the printing system such that the operation of performing droplet measurement can be done without impacting the substrate processing area or any substrate that might be positioned thereon. 
     The droplet measurement apparatus generally has a light source for illuminating a droplet in a droplet illumination zone and an imaging device for receiving light from the droplet illumination zone after the light interacts with a droplet. The light source includes collimating optics, and the droplet measurement apparatus includes optical components to direct the light from the light source on an optical path through the droplet illumination zone to the imaging device. 
     The imaging device has a variable focal plane. In one instance, the imaging device is disposed on a linear positioner that can move the imaging device to position the focal plane of the imaging device at a droplet illumination location in the droplet illumination zone such that an image of the droplet is sharply focused. The focal plane of the imaging device is variable because droplets may be dispensed from nozzles at various locations in an ejection face of the dispenser, and in the case where multiple dispensers are used, the dispenser assembly may need to move to bring a target dispenser within the working range of the imaging device. In another instance, the imaging device may have a tunable lens with dynamically adjustable focal length to position the focal plane of the imaging device at different droplet illumination locations within the droplet illumination zone. The linear positioner may also be combined with the tunable lens to further extend the working range of the imaging device if desired. 
     The droplet illumination zone is defined by the ejection face of the dispenser, and a droplet receptacle positioned in the droplet measurement apparatus. The ejection face of the dispenser may have a plurality of ejection nozzles distributed across the ejection face. The receptacle is typically sized to accommodate the areal coverage of all ejection nozzles of a dispenser so that all ejection nozzles of a dispenser can be fired to measure droplet characteristics without having to move the dispenser. In the event multiple dispensers are used in a dispenser assembly, the dispenser assembly may be moved to position each dispenser at the droplet illumination zone in turn. 
     At  404 , the print material dispenser is positioned at a test position. The test position is typically a position wherein the ejection face of the dispenser is located at a minimum distance from the optical path of the light used to illuminate droplets. In some cases the minimum distance is less than 2 mm, for example less than 1 mm. If optical components are used to bring the light into the droplet illumination zone, the ejection face of the dispenser, or the frame or ejection face of the dispenser assembly, may be shaped to accommodate positioning the dispenser or dispenser assembly at the test position. For example, if prisms are used to steer the light through the droplet illumination zone, in order to position a dispenser at the test position, the dispenser may be positioned partly between the prisms. In such cases, if a portion of the prisms extends above a boundary of the optical path, that is to say beyond a spatial extent of the light field used to illuminate the droplets, the ejection surface of the dispenser or the dispenser assembly may be provided with structures to accommodate the prisms so that the ejection surface can be positioned at the test position. In one instance, a dispenser assembly has a flat nozzle plate where the ejection surface of a plurality of dispensers is disposed, and the nozzle plate has notches to position the top points of two prisms used to steer light through the droplet illumination zone. To position the dispenser at the test position, the top points of the prisms are inserted into the notches such that the ejection surface of one dispenser is positioned a minimum distance from the optical path. 
     At  406 , one or more droplets is ejected from the dispenser into the droplet illumination zone. The droplet passes through the droplet illumination zone into the receptacle. A droplet may be ejected from each nozzle of the dispenser into the receptacle without moving the dispenser, if the receptacle is sized to accommodate the entire areal coverage of ejection nozzles in the ejection surface of the dispenser. In such cases, the droplets may arrive in the droplet illumination zone at different droplet illumination locations arising from the areal distribution of ejection nozzles across the ejection surface of the dispenser. In such cases, the positions of the nozzles may be provided to a controller that is coupled to the dispenser assembly and to the imaging device to position the focal plane of the imaging device at the droplet illumination location to be used for imaging a droplet. As the droplet illumination location changes due to ejecting droplets from different nozzles, the focal plane of the imaging device is moved to image each droplet. 
     At  408 , the light source is activated to provide illumination in the droplet illumination zone at the time a droplet passes through the droplet illumination zone. The light interacts with the droplet to produce a signature light field, which propagates to the imaging device. 
     At  410 , the imaging device captures an image of the signature light field, which can then be analyzed to determine droplet characteristics such as volume. In some cases, the light source is pulsed to produce a plurality of images of the droplet as the droplet passes through the droplet illumination zone. The plurality of images can be analyzed to determine speed and trajectory of the droplet. Volume, ejection speed, and trajectory can be used to judge the performance of the individual nozzles of the dispenser. Because the focal plane of the imaging device can be changed to accommodate all the nozzles of a dispenser, the entire dispenser can be tested without repositioning the dispenser. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.