FLUID EJECTION DEVICE TESTING Background [0001] When manufacturing a fluid ejection device such as a print cartridge, the fluid ejection device may be filled with a fluid such as a testing ink or a tinted shipping fluid in order to test the accuracy and operability of the fluid ejection devices. If a testing ink is used, a relatively small subset of the manufactured print cartridges might be filled with ink and tested, because these may disposed of after testing. If a tinted shipping fluid is used, the shipping fluid may remain in the print cartridge during transport to the customer, but a recipient may then elect to purge the shipping fluid and fill the fluid ejection device with a light-colored ink. The tinted shipping fluid can leave a pigment or dye residue that affects qualities of the colored ink when the recipient employs the fluid ejection device for various printing processes.

Brief Description of the Drawings [0002] FIG.1A illustrates a view of an example fluid ejection device testing system. [0003] FIG.1B illustrates a view of another example fluid ejection device testing system. [0004] FIG.2 illustrates a method for testing a fluid ejection device. [0005] FIG.3 illustrates a system for testing a fluid ejection device configured with selected aspects of the present disclosure. [0006] FIG.4 illustrates a system for testing a fluid ejection device configured with selected aspects of the present disclosure. [0007] FIG.5 illustrates a non-transitory computer-readable medium that stores computer-readable instructions for carrying out selected aspects of the present disclosure. [0008] When manufacturing a fluid ejection device, such as a print cartridge, the fluid ejection device may be filled with a fluid such as a testing ink in order to test the accuracy and operability of the fluid ejection devices. Historically, the testing ink can be an actual ink, a special lightly tinted ink, or a tinted shipping fluid. Depending on the intended use of the fluid ejection device, in some cases, the testing ink may instead be a clear shipping fluid. [0009] Filling the fluid ejection device with clear shipping fluid serves multiple purposes. One purpose is to preserve the integrity of the fluid ejection device during shipment and storage. Another purpose is related to when the fluid ejection device is installed in the intended printing system. Once the fluid ejection device is installed in the intended printing system by the end user, the shipping fluid should be purged by displacing it with the intended ink or fluid. By using a clear fluid, the amount of ink (or other ejected fluid) and time to adequately purge the fluid ejection device can be greatly reduced. This is because even small amounts of residual color tint left in the fluid ejection device may affect the initial color accuracy of the printing system. [0010] One drawback of filling the fluid ejection device with a clear shipping fluid at manufacture is that the fluid may not be readily perceivable by a device testing system. Fluorescent additives can enable fluorescence imaging of the clear shipping fluid, without adding noticeable color tint. The fluorescent additive has an excitation light absorption wavelength range which excites the fluorescing molecules, and an emission wavelength range for the light emitted through fluorescence. When a filter associated with a camera allows light within the wavelength range for the emitted fluorescence to pass, but blocks light within the wavelength range used for the excitation, a fluorescence image can be formed on the camera’s sensor. To the degree that excitation wavelength ranges can be kept separate from the emission wavelength ranges, the contrast of the image is enhanced. [0011] The above description is provided as an overview of some implementations of the present disclosure. Further description of those implementations, and other implementations, are described in more detail below. [0012] It should be appreciated that all combinations of the foregoing concepts and additional concepts described in greater detail herein are contemplated as being part of the subject matter disclosed herein. For example, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the subject matter disclosed herein. [0013] Examples are set forth herein for facilitating quality/accuracy testing of a fluid ejection device such as an ink jet printhead or “ink pen” with minimal impact on print quality for end-users. Various examples allow for a shipping or “testing” fluid, which may or may not be clear, to be employed during testing. This reduces or eliminates contamination—with color-altering testing pigments or dyes contained in the testing fluid—of end product inks that eventually replace the testing fluid. This can improve accuracy of printed colors on paper and other printable mediums, which may otherwise contain visible remnants of testing pigments or dyes. [0014] In some examples, testing fluids employed by the testing systems set forth herein can include a light-sensitive clear fluid, such as an invisible ink (e.g., invisible to an unaided human eye) that includes an amount of a fluorescent additive such as tetrabenzo tetraazaporphine, triazine-stilbene, Rhodamine B, Acid Red 52, 4,4'-Bis (2-sulfostyryl)-biphenyl disodium salt, fluorescein, etc. [0015] . Stages of testing can include, for instance, a printing stage, an image- capture stage, and an analysis stage—during which an analysis of print quality (e.g., nozzle presence, nozzle firing location, etc.) for fluid ejection devices can be determined. During the image-capture stage, a testing system can be employed to excite particles of the testing fluid on a surface (e.g., paper) in order to capture an image resembling the excited particles. During the analysis stage, the image can be processed to determine whether any errors occurred, e.g., with a printhead nozzle, during testing. [0016] In some examples, a system for capturing each image can include a light source such as a dome light that extends over a printed sample. The dome light can evenly/diffusely illuminate the printed sample when an image is being captured by a camera of the system, in order to uniformly excite any light-sensitive fluid (e.g., fluid with fluorescent additive) on the printed sample. The dome light can illuminate at a particular wavelength or range of wavelengths, in order that light emitted from the light-sensitive ink will exhibit a particular wavelength or range of particular wavelengths. In some examples, a band-pass filter can be used, e.g., between a lens of the camera and a sensor of the camera, in order to filter out particular wavelengths of light from any images captured by the camera. For example, the testing fluid can have an absorption wavelength of about 650-715 nanometers (nm) and an emission wavelength of about 670-720 nm. Therefore, the dome light could emit light of wavelength about 640-680 nm, and the band-pass filter can allow light having a wavelength of about 695-735 nm to be incident upon a sensor of the camera. [0017] In some examples, during exposure time, the camera can undergo thermoelectric cooling in order to mitigate imaging noise caused by excessive heat in the camera sensor. The thermoelectric cooling can be performed in some examples using a thermoelectric cooling device (sometimes referred to as a “Peltier device”) that can, for instance, contact the camera sensor of the camera when the camera is capturing images of a printed test sample. Long exposure times (e.g., greater than 100 ms) tend to leave images vulnerable to noise, which can be especially impactful when detecting low levels of light signal. Combining thermoelectric cooling with a reduction of exposure time can improve signal contrast. [0018] In some examples, light intensity is increased so that fluorescent emission is strong enough that high contrast images can be captured to minimize exposure time. In some examples, an area of the printed sample can be greater than 3” by 3” and can be captured in approximately 250ms or less of exposure time. In some examples, the camera can include a sensor size of equal to or greater than 6000 by 4000 pixels. In some examples, the camera captures images at a resolution of equal to or greater than 1200ppi in order to acquire an adequate amount of detail for determining nozzle health. Thereafter, a printed sample that has been imaged can be removed from an input tray of the testing system using, for instance, an electromechanical device, and another printed sample can be loaded into the input tray for imaging. Each printed sample can correspond to a particular print cartridge and processing of a captured image can be correlated to the particular print cartridge by, for example, generating testing data that is stored in association with an identifier for the particular print cartridge. [0019] FIG.1A illustrates a view of an example fluid ejection device testing system 100. The fluid ejection device testing system 100 can capture an image of a medium 118 on which testing fluid has been deposited (e.g., printed) in order to determine an accuracy and/or operability of a fluid ejection device that provided the testing fluid onto the medium 118. The fluid ejection device testing system 100 can include a camera 102 that is mounted onto a mounting arm 110 that extends partially over an electrostatic surface 120 that the medium 118 is extending over. The mounting arm 110 positions the camera 102 over the medium 118 in order to receive light that is reflected and emitted from the medium 118. In some examples, a support mount 116 may be provided to support the light source 112 away from the medium 118. In some examples, the fluid ejection device testing system 100 can include the electrostatic surface 120, which applies static charge to the medium 118, such as a paper medium, in order to flatten the medium 118 against the electrostatic surface 120. [0020] The light that is reflected and/or emitted from the medium 118 and received by the camera 102 may originate from a light source 112, such as a curved dome light. For example, the light source 112 can be a curved dome light having a height that is greater than a quarter of a diameter of the curved dome light. However, another example fluid ejection device testing system 140, illustrated in FIG.1B can include a light source 142 that is a flat dome light. A height of the flat dome light can be less than a quarter of a width of the flat dome light or a quarter of a length of a hypotenuse of the flat dome light measured between adjacent corners of the flat dome light. The light source 112 or the light source 142 can illuminate the medium 118, which can have a first portion that includes a testing fluid, and a second portion that does not include the testing fluid. A diameter of a surface of the light source 112 that extends over the medium 118 can be greater than or equal to a length of a hypotenuse defined by adjacent sides of the medium 118, or opposing corners of the medium 118. [0021] Light sources are not limited to those depicted in FIGs.1A and 1B. Other types of light sources may be used in various examples. For example, a broadband point source may be operated to emit its light through a band-pass filter. In some such examples, the broadband point source (and/or band-pass filter) may be luminously coupled to a fiber optic component such as a fiber optic ring light. In some such examples, a camera may capture images with areas between approximately 0.0081 and 0.25 square inches, and may use an exposure time of about two or more seconds. In some such examples, a band-pass filter may be placed on the outside of the camera lens, instead of between the camera’s lens and sensor. Otherwise, these examples may be implemented similarly to others described herein. [0022] As noted previously, in some examples, the testing fluid can be a fluid (which may or may not be clear) with a fluorescent additive. The fluorescent additive can be, for example, tetrabenzo tetraazaporphine or others mentioned previously. A fluid ejection device can print the testing fluid onto the medium 118 in order that the fluid ejection device testing system 100 can determine whether the fluid ejection device is exhibiting any defects. When the fluid ejection device is flushed and thereafter refilled with printing ink, remnants of the testing fluid may not interfere with subsequent printing performed by the fluid ejection device because, for instance, the testing fluid and its fluorescent additive may be perceived as clear (e.g., clear to an unaided human eye). [0023] In some examples, the light emitted by the light source 112 can be incident upon one or both of the first portion of the medium 118 that includes the testing fluid and the second portion of the medium 118 that does not include the testing fluid. Fluorescent light emitted from the first portion of the medium 118—and more particularly, by the fluorescent additive printed thereupon—can pass through a lens 106 of camera 102 and then through a filter 108 of the fluid ejection device testing system 100. Other reflected light from the second portion of the medium 118 that lacks the fluorescent additive may still pass through lens 106, but not through the filter 108. [0024] The filter 108 can be placed between the lens 106 and a sensor of camera 102 where light rays are aligned more perpendicular to a surface of filter 108 and can, in some examples, filter light having a wavelength of about 695 nm and lower. The fluorescent additive deposited on the first portion of the medium 118 can shift a wavelength of light from the light source 112 toward a longer wavelength (e.g., toward a wavelength that is greater than or equal to 695nm). In some examples, the filter 108 can be a band-pass filter. As another example, the filter 108 can be a band-pass filter that permits light having a wavelength of about 695 nm to pass through the filter 108, but that does not permit other light having a wavelength of about 680 nm to pass. [0025] In some examples, light that is emitted from the fluorescent additive deposited on the first portion of the medium 118, and which subsequently passes through the lens 106 and the filter 108, can be incident upon a substrate of the camera 102. The system 100 can include circuitry 104 for generating image data based on light that is incident upon the substrate of the camera 102. Therefore the image data characterizes a two-dimensional or a three-dimensional distribution of testing fluid on the medium 118. For example, the image data can characterize an area of the medium 118 having an area of equal to or greater than 19 square inches, or equal to or greater than 25 square inches. [0026] Circuitry 104 may take various forms, such as a processor executing computer-readable instructions in memory, a field-programmable gate array (“FPGA”), and application-specific integrated circuit (“ASIC”), and so forth. While circuitry 104 is depicted as integral with system 100 in FIG.1A, this is not meant to be limiting. Circuitry 104 can be implemented in whole or in part on computing device(s) that are separate from, but operably coupled to, system 100. [0027] In some examples, circuitry of camera 102 may control a Peltier cooling device or other thermoelectric cooling device for regulating a temperature of the camera 102. In some examples, the thermoelectric cooling device can be operated to reduce a temperature of the camera 102 during a sensor exposure time when light emitted from the fluorescent additive is incident upon a sensor of the camera. [0028] In some examples, the circuitry 104 can control an electromechanical device, such as but not limited to a servo motor, for replacing the medium 118 with another medium after the exposure time for the camera has expired or the camera 102 has captured an image of the medium. For example, the electromechanical device can maneuver the electrostatic surface 120 to discharge the medium 118 and automatically receive another medium. In this way, loading and reloading of each printed medium can be performed autonomously in order to automate the fluid ejection device testing process. In some examples, a printing device can be part of the fluid ejection device testing system 100 depicted in FIG.1A or the fluid ejection device testing system 140 depicted in FIG.1B, so that the system can print the fluid onto the medium 118 prior to the system maneuvering the medium 118 under the light source 112. The circuitry 104 can control the electrostatic surface 120 to place the medium 118 at multiple positions under the light source 112 and the camera 102, so that multiple images of the same medium 118 are taken. In some examples, the circuitry 104 can use information about the location of the medium 118 to combine data from multiple images. [0029] Additionally, the circuitry 104 can process the image data captured by camera 102 and evaluate a quality and/or accuracy of a fluid ejection device that printed the fluid onto the medium 118, e.g., to determine whether the fluid ejection device exhibits any defects during printing. In some examples, this image processing may yield an accuracy metric that characterizes an ability of the fluid ejection device to accurately print fluid onto the medium. [0030] As one non-limiting example, the circuitry 104 can process the image data to determine nozzle health from the fluid with fluorescent additive that is printed on the first portion of the medium 118. In some such examples, the accuracy metric may correspond to a count or percentage of healthy nozzles that are observed based on implementation of techniques described herein. In some examples, if this accuracy metric corresponding to a count or percentage of healthy nozzles satisfies a quality or accuracy threshold (e.g., minimum count or percentage), the circuitry 104 can generate output indicating that the fluid ejection device passed. When the accuracy metric does not satisfy the threshold, on the other hand, the circuitry 104 can generate output indicating that the fluid ejection device did not pass. [0031] As another non-limiting example, the circuitry 104 can determine a similarity between a captured image of the testing fluid on the medium 118 and a reference image stored by, or accessible to, the circuitry 104. The circuitry 104 can determine an accuracy metric in the form of a degree of similarity between the images. In some examples, the degree of similarity can be compared to a threshold (e.g., minimum degree of similarity) and, when the threshold is satisfied, the circuitry 104 can generate output indicating that the fluid ejection device passed an accuracy test. When the degree of similarity does not satisfy the threshold, on the other hand, the circuitry 104 can generate output indicating that the fluid ejection device did not pass. [0032] FIG.2 illustrates a method 200 for testing a fluid ejection device. The method 200 can be performed in whole or part manually, by a computing device, circuitry, an application, and/or any other apparatus or module that can be associated with a fluid ejection device. The operations depicted as part of method 200 are not so limited, and in some examples operation(s) may be reordered, added, and/or omitted. [0033] At block 202, light may be emitted, e.g., by light source 112/142, within a predetermined wavelength range onto a medium (e.g., 118) that includes a testing fluid. The testing fluid may have been printed or deposited onto the medium by a fluid ejection device. The testing fluid may include a fluid (clear or otherwise) and a fluorescent additive (e.g., tetrabenzo tetraazaporphine, or others mentioned previously) with an absorption wavelength range that corresponds to the predetermined wavelength range. The medium 118 can be, for instance, a paper medium that is positioned below the light source 112/142, and can be secured in place by an electrostatic voltage or vacuum pressure. [0034] At block 204, an image may be captured, e.g., using camera 102, where the image includes light emitted from the fluorescent additive. In some examples, a filter 108 (e.g., a band-pass filter positioned between the camera’s lens and sensor) may be used to filter light that is being reflected from a portion of the paper medium that does not include the testing fluid with the fluorescent additive. As an example, light from the light source 112/142 that is incident upon the medium 118 can be reflected/emitted and filtered by the band-pass filter 108. The reflected or emitted light can include a first portion of light having a first wavelength and a second portion of light having a second wavelength. The first portion of light, which may be emitted by the fluorescent additive, can be permitted through the band-pass filter. The second portion of light, which may be reflected from the part of the medium lacking the fluorescent additive, may not be permitted to pass by the band-pass filter. The band-pass filter can, for example, operate to reflect light having a wavelength that does not correspond to a wavelength of light that is emitted from the fluorescent additive, which can be, but is not limited to, tetrabenzo tetraazaporphine or others listed herein. Light that is emitted from the fluorescent additive can be permitted through the band-pass filter, while other light is not permitted through the band-pass filter. [0035] At block 206, image data characterizing the image captured by camera 102 at block 202 may be processed. This image data may, for instance, represent a distribution of the testing fluid on the medium 118. In this way, the image data can be used to determine the efficacy of the fluid ejection device with respect to printing ink onto the medium. Based on the processing of block 206, at block 208, an accuracy metric associated with the fluid ejection device may be determined. This accuracy metric may subsequently be used, for instance, to determine whether the fluid ejection device passes or fails, e.g., by determining whether the fluid ejection device is exhibiting a malfunction. For example, a defect can be exhibited when a fluid ejection device does not eject fluid according to an expected pattern of nozzles that is indicated by a specification of the fluid ejection device. In another example, a defect can be exhibited when a fluid ejection device does not eject fluid in a spatial arrangement that is specified by printable image data, which is accessed by a computer that controls the fluid ejection device. The accuracy metric alternatively may be output to a user, e.g., on a case-by-case basis, as part of a report on a batch of fluid ejection devices, etc., so that appropriate investigation and/or remedial action may be performed. [0036] FIG.3 schematically illustrates one system 300 for testing a fluid ejection device. The system 300 can include a fluid ejection device testing system 302 that can operate a camera 308 (or, e.g., 102 in FIGs.1A and 1B) for capturing an image of a medium (e.g., 118) having fluid that is printed on a portion of the medium by a fluid ejection device. The image can be captured when a light source 310 (sometimes referred to as “source light(s)”) projects light onto a surface of the medium. The light source 310 can be, for example, a flat dome light 142, a curved dome light 112, a broadband point light source (e.g., with its own band-pass filter), a line light, or any other light source having properties that are controllable by a processor 304 of the fluid ejection device testing system 302. For example, a memory 306 of the fluid ejection device testing system 302 can store computer- readable instructions for implementing a source light controller 318, which can be executable by the processor in order to control operational parameters of the light source 310. In examples in which a line light is employed as the light source, the camera may take the form of a line-scan camera. [0037] Memory 306 of fluid ejection device testing system 302 can also include computer-readable instructions for implementing a mount controller 312 that can control an orientation and position of a mount (e.g., mounting arm 110 in FIG.1A) that the fluid ejection device testing system 302 can be connected to. For example, the mount can be connected to the camera 308 in order control a position of the camera 308 relative to a printed medium that is being illuminated by the light source 310. The position of the camera 308 can thereby be controlled using the mount controller 312. [0038] In some examples, memory 306 of the fluid ejection device testing system 302 can include computer-readable instructions to implement an electrostatic controller 314 to control an amount of electrostatic charge that is applied to the printed medium (e.g., paper with fluid printed thereon). For example, the processor 304 can implement the electrostatic controller 314 to control an amount of electrostatic charge that is applied to the printed medium when the camera 308 is capturing an image of the printed medium. For example, the processor 304 can employ a camera controller 320 (described below) to control operational parameters of the camera 308 simultaneous to the processor 304 employing the electrostatic controller 314 to control the amount of electrostatic charge. [0039] During image capture, the processor 304 can execute computer-readable instructions in memory 306 to implement a filter controller 316 that can control a setting (e.g., a wavelength or range of wavelengths) of a filter (e.g., 108) in order to filter light being reflected from the printed medium. For example, the light source 310 can emit light onto the printed medium, and separate portions of the printed medium can emit/reflect light in response. A first portion of the printed medium that includes testing fluid with fluorescent additive may emit light at a first wavelength. A second portion of the printed medium that does not include the fluorescent additive may reflect light at a second wavelength that is different from the first wavelength. In some examples, the first wavelength can be greater than the second wavelength, and the filter controller 316 can operate the filter in order to limit light having a wavelength that is less than the first wavelength from being incident upon a substrate of the camera 308. The filter controller 316 can be employed by the processor 304 to control a filter, such as a band-pass filter, a high pass filter, or a low pass filter. For example, the filter controller 316 can control a band-pass filter that allows emitted light having a wavelength of 695nm and greater to be captured by the camera 308, and limits reflected light having a wavelength of 680nm and less from being captured by the camera 308. [0040] In some examples, memory 306 includes computer-readable instructions for implementing the aforementioned camera controller 320. The camera controller 320 can control the camera 308 to capture an image of the light emitted from first portion of the printed medium (e.g., emitted from the fluorescent additive). Memory 306 may also store computer-readable instructions to implement an image processing engine 322 to process the image. The image processing engine 322 can process the image as image data and determine different properties of the first portion of the printed medium. For example, the image processing engine 322 can determine individual nozzle health based on a dimension, shape, or other feature of the clear fluid printed on the printed medium. In another example, the image processing engine 322 can compare the image data to reference image data characterizing a reference image that was used by a printing device to print fluid onto the paper medium. [0041] In some examples, memory 306 may store computer-readable instructions to implement a fluid ejection device analysis engine 324 that can determine, e.g., using an accuracy metric output of the image processing engine 322, whether a fluid ejection device, which ejected fluid onto the printed medium, is defective. For example, when a total number of healthy nozzles determined by the image processing engine 322 does not satisfy a total healthy nozzle threshold, the fluid ejection device analysis engine 324 can generate data (e.g., an accuracy metric) indicating that the fluid ejection device is exhibiting an inaccuracy or other malfunction during operation. [0042] In another example, the image from the camera 308 can be processed by the image processing engine 322 to determine a location of a dot of fluid in the first portion of the printed medium. The fluid ejection device analysis engine 324 can determine whether the location of dot is within a threshold distance of a corresponding nozzle that provided the dot onto the printed medium. When the determined location satisfies the threshold distance, the fluid ejection device analysis engine 324 can indicate that the fluid ejection device (e.g., a printhead nozzle) is exhibiting satisfactory performance. Otherwise, when the determined location does not satisfy the threshold distance, the fluid ejection device analysis engine 324 can indicate that the fluid ejection device is not exhibiting satisfactory performance or should undergo maintenance. [0043] FIG.4 illustrates a fluid ejection device testing system 402. The fluid ejection device testing system 402 can include a light source 404 to illuminate testing fluid printed on a medium by a fluid ejection device under consideration. The testing fluid can include a fluid and a fluorescent additive. The testing fluid can be printed onto a portion the medium using a fluid ejection device. The fluid ejection device testing system 402 can include a camera 408 with a lens to capture light that is reflected and emitted from the portion of the medium. The fluid ejection device testing system 402 can also include circuitry 410 to determine, based on processing image data that is based on the captured light, an accuracy metric that characterizes an ability of the fluid ejection device to print fluid onto the medium. [0044] FIG.5 depicts a non-transitory computer readable medium 506 (which may share characteristics with memory 306 of FIG.3) that stores computer-readable instructions 502-504 that are executable by a processor 304 to perform selected aspects of the present disclosure. For example, instruction(s) 502 may be executable by processor 304 to obtain, via a camera (102/308), an image of a medium (118) that includes testing fluid deposited by a fluid ejection device. The testing fluid may include an additive (e.g., fluorescent) that has an absorption wavelength range, e.g., of about 650-715 nm. The medium may also be illuminated using a light source (112, 310) that emits light within the absorption wavelength range. Consequently, the image includes light emitted by the additive. In some implementations, processor 304 may execute instructions 502 to operate the camera itself, e.g., automatically or in response to user input received at a graphical or voice user interface. In other implementations, the camera may be operated manually or otherwise outside of the control of processor 304, and processor 304 may obtain the imagery captured by the camera, e.g., from memory. [0045] Instructions 504 may be executable by processor 304 to process image data characterizing the image to determine an accuracy metric that characterizes an ability of the fluid ejection device to print fluid onto the medium. In some examples, non-transitory computer-readable medium 506 may include additional instructions to filter pixels of the image data (i.e. image processing) that correspond to the absorption wavelength range. By contrast, in other implementations, a band-pass filter (e.g., 108) may be employed to filter these pixels. [0046] The following are non-limited examples of how techniques described herein may be implemented. In some examples, a system may include: a light source to illuminate testing fluid printed on a medium, wherein the testing fluid comprises a fluid and a fluorescent additive, and wherein the testing fluid is printed onto a portion the medium using a fluid ejection device; a camera to capture image data that includes light emitted from the portion of the medium; and circuitry to determine, based on processing the image data, an accuracy metric that characterizes an ability of the fluid ejection device to accurately print fluid onto the medium. [0047] In some examples, the portion of the medium may include a first portion, and the system comprises a band-pass filter to filter light reflected from a second portion of the medium that does not include the fluorescent additive. In some examples, the band-pass filter is located between a lens of the camera and a sensor of the camera, and the band-pass filter is to filter reflected light having a wavelength of about 680 nanometers, and the band-pass filter does not filter other emitted light having another wavelength of about 695 nanometers or greater. [0048] In some examples, the system includes: a thermoelectric cooling device that is attached to the camera, wherein the thermoelectric cooling device is to reduce a temperature of a sensor of the camera during a sensor exposure time when light emitted from the medium is incident upon the sensor. In some examples, the light source evenly illuminates the medium with light having a wavelength of 640 to 680 nm. [0049] In some examples, a diameter of a surface of the light source that extends over the medium is greater than a length of a hypotenuse defined by adjacent corners of the medium. In some examples, the system includes: an electromechanical device that is controlled by the circuitry, wherein the circuitry is to operate the electromechanical device to replace the medium with another medium after the camera has captured the image. [0050] In another aspect, a method may include: emitting light within a predetermined wavelength range from a light source onto a medium that includes a testing fluid, wherein the testing fluid includes a fluorescent additive with an excitation absorption wavelength range that corresponds to the predetermined wavelength range, and wherein the testing fluid is printed onto the medium using a fluid ejection device; capturing, using a camera, an image that includes light emitted from the fluorescent additive; processing image data characterizing the image; and determining, based on processing the image data, an accuracy metric associated with the fluid ejection device. [0051] In some examples, the method includes filtering, using a band-pass filter, reflected light from the medium that does not correspond to an emission wavelength range of the fluorescent additive. In some examples, the image characterizes an area of the medium having an area of equal to or greater than 19 square inches. In some examples, the method includes: operating a thermoelectric cooling device to reduce a temperature of the camera during a sensor exposure time when light emitted from the medium is incident upon the lens. In some examples, the image data characterizes the image with a resolution of equal to or greater than 1200 pixels per inch (ppi), and each pixel is less than or equal to about 5.5 micrometers. [0052] In another aspect, a non-transitory computer-readable medium may store instructions that, when executed by a processor, cause the processor to: obtain, via a camera, an image of a medium that includes testing fluid deposited by a fluid ejection device, wherein the testing fluid includes an additive that has an absorption wavelength range, the medium is illuminated using a light source that emits light within the absorption wavelength range, and the image includes light emitted by the additive; and process image data characterizing the image to determine an accuracy metric that characterizes an ability of the fluid ejection device to print fluid onto the medium. In some examples, the medium includes instructions to filter pixels of the image data that correspond to the absorption wavelength range. [0053] Although examples described herein have related primarily to testing fluid ejection devices, this is not meant to be limiting. In various examples, techniques described herein may be applicable in other contexts in which fluids with fluorescent attributes are deposited on media. As one example, particular colors of ink such as yellow or light green may include fluorescent additive, e.g., to make their ultimate appearance more brilliant. These inks may not be clear, but instead may include both pigment (e.g., yellow, green) and fluorescent additive. The presence and/or uniformity of the fluorescence in these inks may be difficult to detect with the naked eye. Using techniques described herein may facilitate isolation of the fluorescent light emitted from the ink, e.g., to detect its strength and whether it has being deposited as expected. [0054] Additionally, some examples described herein, such as fluid ejection device testing system 302 and 402 in Figs.3-4, have been described in isolation, e.g., almost as standalone devices and systems. However, this is not meant to be limiting. In various example, techniques described herein may be implemented as part of a printing system. Put another way, fluid ejection device testing systems 302 and 402 may be incorporated into various types of printers, e.g., to test fluid ejection devices such as printheads upon installation into the printers. As one non-limiting example, a web press may have a fluid ejection device testing system (e.g., 302, 402) incorporated, e.g., as part of a print bar, so that it can confirm that newly- installed printheads function properly. In some cases, these printheads may eject onto a medium during a production print job. Because the testing fluid is, in many cases, clear, the result would not be visible to the naked eye. And yet with the fluorescent additive, techniques described herein could still be implemented. [0055] While several implementations have been described and illustrated herein, a variety of other means and/or structures for performing the function and/or obtaining the results described herein may be utilized, and each of such variations and/or modifications is deemed to be within the scope of the implementations described herein. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific implementations described herein. It is, therefore, to be understood that the foregoing implementations are presented by way of example and that, within the scope of the appended claims and equivalents thereto, implementations may be practiced otherwise than as specifically described and claimed. Implementations of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present disclosure.