Abstract:
According to one example, there is provided a printing system. The printing system comprises a support having a plurality of spaced apertures and a color sensor moveable to measure light from each aperture. The printing system further comprises a controller to control the color sensor to measure characteristics of light emitted through each aperture, and to determine, for each aperture, light calibration data. The printing system is particularly adapted for use with substrates that are intended to be viewed when backlit, that is in conjunction with backlighting.

Description:
BACKGROUND 
     Increasing use is being made of printed items that are intended to be viewed in conjunction with backlighting. Such printed items include advertising signage, banners, and posters, etc. that are designed to be viewable at night or in low-lighting areas. 
     To ensure that images printed on substrates that are intended to be viewed when backlit (hereinafter referred to as backlit substrates) are rendered accurately printers used to print on backlit substrates need to be correctly calibrated. 
    
    
     
       BRIEF DESCRIPTION 
       Examples, or embodiments, of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a simplified section view of a portion of a printing system according to one example; 
         FIG. 2  is a simplified plan view of a portion of a printing system according to one example; 
         FIG. 3  is a simplified section view of a portion of a printing system according to one example; 
         FIG. 4  is a flow diagram outlining an example method according to one example; 
         FIG. 5  is a simplified top view of a portion of a printing system according to one example; and 
         FIG. 6  is a flow diagram outlining an example method according to one example. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , there shown a simplified section view of a portion of a printing system  100  according to one example.  FIG. 2  shows a simplified plan view of a corresponding portion of the printing system  100 . For clarity it will be appreciated that in the accompanying drawings not all elements of a printing system  100  are shown. 
     The printing system  100  is controlled by a printer controller  102  in accordance with computer or controller readable instructions stored in a memory  103 . The controller  102  is coupled to the memory  103  by a suitable communication bus (not shown). In one example the printer controller  102  may be a microprocessor. Execution of the computer readable instructions by the controller  102  operates elements of the printing system  100  in accordance with a method or methods described herein. 
     The printing system  100  comprises a carriage  104  that is moveable along a carriage bar  106 . The carriage  104  may be moveable, for example, through use of a motor, belt, or other suitable means. 
     On the carriage  104  is mounted a colour sensor or colour sensing device  108 , such as a spectrophotometer, colorimeter, or the like. In one example the carriage  104  additionally comprise a print engine  202 . The print engine  202  may comprise one or multiple printheads, such as inkjet printheads, controllable to eject ink drops onto a substrate installed in the printing system  100 . In another example the print engine  202  is provided on a separate carriage (not shown) distant from the carriage  104 . In a further example the print engine may be an inkjet page-wide array print engine operable to print on the whole width of a substrate without scanning across the substrate. In a further example the print engine may be a dry ink or liquid ink toner print engine. 
     The colour sensing device  108  measures characteristics of light received thereat. For example, measured characteristics may include: the wavelength or wavelengths of received light; and the intensity of received light. If a spectrophotometer is used as the colour sensing device  108 , for example, it may measure the different wavelengths of received light and calculate CIE Lab or XYZ values. 
     The colour sensing device  108  is mounted on the carriage  108  such that it sits above a substrate support or platen  110 . In one example the distance of between the colour sensing device  108  and the platen  110  is in the range of about 1 to 3 mm. In other examples other distances may be used. 
     The printer platen  110  comprises a number of apertures  112  at a number of predetermined locations therein. In the present example the apertures  112  are arranged in a linear manner and are substantially equally spaced. However, in other examples other arrangements or spacings of apertures may be used. 
     In one example each aperture is covered with a cover (not shown). In one example a transparent cover may be used, such as glass. In another example each aperture is covered with a light diffuser. 
     The colour sensing device  108  and apertures  112  are arranged relative to one another such that the colour sensing device  108  on the carriage  104  may be selectively positioned vertically above each one of the apertures  112  and may measure characteristics of light emitted through each aperture. In one example the colour sensing device  108  and the platen  110  are shielded from external light sources such that the colour sensing device  108  does not measure characteristics any external light sources. In one example, the colour sensing device  108  may be used to measure characteristics of light emitted from less than the total number of apertures  112 . 
     In one example the apertures  112  may be spaced apart in the range of about 2 to 5 cm, although in other examples different aperture spacings may be used. In one example 16 apertures are provided in the platen  110 , although in other examples a greater or smaller number of apertures may be provided. 
     In one example, as shown in  FIG. 1 , below each of the apertures  112  is positioned an individual light source  114 . In one example each individual light source  114  is a light emitting diode (LED). In one example each of the LEDs  114  are so-called white LEDs that emit light in about the 400-700 nm wavelength range. In other examples other individual light sources may be used. In one example each of the individual light sources  114  are optically shielded from each other such that through each aperture  112  light is only emitted from a single one of the light sources  114 . 
     In a further example, as shown in  FIG. 3 , below the apertures  112  is positioned a single light source  116 . In one example the single light source  116  is a tube lamp, such as a white fluorescent lighting tube. 
     In a yet further example, one light source may be positioned below a group of apertures  112 , such that multiple light sources are used to emit light through the total number of apertures  112  in the platen  110 . 
     Apertures may be chosen to be any suitable shape, such as circular or rectangular. In one example the width of each aperture is in the range of about 5 to 10 mm, although in other examples other apertures sizes may be used. 
     Before using the printing system  100  of the present examples to print on a backlit substrate the controller  102  controls elements of the printing system  100  to perform a calibration step, as described below in further detail with additional reference to the flow diagram of  FIG. 4 . 
     At block  402  the controller  102  turns on each of the light sources  114  or light source  116  below the platen  110 . At block  404  the controller  102  waits for a predetermined time period to allow the light source(s) to stabilise. This period enables the light source(s) to suitably warm up and helps ensure that the light output has substantially constant characteristics. 
     In one example the predetermined time period is between about 1 minute and 5 minutes depending on the type of light source(s) used. In other example a shorter or longer time period may be chosen based on the characteristics of the chosen type of light source. 
     At block  406  the controller  102  controls the carriage  104  on which is mounted the colour sensing device  108  to position the colour sensing device  108  vertically above a first aperture  112  in the printer platen  110 . 
     Once the colour sensing device  108  is suitably positioned, at block  408  the controller  102  uses the colour sensing device  108  to measure characteristics of the light emitted through the first aperture  112 . At block  410  the measured light characteristics are stored in a memory  103 . 
     At block  412  the controller  102  controls the carriage  104  to position (block  406 ) the colour sensing device  108  vertically above a second aperture  112  in the printer platen  110 , and blocks  408  and  410  are repeated until characteristics of light emitted through each of the apertures  112  have been made and the characteristics stored in the memory  103 . 
     Once characteristics for light emitted through each of the apertures  112  have been measured by the colour sensing device  108  and stored in the memory  103  at block  414  the processor  102  determines, or calculates, light calibration data for light emitted through each of the apertures  112 . 
     The light calibration data determined for light emitted through each of the apertures  112  enables light emitted through each of the apertures to be normalised. In one example the light calibration data may be determined by comparing the measured characteristics of light emitted through an aperture with a set of predetermined light characteristics. The controller  102  may then determine one or multiple coefficients that when applied to measured characteristics of light emitted through an aperture generate a ‘normalised white light’ having predetermined light characteristics. The determined light calibration data enables uniform light measurements to be made across each of the apertures, even though the characteristics of light emitted through different apertures may vary. 
     Advantageously, determining individual calibration data for each aperture  112  enables a low-cost light source  116  or light sources  116  to be used. For example, a low-cost fluorescent tube  116  may be used that does not exhibit constant light characteristics along the length of the tube  116 . Similarly, low-cost LED lamps may be used, even though the characteristics of light emitted by each lamp may be different. 
     Once light calibration data has been determined for light emitted through each of the apertures  112  the processor  102  may perform a further calibration process to determine printing system colour calibration data, as will now be described with further reference to  FIGS. 5 and 6 . 
       FIG. 5  shows a simplified plan view of the printing system  100  in which a sheet of backlit substrate  502  is inserted. 
     At block  602  ( FIG. 6 ) the controller  102  controls the printing system  100  to print a set of colour patches  504   a  to  504   n  on the substrate  502 . 
     In one example one or multiple rows of colour patches may be printed. For example, a first row may comprise a set of single colour colour patches printed at a range of colour densities. For example, a leftmost colour patch may be printed using a first single colour ink and have a colour density of 100%, and a rightmost colour patch may be printed using the first single colour ink and have a colour density of 5%. The patches in-between may have graduated ink densities, for example depending on the number of apertures  112  in the platen 
     In one example, in a printing system capable of printing with N different single colour inks N rows of graduated colour patches may be printed, with each row being printed using a different one of the N colour inks. 
     The substrate  502  is advanced through the printing system  100  in a substrate advance direction  506 . The substrate  502  is advanced using a substrate advance mechanism  508  that may include, in some examples, a powered roller, a belt, or the like. In some examples the substrate  502  may be moved both forwards and backwards in the substrate advance direction. 
     In one example the colour patches  504  may be printed by the print engine  202  located on the carriage  104  using one or multiple inkjet printheads. In another example the print engine  202  may located distant from the carriage  104 . Each colour patch is printed such that when the substrate is suitably positioned over the platen  110  each colour patch  504  is positioned vertically above one of the apertures  112 . 
     At block  604  the printer controller  102  controls the substrate advance mechanism  508  to move the substrate  502  in the substrate advance direction  506  to align each of the printer colour patches  504  with a corresponding platen aperture  112 . In one example, the printer controller  102  may wait for a predetermined length of time before positioning the substrate  502  to allow the ink used to print the colour patches  504  to dry or cure. 
     At block  606  the printer controller  102  controls the light source  116  or light sources  114  to switch on. At block  608  the printer controller  102  waits a predetermined length of time to allow the light source  116  or light sources  114  to stabilise or warm up. 
     At block  610  the printer controller  102  controls the carriage  104  on which is mounted the colour sensing device  108  to position the colour sensing device  108  vertically above a first aperture  112  in the printer platen  110 . 
     Once the colour sensing device  108  is suitably positioned, at block  612  the controller  102  measures, using the colour sensing device  108 , characteristics of the light emitted through the first aperture  112  and through the colour patch  504  positioned above the aperture  112 . At block  614  the printer controller  102  obtains the previously determined light calibration data for the current aperture from the memory  103  and applies the calibration data to the measured light characteristics to determine calibrated colour patch light characteristics. 
     At block  616  calibrated colour patch light characteristic data is stored in the memory  103 . 
     At block  618  the controller  102  controls the carriage  104  to position (block  610 ) the colour sensing device  108  vertically above a second aperture  112  in the printer platen  110 , and blocks  612 ,  614 , and  616  are repeated until characteristics of light emitted through each of the apertures  112  and through the colour patch  504  positioned above each aperture have been made and the light characteristics stored in the memory  103 . 
     In examples where multiple rows of colour patches are printed on the substrate  502  the controller  102  may advance the substrate  502  such that a further row of colour patches are positioned above the apertures, enabling colour measurements to be taken for each of the rows of colour patches. 
     Once calibrated colour patch light characteristics for light emitted through each of the apertures  112  and through the colour patches  504  have been measured by the colour sensing device  108  and stored in the memory  103  at block  620  the processor  102  determines, or calculates, colour calibration data for the printing system  100 . 
     In one example the colour calibration data is determined by comparing the stored calibrated colour patch light characteristics for each colour patch with known characteristics of each printed colour patch, such as the ink colour used and colour density printed. In this way, if it is determined that calibrated colour patch light characteristics of a particular printed colour patch do not correspond to known characteristics of the colour patch adjustments within the printing system  100  may be made. In one example the determined colour calibration data may be used to feed a closed-loop colour calibration system. 
     One advantage of examples described herein is that by integrating the platen  110 , apertures  112 , and light source(s) within a printing system enables the printing system  100  to perform printing system colour calibration for use with backlit substrates in a substantially automated manner. This avoids having to resort to use of separate colour calibration systems. 
     A further advantage of the present examples is that the cost of adding one or multiple light sources below a printer platen having a series of apertures is low. 
     It will be appreciated that examples and embodiments of the present invention can be realized in the form of hardware, software or a combination of hardware and software. As described above, any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are examples of machine-readable storage that are suitable for storing a program or programs that, when executed, implement examples of the present invention. Examples of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and examples suitably encompass the same. 
     All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.