Illumination utilizing a plurality of light sources

Illumination utilizing a plurality of light sources is disclosed. In one embodiment, an illumination level is set to result in a desired contrast level. Calculations are made to derive electrical current values for each of the plurality of light sources which will illuminate a plurality of selected locations at the set illumination level.

RELATED APPLICATIONS

This patent application is related to, and claims priority from, EPO Patent application Ser. No. 04105368.7, entitled “Illumination Utilizing a Plurality of Light Sources”, filed on 28 Oct. 2004, commonly assigned herewith, and hereby incorporated by reference.

BACKGROUND

In the course of performing a print job, printers periodically or continuously advance print media (e.g. paper) along a paper path. Such advancement must be made with precision; inaccuracies in the advancement will result in lessened print quality.

In some applications, one step in the advancement of the print media involves recognizing features defined on the print media, such as specific fibers which form the surface of the print media. Once recognized, a specific fiber can be used as a landmark, allowing the print media to be advanced with great precision.

However, where the print media is illuminated with poor uniformity, print media advancement which is based on recognition of features defined on the print media is impaired. For example, a feature defined on the print media may be recognized under a first lighting condition. Subsequently, the print media may be advanced to a degree that the recognized feature is located in a region having different lighting characteristics. Under the different lighting characteristics, a feature recognition module or feature recognition algorithm may be unable to locate the recognized feature. This may result in less precise control over the advancement of the print media.

Compound lighting systems, using more than one light source, have been developed in an attempt to provide more uniform lighting. However, such lighting systems have failed to provide the uniformity desired to better control the advancement of print media along the paper path in a printer. Accordingly, systems and methods which result in more uniform lighting are needed.

SUMMARY

Illumination utilizing a plurality of light sources is disclosed. In one embodiment, an illumination level is set to result in a desired contrast level. Calculations are made to derive electrical current values for each of the plurality of light sources which will illuminate a plurality of selected locations at the set illumination level.

DETAILED DESCRIPTION

An illumination system utilizing a plurality of light sources may be configured for operation in a variety of environments, such as an exemplary environment comprising a printer or other hard copy output device. In the printer environment, print media traveling through portions of a paper path defined through the printer is illuminated by the illumination system. Illumination of the print media promotes the successful recognition of features (such as paper fibers) defined on the print media. Such features, when recognized, can be used as landmarks when performing very precise advancements of the print media. In one embodiment, the illumination level is set to a level which results in a desired contrast level, such as a contrast level which facilitates recognition of features. Using the selected illumination level setting, calculations are made to derive electrical current values for each of the plurality of light sources which will illuminate a plurality of selected locations at the desired level. An example of the illumination system utilizing a plurality of light sources is discussed below.

FIG. 1is block diagram showing an example of a printer100within which a print media illumination system102is configured. The illumination system102illuminates portions of the paper path104. A sensor106is configured to sense light received from print media108moving through the paper path104. A feature recognition module110is configured to analyze images obtained from the sensor106, and to thereby recognize features, such as fibers or other irregularities, present on print media108. The location of such recognized features is utilized as an input by a print media advancement module112. Such input is useful in moving the print media through the paper path104with a desired precision.

In a typical embodiment, the printer100may be an inkjet printer, which advances print media (e.g. sheet paper, rolled paper, envelopes, etc.) in incremental steps in a stop-and-go fashion consistent with periodic movement of a printhead across the print media. In an alternative embodiment, the printer may be a laser printer, configured to advance media in a smooth and uniform manner.

In the example ofFIG. 1, the illumination system102includes a light source114and a controller116. The illumination system102is configured to derive an appropriate contrast level, and then using an illumination level which resulted in the contrast level, to derive voltages to be applied to each light source included within the compound light source114. The selected voltages should result in a desirable level of uniformity in lighting intensity between several areas of interest.

The light source114may be based on any technology, such as LEDs, incandescent devices, florescent devices, etc. However LEDs are preferred, and will yield superior results in most applications. InFIG. 2, an example of an implementation of the light source114is illustrated. In this example, the light source114includes four LED groups202which are located about a hole204through which print media108may be seen. A feature206, in this case a paper fiber enlarged to show its detail, is representative of features detectable by the sensor106(FIG. 1) and feature recognition module110. In the example ofFIG. 2, each LED group202includes four individual LEDs208. The individual LEDs208within one group202typically receive power supplied by a single driver circuit (not shown), although each individual LED208could be provided with a separate current supply.

The sensor106(seen inFIG. 1, but removed from the view ofFIG. 2to better reveal the print media108) is located to receive light from the light source114which has reflected off or otherwise received from the print media. The sensor106should be selected in part for the ability to monitor a region of a size greater than the amount by which the print media is periodically advanced. Accordingly, a feature identified within the region monitored by the sensor may be observable in a first position within the region before print media advancement, and in a second position within the region after advancement. As a result, the degree of the advancement may be more precisely controlled. Additionally, the sensor106should be selected in part for the ability to define and monitor individual pixels within the monitored region. In one example, the sensor is able to assign values within a range, such as 0 to 255, to represent gray values sensed for each pixel, within a 64 pixel by 64 pixel region.

The controller116may be configured as a software, firmware, hardware or hybrid module or device. For example, the controller116may be configured as a software procedure executed by a microcontroller, or as an application specific integrated circuit (ASIC). Additionally, the controller may include, be a part of, or be contained within, a module, such as the print media advancement module112. The controller116is configured to perform a variety of functions, such as monitoring output signals from the sensor106and controlling the light sources114. In one embodiment of the controller116, its functionality is described byFIG. 3.

FIG. 3is a flow diagram that describes an exemplary method300to illuminate print media by utilizing a plurality of light sources. The method300may be implemented by controller116or another device, based on hardware, software or firmware. At block302, an illumination level is set to result in a desired contrast level. In the example ofFIG. 1, wherein a print system100is implemented, the contrast level would be based on data from the sensor106resulting from light reflected by or otherwise received from the print media108. Setting the illumination level to result in a desired contrast level may be performed in a number of ways, one of which is listed here for purposes of illustration. At block304, the light source114(FIG. 1) is made progressively more luminous, thereby progressively illuminating the print media108. In one implementation, all LED groups202(FIG. 2) of the light source114are used to provide progressive levels of illumination. Optionally, the progressive illumination of the print media108may be repeated while using only a subset (such as half) of the LED groups202and/or individual LEDs208within the light source114. Some media yield better contrast where, for example, only the LEDs on one side are used in the illumination. Accordingly, additional checks of the contrast using larger and smaller numbers of the available LEDs may indicate a preferred lighting strategy. The progressive illumination may be performed by incremental increases (or decreases) in the current supplied to each element within the light source114, typically in a step-function. At block306, contrast of an image detected by the sensor106is monitored for contrast during the progression.

Referring toFIGS. 4 and 5, a method by which the contrast of the image detected by the sensor106may be monitored for contrast can be understood.FIG. 4shows graphical information400obtained from the sensor106(FIG. 1) after viewing a portion of the print media108under a lower illumination level. The horizontal axis402is associated with gray scale values, expressed numerically from 0 (black) to 255 (white). The vertical axis404shows the number of pixels within the image obtained from the sensor106under a lower illumination level having that gray level. Accordingly, any image can be graphically represented by charting the number of pixels in the image taken by the sensor having each of the 256 gray levels. Obviously, a sensor configured to sense a different number of gray levels would result in a somewhat different graph. Referring toFIG. 4, it can be seen that at a lower illumination level, most of the pixels have approximately the same dark gray values between 0 (black) and 50 (dark gray). Accordingly, the image expresses little information, and has low contrast.

In distinction,FIG. 5shows graphical information500resulting from a sensor image taken under higher illumination. In this case, the pixels in the image are spread over a larger range, wherein fewer pixels have any particular value and more pixels are distinguished from adjacent pixels. Accordingly, the image associated with the graph ofFIG. 5has more information and more contrast than the image associated with the graph ofFIG. 4.

Returning toFIG. 3, block308shows that the illumination level can be set at a level wherein contrast is sufficient to allow features206(FIG. 2) on print media108(FIGS. 1 and 2) to be viewed. In particular, a higher level of illumination associated with a higher contrast level (such as that seen inFIG. 5) is selected, rather than a lower level of illumination which results in less contrast (such as that seen inFIG. 4).

Note that while block302discloses one implementation, other embodiments can also be implemented. For example, the block302may alternatively set the illumination level to result in a desired light intensity, or to result in a desired color light intensity.

At block310, electrical current values for each of the plurality of light sources114(FIG. 1) are calculated. This may be performed in a number of ways, one of which is listed here at blocks312-314for purposes of illustration.

At block312, a system of equations is formed. Typically, the system may include six to eight equations; however, this number is flexible, and could easily be adapted according to the application. Generally, the number of equations may be influenced by the number of light sources and other factors. An example of the system of equations follows:
SUM—1=(a1*Q1)+(b1*Q2)+(c1*Q3)+(d1*Q4)+ . . .
SUM—2=(a2*Q1)+(b2*Q2)+(c2*Q3)+(d2*Q4)+ . . .
SUM—3=(a3*Q1)+(b3*Q2)+(c3*Q3)+(d3*Q4)+ . . .
etc.

Each equation may be understood by a discussion of the first equation. In the first equation, SUM_1represents an illumination level at a first location. The illumination level is the sum of the illumination from a plurality of groups of LEDs202or individual LEDs208which make up the light source114(FIG. 1). Accordingly, each equation is the sum of products, wherein each product is associated with an individual light source from within the plurality of light sources. The first location—associated with the first equation—may be a specific pixel, located on the print media108(FIG. 1), and represented by data contained within a location of a file which results from examination of the specific pixel by the sensor106(FIG. 1).

The coefficients a1, b1, c1, d1, a2, b2, c2, d2, a3, b3, c3, d3, etc. are numeric values determined by measurement during a period in which a plurality of current levels are applied to the associated light source. For example, a step-function of current values could be applied to a first light source (e.g. an element202or208within the compound light source114) associated with current variable Q1and the coefficient a1. The gray levels (e.g. values from 0 to 255) associated with each current level could be measured at the location associated with SUM_1. These values could then be totaled, averaged or otherwise manipulated to form the coefficient a1. Typically, the exact values of the coefficients are not important; instead, it is their relative size that is important. Therefore, by deriving each coefficient in a similar manner, based on measurements associated with the location associated with the equation and the light source associated with the term of the coefficient, appropriate coefficients will be derived. Accordingly, coefficient b1could be determined by stepping current values through a second light source associated with the variable Q2, with measurements taken at the location (e.g. a pixel within the image of the sensor) associated with SUM_1. And similarly, coefficient d3could be found in a manner similar to coefficient a1, but with measurements taken at the location associated with SUM_3while current was stepped through light source4associated with current variable Q4.

Note that the locations within the image taken by the sensor106are typically distributed somewhat evenly through the image, and that while six or eight locations may be typical, a greater or lesser number of locations (and therefore equations) could be selected.

Prior to solving the equations, the values for SUM_1, SUM_2and SUM_3, etc. are set to a value which resulted in the desired contrast (see block302). For example, if the sensor106measures gray on a scale of zero to 255, the desired contrast may be (as illustrated inFIG. 5) around 200. Thus, the values for SUM_1, SUM_2and SUM_3, etc. could be set to 200.

At block314, the system of equations is solved. Note that the number of equations and unknowns may not be equal. Thus, the system could be over-constrained or under-constrained, i.e. there could be more or less equations than variables (the Q's). However, it is generally best to have fewer locations (that is fewer equations, since each SUM_x is associated with a location) than there are light sources (i.e. the Q's). This allows more flexibility in the solution to the equations; that is, one or more values for Q can be arbitrarily set, and the others derived. For example, in most applications the solution is more stable if one of the currents is forced to the level of maximum contrast.

At block316, features204(FIG. 2) defined on the print media108(FIGS. 1 and 2) and illuminated by the light sources114are recognized. Features are typically fibers, which form the paper of the print media. Recognition of a feature204provides a “landmark” on the print media, which helps to advance the print media by a precise amount. Accordingly, at block318the print media is advanced along a paper path within a printer, using the recognized feature(s). Note that blocks316and318may be repeated more frequently, while the earlier blocks may be repeated only upon loading the media or upon changing the media.

In general, the process300ofFIG. 3is performed whenever new print media is used. For example, where different print media is located in different trays within the printer, and frequent changes of print media are performed, it is advisable to reset the illumination level (e.g. block302) and recalculate the current levels (e.g. block310). Such calculations should additionally be performed at intervals due to LED replacement or aging. However, these events typically occur much more infrequently than changes in print media.

Although the above disclosure has been described in language specific to structural features and/or methodological steps, it is to be understood that the appended claims are not limited to the specific features or steps described. Rather, the specific features and steps are exemplary forms by which this disclosure may be implemented. For example, while actions described in blocks of the flow diagrams may be performed in parallel with actions described in other blocks, the actions may occur in an alternate order, or may be distributed in a manner which associates actions with more than one other block. And further, while elements of the methods disclosed are intended to be performed in any desired manner, it is anticipated that computer- or processor-readable instructions, performed by a computer and/or processor, typically located within a printer, reading from a computer- or processor-readable media, such as a ROM, disk or CD ROM, would be preferred, but that an application specific gate array (ASIC) or similar hardware structure, could be substituted.