BACKGROUND OF THE INVENTION In printing a composite color image, the image is typically separated into component colors: cyan, magenta, yellow and black, and one or more images, each using a different component toner color, are formed, for example, on one or more photoreceptor drums. The color image is optionally transferred therefrom to one or more intermediate image drums and then to a media sheet. A composite color image on the media sheet is formed when all component toner colors have been overlaid upon each other on the media sheet.

After a component color, for example black, is transferred from the one or more drums and/or a composite image is produced, remaining black toner is removed from the one or more drums in preparation for receiving another component color, for example yellow. The black toner removed from the drum is optionally returned to the black toner reservoir, to be used in forming further color images.

Contamination of a toner color caused by, for example a first toner color mixing into a second toner color, must be kept low in order to realize consistent print quality on the media sheet. Black toner (or pigment) in a relatively light color such as yellow causes an often- troublesome color contamination where visually negligible amounts of black can effect the yellow toner in a way that seriously disturbs the color balance on the printed media sheet.

A prior art method for determining contamination by particles of a second color toner in a first toner color is described by Landa, et al. in US Patent 5,579, 193, hereby incorporated by reference. This contamination detection method comprises measuring the attenuation of light passing through a given path in a liquid toner to determine if the toner is contaminated.

U. S. Pat. No. 4, 579, 253 describes a system in which the beam of light is split into two components, only one of which is attenuated by a toner. The contamination is determined from the ratio of the attenuated and unattenuated beams.

Japanese Patent Publication Kokai 1-48943 describes a system in which the attenuations of beams of light passing through two different toner colors are sequentially measured and used to determine color contamination.

Ott in United States Patent 5,068, 810 and Amory, D. in FR WO 02/08730 Al describe methods of determining color density of a color image printed on a media sheet.

SUMMARY OF THE INVENTION An aspect of an embodiment of the present invention relates to determining toner or other pigment contamination using spectral analysis of one or more printed color images formed on or printed by a color printer.

In an exemplary embodiment the invention is applied to a digital liquid toner imager.

However, the invention can be applied to other types of imagers, including substantially any type imager in which color contamination can occur.

In an exemplary embodiment, light provided by a light source and/or ambient lighting, is reflected from one or more formed or printed color images, for example cyan, magenta, yellow or black, and collected using a spectrophotometer. The spectrophotometer provides spectral data of the one or more reflected colors, for example, to computing circuitry that compares the reflected spectral data to known spectral data and determines the existence and/or level of color contamination in the respective color toner.

Alternative to using a spectrometer, in some embodiments of the invention, a densitometer with a plurality of selectable filters and/or a plurality of densitometers each with a fixed or selectable filters can replace the spectrometer. As used herein the tenns"optical spectral analyzer"or"spectrometer"mean a device such a spectrometer or densitometer (s) with filter (s) which can measure optical energy selectively at more than one wavelength.

In an exemplary embodiment, the computing circuitry determines whether the contamination represents 1) non-obvious change, for example not readily noticeable to the human eye ; 2) a color change that is obvious but that will respond to compensatory measures; and/or 3) a color change that is obvious and requires replacement of the respective toner.

Determination of level 2 is optional and in some embodiments only level 1 and 3 will be determined.

In an exemplary embodiment, the determination of contamination and/or a contamination level by the computing circuitry, for example, is made by assessing a difference between : 1) a ratio of light reflected at two or more wavelengths from said reflected spectral data ; and 2) a ratio of light reflected at two or more wavelengths from said known spectral data.

In an exemplary embodiment, the computing circuitry is connected to a feedback system that compensates for obvious color toner contamination, for example, by lowering and/or raising the amount of one or more other toner colors used in achieving a specific printed shade of color. Alternatively, the correction may require correction of the color conversion.

For example, when yellow toner is contaminated by red toner, the feedback system may reduce the amount of red toner used to form a shade of orange in the printed image, thereby compensating for the excess red toner already in the yellow toner. Alternatively or additionally, the feedback system adjusts a volumetric ratio between two or more toner colors in an image to balance out the change in color.

In an exemplary embodiment, in the presence of a color change that is obvious and requires replacement of the respective toner, an operator and/or the feedback system will provide one or more of the following actions: 1) stopping said printer ; 2) removing a toner requiring replacement; 3) marking said removed toner as being contaminated ; and 4) replacing said contaminated toner color with an uncontaminated color.

It may be necessary, in this case, to have the printer serviced, since such high levels of contamination may indicate a malfunction which should be corrected or repaired.

Optionally, the computing circuitry and/or feedback system provide visual and/or auditory indicators that apprise an operator of data collected, contamination levels, actions to initiate and/or a history of initiated actions.

As used herein, "printer"refers to any type of printing device as is known in the art, whether a stand-alone printer or a part of a multifunction device, for example a copier or facsimile machine.

As used herein toner refers to either a liquid toner comprising a liquid carrier in which a colored pigment is suspended and/or a dry toner comprising a dry carrier in which colored particles are suspended.

As used herein,"printed image"refers to: 1) one or more color toner patches or images on one or more photoreceptor drums ; 2) one or more color toner patches or images on one or more intermediate color drums that act to transfer the photoreceptor image to one or more media sheets; and/or 3) one or more color patches or images on one or more media sheets comprising, for example, sheets of paper.

There is thus provided, in accordance with an exemplary embodiment of the invention, a method for determining color toner contamination from a printed image, comprising : a) printing one or more areas using one or more toner colors ; b) collecting light reflected from said one or more color areas; c) comparing spectral data from said reflected light to known spectral data for printing with uncontaminated toner; and d) determining color contamination in said one or more toner colors based on said comparing.

Optionally, the determination is based on a comparison of : a) light reflected at two or more wavelengths from said reflected spectral data; and b) light reflected at two or more wavelengths from said known spectral data.

Optionally, the determination comprises determining whether said contamination can be compensated without replacing said one or more toner colors.

Optionally, the method includes compensation, said compensation comprising at least one of : a) revising a volume of toner color deposition per square area of image; and b) adjusting a ratio between two or more toner colors in an image.

Optionally, said determination comprises determining when replacement of said one or more toner colors is necessary.

Optionally, said replacement comprises at least one of : a) automatically stopping printing; and b) removing a toner requiring replacement and replacing said contaminated toner color with an uncontaminated color.

There is further provided, in accordance with an embodiment of the invention, apparatus for detecting color contamination in one or more colors of toner, comprising : a) a printer that prints one or more images using one or more toner colors; b) an optical spectral analyzer that: i) receives light reflected from said one or more printed images; and ii) generates spectral data, from said one or more printed images; and c) computing circuitry that determines contamination of said one or more toner colors, based on said generated spectral data.

Optionally, the reflected light is provided by a light source associated with said printer.

In an embodiment of the invention, the computing circuitry comprises a memory containing at least one spectral data characteristic of an area formed from uncontaminated toner and wherein said determination is made by comparing said generated spectral data and said characteristic. Optionally, the computing circuitry comprises a memory in which at least one spectral data characteristic of at least one non-standard toner is stored. Optionally, the computing circuitry comprises a memory in which at least one spectral data characteristic of said one or more toner colors printed on a drum in said printer is stored. Optionally, the computing circuitry comprises a memory in which at least one spectral data characteristic of said one or more toner colors printed on at least one of a non-white colored media sheet and a textured media sheet, is stored. Optionally, thecomputing circuitry comprises a memory in which at least one spectral data characteristic of at least one media sheet from which at least one spectral data characteristic of said one or more toner colors printed on at least one of a non- white colored media sheet and a textured media sheet can be computed, is stored.

In an embodiment of the invention, the apparatus includes a feedback mechanism responsive to said computing circuitry, that compensates for contamination of said one or more printed images with one or both of : a) revising the volume of color deposition per square area of one or more colors in said image; and b) adjusting a ratio between two or more colors in said image.

Optionally, when said computing circuitry determines contamination above a pre- determined level, the circuitry performs one or both of : a) automatically stopping said printer ; and b) removing a toner requiring replacement and optionally replacing said contaminated toner color with an uncontaminated color.

BRIEF DESCRIPTION OF THE DRAWINGS Exemplary non-limiting embodiments of the invention that are described in the following description read with reference to the figures attached hereto. In the figures, identical and/or similar structures, elements and/or parts thereof that appear in more than one figure are generally labeled with the same or similar references in the figures in which they appear.

Dimensions of components and/or features shown in the figures are chosen primarily for convenience and clarity of presentation and are not necessarily to scale. The attached figures are: Fig. 1 is a cross-sectional schematic representation of a printer known in the art having a color contamination detecting system, in accordance with an exemplary embodiment of the invention; Fig. 2 is a cross-sectional representation of a cleaning station known in the art, in accordance with an exemplary embodiment of the invention; Fig. 3 is a color contamination detecting system, in accordance with an exemplary embodiment of the invention ; Fig. 4 shows exemplary spectral absorbance curves of black, cyan, magenta and yellow toners printed on white paper for an exemplary system in which the invention can be utilized ; and Figs. 5A and 5B are exemplary embodiments of printed color images used in the color contamination detecting system of Fig. 3, in accordance with an exemplary embodiment of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Fig. 1 is a schematic representation of a liquid toner color printer 100 having a color contamination detecting system 300, in accordance with an exemplary embodiment of the invention. Printer 100 comprises a photoreceptor drum 102, a charger 104 that charges photoreceptor drum 102 and an image-wise discharge system, such as a scanning laser 106, that discharges photoreceptor drum 102 to form a latent image on photoreceptor drum 102.

Situated about photoreceptor drum 102 is a developer 108 comprising multiple component color reservoirs, each containing a single liquid toner color of yellow 1510, magenta 1512, cyan 1514 or black 1516 (Fig. 2).

Developer 108 develops an electrostatic image on photoreceptor drum 102, according to the image discharged on photoreceptor drum 102 by scanning laser 106 and one or more toner colors 1510, 1512, 1514 and/or 1516 are deposited on photoreceptor drum 102. In an exemplary embodiment of the invention, a composite image, comprising multiple component colors, is formed on a transfer drum 110 and transferred to a media sheet 148, for example a neutral sheet comprising smooth white paper. Alternatively or additionally, transfer drum 110 rotates once in a direction 142 for each component color and the different component color images are transferred serially to media sheet 148.

Following transfer of an image from photoreceptor drum 102, each individual toner color, yellow 1510, magenta 1512, cyan 1514 and/or black 1516 is removed individually by developer 108 and/or by a cleaning station 112 and returned to its respective component color toner reservoir in developer 108.

For high-speed printing, a dryer 114 may be used to dry the image on transfer drum 110.

Alternatively or additionally, after forming an image on sheet 148, a prep unit 116, may be used to remove liquid that remains on or is solvated by transfer drum 110.

Fig. 2 illustrates a multicolor liquid toner printing system 200 as described in United States Patent 5,745, 829, incorporated herein by reference. Printing system 200 comprises developer 108 that recycles color toner by sending it to its corresponding color reservoir in a multicolor liquid developer spray assembly 20. Developer 108 further comprises color specific cleaning blade assemblies 34. Cleaning station 112 comprising a background cleaning station 24, and an electrified squeegee 26.

Color specific cleaning blade assemblies 34 are operatively associated with a developer roller 38 for separate removal of residual amounts of each colored toner remaining thereon after development. Each of blade assemblies 34 is selectively brought into operative association with developer roller 38 only when toner of a color corresponding thereto is supplied to a development region 44 by spray assembly 20. The construction and operation of cleaning blade assemblies 34 are described in PCT Publication WO 90/14619 and in U. S. Pat. No. 5,289, 238, the disclosures of which are incorporated herein by reference.

Each cleaning blade assembly 34 comprises a toner directing member 52 which serves to direct the toner removed by cleaning blade assemblies 34 from developer roller 3 8 to separate collection containers for toner colors yellow 1510, magenta 1512, cyan 1514 and/or black 1516 to prevent contamination of the various developers by mixing with one another. A final toner directing member 62 engages developer roller 38 and the toner collected thereat is supplied into a collection container 64 and thereafter to the black reservoir or is removed from the system.

Many aspects of the present invention have application in a wide range of printers using ink, liquid toner and/or powder toner as are known in the art. For example, an alternative developer 108 and multicolor liquid developer spray assembly 20 assembly is described in WO 96/29633, incorporated herein by reference. The present invention is also applicable to powder toner systems.

Color contamination, for example, when black toner 1516 mixes into yellow 1510, magenta 1512 and/or cyan 1514, may result from any number of causes in printer 200, including, but not limited to improper movement of cleaning blade assemblies 34, malfunction of background cleaning station 24 and/or improper toner dispensation by spray assembly 20 to name a few.

In other printers, other assemblies associated with color dispensation and/or removal from transfer drum 110 and/or photoreceptor drum 102 may malfunction and contribute to toner contamination. Contamination of a toner color that results in noticeable color change in the printed image may occur at any time during the printing process. Comparing a spectrum of a printed image to known spectral data provides accurate assessment of the extent of color change prior to and/or during the printing process.

Fig. 3 is color contamination detecting system 300 in which a source of light 302, for example an incandescent lamp and/or natural environmental light, driven by a current or voltage source 304, provides lighting 306 on a media sheet 148 comprising printed color images of yellow 510, magenta 512 and/or cyan 514.

Lighting 306 incident on images of yellow 510, magenta 512 and/or cyan 514 form reflected rays 326 that are received by a spectrophotometer 322. Spectrophotometer 322 analyzes the spectra of reflected rays 326 and sends spectral data to computing circuitry 324 that compares spectral data from spectrophotometer 322 to known color spectral data. In an exemplary embodiment, printed images of yellow 510, magenta 512 and/or cyan 514 that were printed with uncontaminated toner colors yellow 1510, magenta 1512 and/or cyan 1514, are used as a basis for the known spectral data.

With minute amounts of a foreign color in one or more component reservoirs (Fig. 2) containing colors 1510,1512, and/or 1514, the degradation in the quality of the printed color image 510,512 and/or 514 may not be immediately obvious to the human eye.

At an intermediate level of contamination, wherein color changes become obvious to the human eye, compensation of one or more color toners 1510, 1512 and/or 1514, may be possible. In compensating for a contaminated color, for example, the dispensed volume of one or more color toners 1510,1512 and/or 1514 vis-a-vis the other toner colors is adjusted to change the color mix in the printed image. For example, if yellow 1510 is contaminated with cyan 1514, when mixing the two colors to form a specific color tone of orange, toner spray assembly 20 may be programmed to apply less cyan toner 1514 to achieve the same tone of orange. With contamination of yellow toner 1510, however, developer 108 can no longer provide a composite color image that requires pure yellow toner 1510.

Optionally, an operator initiates a compensation activity, e. g. , recalibration of the color tables, following notification by an indicator station 340 that is based upon the contamination level that is presented. Alternatively or additionally, a feedback system 330 automatically changes the amount of dispensed toners 1510, 1512, 1514 and/or 1516 to compensate for the contamination.

At a higher level of contamination compensation of the toner may be impossible and replacement of one or more color toners 1510,1512 and/or 1514, may be necessary.

Alternatively or additionally obvious contamination may occur at a lower contamination level, with sensitive color blends, for example when a given composite color image requires a pure toner color, for example yellow toner 1510 and/or a given color image has an area with shades of printed yellow 510 that specifically exclude a contaminating color, for example printed cyan 512.

In cases where contamination becomes obvious, feedback system 330 and/or the operator stop printer 100 from further printing, the contaminated toner is removed, appropriately marked and replaced with a fresh, uncontaminated toner. Alternatively or additionally, when the contamination is at a lower level in a color-sensitive image, the level of contamination is marked on the toner so that it can be optionally used in printing images comprising shades where such contamination will not be obvious.

Optionally, the process of color contamination detection is made during a print setup prior to running printer 100 continuously. Alternatively or additionally, color contamination detection may be made at any time during a continuous printing run. In an exemplary embodiment, following completion of the confirmation process, printer 100 is set to print serial composite color images on one or more media sheets 148 in its usual manner of operation.

Fig. 4 show exemplary spectral absorbance curves of black, cyan, magenta and yellow toners printed on white paper for an exemplary system in which the invention can be utilized.

Variations from these absorbance curves generally are caused by either an undesirable amount (either too little of too much) of toner particles deposited or by contamination of one of the colored toners with particles from another toner.

These two causes, while they both change the absorbance, do not change it in the same way. For example, if too much magenta toner is printed (for example a double amount is printed), such that at 475 nm the absorption is 44% instead of 25%, then at 540 nm, the absorption will be 84% instead of 60%. On the other hand, if the increase in absorption is caused by black toner contamination then, if the absorption is 44% at 475 nm, the absorption will be only about 70% at 540 nm. This calculation is based on the black contaminant absorbing a same proportion of the reflected light at the two wavelengths. This is in contrast to the double printing case, in which a higher proportion of the reflected light is absorbed by the"additional" colored toner at 540 nm than at 475 nm.

A large amount of contamination has been used as an illustration, however, this is for illustration only. Generally, quite small amounts of contamination, especially of black in the colors, and most especially in yellow, can cause serious changes in color balance.

This change in color balance is caused, in general, more by a reduction in the amount of colored toner that is printed and not so much by an increase in black. This somewhat paradoxical result is caused by the fact that the amount of toner to be printed in a given color is controlled by keeping the absorbance or reflectance of the printed color within a narrow range of values. If the colored toner is contaminated by black, it will have an abnormally high absorbance, even when the amount of color is correct. Any system calibration will then "correct"the amount of color printed by reducing the coverage of the toner. However, since the abnormally high reading was caused by contamination, not by too much colored toner, the amount of color will be incorrectly reduced.

In this way, small amounts of black, in yellow for example, can cause a relatively objectionable decrease in yellow coverage, even when the black itself is very small, for example, where it absorbs 5% or even 1% or less.

In an embodiment of the invention, for small amounts of black (or other) contaminant, the amount of colored toner is increased to compensate for the error in absorption caused by the contaminant. Thus, if a relatively small amount of the contaminant is present, then the toner need not be replaced. A relatively small amount is defined as an amount for which the unwanted contaminant does not cause an objectionable amount of color imbalance. For somewhat larger amounts, more imperfect correction can be achieved by reducing the amount of the contaminant toner that is printed.

In general, when n types of toner are used, the cross contamination for any three contaminants requires measurement at n wavelengths. However, in general, the only important contaminant is black. Furthermore, the black toner, being highly absorbent, is not generally contaminates in an objectionable manner by the other toners. Thus, in many cases, measurement at two wavelengths is sufficient. However, better results are obtained by wider ranges of measurement or by spctrum matching and decomposition.

In an exemplary embodiment, computing circuitry 324, for example, determines that a measured contaminated ratio represents a level of contamination that cannot be compensated and sends commands to feedback system 330 to replace the contaminated toner with a non- contaminated yellow toner 1510.

Optionally, computing circuitry 324 contains comparison spectral data allowing printed colors 510,512 and/or 514 to be assessed when printed on a non-standard media sheet 148, for example having a non-smooth texture, comprising materials other than paper and/or having a color other than pure white. Alternatively or additionally, computing circuitry 324 contains comparison spectral data allowing non-standard colors, for example special toner colors, to be assessed.

In an exemplary embodiment, color contamination detecting system 300 contains a spectrophotometric input and/or one or more alternative data input systems known in the art, whereby spectra of non-standard printed colors 510,512 and/or 514, spectra from non-standard media sheets and/or spectra demonstrating alternative color densities, are input into computing circuitry 324 to provide additional known color spectral data. Optionally, such additional known color spectral data may be input prior to a given printing run.

Fig. 5A is an exemplary embodiment of printed colors yellow 510, magenta 512 and/or cyan 514 on print area 500 that are used as determinants of color toner contamination. In an exemplary embodiment, an individual image, for example yellow 510, is printed on a single area 502 (Fig. 5B), another image, for example magenta 512, is printed on a single area 504, and cyan 514 is printed on a single area 508.

Print areas 500,502, 504 and/or 508 may, for example, comprise single media sheet 148 (Figs. I and 2), photoreceptor drum 102 and/or transfer drum 110 as noted above. While troublesome color changes often occur in printed images 510, 512 and/or 514 ; black image 516 does not usually become contaminated to a noticeable degree.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Also, combination of elements from variations may be combined and single elements may be used. As an example, light source 302 may comprise multiple sources, each with a different frequency and/or intensity. Such variations and modifications, as well as others that may become apparent to those skilled in the art are intended to be included within the scope of the invention, as defined by the appended claims. Further, variations in the embodiments can vary even further based upon a variety of engineering principles, materials, intended use and designs incorporated into the invention.

In addition, the particular geometric forms and measurements used to illustrate the invention should not be considered limiting the invention in its broadest aspect to only those forms. Although some limitations are described only as method or apparatus limitations, the scope of the invention also includes apparatus designed to carry out the methods and methods of using the apparatus.

The terms"include", "comprise"and"have"and their conjugates as used herein mean "including but not necessarily limited to. "A person skilled in the art will appreciate that the present invention is not limited by what has thus far been described. Rather, the scope of the present invention is limited only by the following claims: