The subject invention relates to digital printing. (As used herein, the term xe2x80x9cdigital printingxe2x80x9d refers to any form of printing wherein print control signals control a print mechanism to produce a matrix of pixels, i.e. picture elements, having two or more intensity values to represent an image.) More particularly it relates to apparatus and methods for the real-time measurement of digital print quality.
Low cost, widely available digital printing technologies such as ink jet, bubble jet, and thermal transfer printing have enabled many new applications where dynamically varying information must be transmitted in printed form. Many of these applications rely upon a consistent level of print quality over time since the failure to capture the unique information on even a single document can have serious consequences.
A particular example of an application of digital printing where a consistent level of print quality is very important is the use of digital print mechanisms in postage meters and mailing machines. As is well known such devices print postal indicia on mailpieces as proof of the payment of postage. Upon payment to a proper authority such meters or machines are xe2x80x9cchargedxe2x80x9d with a representation of an equivalent amount of funds. As postal indicia are printed the funds in the meter are debited accordingly until exhausted. Since postal services accept indicia printed by postage meters or mailing machines as conclusive proof of payment of the amount of postage indicated such devices are in effect machines for printing money. As a result postal services have imposed high standards both on the print quality of indicia produced by such machines, and on the design of the machines themselves to assure that the appropriate amount is debited from the amount charged into the machine for each indicia printed.
Low cost digital print technologies have greatly simplified and improved the design of postage meters and mailing machines in many respects. Prior postage meters and mailing machines relied upon impact printing techniques which required complicated and expensive mechanisms to print varying postage amounts, which can now be printed in a simple, conventional manner with digital print mechanisms. More importantly, digital print mechanisms can be easily programmed to print other information such as security codes or addressing or tracking information with the postal indicia to facilitate automated mail handling. However, such low cost digital print mechanisms can not easily provide consistent print quality as their mechanisms tend to degrade over time as ink dries up, small print nozzles clog or one or more of a number of small, rapidly cycling print elements fails. Such failure can cause substantial losses to a mailer since a large number of mail pieces of substandard print quality may be rejected by a postal service after the cost of the postage has been debited from the prepaid amount charged to the machine.
U.S. Pat. No. 4,907,013; to: Hubbard et al.; issued: Mar. 6, 1990 is believed to be the prior art closest to the subject invention and relates to circuitry for detecting failure of one or more nozzles in an ink jet printhead. In Hubbard et al. a line containing one dot printed by each nozzle in the printhead is scanned to detect the possible absence of a dot. The line can form either a test pattern run before the start of a printing operation or can be incorporated into the image to be printed.
U.S. Pat. No. 5,038,208; to; Ichikawa et al.; issued: Aug. 6, 1991 teaches an ink jet printer which stores the image forming characteristics of an ink jet printhead and which corrects the image forming signals in accordance with the stored characteristics to maintain uniform print density.
U.S. Pat. No. 5,126,691; to: Millet et al.; issued: Jul. 7, 1992 is similar to Hubbard et al. in that it teaches a method for monitoring print quality by the use of a specially printed control frame.
U.S. Pat. No. 5,321,436; to: Herbert; issued Jun. 14, 1994 teaches a postage meter in which the operation of an ink jet printhead is checked by printing a predetermined bar code and then scanning the bar code to determine if it was correctly printed.
U.S. Pat. No. 5,473,351; to: Heterline et al. teaches a method and apparatus for monitoring print density by measuring printed line width and modifying the energy of the pulses applied to each ink jet nozzle to correct the line width.
Commonly assigned U.S. Pat. No. 6,000,774; titled: Mailing Machine Including the Prevention of Loss of Funds; issued Dec. 14, 1999, which is hereby incorporated by reference, teaches a postage meter or mailing machine having a capability for generating a test pattern; where the test pattern includes pseudo-random information unknown to an operator. Failure of the operator to correctly input the information causes the postage meter to be disabled; and correct input of the information enables the postage meter to continue operation.
While perhaps suitable for their intended purpose the print quality monitoring and control techniques found in the prior art did not provide a simple and inexpensive way to monitor print quality in real-time. Hubbard and similar prior art require special test patterns and so lack the immediate ability to detect a failure of print quality and/or the flexibility to monitor arbitrary print images; while other techniques taught in the prior art require expensive apparatus for measuring line width or printhead characteristics together with complicated control of the printhead drive signals.
Thus it is an object of the invention to provide an improved apparatus and method for the prompt, real-time monitoring of print quality so that prompt corrective actions can be taken.
The above object is achieved and the disadvantages of the prior art are overcome in accordance with the subject invention by means of a method and apparatus for real-time monitoring of digital print quality produced by a digital printing mechanism; by providing predetermined print control signals to the digital printing mechanism, the printing mechanism responding to the print control signals to print an image on a substrate; providing a background reflectance signal representative of the background reflectance of said substrate; scanning the image to generate a post-print reflectance signal; comparing the background reflectance signal with the post-print reflectance signal; and, if the post-print reflectance signal is greater than a predetermined fraction of the background reflectance signal, generating an output signal indicative of poor print quality.
In accordance with one aspect of the subject invention, the output signal indicative of poor print quality is also generated if the post-print reflectance signal is less than a predetermined minimum value of the background reflectance signal.
In accordance with another aspect of the subject invention, the image is scanned synchronously with movement of the substrate relative to the printing mechanism.
In accordance with another aspect of the subject invention, the print mechanism is comprised in a postage metering system and the image includes a postal indicia.
In accordance with another aspect of the subject invention, the postage meter is responsive to a signal generated as a function of the output signal to inhibit further printing of postal indicia.
In accordance with another aspect of the subject invention, the printing mechanism comprises a plurality of printheads, each of the printheads printing a portion of the image.
In accordance with another aspect of the subject invention, the post-print reflectance signal includes a plurality of component signals, each of the component signals corresponding to one of the portions of the image.
In accordance with another aspect of the subject invention, each of the component signals is compared separately with the background reflectance signal and, if any of the component signals is greater than the predetermined fraction of the background reflectance signal, the output signal is generated.
In accordance with another aspect of the subject invention, each of the component signals is generated by a separate linear array of photosensors, the arrays being aligned end-to-end to form a single linear array, the single array spanning the image transversely to the direction of motion of the substrate relative to the printing mechanism.
In accordance with another aspect of the subject invention, each of the separate arrays scans the corresponding one of the portions a plurality of times so that a predetermined number of scans of the image are made and the scans are integrated for each of the corresponding portions to generate the component signals.
In accordance with another aspect of the subject invention, the integrated scans are divided by the predetermined number, whereby the component signals represent an average over the plurality of scans.
In accordance with still another aspect of the subject invention, the background reflectance signal is compared with the post-print reflectance signal to classify the post-print reflectance signal as being satisfactory, unsatisfactory, or doubtful; and if the post-print reflectance signal is unsatisfactory, generating an output signal indicative of poor print quality; and if the post-print reflectance signal is doubtful, printing a test pattern and waiting for an operator response; and then if the operator response indicates the test pattern is acceptable, accepting the indicia and continuing operation of the printing mechanism; and if the operator response indicates the test pattern is unacceptable, rejecting the indicia and generating the output signal indicative of poor print quality; and if the operator response indicates the test pattern is acceptable, adjusting the comparison to classify a greater portion of post-print reflectance signals as satisfactory; and if the operator response indicates the test pattern is unacceptable, adjusting the comparison to classify a greater portion of post-print reflectance signals as unsatisfactory.
In accordance with still another aspect of the subject invention, the comparison is adjusted so as to classify a lesser portion of the post-print reflectance signals as doubtful.