Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1998-11-24
2001-04-10
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S015000, C347S017000, C347S019000
Reexamination Certificate
active
06213579
ABSTRACT:
TECHNICAL FIELD
The present invention relates to systems and methods for compensating for thermally induced variations in the size of droplets ejected from an ink-jet print head and, ore particularly, to a control method and apparatus in which halftone levels are adjusted based upon the temperature of the print head, so that the number of dots printed per unit area is adjusted based upon the temperature to thereby compensate for thermally-induced variations in the printed image.
BACKGROUND OF THE INVENTION
Today, thermal ink-jet printers have gained wide acceptance for use in printing computer images due to their small size, portability, efficiency, and ability to produce high quality print. The print head of a typical thermal ink-jet printer has a plurality of precisely-formed nozzles, each nozzle being in fluid communication with a chamber that receives ink from an ink reservoir. Each chamber is adjacent to an electrical resistance element, known as a thermal ink-jet resistor, which is located opposite the nozzle so that ink can collect between the nozzle and the resistor. Electric printing pulses heat the thermal ink-jet resistor, causing a small portion of the ink adjacent to the resistor to become vaporized, thereby imparting mechanical energy to a quantity of the ink and propelling the ink through the nozzle of the print head toward a print medium. The ejected drops collect on the medium and form printed characters and/or images thereon. The printing is generally accomplished by incrementally moving the medium in a first direction relative to the print head and moving the print head in a second direction which is perpendicular to the first direction. A number of nozzles may be supplied across the print head so that a number of droplets may be fired from the print head at once. The spacing of the nozzles and the incremental stepping of the print head and the medium define the resolution of the printed image. A printer with 600 dots per inch resolution can print 600 dots per square inch or, in other words, each dot printed from the print head can cover approximately {fraction (1/600)}th×{fraction (1/600)}th square inch area. This small area which may be covered by one dot is generally referred to as a picture element, or “pixel”.
Halftone printing is a type of printing that can be used to print text but is particularly useful for printing graphical images, such as in desktop publishing, so that more detailed images can be created. The method takes advantage of the tendency of the human eye to blur together groups of dots and merge them into a single perceived shade. The more dots per unit area, the darker the shade will appear.
Typically, such printing entails dividing the source image into a number of pixels which corresponds to the resolution of the printer, and then assigning each pixel a halftone level, depending on the shade of gray of the particular pixel from the source image. The shade falls within a gray scale which is a progressive series of shades, ranging from black through white. The number of shades of gray which can be used to describe each pixel in the source image (and thus the number of halftone levels) depends upon the amount of memory allocated for each pixel to be printed. The more bits used in coding shades of gray, the more gradations which become possible. For example, the use of two bits per pixel allows for four halftone levels per pixel, six bits allows 64 levels, and 8 bits allows 256 levels. However, as the number of bits increases, so does the need for storage capacity. For example, with 256 shades of gray, one byte of storage is required for each pixel in the image. Accordingly, a small image 100 pixels wide by 100 pixels high requires 10,000 bytes of storage. As a result, detail and storage requirements are usually balanced to provide the best possible image at the least cost in storage capacity.
The halftone level of each pixel corresponds to a probability of printing a dot at that particular pixel. For example, in one embodiment, a halftone value of zero means that a black dot absolutely should not be printed at that pixel, a value of 255 means that a dot absolutely should be printed at that pixel, and a value of 128 means that there would be slightly more than a 50% chance ({fraction (128/255)}) that a dot would be printed at that particular pixel. Thus, for example, a source image which consists of an entire page colored a “medium” gray color would typically be mapped such that all the pixels to be printed on the printed medium are assigned a halftone level of 128, thereby creating a 50% chance at each pixel that a dot will be printed. Thus, on that page, dots will be placed at approximately 50% of the available pixels. When printed, the dots visually blur together to appear as a “medium” shade of gray.
However, the quality and consistency of the halftone images printed by ink-jet printer is affected by the temperature of the print head. As the temperature of the print head rises, the drop size produced by each nozzle increases, because the rise in temperature lowers the viscosity of the ink in the chamber, causing larger vapor bubbles to be produced, thereby ejecting more massive droplets. Accordingly, the larger droplets make bigger spots on the page upon which the printer is printing, and these larger spots, in turn, cause the image that is produced to look different than the image that is produced when the print head is operating at a lower temperature. Moreover, more massive droplets mean that more ink is being deposited on the page, increasing the chances that paper cockling and ink smearing may occur.
The temperature of the print head rises most significantly when a very dark image is being printed, because numerous dots are being ejected from the print head to produce the dark image, and numerous resistors are being heated to produce the dots, the accumulated heat of the resistors raising the temperature of the print head. Thus, as the print head finishes a line of print, or “swath”, the print head will often be at a higher temperature than when it began the swath. Also, the print head may remain at this higher temperature during the next swath to be printed. For example, a page that is intended to be printed as a uniform shade of gray may turn out with variations in the shade of gray produced at various locations on the page. If the print head prints from left to right, and from top to bottom, the right part of the page may be at a darker shade of gray than the left part of the page and, similarly, the bottom half of the page may be a darker shade of gray than the top half of the page. Moreover, if the print head temperature during actual printing is different than the print head temperature assumed in generating the halftone levels, then the shades of the printed image will not correspond well with the source image data.
Previously, the problem of poor print quality due to temperature variations has been addressed by controlling the temperature of the print head. However, because active cooling, such as through the use of thermo-electrical cooling, circulation pumps, and the like, is not cost effective, passive cooling has been the method utilized by most thermal ink-jet manufacturers. One method of passive cooling is to utilize an aluminum heat sink to absorb the heat generated by the print head. In utilizing this method, after each print line, the micro code of the printer will command the print head to wait in the margin until the print head cools to the desired temperature range through dissipation of heat to the heat sink.
However, passive cooling of the print head is not without disadvantages. The additive affect of the delay while the print head cools can become quite noticeable over time and have a substantial impact on the throughput of the printer. In addition, if the print head is not cooled to nearly the same temperature prior to each print line, the print head will produce a variable droplet size, and therefore a variable output, from swath to swath. Such swath to swath variations are extremely problematic b
Cornell Robert Wilson
Heydinger Scott Michael
Powers James H.
Barlow John
Do An H.
Harmeyer, Esq. John
Lambert D. Brent
Lexmark International Inc.
LandOfFree
Method of compensation for the effects of thermally-induced... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of compensation for the effects of thermally-induced..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of compensation for the effects of thermally-induced... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2451836