Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2000-09-25
2002-07-16
Hilten, John S. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06419342
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a module for monitoring the print quality of a dot printer, which monitor is to be fixed to a carriage for moving a printing device of the printer, said module comprising:
an illumination system for supplying a monitoring radiation beam to illuminate an area of a printing medium;
an imaging system for collecting monitoring radiation reflected by the illuminated area and forming an image in an image plane, and
a radiation-sensitive detection system for converting said image into electric signals representing quality parameters of the print.
The invention also relates to a dot printer provided with such a monitoring module.
A dot printer is to be understood to be any kind of apparatus for writing an image on a recording medium whereby the medium produces marks, in the form of dots when exposed, for example, to energies above a threshold level. A number of such dots jointly constitutes a character, a sign, a graphic representation, etc. Well-known dot printers are laser printers, inkjet printers and thermal printers.
U.S. Pat. No. 5,633,672 discloses such an apparatus which comprises a communication channel for carrying an information signal representing the image to be written, a writing assembly, or writing device, coupled to said channel and modulated by the information signal for making marks, representing the image, on a printing medium in response to the signal, a device for producing a relative movement in one direction between the printing medium and said writing assembly to establish a pattern of marks in the direction of the relative movement.
The apparatus described in U.S. Pat. No. 5,633,672 further comprises a calibration instrument that reads the pattern of marks and adjusts the parameters of the apparatus in real time to calibrate the writing device and, when necessary, modify the characteristics of the marks and their pattern. The calibration instrument can be fixed to the drive, or carriage, for the writing device so that it is moved simultaneously with this device. By using this built-in calibration instrument, it is no longer necessary to remove printed images from the apparatus and analyze them off-line for the characterization of the marks and their pattern, which is a cumbersome and time-consuming process. The controlled writing parameters are the size, the relative position and the density of the dots. In the printer described in U.S. Pat. No. 5,633,672, the calibration instrument comprises an electronic (CCD) camera having an optical axis, and the marks are illuminated co-axially along the same axis. The camera receives. a two- dimensional image of the recorded dots and their pattern, for example, if the recording medium carrier is a drum at each rotation of this drum, so that all dots of a print line, or of a number of print lines if the writing device is a multi-channel device, are taken in simultaneously. The camera reads the spatial and density information from the pattern.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a monitoring module of the kind described in the preamble, which module allows measurements on a dot-by-dot basis and, moreover, has a simple and compact construction. The module is characterized in that the detection system comprises at least a linear array of discrete detector elements, and in that means are provided for imaging, in the plane of the detector elements, both a medium area of a first kind, comprising a number of printed dots, and a medium area of a second kind, comprising one printed dot.
By means of this module it is not only possible to image a portion of a print line on the detection system and observe it, to monitor the density or gray scale of the print line portion and to detect the edges of the recording medium, or print paper, but it is also possible to image an individual dot on the detection system. The detection system has detector elements which are so small relative to the image of a dot that such an image covers several detector elements, or pixels, so that characteristics of an individual dot, like its size, can be measured.
The monitoring and calibration can be carried out during a test before recording the desired image, or concurrently with image recording.
It is to be noted that EP Patent 0.186.651 discloses a thermal printer wherein individual dot areas are monitored, however during dot formation and not after the dot formation has been completed. The printer described in EP Patent 0.186.651 is used to carry out a special method of thermal printing. First, initial energy is applied to selected areas of the recording medium to form, in each area, a dot having an initial size which is smaller than necessary to achieve the desired density in the selected area. This area is monitored against a reflective background, in the form of a reflective layer on the rear side of the transparent record medium, by illuminating the area and capturing radiation reflected by the area by a single detector element. This element supplies a signal which is indicative of the density of the area. This signal is compared with a desired image signal and the result of the comparison is used to regulate further supply of thermal energy to progressively increase the dot size in the area until a predetermined value of density is achieved. In the thermal printer described in EP Patent 0.186.651, a dot is not monitored by a number of detector elements and no image of a portion of a print line is formed on the detection system.
A first-embodiment of the module is characterized in that said means for imaging comprises an actuator for positioning the imaging system in either one of two fixed positions along the axis of said imaging system.
In this embodiment, the same lens element(s) of the imaging system is (are) used to image both an individual dot and a print line portion on the detection system. When the imaging system occupies a first one of said positions, a dot is sharply imaged on the detection system and when the imaging system occupies the second position, a portion of the print line is imaged on the detection system. The detection system then receives either an image of an individual dot or an image of the print line portion.
It is also possible to image the individual dot and the print line portion simultaneously on the same detection system. This can be realized with an embodiment of the module which is characterized in that said means for imaging comprises a first and a second sub-lens system of the imaging system, the first sub-lens system having such a power and position that it image a medium area of the first kind on a first region of the detection system, and the second sub-lens system having such a power and position that it image a medium area of the second kind on a second region of the detection system.
Each sub-lens system can now be optimally adapted to the object it has to image and no movement of the imaging system is required. The images of the individual dot and of the print line portion can then be evaluated simultaneously.
Especially for monitoring an individual dot, it is essential that the recording medium, or print paper, remains in the focal plane of the imaging system or of the sub-lens system which images the dot. In practice, it has been found that, during the printing procedure, the medium, at the position of the module, may wobble over a range of +0.5 mm to −0.5 mm relative to the nominal plane of said medium. A lens system with which a dot can be measured at an accuracy of +10 &mgr;m and fits in the small module, i.e. having a sufficiently small object-to-image distance, has a depth of focus which is smaller than said wobble range. An embodiment of the module which provides a solution to the important problem of the wobbling medium is characterized in that it is provided with an auto-focus system comprising a focus error detection system coupled to a focus correcting actuator.
The auto-focus system ensures that optics the medium is in focus in the field of view of the imaging.
The embodimen
Bronswijk Tako K. A. M.
Lindenhovius Karianne H.
Timmers Wilhelmus A. G.
Hilten John S.
Koninklijke Philips Electronics , N.V.
Stewart Jr. Charles W.
Waxler Aaron
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