Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1997-12-23
2001-07-24
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S237000, C347S130000
Reexamination Certificate
active
06266077
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of compensating for uneven output in a printing head employing an array of printing elements, and to a printing head in which this method is employed.
One type of printing head having an array of printing elements is the LED printing head used in some electrophotographic printers. The printing elements are light-emitting diodes (LEDs) that illuminate a photosensitive drum to create an electrostatic latent image, which is developed by application of toner particles, transferred to a printing medium, and fused onto the printing medium. An LED printing head has the advantage of providing high throughput, because the drum can be illuminated by all light-emitting diodes (LEDs) in the array simultaneously, forming an entire line of latent image dots at once.
An attendant disadvantage, however, is that the optical output of the array is, in general, not uniform. Without compensation, different LEDs would produce different amounts of light, leading to printing irregularities. The usual solution to this problem is to measure the optical output of each LED when the array is manufactured, and compensate for the differences by varying the driving energy supplied to each LED during printing operations.
One conventional LED printing head, for example, has a control unit storing four bits of compensation data for each LED. When driven, each LED receives an amount of driving energy determined by the compensation data. This arrangement enables the optical output of each LED to be adjusted in sixteen steps.
A problem is that some LEDs may require more compensation than four bits of compensation data can provide. This problem could be solved by adding more bits of compensation data, but that would require expensive alterations to the LED driving electronics in the printing head. In practice, an LED array having an LED that is outside the four-bit compensation range is discarded. This practice lowers the yield of the LED array manufacturing process, consequently increasing the LED array cost and the cost of the printer.
A more detailed description of this problem will be given below. Similar problems can occur in thermal printing heads and ink-Jet printing heads, in which the printing elements are heating elements or piezoelectric elements.
SUMMARY OF THE INVENTION
An object of the present invention is to compensate for nonuniform output in a printing head having an array of printing elements, using a limited number of bits of compensation data, without impairing manufacturing yields.
A further object is to improve the appearance of pages printed with a printing head having an array of printing elements.
The invented compensation method comprises the steps of:
measuring the output of each printing element in the array;
from the results, calculating a target value for each printing element, the target values varying gradually across the array;
determining compensation data for each printing element according to the difference between the output of the printing element and the target value of the printing element;
storing the compensation data in the printing head; and
driving each printing element with an energy corresponding to the compensation data.
Each target value can be calculated by, for example, linear interpolation or moving averages.
The invented printing head has an array of printing elements and a non-volatile memory storing compensation data used in driving the printing elements. The compensation data cause the outputs of the printing elements to vary across the array, output variations between adjacent printing elements not exceeding a certain limit, but this limit does not apply to variations between printing elements that are not adjacent.
Given a limited number of bits of compensation data, allowing the target values to vary Increases manufacturing yields, by increasing the probability that the output of every printing element in the array can be successfully adjusted to the target value.
For a given manufacturing yield, the invented method allows the size of the compensation steps to be reduced, thereby reducing output variations between adjacent printing elements and improving the appearance of the print
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Le N.
Nguyen Lamson D.
Oki Data Corporation
Rabin & Champagne, P.
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