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A FOCUSING TECHNIQUE FOR RASTER INPUT SCANNERS USING SENSOR ARRAYS

IP.com Disclosure Number: IPCOM000024903D
Original Publication Date: 1982-Oct-31
Included in the Prior Art Database: 2004-Apr-04
Document File: 2 page(s) / 101K

Publishing Venue

Xerox Disclosure Journal

Abstract

A typical raster input scanner uses a CCD image sensor array and a 5 to 1OX object to image reduction lens system. The focusing of the image onto the sensor array is quite critical and is very time consuming. Typically, one looks at the output of the sensor array on an oscilloscope and tries to maximize the signal output cor-responding to a test pattern. This method is not very precise and can be time consuming. A technique is proposed here which can be both fast and precise.

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XEROX DISCLOSURE JOURNAL

A FOCUSING TECHNIQUE FOR RASTER INPUT SCANNERS USING SENSOR ARRAYS Jagdish C. Tandon

Proposed Classification
U.S. C1. 250/204 Int. CI. GOlj 1/36

FIG. 1

(//

SENSOR PIXEL CLOCK>

COMPARATORS

I

6 f5

1 SENSOR

ARRAY

-

1

  TO COUNTER

Volume 7 Number 5 September/October 1982 30 1

FIG. 2

(11) CLOCK PIXEL SENSOR 1

COUNTER INPUT (@AN0 GATE 10) 1

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A FOCUSING TECHNIQUE FOR RASTER INPUT SCANNERS USING SENSOR ARRAYS (Cont'd)

A typical raster input scanner uses a CCD image sensor array and a 5 to 1OX object to image reduction lens system. The focusing of the image onto the sensor array is quite critical and is very time consuming. Typically, one looks at the output of the sensor array on an oscilloscope and tries to maximize the signal output cor- responding to a test pattern. This method is not very precise and can be time consuming. A technique is proposed here which can be both fast and precise.

A uniform ladder chart is imaged on the sensor array. The ladder chart frequency is not critical, but it is much lower than the sensor array nyquist frequency (at least by a factor of 5 to 10). At the point of best focus, the ladder chart square wave edges as seen at the sensor output, are the sharpest and thus the number of pixels sampling the edges of the ladder chart are a minimum. This fact is made use of in this proposal.

Figure 1 shows a block diagram for implementing this proposal. The output of CCD image sensor 5 is passed through a sample and hold circuit 6 and fed into comparators 7, 8 as shown. Th...