Method and apparatus for image nonlinearity compensation in scanning systems

• US 5,910,651 A
• Filed: 07/15/1997
• Issued: 06/08/1999
• Est. Priority Date: 07/15/1997
• Status: Expired due to Term

• Current Assignee: Gerber Systems Corp (Barco NV)
• Original Assignee: Gerber Systems Corp (Barco NV)
• Inventors: Ryvkin, Mark
• Primary Examiner(s): Le, Thien Minh
• Application Number:
  US08/892,821
• Time in Patent Office: 693 Days
• Field of Search: 235/462.01, 235/462.06, 235/462.07, 235/462.1, 235/462.11, 235/462.12, 235/462.15, 235/462.25, 235/462.27, 235/462.28, 235/462.29 , 235/472.01
• US Class Current: 235/462.25
• CPC Class Codes: G02B 26/10 Scanning systems
• CPC Class Codes: H04N 1/053 in main scanning direction, e.g. synchronisation of line start or picture elements in a line
• CPC Class Codes: H04N 1/0607 Scanning a concave surface, e.g. with internal drum type scanners H04N1/0657 takes precedence
• CPC Class Codes: H04N 1/0635 using oscillating or rotating mirrors
• CPC Class Codes: H04N 1/0671 with sub-scanning by translational movement of the main-scanning components
• CPC Class Codes: H04N 2201/04755 by controlling the position or movement of a scanning element or carriage, e.g. of a polygonal mirror, of a drive motor
• CPC Class Codes: H04N 2201/04774 using a reference clock or oscillator H04N2201/04772 takes precedence
• CPC Class Codes: H04N 2201/04775 using a counter H04N2201/04772 takes precedence
• CPC Class Codes: H04N 2201/04793 using stored control or compensation data, e.g. previously measured data

Abstract

A method and apparatus for use in improving the accuracy of scanning systems is disclosed and claimed. Systems such as internal drum photoplotters include a raster scanner responsive to control signals for advancing relative to a substrate an optical beam across a substrate surface in a first direction forming a scan line of pixels and for cooperatively advancing relative to the substrate the optical beam in a second direction substantially perpendicular to the first direction. The clock signal which is provided to a spinner to advance the optical beam across the scan line is independent from the pixel clock signal which is provided to control the generation of pixels. There is a memory for storing error compensation signals indicative of compensation needed to remove deviations of the separation between adjacent pixels in said scan line from preferred values thereof. The apparatus is characterized by a controller for generating control signals in dependence on the error compensation signals such that the phase of the clock signal is adjusted, thereby removing the pixel separation error for a segment of the scan line.

Claims

1. An apparatus for providing compensation for inaccuracies in a scanning optical system that has a platen for receiving a substrate having a surface and a raster scanning means responsive to control signals, having a mirror for advancing relative to said substrate an optical beam across said substrate surface in a first direction forming a scan line that includes a linear array of pixels formed by optically modulating the optical beam, said apparatus comprising:

a signal generator means for generating a master clock signal;

a pixel clock signal generator for receiving said master clock signal and for providing a pixel clock signal in response to pixel control signals for governing the separation between adjacent pixels in said scan line;

a mirror clock signal generator for receiving said master clock signal independently of said pixel clock signal generator, and for providing a mirror clock signal in response to mirror control signals for governing the rotational speed of the mirror, thereby determining the advancement of said optical beam in said first direction;

a memory means for storing error compensation signals indicative of compensation needed to remove deviations of the separation between adjacent pixels in said scan line from preferred values thereof; and

a controller for generating said pixel and mirror control signals in dependence on said error compensation signals such that the phase of said pixel clock signal is adjusted relative to the phase of said mirror clock signal, thereby removing said deviations for a segment of said scan line.

2. The apparatus of claim 1 wherein said platen comprises an internal drum of a raster photoplotter.

3. The apparatus of claim 2 further comprising a means for generating said optical beam.

4. The apparatus of claim 2 further comprising a means for cooperatively advancing relative to said substrate said optical beam in a second direction substantially perpendicular to said first direction and a linear encoder means for generating signals indicative of a current position of said raster scanning means along a drum longitudinal axis.

5. The apparatus of claim 2 further comprising a rotary encoder means for generating signals indicative of a current position of said optical beam along said scan line.

6. The apparatus of claim 2 wherein said raster scanning means comprises a mirror configured to move substantially along a internal drum longitudinal axis for providing said optical beam to said substrate within an included angle, thereby exposing a scan line.

7. The apparatus of claim 6 wherein said mirror is a 45°

turning mirror.

8. The apparatus of claim 6 wherein said mirror has a curved reflective surface that comprises a portion of an off axis parabola.

9. The apparatus of claim 1 wherein said controller further comprises an oscillator and wherein said pixel clock signal generator further comprises a N prescaler/divider (Npd) means that directly receives an oscillator signal and outputs a pulsed pixel clock signal and an integer compensation signal generator (Ncomp) which provides a hold signal to said N prescaler/divider means to skip the next pulse in said pulsed pixel clock signal and wherein said mirror clock signal generator further directly receives said oscillator signal and comprises a direct digital synthesizer for presenting a digital clock signal to control the rotation of said mirror.

10. The apparatus of claim 9 wherein said pixel clock signal generator further comprises a multiplier means for receiving said oscillator signals and generating therefrom a signal whose frequency is an integer multiple of the frequency of said oscillator signal.

11. The apparatus of claim 9 wherein said pixel clock signal generator further comprises a divider means for receiving said pulsed pixel clock signal and generates therefrom a pixel clock signal whose frequency is an integer fraction of the frequency of said pulsed pixel clock signal.

12. The apparatus of claim 9 wherein said memory means further comprises an SRAM.

20. The apparatus of claim 1 wherein said platen comprises a planar surface of a raster photoplotter.

21. The apparatus of claim 1 wherein said scanning optical system further comprises a means for receiving said optical beam from said scan line segment and for generating therefrom electrical signal equivalents indicative of intensity variations of said optical beam.

22. The apparatus of claim 21 further comprising a means for illuminating said scan line segment, thereby generating said optical beam.

23. The apparatus of claim 5 wherein said rotary encoder provides an index signal that corresponds to the ideal beginning of a scan line and where said apparatus further comprises a means for delaying the start of the current scan line and adjusts said scan line start relative to a previous scan line start to be equal as compared to an adjacent substrate edge.

24. The apparatus of claim 23 wherein the actual start of a scan line is delayed by approximately 1.0" to evenly position the beginning of each scan line in the slow scan direction.

13. An optical photoplotter comprising:

an optical source for generating an exposure beam;

a platen for receiving a substrate;

a raster scanning means responsive to control signals, having a mirror for advancing relative to said substrate an optical beam across a substrate surface in a first direction forming a scan line that includes a linear array of pixels and for cooperatively advancing relative to said substrate surface said optical beam in a second direction substantially perpendicular to said first direction, said raster scanning means also including a means for providing optical modulation to said exposure beam, thereby forming said pixels;

a signal generator means for generating a master clock signal;

a pixel clock signal generator for receiving said master clock signal and for providing a pixel clock signal in response to pixel control signals for governing the separation between adjacent pixels in said scan line;

a mirror clock signal generator for receiving said master clock signal independently of said pixel clock signal generator, and for providing a mirror clock signal in response to mirror control signals for governing the rotational speed of the mirror, thereby determining the advancement of said optical beam in said first direction;

a memory means for storing error compensation signals indicative of compensation needed to remove deviations of the separation between adjacent pixels in said scan line from preferred values thereof; and

a controller for generating said pixel and mirror control signals in dependence on said error signals such that the phase of said pixel clock signal is adjusted relative to the phase of said mirror clock signal, thereby removing said deviations for a segment of said scan line.

14. The optical photoplotter of claim 13 further comprising a linear encoder means for generating signals indicative of a current position of said raster scanning means along a longitudinal axis.

15. The optical photoplotter of claim 13 further comprising a rotary encoder means for generating signals indicative of a current position of said optical beam along said scan line.

16. The optical photoplotter of claim 13 wherein platen surface is curved and said raster scanning means comprises a mirror configured to move substantially along a longitudinal axis for providing said optical beam to said substrate within an included angle, thereby exposing a scan line.

17. The optical photoplotter of claim 16 wherein said mirror is a 45°

turning mirror.

18. The optical photoplotter of claim 16 wherein said mirror has a curved reflective surface that comprises a portion of an off axis parabola.

19. A method for providing compensation for inaccuracies in a scanning optical system that has a platen for receiving a substrate having a surface and a raster scanning means responsive to control signals, having a mirror for advancing relative to said substrate an optical beam across said substrate surface in a first direction forming a scan line that includes a linear array of pixels formed by optically modulating the optical beam, said method comprising the steps of:

generating a master clock signal;

providing a pixel clock signal in response to pixel control signals for governing the separation between adjacent pixels in said scan line and said master clock signal;

providing a mirror clock signal in response to mirror control signals for governing the rotational speed of the mirror, and said master clock signal received independently of said pixel clock signal generator, thereby determining the advancement of said optical beam in said first direction;

storing error compensation signals indicative of compensation needed to remove deviations of the separation between adjacent pixels in said scan line from preferred values thereof; and

generating said pixel and mirror control signals in dependence on said error compensation signals such that the phase of said pixel clock signal is adjusted relative to the phase of said mirror clock signal, thereby removing said deviations for a segment of said scan line.