1. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Printing

2. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Origin of Printing n Invented by Chinese c.1000 AD –Entire page printed at once wood cuts metal type n Reinvented in Europe, Johannes Gutenburg –Entire page printed at once movable type, could ‘reset’ the plate

3. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Traditional Printing Processes n Intaglio n Letter press / Relief n Silk screen / Porous / Stencil process n Lithography / Planographic

4. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Lithography/Planographic Process n Image and non-image areas are on the same plane. n Difference: Receptivity of ink –Image areas accept ink –Non-image areas repel ink n The plate image is the same as the print. Lithography Plate repels ink Absorbs ink for copying

5. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Intaglio Printing n The image is etched or depressed into the plate as small wells, which contain ink. n Non-image areas are the surface of the plate. n The plate image is the reverse of the final print. plate final print

6. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Letter/Relief Press n The image area is raised and inked. n The non-image area is depressed. n The plate image is the reverse of the final print. plate final print

7. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Silk Screen/Porous/Stencil Process n Image areas are porous allowing ink to pass through. n Non-image areas are non porous. n Metal, nylon, silk screen, or fibrous material is used. n The plate image is the same as the print. Screen Ink goes through Ink does not go through T-Shirt Ink

8. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Binary Printing n The traditional printing processes described maintain only two tones: color no color ink no ink image no image All printing methods from this point on, unless otherwise noted, are binary.

9. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Binary Printing n How do printers produces gray tones between the traditional ink and no-ink areas without many gray inks? n Challenge: Find a way to print continuous tone images:

10. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science n Make a grayscale … with discrete levels of gray. The Halftone Idea

11. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science The Halftone Idea n Early artists used the Halftone Idea and alternatives to make gray with dots or lines: –Stippling, lithography, mezzotint, etc. A work by Albrecht Dürer employs both lines and dots.

12. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science The Halftone Idea n Levels of Gray –Decrease the amount of black concentrated in an area to make it look lighter: spaced lines and dots –Increase the area covered in black to darken it: condensed lines and dots

13. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science The Halftone Idea n The spacing and size of markings used to simulate gray depend on: –Human Visual System: Don’t want people to distinguish lines / spots Roughly, people see up to 60 lines per inch (lpi) at 2 ft. –Viewing Distance Zoom farther out and the dots appear smaller.

14. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science The Halftone Idea n Two Types of Halftone Spots: –AM: Amplitude (or Area) Modulated spots have a fixed screen ruling and vary in size –FM: Frequency Modulated spots have a fixed size and vary in line spacing.

15. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Impact Printers n Impact Printing –A needle/head physically hits an ink ribbon onto paper leaving an impression behind. n Typewriter, 1874 –QWERTY keyboard becomes standard –Text only; No halftones –1 color ribbon n Dot Matrix Printers –Set of 9 or 24 pins form spots –Text and low quality imagery; FM halftones –1 or 4 color ribbons

16. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science The Halftone Idea Conventional/ AM/Clustered Stochastic/ FM/Dispersed n Structures of Digital Halftone Cells

17. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Spatial Resolution n dpi = dots per inch –Each dot that makes up a halftone spot –Associated with optical resolution of scanners and addressability of digital printers n lpi = lines per inch –Each spot or entire halftone cell –Usually associated with printer capabilities 1 spot of ink = 1 cell 1 inch 14 dpi2 lpi

18. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science n Digital Halftone Cells –Based on a grid of spots (halftone cell): –Change the number of dots in a spot to make different tones of gray Tone Resolution A 2x2 matrix can produce 5 tones. A 3x3 matrix can produce 10 tones. A NxN matrix can produce N 2 + 1 tones

19. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Lots of Dots n People cannot see dots greater than 133 lpi at a reasonable reading distance. n Publications set in 133-150 lpi –magazines –books

20. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Lots of Dots n People can see dots less than 133 lpi at a reasonable reading distance. n Publications set in 65-100 lpi –newspapers

21. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Summary n The Halftone Idea –The creation of gray levels utilizing dot formations of color on a white background. –Dot size and spacing are varied to create different levels of gray.