5 - Printing in Color (and B&W)

1. 5 - Printing in Color (and B&W)

2. How does one go about laying down the ink in such a way as to render a picture that is as close as possible to what is intended?

3. Black &White Continuous Tone– in principle, one could lay down a layer of “gray” reflectance by simply altering the opacity of black ink placed on a white surface. But suppose you wanted to make 256 shades of “gray”

4. Photography negative positive

5. A photographic print utilizes an approximation to continuous tone - but it is actually more “granular” A positive print of a subject, scanned at 300 dots/inch. This image exhibits a range of tone and seems “continuous” at first.

6. A portion of the negative transparency scanned at 2400 dots/inch. A closeup of the fingertip. The “texture” above the finger is due partly to skin texture. The fingernail shows the actual silver grains.

7. Halftones • A series of ruled screens are introduced to make evenly-spaced “holes” through which the light can pass when printing the negative. The “locations” of the dots fixed by the screen, their “size” is still determined by the bright and dark regions of the negative.

8. Reproductions

9. Lithography • Oil & water do not “mix”. Lithography uses this fact: 1. Create the desired pattern in oily substance on stone 2. Cover the stone with water (mixed with gum arabic) which fills in the stone surface not covered by the oil 3. Roll an oil-based ink on the stone which adheres to the oily areas and is repelled by the watery ones. 4. Press the stone to paper, transferring the ink to the paper. Modern photographic lithography, stone is replaced by metal plate with a photographic emulsion. The action of light on the emulsion alters its oil-grabbing capabilities

10. Offset Printing • Rubber roller is used as an intermediary between the plate and the paper – save swear & tear on the plate, & pliability of the rubber roller makes contact with the paper more even.

11. Letterpress - Here the ink is placed on raised letters, and then pressed onto the paper. Manual typewriters (if you have ever seen one!) simply strike the raised letter through an inked ribbon onto the paper. Gravure– uses the opposite technique as letterpress. Here ink sits in etched valleys on the plate. The earliest photographic prints from negatives used a photogravure process. Here, light striking a photoresistive surface would allow chemical processes to occur which would resist etching by an acid. After etching away the unprotected zones, inked-in valleys would produce a print. A similar process is used in silkscreening.

12. Xerographic Photocopiers

13. Color Printing • In color printing: • Get image of “scene in 3 colors • Print each color as “registered” halftone (but may offset by a fraction of a grid spacing) • Rotate halftone screen between colors

14. Three-Color Printing:CMY (Cyan, Magenta, Yellow) • BUT: hard to get a good Black from just these three • Four-Color Printing:CMYK (Cyan, Magenta,Yellow, and black or Key) Still, limited range of colors. • For more: 5, 6 color printing

16. High magnification Same region scanned at 9600 dots/inch

17. Modern Computer Printers • Inkjet Printers • Modern inkjet printers: • squirt B and/or CMY • Ink onto paper • Laserjet Printers • Similar to xerographic printing - lasers are used as powerful and controllable light sources to produce the image of electric charge.