Friday, April 24, 2009

Rosettes – everything you didn't realize you needed to know - part 2 of 2

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4 comments:

  1. By far and without question, the best explanation of screening I have ever read. Brilliant. Thank you.

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  2. Just as the Title promised,

    "Rosettes – everything you didn't realize you needed to know"

    Gordo never fails to deliver! Even for folks like me who have been around the block a time or two!

    Keep up the Great work!
    Otherthougts

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  3. i do believe the "reduced in size for clarity" picture could also be an example of rational screening where that change in rosettes exists just too big to fit in the plate area so you don't see any pattern while irrational aims to not to have a pattern indefinitely

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  4. To Anonymous:

    Correct, that graphic could be an example of Rational Screening since that type of screening cannot align dots at the required irrational angles (angles other than 45 and 90 degrees) nor can it deliver the required screen frequency (lpi).
    "IS" or "irrational tangent screening" as well as "Supercell" screening (which has largely replaced IS screening in vendor offerings - despite marketing claims) can more accurately place dots at the required irrational angles and correct frequencies.
    However, there is still a degree of angle and frequency error - the effect of which grows with distance.

    Some examples - for 2540 dpi imaging a 133 lpi screen the required angles and frequencies are:
    C - 15º, 133 lpi
    M - 75º, 133 lpi
    Y - 0º, 133 lpi
    K - 45º, 133 lpi

    RT screening would deliver:
    C - 18.435º, 133.871 lpi
    M - 71.565º, 133.871 lpi
    Y - 0º, 127 lpi
    K - 45º, 119.737 lpi

    IS/Supercell screening (depending on vendor implementation) would deliver:
    C - 15.0037º, 138.142 lpi
    M - 74.9963º, 138.142 lpi
    Y - 0º, 138.545 lpi
    K - 45º, 138.158 lpi

    The errors are small, and for most printing plate sizes should not cause significant color issues (although they do for projects like car brochures that have large expanses of 4/C neutral grey). However, with very large plate sizes the small errors can cause visible color and tone shifts. A well designed AM/XM screen will hold an open rosette across the largest diagonal length of plate size that the workflow/CtP device will be expected to image.

    best, gordo

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