Gradations: Do not confuse the terms dpi and lpi

Dpi (dots per inch) show the number of distinct pixels that can be created on each linear inch of output on screen or in printing. Dpi is a measure of resolution. Lpi (lines per inch) measure frequency of screen. The normal range of half tone screens is from 75 to 200 lpi or even higher. Postscript (by Adobe) can produce 256 shades of each font color (Cyan, Magenta, Yellow, Key Color-Black). The number of shades depends on a combination of output device’s dpi and halftone screen lpi. For calculating the number of shades, we use the creating color blend on gradation (known as degrade): Total shades=(Dpi/Lpi)˛. Using this we calculate the upper limit of shades (256 for Adobe Postscript). Using a higher dpi/lpi combination, will enable you to distribute those 256 possible shades across a small percentage change, giving you much more subtle shifts. For example:
For low-quality printing:

  1. (300dpi/53lpi)˛ gives 32 shades
  2. (300dpi/90lpi)˛ gives 10 shades
  3. (300dpi/150lpi)˛ gives 5 shades

For medium-quality printing:

  1. (1200dpi/53lpi)˛ gives 512 shades
  2. (1200dpi/90lpi)˛ gives 178 shades
  3. (1200dpi/150lpi)˛ gives 65 shades

For high-quality printing:

  1. (2400dpi/53lpi)˛ gives 2051 shades
  2. (2400dpi/90lpi)˛ gives 712 shades
  3. (2400dpi/150lpi)˛ gives 257 shades.

To design a gradient follow these steps (unless the software you use creates a blend automatically).

  1. Calculate the number of shades available using the equation (dpi/lpi)˛.
  2. Measure the distance in point, between the end ponts of your blend.
  3. Calculate the size of each step in points by dividing the distance with the level of gray (shades).

Finally if the result is greater, you have banding effect or visible stairstepping on the blend. To avoid banding, you need to change one or more parameters:

  1. Decrease the distance
  2. Increase the percentage of gradation
  3. Raise the output resolution
  4. Lower the screen frequency.

P.S. These apply to laser printers and to image setters.

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