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Why Color Looks so Good – FM Screens Explained

FM, AM, XM, Hybrid, Staccatto, Segundo, stochastic, conventional, 200 lpi, 175 lpi, 20 micron, 10 micron, first order, second order…What does this all mean?

NOTE: Screening is the process after rasterizing PDF files in the RIP (raster image processor) during prepress. Halftone screening is done through software and creates very small dots, or cells, that are imaged onto a printing plate. The tiny dots create the illusion of continuous tone photographs when printed on press.

FM Screening comparison to AM screening - stochastic versus conventional screening
Top: 200 lpi conventional (AM) halftone screen magnified 200x.
Bottom: 20 micron (FM) stochastic screen magnified 200x.
Image: Gordon Pritchard

Advantages to printing with FM screening versus AM screening:

  • Continuous tone photograph reproduction
  • Produces a larger CMYK color gamut on press
  • Renders greater detail
  • Eliminates moire patterns
  • Reduces ink consumption by as much as 10% – notice the ‘pooling’ of ink in conventional dot
  • Produces smoother gradients
  • More consistency in color throughout pressrun
  • Faster ink drying

Conventional screening (150 lpi, 175 lpi, 200 lpi) refers to AM screens, or amplitude modulation. This refers to halftone dots that are fixed on a grid, angled in 30 degree increments (except yellow: 15 degrees) and grow in size based on tonal value.

LPI = lines per inch

Stochastic screening (Staccato) refers to FM screens, or frequency modulation. This refers to micro-dots (20 micron, 10 micron) that are FIXED in size and tone values increase by adding more dots. The dots are rendered in a psuedo-random algorithm making them ideal for high definition details in photography and artwork. The micro-dots are rendered in a ‘weave’ to create very smooth tonal transitions.

Micron =1/1,000,000 of a meter

It’s important to note that FM screens produce a larger CMYK color gamut than AM screens. This occurs because light reflecting off the paper is filtered more efficiently, resulting in less ‘whiteness’ from the paper reflecting into the eye.

Also, reprints are less likely because of the stability in controlling color on press. FM screens are much less likely to be impacted by ink density variations on press. The ink film on press is much thinner and less likely to be affected. Notice in the enlargement photo above the ‘pooling’ of ink in the conventional halftone dots. This causes the press to use more ink than is necessary. utilizes 20 micron Staccato screening for all color printing.

Have you ever seen printing with FM/stochastic screening? Please leave your comments below…


3 replies on “Why Color Looks so Good – FM Screens Explained”

Just a small clarification to your statement that FM screens produce a wider color gamut than AM screens because light reflecting off the paper is filtered more efficiently, resulting in less ‘whiteness’ from the paper reflecting into the eye.

We see color in print because the ink filters the light that passes through it and reflects off the paper below the ink. With FM screens there is less unprinted area around the individual halftone dots for a given tone. Hence there is less light being reflected off of unprinted paper. It is that unfiltered light between the dots that contaminates the color that you see with an AM screen reducing its gamut. You can see that difference in ink/paper area in the graphic in your post.

So, to put it more accurately – FM doesn’t produce a larger gamut. Instead, AM screens reduce your potential gamut on press. Theoretically, if you had continuous tone lithography where the paper is completely covered in ink you would achieve the maximum gamut that your inks are capable of.

All halftone screening reduces gamut. It’s just that different screen patterns reduce it by different amounts.

BTW, this is also one of the reasons that the near continuous inkjet printers have a larger gamut than an offset. press.

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