Screen Angles: Why are they 15, 45, 75 and 90?

[pcmodem]

Since there can be a moire between angles 75 and 90. Would it make more sense to have the 4 colors evenly divided among the 90 degrees. ie 22.5, 45, 67.5, 90?

 

[maynardsayswhat]

Quality in Print did a better job that I would at the explanation. Clicky: Quality In Print: Why use halftone screen angles?

 

[C Schmidt]

Since there can be a moire between angles 75 and 90. Would it make more sense to have the 4 colors evenly divided among the 90 degrees. ie 22.5, 45, 67.5, 90?

My understanding is that it's a compromise on the yellow to make the angles between the other inks bigger because yellow would be the least noticeable moiré. Although in your case the cyan and black are only 15 degrees off too, so I don't know. That's just what I remember hearing in school; someone with more experience actually running or managing a press could probably give you a more definite answer.

 

[gordo]

The link that maynardsayswhat provided goes into more depth with illustrations: Quality In Print: Why use halftone screen angles?

But basically...

Black is angled at 45° because that angle results in the tone are the dots represent being visually the smoothest. This is because of a phenomenon of the human visual system known as directional visual integration of tone - explained somewhat in this thread: http://printplanet.com/forums/prepress-workflow-discussion/23033-black-screen-angles

Yellow is placed at 0°/90° because it is the lightest color and hence will not show the lack of smoothness caused by the directional visual integration of tone (see black angle explanation above).

When two or more screens overlap they create an interference pattern called a moiré.

The moiré of black and yellow is not usually visible because the yellow is so light relative to the black. Also the yellow is typically run at a higher frequency/lpi (usually 108% of the black).

So the cyan and magenta angles must fit between 0° and 45°.

Because halftone screens are quadratic grids, positioning them at 30° offsets relative to the black results in the least visible and most stable moiré. That type of moiré is called a rosette. A 30° offset from the K 45° gives you the 75° for magenta angle. A 30° offset from magenta gives you the 15° for cyan angle.

Because cyan ends up only 15° from yellow the moiré is visible. However because the yellow is quite pale relative to the cyan and run at a higher lpi the moiré is not normally visible.

Some screening systems will rotate the entire grid by 7.5° so each screen angle is off from the 0°/15°/75°/45° standards by 7.5°. This is done to introduce a small degree of "noise" in the screen in order to avoid single channel moiré. I.e. A moiré within one screen caused by a repeating pattern of pixels on the edge of individual dots.

As long as the screen angle of magenta, cyan, and black are offset by 30° then they can be interchanged. I.e. Black can use the magenta angle if magenta uses the black. Usually the angle interchange is made when there is a high degree of the likely hood of visible moiré occurring because of a clash with yellow. So, for skin tones magenta may be exchanged with black. Where pastel greens predominate, magenta may be exchanged with cyan.

The following screen angle sets are all valid and are in common use. The sequence for the screen sets listed below is C, M, Y, K (i.e. the first screen set on the list is: 15C, 45M, 0Y, 75K). Remember that screen angles have quadratic symmetry so 0 degrees is the same as 90, 180, and 270 degrees.

Standard 4/C U.S. screen angle set:
15, 75, 0, 45 (possible moiré in greens since C and Y are only 15º apart)

Standard 4/C European screen angle set:
15, 45, 0, 75 (possible moiré in greens since C and Y are only 15º apart)

Other usable screen angle sets: Keep in mind that when two colors are less than 30º apart there is a risk of visible moiré
15, 45, 0, 75
15, 75, 0, 45
15, 45, 30, 45
45, 15, 0, 75
45, 75, 0, 15
75, 15, 0, 45
75, 45, 0, 15
75, 15, 60, 45

best, gordon p

 

[oxburger]

Dang Gordo, that made my brain hurt. You should have went with my answer "because I said so" It always worked on me growing up.

 

[gordo]

Dang Gordo, that made my brain hurt. You should have went with my answer "because I said so" It always worked on me growing up.

LOL - you're right!

best gordon p

 

[DavidMa]

Some screening systems will rotate the entire grid by 7.5° so each screen angle is off from the 0°/15°/75°/45° standards by 7.5°.

We use this here. It was called "shifted" angles back in the old Scitex days. Still works great.

 

[gordo]

We use this here. It was called "shifted" angles back in the old Scitex days. Still works great.

Yes, it works well for its purpose - to introduce a small degree of "noise" in the screen in order to avoid single channel moiré. Today, other vendors will use other techniques to achieve the same result without the 7.5° shift.

best, gordon p

 

[evegtca]

Yes, it works well for its purpose - to introduce a small degree of "noise" in the screen in order to avoid single channel moiré. Today, other vendors will use other techniques to achieve the same result without the 7.5° shift.

best, gordon p

To add to this, the whole set is sometimes shifted by 7.5° or 15° to avoid moiré caused by the anilox rollers in flexo printing.

Cheers,
Ernst Vegt

 

[David Ingram]

You can also Eliminate the Moire Completley

You can also Eliminate the Moire Completley

Had a Cust that could not STAND to see Moire EVEN on the light table stripping up negs
SO we had to run their yellow plates at 105 degrees relative to the rest. Making 30 degrees of separation all around 15, 45, 75, and 105 vs 90. Ink on paper you could not see it, but they insisted anyway- And Gordo beat me to the brilliantly comprehensive answer. This is a damn good forums.

 

[evegtca]

Had a Cust that could not STAND to see Moire EVEN on the light table stripping up negs
SO we had to run their yellow plates at 105 degrees relative to the rest. Making 30 degrees of separation all around 15, 45, 75, and 105 vs 90. Ink on paper you could not see it, but they insisted anyway- And Gordo beat me to the brilliantly comprehensive answer. This is a damn good forums.

Sorry, David, 105 degrees IS the same as 15 degrees when measuring the angles through the corners of the halftone dot, it's now just through the other two corners. Unfortunately, there's just no getting away from placing the yellow halftone dot at 15 degrees between two other colors. Usually, we can stand a bit of moiré in greens such as busy grass and forest scenes and such if we have a bit of slur on the press, for example. Where we especially don't want them is in smooth skin-tones in fashion work (unless it's the cover of the National Inquirer where everyone is made to look bad, every pore sharpened, etc.)

Best regards,
Ernst Vegt

 

[William Campbell]

30 degrees of separation all around 15, 45, 75, and 105 vs 90.

Screens extend four directions within the total 360 degrees; The "line" of dots repeats every 90 degrees ( 360/4 directions = 90 ). Putting a screen at 105 degrees is no different than 15 degrees (typical C angle). Even so, this practice of putting C and Y at the same angle is not unusual. I've done it for loose presses having trouble with skin tones. Swapping K and M is also not uncommon.