has anybody ever measured angles before? the plate that has the most detail needs the least amount of frequency interference. what angle is that? mesh count is meaningless unless you measure the interference on your screen. then work down. yellow almost always has the most interference due to it's use as a tonal correction color.
Those photoshop numbers listed are for 300 DPI separation. When the default resolution is changed to 1200 DPI they become.
C = 54.2/108.4
M = 54.2/161.6
Y = 57.1/90
K = 60.6/45
In the photoshop screen dialog, there is an option to "accurate screens". This option is only for use with accurate screen technology. This is a feature where the RIP can dynamically modify its resolution to allow for the creation of the requested screen values.
Even for programs like Corel, when printing on a postscript printer the halftone screens will be changed to an adobe screen nearest equivalent. This is what makes entering screen values rather than accepting the values of the PPD or Adobe automatic selection gets tricky. While the numbers listed in books might make sense, it is very unlikely that those numbers will be maintained unchanged as the file is sent to a postscript printer. Postscript will always do what it is going to do because mathematics forces it to be that way.
A simple way to think of this imagine a staircase with the steps rising at a 45 degree angle. The only option to climbing the steps is to take the steps 1 at a time, 2 at a time, 3 at a time... Trying to climb the steps in 10 steps can only happen if the number of steps is divisible by 10.
That's right. I design everything at 300PPI. Should I be using a higher resolution for designing process printed artwork?
You are confusing the editing resolution and the output resolution.
Editing the file at 300 DPI is fine, but probably excessive for a 55 LPI output. If you drop the resolution down to 200 or 150, you won't be able to tell the difference in the final output. Editing at 300 isn't high enough to hurt anything, so if you think in the future you might want to go bigger, having extra DPI is a good thing.
For outputting, you need to enter the DPI value of the printer. The C8800 is 1200x600, so you would enter 600 DPI. Print using the printers highest printer resolution.
For outputting, you need to enter the DPI value of the printer. The C8800 is 1200x600, so you would enter 600 DPI. Print using the printers highest printer resolution.
fred
Where would I enter that? I don't see it anywhere in the print dialog. I'm not doing the grayscale to bitmap conversion thing mentioned earlier. I'm printing directly from Photoshop to the postscript printer. I set the output to separations, in the driver select Black/White and it prints a film for each channel with the dot attributes defined under "screen" in Photoshop.
In the print dialog, with "output" selected, click on screen, then in the "halftone screen" dialog, click auto, then in the "auto screens" dialog enter the DPI and the LPI and click ok, and ok again.
yellow and black screens never have issues. It is usually witht he magenta and cyan where problems lie with cmyk inks. Pretty much wehat all the industry articles say.
I got my angles from an article by Marc Coudray about 10 + years ago.
Most PS defaults will work well yet every now and them you need to modify. Just converting to cmyk and using the channels seems to work ok for most jobs.
tweaking color before and after (individual channels) cmyk conversion will make them that much better
In the print dialog, with "output" selected, click on screen, then in the "halftone screen" dialog, click auto, then in the "auto screens" dialog enter the DPI and the LPI and click ok, and ok again.
fred
Thanks Fred, I see it now. So "printer" should be set to 600 or whatever the printer resolution is and "screen" should be set to 55? It almost sounds like screen should be set to the LPI of your screens. IE: 305 in this case, but that sets the frequency too high. 600 and 55 seem to work, but I'm unclear as to what that 55 actually refers to.
Most PS defaults will work well yet every now and them you need to modify.
With a postscript printer, you have it backwards. Postscript will ALWAYS use a postscript screen. Trying to override the postscript defaults will still result in postscript choosing its own values.
Thanks Fred, I see it now. So "printer" should be set to 600 or whatever the printer resolution is and "screen" should be set to 55? It almost sounds like screen should be set to the LPI of your screens. IE: 305 in this case, but that sets the frequency too high. 600 and 55 seem to work, but I'm unclear as to what that 55 actually refers to.
55 refers to 55 halftone cells per inch. When a 55 line screen is applied to a 305 mesh, the result is that each halftone cell will be made of up of a mesh matrix 5.54x5.54 (305 divided by 55 x 305 divided by 55) mesh squares.
I got my angles from an article by Marc Coudray about 10 + years ago.
I got my knowledge of screening comes from the adobe developers society, which i used to be a member. I became a member prior to the Lino 300 and continued to be a member till after postscript level 2. Back in the day, being a member of the society was the only way to talk to a technical person at adobe.
Strobing is a macro version of a moire. Rather than forming a repeating pattern, strobing creates a single interference pattern across the entire image.
Using your suggested line setting, i setup this quick simulation of strobing. The first image does have yellow, it is just positioned under the black. In the second image the yellow peaks out and the third image the yellow is fully visible. These 3 variations can all result from the movement a screen less than 1/30". By less than a 1 degree of rotation, the image will demonstrate all 3 variations.
It is simply impossible to produce 15 & 75 degree angle on a raster image device with the exception of high end imagesetters which can produce "accurate screens". While it is claimed that it is indistinguishable when a yellow and a black are printed in this fashion, i question the opinion of anyone who actually believes they are using a 15 & 75 degree film. Get out your protractor and measure. 15 is 108.4 (-18.4) 15 is 161.6 (-71.6).
Another way to look at it is that raster devices are like lego blocks. No matter how you stack the blocks, only a limited number of angles will be able to be produced by stacking them. The big duplo blocks are like a low resolution output device and allows half as many potential angles to be formed as the smaller blocks would allow. Traditional halftone angles can't be created with lego blocks.
Digital imagesetting rewrote the book on halftone screening.
Re: Process screen printing - Photos 
