Grey Levels

[aqazi81]

Hi All,

New to prepress, want to know the optimum grey levels that we should set in rip according to our CTP DPI and job LPI, Currently, we are ripping files at 4800 DPI and Same DPI is set in our Flexo CTP. Grey levels are set at 4096.

 

[chriscozi]

Everything downstream of the original file (ripping) is predicated on the final printed dot size and shape.
Please provide your press maximum dpi and shape.
This will define the required rip dpi and THAT will ~mostly define the required grey levels.
Too low of a grey level produces banding in a high dpi environment.
Too high of a grey level wastes processing time for no real image improvement in ~most circumstances.
YMMV

ps - there are many others here who can be very specific with answers if they have the condition requirements.

 

[SteveSuffRIT]

With traditional AM dot (Amplitude Modulate, round cluster), #GL = (spi CtP / LPI screening)^2.
ie: 4800 spi / 133 lpi (flexo) = 36^2 = 1,296 gray levels
Formula does not apply for FM stochastic.

 

[gordo]

With traditional AM dot (Amplitude Modulate, round cluster), #GL = (spi CtP / LPI screening)^2.
ie: 4800 spi / 133 lpi (flexo) = 36^2 = 1,296 gray levels
Formula does not apply for FM stochastic.

That formula "(dpi/lpi) squared + 1 = number of grey levels" is only true for the tone represented by a single, isolated, halftone dot based on an individual halftone dot cell - something that never occurs in real production environments.
However, modern halftonenscreens (since about 1989) take a different approach. The approach is based on the fact that we don't care about individual halftone dots. What is important is the tone represented in an area. For example, let's say that we want to see a 17% tone patch value in the presswork. However, if we cannot represent that area with individual 17% dots – because of that classic formula limitation – we can still create the 17% value by alternating 16% dots and 18% dots (this is called "dithering"). The eye (and instruments) integrate the alternating 16% and 18% dots and the result is the average value - in this example 17% – our desired tone value.
So, unless there is a specific reason to increase your dpi, other than for grey levels, there is no need to RIP at 4800 dpi.

 

[maxon]

Aqazi, the number of available grey levels depends on the rip software your customer uses. You didn't mention what software it is. All modern 'super cell' screening solutions based on Adobe library or harlequin or else do minute changes to dot shape as gordo described already, we perceive this as either dithering or pseudo-randomizing but in fact it's the supercell processing algorithm you can learn about from the rip documentation or internet. Some rip products allow the operator to choose the number of desired grey levels. The 4096 setting is generally adequate for offset and flexo. Selecting a higher number of grey levels does not necessarily increase the output image quality on plate or final print, it just takes more time to process the rip separations. It's safe to use a value above 4096 if the rip platform is higher spec and no performance penalty.

 

[aqazi81]

Thank you all for your valuable input, we are working as a plate bureau house and we do not have our own printing facility.
We are using Eagle Rip and it is capable of ripping a job at 9600 dpi.

 

[Macmann]

Yay Gordos back ;-) I'm not in the Flexo universe but 4800 or 4096 seems high to me. Offset usually is ripped at 2000, 2400, or 2540 depending on the RIP manufacturer. The higher settings add considerable processing time for little value. That being said there is always a trade-off. We do some work at 4000 or 5080 but rarely. I believe printed circuit boards are done at over 10,000dpi.

 

[chriscozi]

Thank you all for your valuable input, we are working as a plate bureau house and we do not have our own printing facility.
We are using Eagle Rip and it is capable of ripping a job at 9600 dpi.

Just because you CAN do something doesn't mean you should. (think about running a red light)
Your plate customers should be requesting a certain LPI and dot style/shape.
THAT is what will determine the processing DPI.
Good on you for asking.
You could see a significant speed increase with appropriate configuration.
YMMV