Film Line Screen

Paul

Under the right conditions photopolymer plates are capable of handling 200 lpi. Depending upon what you are trying to achieve a 120lpi (and even 150 lpi) for letterpress can provide a half tone that has sufficient detail and sharpness (assuming everything is set up correctly).

Gerald

Paul:

I think you might be confusing two very different measuring systems.

I have used 133-line (dpi) halftones with polymer plate material when printing on coated paper, but a rougher surface stock would require a coarser screen. 100-line might work for a fairly smooth uncoated stock. Newspapers generally used from 65-line to 85-line screen for photo reproduction on newsprint from metal or polymer relief plates.

Those are settings in dots/inch (dpi) for halftone resolution. I think when you are talking about “lpi”, that is the lines/inch resolution of the printer or imager you are using to produce the film. In that case, 100 lpi is nowhere near enough resolution for good quality output. The bare minimum would be 600 lpi, and I would say that you would be much happier with film at 1200-3200 lpi if fine lines or small type images are involved.

The terminology is easily confused, and you would want to know what the maximum lpi is for the device you where will be outputting the film and unless speed is important, choose the highest resolution you can for the film output.

John Henry
Cedar Creek Press

I second John’s sentiments. Nuff said.

These comments didn’t make sense to me so I submitted them to my trusted film maker and here is his reply:

” :) what.

the film i make for you is 133 LPI.

yes i can change the Line Screen or LPI to anything you want. well there is a limit of 200 LPI. I have never heard of 600 LPI that is so fine.

I would think you could probably use that for offset, but even for that its way high. give you an example i use 175 LPI for 4 color jobs for offset printers. One of them prints the Tahiti booklets fine print at 175 LPI.

You can easily confuse DPI and LPI. DPI is the dots per inch in Photoshop. Rule of thumb, use to be DPI would be double the line screen. 300 DPI to 150 LPI supposedly gave you the best result. But no one pays attention to that anymore.

So 600 DPI or 1200 DPI would generally be used for bitmap files. I would not go that high on greyscale or color images.

Let me know if this helps.”

Gerald

At the risk of sounding like an errant fool- why is it that the specified output value of a the imagesetter can be 2400 DPI?

For example, when submitting typography from illustrator in PDF format, I I routinely forward it to my filmsetter company and request a res of 2400. When working with Vector and line art from a vector program, it is exceedingly important you request a res (or so I have learned).

The RIP (Raster Image Processor) then adjusts the vector file (the type you set in duh kompootah via illustrator) to become 2400 DPI rather than a bunch of calculations and algorythms that represent shapes and lines (this is how the data is formed in the program- algorythms that reflect shape, not ‘dots’ or ‘pixels’).

After the file is ripped, the rip sends it to the memory in the imagesetter which breaks it down into lines of dots on/off. Binary. 2400 per inch in length, 2400 across each inch. As the film is passing through the imagesetter, a laser is firing at it, exposing the film emulsion to create areas that are sensitive to development and not sensitive to development.
This gets washed with developer, then dried, then coated with fixer. Out it comes, ready to work with.
The film comes back perfectly smooth with indistinguishable dots if I ask for 2400. At 1440, I can see the edges and sawtoothing. At 1000, naked eye, it’s distinguishable. At 100 or 70, it is glaring!

John, I think you have the terms backwards. 133 lpi requires 266 dpi for a CMYK raster image. For lineart/vector, you want 1200dpi. The imagesetter/laser printer etc outputs at DPI, 600, 1200, 2540, and if it’s a grayscale image, you have to specify the linescreen to break it down, 80, 133, 150 etc.

fwiw the whole thing about everything having to be 300dpi effective resolution is a funny thing because for the most part, 133 lpi is the most common lpi, so really you’d only need 266dpi. I alway imagined somebody somewhere said “let’s just call that 300 dpi to be safe, give them designers a bit of leeway”. But with 4c art you can go lower and still look fine. That’s what matchprints are for.

When I used to output lino 330 film, the options were 1200lpi or 2540lpi. One typical project was printing film that would be used in lobbies of buildings, as backlit signage.

I don’t totally understand why a CMYK image needs an effective resolution twice that of the linescreen (again, usually 300 dpi) while lineart/bitmaps need to be 1200. Nor do I understand why lineart with an effective res of 1200 would look better imaged at 2400 dpi than 1200.

I think two distinct terms are being conflated.

LPI refers to the dot density of the halftone line screen. We don’t use halftones with letterpress type, so that’s only relevant if you’re printing a photo.

DPI, in the context of an imagesetter, refers to the resolution of the laser that exposes the film.

The vector art is rasterized into a series of dots by the RIP software.
The smooth curves of the type then must be represented on a grid. The tighter the grid, the smoother the curves will appear.

That’s why 600DPI looks jaggy to the naked eye - while 3000DPI requires magnification to see the stair-step.

In the past I have ganged up 1200dpi bitmap artwork with vector artwork and printed that at 3000dpi on the imagesetter. I did not try setting the imagesetter to 1200dpi. It would be interesting to compare!

Let me see if I can explain this clearly. The dpi (dot per inch) is the line count of an inch wide measure. Data from vector line art from a high end imagesetter is usually around 2540 dpi or ppi (pixels per inch). The default dpi for an image or photo is 300-350. Think Photoshop. The amount of data in a Photoshop image can be very large, thus the low dpi requirement. A 1200 dpi raster image would be multi-MBs. As has been discussed before on this forum, letterpress and offset printing has been done at 85, 100, 133, 150, 175, 200 and yes even 600dpi. From 150 up is usually offset because most printers can’t print clean dots on a letterpress at 150. When it is up to 600 it is sometimes stochastic screening which is a bit like a extremely high, dither Photoshop file.

To answer the 1270 versus 2540 issue, if you enlarge a neg with type 400 percent, the 1270 type will have a stair step pixel edge that does not show so much at 100%. At 2540, the resolution is doubled so you have twice as many pixels making the edge of each letter of type and it will be tack sharp under a magnifying glass.

PS. Less than a dozen printers were known for their 600dpi printing and now I would imagine the playing field has leveled quite a bit since almost every commercial printers outputs directly to plate at 175-200lpi or higher for offset printing.

I hope I have not offended any one or caused any consternation with this explanation.

How many printers does it take to exlain a linescreen?

130.
No wait.
2400.

No, wait.

1200.

No, wait. 2556.

Longdaypress- nobody’s talking about a CMYK or RGB 1200 dpi raster image, though I have worked with images of that quality for high-fashion large format printing. And yes, those files can be a couple of gigs, not just multi-MBs.

1200 dpi images are for bitmap lineart, and since the image is bitmap, i.e. black and white, the file sizes are not very big at all. The use of 1200dpi lineart has been discussed a few times, usually to the confusion of people who are told they have to convert all art to vector for it to be made into a polymer plate.

And again, when you list 85, 100, 133 etc…those are linescreens, LPIs! Nobody’s using an 85dpi image, except for web designers working on their first print project.

Thank you everyone for your input. To clarify.

I simply asked the rep. ” could I use this system ( AV - AV Star ) to produce negatives that would be used to make photopolymer plates for quality letterpress printing? “

He Replied, ” as long as you do not need more than 100 LPI”

I do understand Lines Per Inch, Dots Per Inch, Pixels Per Inch etc. I just never really put much thought into what was going on at the film output stage. It has been about 20 years since I last used an image setter, it was used for newspaper production.

Sorry for starting such a rant, I should have just asked, ” hey anyone think this AV system could make decent negs?” I was excited about the possibility of finding a quick solution to what is becoming a daunting problem.

Widmark:

You are absolutely correct, I had my acronyms switched.

John Henry