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I would need to measure the quality of a printed material in an offset printing procedure. There is any kind of points per square inch or something similar? I cannot find any kind of information regarding the quality of offset printing, that's why I'm questioning this here.

Thanks for your time, (:

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Measure quality to what end? In other words, why? What flaws would you expect to find in lower quality pieces which aren't readily apparent by looking? –  Scott Jul 17 '13 at 19:20
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You'd measure it in the ways you describe: fidelity, color accuracy, color depth, trapping, registration, consistency, etc, etc –  DA01 Jul 17 '13 at 19:45
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IMO, quality printing is about how well the job matches what I gave them. If I get a job that is more intense/contrasty/etc than what I gave them, then my face quickly becomes more intense. –  horatio Jul 17 '13 at 20:02
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Good quality printing is good quality printing. If the job is done correctly, there will be no "better than average" reproduction. There's good, then there is sub-par. There is no "above-average". Any printer below average doesn't last long in the industry. I wouldn't describe print services as being "better than average". I'd describe them as accurate which is what is required. –  Scott Jul 17 '13 at 23:09
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@Totty the amount of ink or the amount of dots you use aren't really any sign of particular quality. If you're getting your house painting, a bad painter can use two coats and still do worse than a good painter with one coat. –  DA01 Jul 19 '13 at 5:35

3 Answers 3

In terms of spatial resolution, the analogue of DPI in offset printing is lines per inch (LPI). This describes how fine or coarse the halftone screen is. The greater the LPI, the more fine the details that can be represented in the printed image (and also, the higher the DPI that is required in the source file if you are starting from a digital image; rule of thumb is to multiply the print LPI by 1.5-2.0 to get the minimum source DPI).

LPI will affect the quality of photographic material, shaded regions, gradients, etc.

Here's a page I found in a quick search that explains this concept more fully and shows some examples: Understanding Halftones

I think that answers the question as you worded it. However, if you're generally interested about what will affect offset print quality....

The type of surface you're printing on will affect the fidelity of the halftone dots as well as how high an LPI you can practically achieve. For example, ink dots spread out in newsprint (leading to "dot gain"), whereas this is less of an issue on coated stock.

The brightness of the paper will affect how vivid the bright regions of your design can be.

Using spot colors can allow you to have a specific color appear perfectly solid instead of halftoned if it's not one of the CMYK inks.

All of these factors will affect the fidelity/accuracy of printed product, which as others have noted is what clients care about in the end. (Such as color matching, as vector said, and contrast as horatio said)

I suggest you talk with whoever will actually be doing your offset printing to find out what quality options they can offer in terms of the printing process, the material you're printing on, etc.

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DPI has no effect on print quality. It is a specification of resolution not on the actual resolution achieved or potential resolution possible, only. –  Stan Sep 7 '13 at 19:11
    
@stan, true, though one could argue that the higher line-screen the press can support, the higher DPI images it can reproduce, ergo, the higher "quality" (subjective, of course) it can achieve. –  DA01 Sep 7 '13 at 21:51

There is no way that I know of to accomplish this in the manner you're looking for. What printshops brag about is their equipment quality and facilities, experience and technical expertise. The stuff you're talking about makes no sense. Jobs get printed at 300 dpi or 130 dpi for a reason. From your question you're projecting an extremely low level of familiarity with printing in general. Not sure if that's really the case, that's how you're coming off. If you attempt to conjure up those metrics and put them on the site, you'll convey the idea that that print shop is either a fraud or people there don't know what they talking about. If I saw something like that I'd not trust any important job to a place like that. Any printshop that wants to stay in business from quality perspective has to meet certain basic expectations. For example if I'm getting a letterhead printed with PMS 185, when I go to accept the job, I'll take my swatch book and expect the your 185 match my swatch book. Not lighter, not darker not washed out.

One of the things that can separate you form your competition is your level of customer service. Do you deliver? What other services do you offer? High speed envelope printing? Do you offer bindery services that are unique in your area? Some places offer very limited services and they emphasize that what makes them the best in the area is their expertise in that narrow field.

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So the most important fact is the color? I've seen in a previous work that on large black fills there were some white random spots, that were not expected. This for me is a lower quality service. Also if the density of the dots (dpi) is too low then the images/lines/text will not be as sharp as it would with a higher one. I'm I wrong with this? I don't understand why you don't focus on those aspects too... –  Totty Jul 18 '13 at 14:35
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Well, I have no idea how to quantify dirty blankets, etc. Those things happen on some sheets. If the whole job is affected, the client may very well reject it. Dpi does not affect line work. Not unless you're printing linework as halftones. If a job specifies 60 lb text Cougar and uses halftones, there's no need to bother with 300 dpi images, 130 dpi would be sufficient. –  vector Jul 18 '13 at 15:27
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@Totty there is no 'most important factor' when it comes to printing--or more specifically, the quality of a printing service. It's a total package issue. Color. Consistency. Reliability. Speed. Price. Customer service. Guarantee. Prepress capabilities. Proofing capabilities. Paper options. Press options, etc, etc. –  DA01 Jul 19 '13 at 5:37

Measuring Print Quality

Sheet fed Offset and Gravure are high-quality printing presses that are used to achieve the highest precision registered images.

The quality of a printed image cannot be easily judged. The precise technology that is used for colour is not reproducible. This might seem strange; but, it's true. No two manufacturers of colour quality measuring equipment can reproduce another's findings. No two instruments agree. It's troubling.

The best attempts result in some close readings.

Printer's (the people) use test targets which are also called printer's marks. There are various ones in addition to finishing (trim, fold, score, punch, emboss, etc.) for different purposes.

A geneva star is a pattern that can be visually examined to determine resolution, press sheet movement during the impression, direction of the unwanted movement (different controls for different directions) and a couple of other esoteric things like double-impression and slur (twist).

4C Geneva Star pattern series

When you see one, look to see if there is a small clear spot in the centre. No, then there was a printing problem. There's a specific pattern for each problem diagnosis.

EDIT: Here's what to look for, close-up. enter image description here

This must be repeated for each plate.

Colour bars are series of small ink patches of 25%, 50%, 75%, and 100% (solid) for each plate which are read by machine to tell the variation from ideal. Dot gain (change in contrast) is one of their purposes.

Register marks indicate how well the various plate images fall on top of each other. Misalignment of the printing plates is one of the basic problems with colour. As a matter of practicality, If you are ever asked to approve a press proof, the first thing you should check is that the alignment (registration) is PERFECT. Nothing can be judged until that detail is nailed-down. Any time you hear, "We'll get the registration. Just approve the colour." Pack up and find another shop. They don't know what they're doing and have just told you so in as many words.

There are other targets developed for specialized troubleshooting of various press problems.

We rely on technology today for mechanical press control. The early machines were ink densitometers, now, spectrophotometers are used that are more accurate. There are various alignment and calibration procedures that have been developed to ensure the reliability of the devices used.

Here are some other ways that we control printing quality "By the numbers."Printing Industries of America

EDIT: I forgot to mention that since printers have no control over what they're asked to print, they have adopted indirect methods to assure that whatever you want to have reproduced will be done faithfully even if it means that if you give them garbage, it will be reproduced exactly as you gave it to them.

"All mistakes will be faithfully reproduced." hung over my desk for years.

This is why you'll find various printer marks put into the live area of the printing plate but outside of the trim (discarded part) of the flat (the printed piece before being finished).

These test target originals are perfect images that will degrade when reproduced. The amount and kind of degradation indicates the corrective action that must be taken.

What does all this mean?

While I've been writing about how press operators judge print quality, you can use the same techniques they use. Learn how to use the printer's marks. Learn how to do a press check and what to look for in proofs, colour management, get yourself a loupe, or borrow one to look closely at registration. Check to see how closely the proof matches the press proof/run.

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Y'know, after all this, I think the OP wants to know how to EXPRESS print quality to the client, to SELL or MARKET print quality, not judge it or measure it. Different. –  Stan Sep 9 '13 at 5:45

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