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, (:
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.
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).
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.
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.
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.
English is not my native language, so feel free to comment on the exact term used.
Long answer
Offset print is a process of several steps, which involves 2 basic steps, prepress and press.
I will start with the press which is the direct question.
I will also mention some errors in the file itself, which is not part of the print but the design.
And finally, I will mention some things that regard administrative decisions like the budget.
I am trying to simulate the errors with some images.
Some of these errors are part of the process, meaning that they are tuned at the start of the project and corrected as the press rolls. If they persist is when we have "bad quality".
Although I put these in the press chapter, if an error exists, you need to look at the plates, which are part of the prepress part.
1. Alignment, registry
When one ink (or more) does not match the exact location where it is supposed to be.
We use registry marks to facilitate the alignment.
But sometimes, on old machines or too much force is applied to the rubber cylinder, a bulge is formed in the middle, and, although the registry marks are aligned, the center is not.
This happens more on big press machines because the rubber plate is bigger.
It is very rare that the plates do not align, but it can happen.
2. Dust
Either the environment is not clean or the paper has dust, sometimes is dust from the paper itself. If the press buys the paper, they should talk to the paper provider to change the stock.
Check also the plate to see if the information is washed out.
This is a really bad image.
3. Uneven flat zones
Probably it is as simple as adding more ink.
Check also the plate to see if the information is washed out.
4. Gain
The zones look smeared around the edges. This can happen when you have too much pressure when the plate does not have the right amount of water or for some other reason.
Also, compare the plates if the problem persists.
5. Ink density
You either have too much ink or too little.
To control the gain and Ink density we use color bars. The specific model depends on what the provider uses. There are some that need to be read by a densitometer and others can be used with the bare eyes.
https://stock.adobe.com/es/images/cmyk-print-test-control-scales-vector-set-color-bar-cmyk-and-test-chart-offset-print-control-strips-color-cmyk-for-prepress-and-print/348479448
6. Uneven ink on the horizontal axis
It is important to differentiate the horizontal axis regarding the press itself. Each zone has ink regulators that need to be opened or closed.
7. Washed out on the vertical axis
The design is demanding much ink. Probably the machine is running too fast.
I mentioned that you need to check the plates to detect any of the above errors.
8. Bad lineature
The lineature depends on the paper used and the project itself, but there can be a case where the specification needed let us say 150 LPI, and the prepress delivered 133 LPI for example.
9. Overprint
This is not an error by itself. It is an instruction that needs to be clearly stated and most of the time is overlooked. (see point 13)
10. Wrong color profile
Again this is widely overlooked. The color profile must ba defined by the press, and informed to the prepress and the designer. I put this on this chapter because the main actor that can check it is the prepress department.
These are not part of the press or prepress process.
11. Pixelation
The images have insufficient resolution.
12. Rich black
Especially when you do not want it. For example on normal black text. It demands a perfect registry wich is almost impossible.
13. Overprint
When you add blocks of color, especially black, over a photo. The black will be overprinted so a ghost of the photo below can be shown.
13. Thin lines
Which will not be printed consistently. Specially white or light colors over dark.
14. Many others
I can go on and on with this chapter. :)
Some things that could be considered "bad quality" can be attributed to cutting corners or budget constraints. For example the quality of the paper.
This will produce a "bad" result. It can be compensated in some steps, like choosing the correct color profile, but sometimes the low quality is "inevitable".
And if we want to aim at great quality, every step is important. The design, the providers, the paper, the press, the prepress, and the finishing ones.
Short answer
A numeric answer considering all of the above, can be if the printer can print "High lineature". On coated paper, a normal number is 150LPI. Great quality can print let's say 200LPI.
Shorter answer
If it looks good it is good!
