Identifying the Printing Technology: Halftone Dots

From time to time I find a printed product I like, and I want to identify the commercial printing technology used in its production, whether offset, digital, flexographic, or some other process.

If you have similar questions, here are some tips you can use in your print buying work to make this determination.

First of all, I’d encourage you to Google each technology using Google Images. Use search terms such as "halftones" and "halftone dots." Nothing I will tell you here will be as clear as photos showing sample halftones used in each printing process. That said, here’s a cheat sheet. Use a 12-power printer’s loupe to magnify the halftones and make the halftone dots visible.

1. Offset Printing: Look for "rosettes" in offset printed halftones. Halftone screens used for process colors in offset printing are turned at slight angles to one another. This keeps the halftone dots from printing on top of one another. Instead they print in circular patterns (like flower blooms, hence rosettes). When I look for rosettes, I check the areas with more ink, because this is where the halftone dots are larger and hence where the rosettes are more evident. Coarser halftone screens yield larger, more visible dots (fewer per inch, although their size will still vary depending on the amount of ink required). The preceding information pertains to area screens and gradations of a color as well as to photos.

2. Digital Printing (Laser Printers): When I look at laser-printed halftones, I notice two things. First, the halftone dots (as well as any line art, for that matter) are surrounded by a dusting of laser toner. Remember that electrophotography (also called xerography or laser printing) uses tiny bits of dry toner, and unlike offset printing ink these can wind up elsewhere than precisely on the halftone dots. In addition, I have noticed that the halftoning algorithms used in laser printing seem to differ from those used in offset printing. Whereas the rosettes are visible in offset printing, I have not really seen them be as pronounced in laser printing (if visible at all). The rows of halftone dots are still larger or smaller depending on the color density, but they seem to overlap rather than to create the characteristic rosettes of offset printing. (The rosettes may just be less evident, but I personally haven’t seen them, so this is one way I identify samples of this printing process.)

Finally, there seems to be more of a glossy effect in the toner (but not necessarily in the paper surrounding the image areas), presumably due to the high heat and roller pressure applied to fuse the toner to the substrate. I would state as a caveat that samples I’ve seen from the HP Indigo press (which uses laser technology but which suspends the bits of toner in an oil rather than applying them in a completely dry process) are almost indistinguishable from samples of offset printing.

3. Digital Printing (Inkjet Printers): Here’s an easy one. When you look closely through a printer’s loupe, you will see only minuscule spots of color in an inkjet printed halftone or for that matter in any area of color not entirely cyan, magenta, yellow, or black. This is similar to FM screening or stochastic screening in offset printing work, in which more spots of a uniform size of a particular color will be applied to the substrate to create a denser color or a color build (as opposed to using an AM screening algorithm, in which there are the same number of halftone dots in a given area, and they just get larger or smaller depending on the amount of a certain color you want to print).

The best way to remember this is to look for a "spray" of minuscule spots in an inkjet printed image. An inkjet image almost looks like a continuous tone photo rather than a halftone-screened photo.

4. Flexography: Halftone dots in a flexo-printed image or area screen will sometimes be elongated. This is because the rubber relief plates will move slightly when depositing ink.

Another point of observation unrelated to halftones is the slight difference in line work between the edge of a letterform and its interior. The edge of a letter is often clearly lighter than its center (it has a halo), because flexography is essentially a "stamp-pad" operation, using rubber relief plates (i.e., raised printing) pressed against a paper substrate.

5. Gravure: In gravure printing, type, screens, and images are all composed of tiny, visible dots. This is because the entire plate image contains tiny recessed wells that carry the ink for deposit on the substrate.

6. Screen printing: In addition to the thick laydown of ink, look for more prominent halftone dots (fewer rows of dots per inch) in screen-printed images. Halftone screens must be coarser than in offset printing. Otherwise, the thick ink will plug up the halftone screens.

If all of this information confuses you, here is a suggestion. Ask your printer for samples and check them closely under magnification. If you know at the beginning exactly which press technology was used, you can start to both understand and compare the appearance of halftones from one sample to another. The hardest technology I have found to identify is laser printing, because I’m always looking for the rosettes as a way to differentiate laser printing from offset printing. Both online photos of enlarged halftone dot patterns and printed samples including halftones will make this process easier. It will take time. That said, this is a useful skill, and it will often help you choose the ideal printing technology for a specific project.

Saving Money by Ganging Up Jobs

You pay for all wasted printing paper. The printer may not say that, but it’s true. For this reason, if you’re designing a pocket folder, for example, and you’re using 12pt. coated stock to provide a durable product that will protect its contents, you may be able to gang up some other jobs that would be ideal for this exact paper stock.

For instance, let’s say you need more business cards, or perhaps a 6" x 9" mailer postcard for a marketing initiative. Depending on the size of the press sheet your printer will be using, there may be room on the press sheet (in addition to the flat pocket folder blank including pockets and glue tabs) for either the postcards, business cards, or both. Moreover, you may even be able to include two postcards or more.

The way to figure this out is to ask your printer the size of the press sheet he will be using, how many pocket folders (as per this example) he will be imposing (or laying out) on this press sheet, and what the dimensions will be for the flat/unfolded pocket folders, the unfolded pockets, and the attached glue tabs.

Also ask about the gripper margin (an unprinted area on the press sheet at which point the press gripper will grab the press sheet and pull it into the press for printing). Also ask about the space needed for printers’ marks, color bars, and such.

Perhaps an even easier way to do this is to ask for a drawing of the press sheet showing any free space you can use for the additional printed products.

And remember, everything will be on the same paper. This can be a good thing (ideal for giving a sense of coherence to the separate marketing pieces), but you probably wouldn’t print the postcards, pocket folder, and business cards on different press sheets once you’ve ganged them up like this (it would defeat the purpose). And all of these jobs need to go to press at the same time to be processed simultaneously.

Ask your printer about this option. It can potentially save you a lot of money.

[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]