The choice of halftone dot shape is important because it has an impact on the aesthetics of the final presswork as well as on the lithographic performance of the press. This post describes the basic dot shape options available with conventional "AM" halftone screening. The descriptions apply to all the various vendors' offerings – though there will be some very subtle variations between their various implementations.
When dots grow in size from highlight to shadow there is a point at which the dots first touch each other. When that happens there is an effect called the “optical bump.” In a gradient blend this may show up as an artifact of a dark line in an otherwise smooth blend. Another issue is that, because paper moves through a press from printing unit to printing unit, presses are effectively directional imaging devices. This means that halftone dots shapes that are also directional, can interact with the directionality of the press and may exaggerate some issues such as slur and doubling.
In an AM screen each dot is formed in a halftone cell, typically based on a grid of 16 by 16 pixels. The pixels within the cell are "turned on" in order to form the dot shape. The cells are then stitched together, like a mosaic, in order to form an area of dots.
On the left (enlarged) is a single halftone dot within its 16x16 pixel cell. On the right (reduced) is an area of dots within their 16x16 pixel cells.
Following are the basic AM screening dot shapes and their performance characteristics in use.
Click on the images to enlarge.
Round dot: Dots are round through the tone range
Benefits: Dot shape is the same for all screen angles and frequencies, optical bump is hidden in the shadows at the 75% tone, dot is non-directional so it is less affected by press problems. Reduces single channel moiré issues. Dot is non directional, i.e. all screen angle dots react the same to directional press issues such as slur and doubling
Issues: Not suited for film-imaged plates because the diamond shape that results at 75% and darker tones is very sensitive to dot gain and sudden loss of shadow detail. Excellent for computer-to-plate imaging because of the greater integrity of halftone imaging as well as the ease of dot gain compensation with tone reproduction curves.
Euclidean dot (a.k.a. Transforming Round Dot): Round/Square/Round
This is the classic dot shape that resulted from the original etched glass screens from the 19th century and is now replicated in digital form.
Benefits: Dot shape, except for 50% tint, is the same for all screen angles and frequencies, dot is non-directional so it is less affected by press problems. Shadow dots are less prone to dot gain - especially in a film workflow – than the simple Round dot.
Issues: optical bump occurs at 50% midtone tint which puts it in the most important and visible image tones.
Elliptical dot (a.k.a. Transforming Elliptical Dot, Chain Dot): Rounded corner diamond shape
Benefits: Optical bump is moderated by being split into two – when the dots first touch at the long width at the 40% tint and then again at the short width at 60%.
Issues: dot shape varies at different screen angles which can cause single color moiré and uneven dot gain. Dot is directional, at low lpi frequencies the “chaining” of the dots as two points touch can cause lines to appear as artifacts. Directional problems on press such as slur and doubling can cause strong tone and color shifts depending on the angle of orientation of the dots relative to the angle of the paper as it travels through the press.
Square dot: Square
Historically used for catalog work, letterpress, and specialty work.
Benefits: Gives an impression of a "sharper" looking image on press thus reducing the need for excessive sharpening in image editing applications.
Issues: Very prone to loss of shadow detail due to very thin spaces between shadow dots.
Line dot: Line
Historically used for newspaper work, lenticular, and specialty work.
Benefits: At lower screen rulings can have a strong graphic effect.
Issues: Directional problems on press such as slur and doubling can cause strong tone and color shifts depending on the angle of orientation of the lines relative to the angle of the paper as it travels through the press. Directional lines can introduce very visible subject moiré. Image edges can appear ragged.
Specialty dot:
Any custom dot shape designed to meet specific requirements.
Benefits: Halftone dot optimized for specific applications.
Issues: Requires extensive knowledge, development time, and testing in prepress and pressroom in order to implement.
Here are two examples:
Pepper Dot:
Uses small dots within larger conventional dots in order to reduce ink usage on press – especially for newspaper work. (Click on image to enlarge)
Novelty Dot:
Uses a recognizable image or graphic to form the dot. (Click on image to enlarge)
The choice of FM halftone dot shape is important because it has an impact on plate choice, plate imaging, the aesthetics of the final presswork as well as on the lithographic performance of the press. Because there can be a wider variation of FM dot shapes than AM, this post will just illustrate several representative dot shape options. Each vendors' offerings can then be evaluated by using the information in this post as a guide.
Benefits:Image has a photographic quality even when viewed under a loupe. Often used for fine art reproductions.
Second order FM screens are the de facto standard in today's print production. With this type of halftone screen the dots grow in size through the tone scale. Dot growth can be in one direction – perhaps forming worm-like features as in the first example (Kodak Staccato), or grow in both directions – forming more conventional looking dots as in the second example (Screen Spekta).
Dots have a strong directionality. Sometimes this dot shape in a vendor's FM offering is used for only one of the process colors in order to reduce "clumping" or secondary patterns when process colors overprint.
This graphic is just intended to emphasize the fact that there is a great variety in FM screen patterns. Therefore one should not apply general statements such as "FM screening is grainy" – quality will vary according to each vendor's implementation.
The choice of halftone dot shape is important because it has an impact on the aesthetics of the final presswork as well as on the lithographic performance of the press. This post describes the basic dot shape options available with conventional "AM" halftone screening. The descriptions apply to all the various vendors' offerings – though there will be some very subtle variations between their various implementations.
Benefits: Dot shape is the same for all screen angles and frequencies, optical bump is hidden in the shadows at the 75% tone, dot is non-directional so it is less affected by press problems. Reduces single channel moiré issues. Dot is non directional, i.e. all screen angle dots react the same to directional press issues such as slur and doubling
This is the classic dot shape that resulted from the original etched glass screens from the 19th century and is now replicated in digital form.
Benefits: Optical bump is moderated by being split into two – when the dots first touch at the long width at the 40% tint and then again at the short width at 60%.
Historically used for catalog work, letterpress, and specialty work.
Historically used for newspaper work, lenticular, and specialty work.
Uses small dots within larger conventional dots in order to reduce ink usage on press – especially for newspaper work. (Click on image to enlarge)
Uses a recognizable image or graphic to form the dot. (Click on image to enlarge)
Gather a group of printers around the table to discuss the business issues they face and one topic that's sure to come up is the problem of today's graphic arts training - or, more often, the lack of training in their typical customer. They'll complain that today's creatives don't know anything about what it takes to prepare art for their presses. They will share tales of designers who supply art for a company's Annual Report in PowerPoint format, or the 200-page, fully illustrated catalog created in a shareware word processor and saved across 30 floppy discs.
Once in a while, even in the best-run printshops, production can go sideways, go south, foul up, mess up, screw up, and go wrong. When that happens a well run printshop turns to their "Golden Reference" as one tool to diagnose the problem and determine a solution. A Golden Reference typically it consists of a file, press sheets, proofs, plates, and documentation. It preserves a record, or "snapshot," of the prepress and print conditions when everything was working correctly. As such, when a problem occurs, the Golden Reference file can be rerun through the workflow and even on press if required. Comparing how the Golden Reference performs at each stage of the current process to the original can help isolate where the differences are and therefore reveal what may be causing the live job to go wrong. The Golden Reference is also useful for validating consumables - such as ink and plates – which may change from batch to batch. It is also useful as the standard by which new processes – such as a change in halftone screening, or a new ink type – can be compared and introduced.
To discover opportunities for building strong relationships with print buyers, try looking at all your customer touch points as if you were an alien explorer. Step out of your office and imagine yourself coming from some far away city with the mission of seeking out new printshop civilizations – boldly going where no print buyer has gone before – deep into the realm of Possible New Print Supplier (PNPS). 
First, a black and white halftone (original size at left and enlargement at right - click on image to enlarge)
Next, is a scan of a color bar which shows a lay down problem with the black printer (original size at left and enlargement at right- click on image to enlarge):
For best results use a scanner with the highest possible native (non-interpolated) resolution. Scans at about 1200-2400 dpi seem to work best. Have the graphic to be scanned placed in line with the direction of travel of the scanning head, and use a black backing to prevent show through. Finally, scale the image in Photoshop by changing the resolution (lowering the dpi) with "Resample Image" deselected.
Long-term relationships between print buyer and supplier have both good and bad aspects. Generally, the benefit is to the seller rather than the buyer, which is why long-term relationship building is worth the investment in time and energy. With a long-term relationship buyers lose their objectivity – personal and emotional factors can overwhelm objective decision-making. Also, as print buyers become dependent on the seller’s systems, they are less likely to seriously look at competitive offerings for fear of steep learning curves and/or compatibility with legacy work. Competing sellers find it harder and harder to effectively penetrate the print buyer’s organization and decision making structure and therefore they end up making fewer calls to buyers who they feel – rightly or wrongly – are “locked-in” to steady vendors. Buyers also become lethargic, waiting until the last minute to submit increasingly less specific requests for quotes based on the assumption that their printer intuitively knows what they really want. This, in turn, makes it more difficult for competing printers to respond with an effective, competitive proposal.
The tool to get rid of the image artifacts while preserving the graphic detail is the "Smart Blur" filter in Adobe Photoshop. Below is its dialog box. You will need to experiment with the settings according to the specific image that you are working with. In general the "Radius" setting will be lower than the "Threshold" setting. The preview window shows the effect of changes you make to those settings. Sometimes, if the adjustments result in degrading the image, it's better to leave some artifacts and remove them afterwords using the clone or other tools. Always use the "High Quality" setting with "Mode: Normal."
The final result – the artifacts in the graphic are cleared away while the detail of the text and art is preserved.
The tool to get rid of the paper texture while preserving the image detail is the "Smart Blur" filter in Adobe Photoshop. Below is its dialog box. You will need to experiment with the settings according to the specific image that you are working with. In general the "Radius" setting will be lower than the "Threshold" setting. The preview window shows the effect of changes you make to those settings. Sometimes, if the adjustments result in degrading the image, it's better to leave a bit of texture and remove it afterwords using the clone or eraser tool. Always use the "High Quality" setting with "Mode: Normal."
The final result – the watercolor paper texture is eliminated while the detail of the painting is preserved.
Watercolor study of a strawberry courtesy of Susan Pritchard. To visit her blog, please click HERE
Unfortunately printers seldom buy print themselves, because if they did, they’d have a better understanding of the customer’s perspective – and it’s that perspective that helps determine who gets the print order and who gets the “thanks for your interest” email. 