Saturday, November 26, 2011

"DPI" and the misuse of graphic arts terminology

The prepress and press worlds are some of the worse misusers of terminology with the all too frequent resulting confusion in sales, marketing, specification, and production. Here is one of the most misused: "DPI" (or as it is spoken of in the rest of the world: DPCM).

"DPI" - Dots Per Inch is a term used for a variety of things that properly speaking it shouldn't.

DPI - when used to describe the resolution of a computer to plate imaging device or filmsetter. E.g. "This is a 2400 dpi CtP device."

"Dots" in this case refers to the laser "Spots" of energy that expose the printing plate or film. However, while DPI, identifies the number of dots per inch - it doesn't actually describe the resolution of the device or size of the spot of energy. Instead it defines the device's "addressability." In other words, dpi tells you how many locations per inch a spot of energy can be focussed on – not the actual size of the spot of energy.

This graphic shows plate media being exposed at 2,400 dpi by six different CtP devices:Note that they are all 2,400 dpi - that is that they all can hit the target location (address) with their beam of energy - however the exposing spots of energy are all different sizes, in this example ranging from about 2 microns on the left to about 30 microns on the right.

Resolution vs addressability is explained in more detail by clicking HERE.

DPI - when used to describe the resolution of an inkjet printer. E.g. "This inkjet proofer prints at 2880 x 1440 dpi."
In the case of an inkjet printer, the clue to this misuse of dpi to wrongly mean resolution is revealed with asymmetrical dpi specifications. So, an inkjet proofer that has the specification that says it prints at 2880 x 1440 dpi does not mean that the resolution is finer, or that the droplets of ink are finer in one direction. Instead it simply means that the paper is moved more slowly in one direction - i.e. the addressability is changed - while the physical size of the droplet of ink, and hence its resolution remains the same.
On the left a symmetrical inkjet addressability grid (600 x 600 dpi). On the right the same printer set at 1200 x 600 dpi. The addressability has changed but not the size of the cyan droplet of ink and therefore the actual resolution of the device remains the same.

In any case, the actual size of the mark the droplet of ink makes on the paper is unknown. For a 600 or 1440 "dpi" ink jet printer it most certainly is not 1/600ths or 1/1440th of an inch in size. As a result, with some inkjet printers, reference is sometimes made to "picoliters" in addition to dpi when the resolution of the device is described in the specifications. A picoliter is a unit of fluid volume. A lower minimum ink volume tends to yield a smaller minimum droplet size of ink allowing more dots of ink to be in the same area thereby yielding higher actual resolution. While picoliter is a better indicator of the relative size of the splat of ink on the paper it is still a unit of volume and not area. So it suggests a difference in resolution but doesn't actually specify it.

DPI - when used to describe the resolution of an image scanner. E.g. "This is a 600 x 2400 dpi scanner."
An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. The resolution of Digital images is usually expressed as dots per inch or pixels per inch. As a result the resolution of scanners is often expressed in terms of dpi (and sometimes "ppi" pixels per inch). The more accurate description is "spi" which stands for "samples per inch" since scanners sample the document they are scanning.

A related issue with defining scanner resolution is that manufacturers typically refer to the scanner's interpolated resolution - which is a software upsampling algorithm method to increase the pixel density - instead of using the scanner's true optical resolution. If the scanner's dpi is asymmetrical (e.g. 600 x 2400 dpi) then the smaller number usually indicates the particular number of individual samples that are taken in the space of one linear inch while the larger number is the interpolated samples.

DPI - when used to describe the resolution of an image. E.g. "This is a 300 dpi image."

Once an image has been digitized, either via scanning or captured with a digital camera, it is in the form of a raster image made up of pixels (picture elements). In graphic arts usage the pixels are typically square in shape and 8-bits (256 grey levels) in depth per channel (greyscale = one channel, RGB = three channels, CMYK = four channels).

Because pixels are generally thought of as the smallest single component of a digital image, the more pixels that are used to represent an image, the closer the result can resemble the original.
As ppi, a.k.a. "dpi", increases so does the amount of image detail that can be rendered creating the impression of greater apparent resolution.
Pixel counts can be expressed as a single number, e.g. an image at 100% reproduction size being 300 "dpi", or as in a "three-megapixel" digital camera, which has a nominal three million pixels, or as a pair of numbers, as in a "640 by 480 display", which has 640 pixels from side to side and 480 from top to bottom (as in a VGA display), and therefore has a total number of 640 × 480 = 307,200 pixels or 0.3 megapixels.
Again, the measures dots per inch (dpi) and pixels per inch (ppi) are sometimes used interchangeably, but have distinct meanings, and although dpi is often used to refer to digital image resolution the proper term is "ppi" - pixels per inch.

Saturday, November 19, 2011

How to extend the life of your inkjet proof

A great deal of effort is put into the making of a contract quality inkjet proof. However, if it is not properly taken care of, its useful life can be shortened and its integrity as a color reference compromised.

How the inkjet media and proofs are stored and handled, as well as their exposure to ambient office or light booth lighting, affect the color of the proof and its color integrity over time.

Here are a few simple things you can do to help maintain, or extend, the color integrity of your inkjet proofs.

• Depending on the surrounding ambient temperature and relative humidity, it can be several minutes, or even hours before an inkjet proof is dry enough to handle. So, before handling a proof, make sure that the ink is actually dry.

• Handle the proof by its edges to avoid leaving fingerprint smudges on the imaged area. Fingerprints may also leave residual oils on the media which may repel ink when the proof is imaged. After the proof is imaged fingerprint oils may dissolve water-based proofing inks leaving unsightly marks on the proof. Try to always use lightweight cotton gloves when handling proofs.

• Avoid bending the corners of the proofing media.

• If you will be stacking proofs, always place a light-weight slip sheet between the proofs.

• Store proofs in cool, dark, low humidity shelves or envelopes away from light or dust as inkjet proofs will fade with exposure to ambient office or light booth lighting.

• Keep all unimaged media in their boxes when not in use and store in a cool dry place away from light and dust.

• Always allow the proofing media to acclimate for 48 hours in the proofing area before printing.

• For optimal results, if available, follow the recommended environmental conditions provided by the proofing media manufacturer.

Saturday, November 12, 2011

Pantone color shifting - the problem with coatings

I think that most of us have been hit with an unpleasant surprise when a Pantone ink color shifts after being UV or aqueous coated. As one example, Reflex Blue (arguably the most difficult color in printing) prints as dark blue but will dry with a reddish sheen that will not dry properly. To try and make it dry faster the printer might run it through the press a second time to apply a coating, either UV or Aqueous, to seal the ink. This causes an immediate color shift which will continue to shift over the course of a few days.

The colors in the table below, as well as any spot color recipes that use them in the formula, will shift color (a.k.a. alkalinity burn) when subjected to the alkalinity of a aqueous or UV coating.
In order to avoid the costs involved with reprinting a spoiled job, when mixing and/or specifying PMS colors it's critical to inform your ink vendor that you will be aqueous or UV coating the job.

Special thanks to Bob Peterson of Superior Ink for his contribution to this post.

Monday, November 7, 2011

Printers and Ink - wherever you go, there you are!

Printers Alley, Nashville, Tennessee, United States
Ink Street, Carletonville, Gauteng, South Africa
Printers Alley, Doylestown, Pennsylvania, United States
Ink Wells Drive, San Antonio, Texas, United States
Printers Place, Darwin, Northern Territory, Australia
Ink Road, Iowa, United States
Printers Lea, Balgrochan, United Kingdom
Ink Street, Rochdale, United Kingdom
Printers Lane, New Haven, Conneticut, United States
Ink Lane, North Melbourne, Victoria, Australia
Printers Alley, Middlebury, Vermont, United States
Ink Drive, Redding, California, United States
Printers Parkway, Colorado Springs, Colorado, United States
Graphics Way, Lewis Center, Ohio, United States
Graphics Drive, Tinley Park, Illinois, United States
Graphic Way, Westerville, Ohio, United States

Wednesday, November 2, 2011

Is this blog being censored? And some housekeeping.

I found this graphic in my blog's visitor history (the company name is blurred out):
I know that this blog is censored in China so visitors from that region of the world have to use methods to get around the great firewall. But I didn't realize that companies in the west censored the site. Has this happened at the company that you work for? If so, I would love to hear about it and how you dealt with it. I'm also interested in how common it is for companies to censor their employee at work internet browsing.

On another note. Comments on blog postings are always appreciated. However, if the comment contains a link to a commercial web site I will consider it as spam - no matter how flattering the comment is.

And a reminder, if you find this blog of value then I'm sure you'll feel great by sending a donation. Don't be shy.