This is a guest post by Jim Perkins, a professor at the Rochester Institute of Technology's medical illustration program. Jim explains the most befuddling concepts in graphics and illustration with clarity and ease - he has written for Symbiartic twice before: first, on why it's a good idea to calibrate your computer monitor and second, on the mysterious settings known as gamma and white point. I'm delighted to be able to post another of his extremely useful explanations, this time on the difference between dots, spots, and pixels.

As a medical illustrator, I'm obsessed with terminology. On a daily basis, I deal with hundreds of anatomical terms, most derived from Latin or Greek roots. And I'm a stickler for accuracy. As with our own language, changing just one letter can completely alter the meaning of a word, e.g., the prefix myo- (meaning muscle) becomes mylo- (molar) which could become myelo- (referring to either bone marrow or the spinal cord). As my students will attest, I'm also a stickler for the arcane plural forms of Latin terms. The plural of ramus communicans is rami communicantes and the singular of phalanges is phalanx. There's no such thing as a "phalange."

I guess it's no surprise that my obsession with terminology has spilled over into other facets of my teaching. One of my pet peeves is the misuse of the word "dot," especially as it is used in the expression "dots-per-inch" or DPI. This simple term is applied to several different (and very distinct) concepts in the graphic arts, leading to widespread confusion. Let's explore these concepts and why it's important to keep them straight.

Halftone Dots

The word "dot" was first used in the graphic arts to refer to the tiny pattern of dots that can simulate a continuous tone image using solid ink. Developed in the mid- to late-1800s, this technique – and the use of the term "dot" – predated the computer graphics revolution by more than a century.

As you can see in the image below, a photograph can create a smooth gradation of values from black to white and all shades of gray in between. This is not the case, however, with most printing methods, including offset lithography and desktop digital printing. These technologies can only print areas of solid ink. The ink is never diluted, nor is white ink added to the mix to make shades of gray. The only way to reproduce shades of gray in print is to break the image up into tiny dots that appear to blend into a continuous tone when viewed with the naked eye. Such an image, composed of a pattern of tiny dots, is called a halftone. The dots themselves are known as halftone dots.

The process begins with a film negative of the original image. Light passes through the negative and then through a screen, usually a plate of glass with a grid of horizontal and vertical lines etched onto its surface. After passing through the screen, the light exposes another piece of film. The screen functions as a diffraction grating, breaking the light into tiny discreet rays, which create the pattern of dots. The result is a duplicate film negative with a pattern of solid dots instead of continuous shades of gray. The duplicate negative is then used to create a plate for the offset printing process.

Lines Of Dots

The halftone process introduces another bit of printing terminology that often gets confused with the others. If you look at Figure 1, you'll see that halftone dots are arranged in orderly rows or lines, usually oriented at an angle to the paper. In the conventional halftone process, the spacing of these lines of dots remains constant throughout the image; only the size of the dots varies to create different shades of gray.

The spacing of lines of halftone dots is known as the screen frequency or line screen and is expressed in lines-per-inch (LPI), i.e., the number of lines (rows) of dots in an inch. Although this is a form of resolution, it is quite different from the resolution of a digital image, which will be discussed below. Remember that this halftone process predates digital imaging by a hundred years.

Although the line screen remains constant throughout a single image (and usually for an entire printed piece) it is possible to use different line screens for different printed pieces (see Fig. 3).

The choice of line screen depends almost entirely on the type of paper being printed on. Newsprint, for example, is highly absorbent, allowing ink to soak into the paper and causing the halftone dots to enlarge, a phenomenon known as dot gain. If the lines of dots are too close together, the ink will bleed together and fine details will be lost. Therefore, printing on newsprint (and other cheap papers) requires very coarse line screens, usually around 85 LPI. With such coarse line screens, the halftone dots are often visible to the naked eye, a fact exploited by pop artist Roy Lichtenstein (Fig. 4). Better quality paper is coated to minimize dot gain and, therefore, supports much finer line screens. Most books, magazines, and other printed materials are printed at 133 or 150 LPI, while some art books and corporate reports may use very fine screens of 200 LPI or more.

Electronic Halftones

The photographic process for generating halftones was the industry standard for nearly 100 years. In the 1970s, companies like Crossfield Electronics and Linotype-Hell developed electronic dot generators that used a laser to translate a scanned image into a halftone film negative. In 1984, Linotype introduced the Linotronics 100 and 300, the first imagesetters that used laser technology to translate a digital image into a halftone. The release of the Linotronics coincided with the introduction of the Macintosh computer running Aldus Pagemaker, the world's first page layout software. Apple and Linotronics were also the first to employ Adobe's PostScript page description language, allowing the computer to send graphical information to the imagesetter (and to Apple's newly-released Laserwriter printer). Thus began the age of desktop publishing.

Imagesetters are still widely used today. They have been supplemented by platesetters that skip the process of making a film negative and use a laser to burn the halftone directly onto a printing plate. Desktop laser printers also use similar technology. However, instead of etching onto film or a plate, the laser creates static electric charges on a rotating metal drum. The drum picks up dry carbon toner and then transfers the toner to a sheet of paper.

Regardless of the specific technology, these electronic halftone systems have one thing in common – they create extremely tiny marks, called spots, printer elements, or even device pixels (not to be confused with pixels on a monitor) which can be combined to create halftone dots of varying sizes.

Imagine that the printing surface (paper, film, or a plate) is divided into a grid of tiny spaces (see Fig. 5). Each of these little spaces corresponds to the smallest possible mark that the laser device can create. If the laser strikes a specific space, it is turned "on" to create a black spot or printer element. In order to generate a halftone pattern, i.e., halftone dots arranged in orderly lines, the printer divides its pattern of spots into a grid of vertical columns and horizontal rows. At the intersection of each row and column is a cluster of printer spots known as a halftone cell. The printer can turn on or off the spots within each cell to create halftone dots of varying sizes. If only a few spots are turned on within each cell, it produces a small halftone dot, which gives the appearance of a light gray. As more spots are turned on within each cell, the halftone dots become larger, producing darker grays.

The spacing of these tiny spots or elements is the printer's resolution. I prefer the term spots-per-inch (SPI), referring to the number of tiny spots or printer elements that the device can lay down in a linear inch. Unfortunately, most printer manufacturers use the more familiar expression dots-per-inch or DPI, a trend that began with the first dot matrix printers in the 1970s. This has lead to significant confusion between printer spots and halftone dots. In fact, many graphics professionals have reversed the terminology, using the word dot (and DPI) to refer to the tiny marks or elements made by the printer and the word spot for what were traditionally called halftone dots. The situation is further complicated by the fact that SPI is also used as a measure of the resolution of a digital scanner (in samples-per-inch).

In spite of current trends, I prefer the traditional use of the word dot to refer to halftone dots that vary in size to create different shades of gray. This was the accepted terminology for over 100 years and is incorporated into other graphics terms such as dot gain, discussed above. I'll continue to use spots-per-inch throughout this blog when referring to printer resolution, but readers should be aware that it is often used interchangeably with the term dots-per-inch or DPI.

Pixels

One of the most common (and inaccurate) uses of the term dots-per-inch is to describe the density of pixels in a digital image. A pixel (short for "picture element") is the smallest editable component of a raster image. Pixels are usually square (except in some digital video formats) and are arranged in a grid of horizontal rows and vertical columns. The proper term for the resolution of a raster image is pixels-per-inch (PPI), a measure of the number of pixels in a linear inch in both the horizontal and vertical dimensions. A one-inch square at 300 PPI will be 300 pixels across and 300 pixels tall, for a total of 90,000 pixels.

Sophisticated graphics software such as Adobe Photoshop uses the proper terminology of pixels-per-inch when describing raster images (for example, check out the Image Size dialog box in Photoshop). However, the inaccurate term dots-per-inch has begun to creep into some graphics software, reflecting the widespread misuse of the term to mean any measure of resolution. Even Adobe Illustrator, Photoshop's close cousin, has fallen victim to the confusion between PPI and DPI. When exporting a PSD, PNG, or BMP file from Illustrator, the resolution options are listed in PPI. But when exporting a JPEG or TIFF file, the output resolution is given in DPI.

Adding further confusion is the term megapixel which is sometimes used as a measure of a digital camera's resolution. This isn't really a measure of resolution, per se, since resolution refers to the number of units (dots, pixels, etc.) within a linear measurement (e.g., the number of pixels per inch). Instead, megapixels refers to the total number of pixels that a digital camera can capture and says nothing about whether those pixels are packed tightly together in a small space (high resolution) or spread across a large area (low resolution).

What's In a Name?

You might ask why all of this matters. Is it just because I'm neurotic about terminology or is there a good reason to keep these terms straight? In my experience as a teacher, I find that my students often confuse these terms, leading to bad decisions about how to construct and print digital images. In my opinion, it's important to keep the terms straight in order to keep the concepts straight.

Probably the biggest problem that my students encounter is that they assume there's a one-to-one relationship between pixel resolution of a digital file (PPI), output resolution of a printing device (SPI or DPI) and halftone screen frequency (LPI). For example, they may assume that, in order to print to a 1200 DPI laser printer, the image itself must be 1200 PPI. And they're usually in the dark about how these numbers relate to halftone screen frequency. There is a relationship, but it's not necessarily one-to-one.

The correct resolution for a digital graphics file (in PPI) depends on the type of artwork being created, specifically whether it is line art or continuous tone (see Fig. 6). Line art refers to any image that consists of solid black lines, stipple dots, or other black objects against a solid white background. There are no shades of gray (i.e., no halftones). When creating (or scanning) a piece of line art, the resolution of the image should be quite high. Most publishers request that line art be prepared at 600-1000 pixels-per-inch. This will ensure that the black lines will appear crisp and smooth when output to a high-resolution printing device.

Continuous tone images are those that contain shades of gray and subtle variations in tone. This includes many types of artwork and virtually all photographs. You might think that a continuous tone image would require higher resolution in order to capture the subtle variations in tone. However, just the opposite is true. Continuous tone images generally lack the hard edges and high contrast of line art, so there's no need for high resolution to create crisp edges. Instead, continuous tone images require only enough resolution to create a decent halftone.

The rule of thumb is to create continuous tone images with a pixel resolution (PPI) that is twice the halftone line screen (LPI) that will be used in printing the final image. For example, if an image will be printed in a newspaper at 85 LPI, a resolution of 170 PPI is sufficient. For a book or magazine printed at 133 or 150 LPI, the resolution of the digital image should be 266 or 300 PPI. Since 133 and 150 LPI are the most common line screens used for the majority of offset printing, many artists make all of their continuous tone images at 300 pixels-per-inch. But this resolution would not be adequate for high-end printing (artbooks, corporate reports, etc.) which use line screens of 200 LPI or more. Therefore, before even starting an illustration, the artist must know how that image will be used. If the final use is in a printed piece, the artist must know the line screen that will be used by the printer.

Once an illustrator sends a piece off to his/her client, it is the client's responsibility to make sure that it prints correctly. This means using the right line screen and selecting the correct printing equipment for the job. The illustrator normally doesn't have to worry about the resolution of the final output device (typically an imagesetter or platesetter). But just to complete this story, I'd like to discuss the relationship between halftone line screen and printer resolution. If you've ever seen noticeable banding in your laser prints, this might explain why.

Recall from above that the little spots produced by a printing device are subdivided into clusters called halftone cells (see Fig. 5). The spots within the halftone cell are turned on or off to create halftone dots of varying sizes, resulting in the appearance of different shades of gray. The number of spots within each halftone cell determines the variation in sizes of halftone dots and, therefore, the number of different shades of gray that can be produced. For example, a halftone cell that is 16 spots wide by 16 spots tall will have a total of 256 spots (16 x 16 = 256) and can produce 256 different sizes of halftone dots and, therefore, 256 shades of gray. It just so happens that the human eye can only discern a few hundred shades of gray, so 256 is roughly adequate to reproduce the full range of visible gray values.

You need two pieces of information to determine the number of spots in a halftone cell – the resolution of the printing device (in spots-per-inch) and the line screen frequency (in lines-per-inch). Simply divide the printer resolution by the line screen and the answer tells you how many spots reside in each halftone cell in the horizontal and vertical dimensions. The square of this is the total number of spots within the cell. For example, say you print a 150 line screen to a 1200 SPI laser printer. 1200 divided by 150 equals 8, meaning the halftone cell is 8 spots wide by 8 spots tall for a total of 64 spots (8 x 8 = 64). Therefore, this combination of line screen and printer resolution will produce a print with only 64 shades of gray. The print will have noticeable banding and will be pretty ugly (see Fig. 7).

The bottom line is that a typical office laser printer with a resolution of 1200 SPI does not have sufficient resolution to print a decent halftone at 150 LPI. You need at least 2400 SPI to produce a halftone cell that is 16 x 16 spots, producing 256 shades of gray with no visible banding. This is why professional printing hardware – imagesetters and platesetters – typically print at a minimum of 2400 SPI. With increasing demand for higher line screens (200 LPI and above), these devices often print at 3600 SPI or even higher.

Summary

The term "dots-per-inch" or DPI is widely misused among graphic artists, illustrators, and photographers, often used in place of more accurate terms like lines-per-inch (for halftone screens), spots-per-inch (for printer resolution), or pixels-per-inch (for the resolution of digital images). This leads to confusion about the relationship between these concepts. Hopefully I've helped to clarify these terms (and how they relate to one another).

Jim Perkins is a Professor in the Medical Illustration program at Rochester Institute of Technology, where he teaches courses in human gross anatomy, scientific visualization, and computer graphics. He is also a practicing illustrator, creating artwork for several best-selling medical textbooks, mostly in the areas of pathology and physiology. For 20 years, he has been the sole illustrator of the Robbins and Cotran series of pathology texts. He is also part of a team of illustrators who carry on the work of the late Dr. Frank H. Netter, considered by many to be the greatest medical artist of the 20th Century. To see examples of Jim's work, visit the following links:

RIT faculty page

Netter art