Setting Halftone LPI: Taming the Beasts of Resolution

Home , Flexography, Halftone, Screen printing

By Jerry Waite, Cheryl Willis, and Garth Oliver attributes that can contribute to, or detract from, the faithful reproduction of the image.

Some of the decisions that must be made about halftones include type (amplitude- or frequency- In addition to an exposition of the modulated), dot shape (elliptical, square, round, and so on), screen angle(s), and screen frequency. The variables involved in choosing LPI, choice of screen frequency, known in the United States this paper provides a concrete as lines per inch (LPI), is complex and based upon numerous aesthetic and mathematical considerations. method for selecting the best screen Although halftone type, dot shape, and screen angle(s) frequency for a given job. are also important concerns, this paper deals solely with LPI decisions. In particular, it is concerned with decisions that need to be made about LPI when reproducing images using conventional amplitude- modulated (AM) screening. Abstract Photographs must be converted to halftone format when In addition to an exposition of the variables involved they are prepared for print reproduction. To faithfully in choosing LPI, this paper provides a concrete reproduce an image using halftone dots, the proper method for selecting the best screen frequency for a resolution must be chosen to match the end use of the given job. printed product as well as the attributes of the process and materials that will be used to reproduce the image. Relevance to Standards for Technological One of the important considerations when making a Literacy halftone is screen frequency, or lines per inch (LPI). The procedures recommended in this paper will The choice of LPI is complex and based upon numerous assist technology teachers, especially those who teach variables. This paper explains the variables and provides information and communication technologies, to meet a step-by-step method for determining LPI. several of the Standards for Technological Literacy (STL) standards. In particular, Standards 2, 11, and 17 Introduction are supported. One of the most fundamental decisions to be made when preparing a photographic image for printed Standard Two (Students will develop an understanding reproduction is the choice of the proper halftone of the core concepts of technology) Benchmark Y (The screen. Whereas lay people simply click “Print” and stability of a technological system is influenced by all let their computer and printer driver make all the of the components in the system, especially those in decisions, professional graphic artists—both designers the feedback loop) is supported by teaching students and printers—must take into consideration halftone how to choose halftone LPI because of the impact

• THE TECHNOLOGY TEACHERe • October 2006 of various variables on the overall appearance and Another way of illustrating frequency would be to usability of a printed project. In particular, relevant consider the compactness of halftone dots within components of the system include aesthetics, end-use a square inch. Higher LPI screens have many more requirements, the printing process, the substrate on halftone dots in a square inch than lower LPI screens. which the image will be printed, and the resolution For example, a 133 LPI screen has 17,689 halftone of both the output device and the digital photograph. dots per square inch (133 x 133) while a 35 LPI screen Choices made regarding any of these components will has only 1,225 halftone dots per square inch (35 x 35). influence the outcome. Some commonly used screen rulings include 65, 85, 100, 120, 133, 150, 175, 200, and 300 LPI. Teaching how to choose halftone LPI also helps students to develop the abilities to apply the design Factors Affecting Screen Frequency process (Standard 11). In particular, the criteria and constraints imposed by the several variables that The choice of LPI is affected by numerous variables must be considered when choosing LPI will affect the including: aesthetics, end-use requirements, the outcome of the design process (Benchmark N). printing process, the substrate on which the image will be printed, and the resolution of both the output Perhaps the most relevant STL standard is 17: Students device and the digital photograph. The graphic artist will develop an understanding of and be able to select must carefully consider each of these variables and and use information and communication technologies. determine which LPI best suits the constraints of Halftoning is an integral part of visual communications each one. Oftentimes, the best LPI resolution for one technologies because reproductions of images using variable is not suitable to another. For example, an either black-and-white or color halftones are a part of output device may be able to handle a 150 LPI screen many printed communications. In particular, students but the substrate may only accept a 50 LPI screen. The who are taught LPI selection will learn about the proper LPI for a given job is the lowest appropriate to inputs, processes, and outputs associated with sending any of the variables. and receiving information (Benchmark L). Aesthetics Screen Frequency Defined Higher LPI screens produce printed halftones using Screen frequency refers to the number of lines, or smaller dots than lower LPI screens. Smaller dots rows, of halftone dots in one linear inch (LPI) or one result in a higher-quality image that more faithfully linear centimeter (LPC). For example, in Figure 1, the reproduces minute detail and variations in density than halftone on the left is reproduced using a 35 LPI screen, the larger dots used in low-LPI screens. In essence, a while the photograph on the right uses a 133 LPI high LPI halftone has greater resolution than a low LPI screen. Notice that the dots in the 35 LPI screen can be reproduction. In this case, “resolution” refers to “the seen easily while those in the 133 LPI screen are almost level of reproduction detail offered by a TV or invisible to the naked eye. computer screen or other image” (Microsoft, 1999, Resolution section, paragraph 5).

Aesthetically-based LPI decisions should best be made by the graphic designer in consultation with the client. One way to make aesthetically based LPI decisions is to peruse printed samples of the same image reproduced using varying screen frequencies. Sources of such samples include the Pocket Pal, which is regularly updated and published by International Paper (Romano, 2003). Designers and clients understandably want their images to contain the highest possible resolution. However, it is important to remember that aesthetic considerations are only one of six variables that must be considered when making LPI decisions. What is desirable is not always achievable. Figure 1. 35 vs. 133 LPI.

• THE TECHNOLOGY TEACHERe • October 2006 End-Use Considerations “The rule of 240 comes from establishing premium Printed products are all designed and produced quality as 175 LPI at 16.5 inches from your eyes with a particular end-use in mind. For example, the (normal magazine high quality printing and viewing). product could be a fashion magazine, daily newspaper, If you extrapolate the 175 quality to a greater distance, direct-mail advertisement, invoice, letterhead, novel, you can use the number 240 and it will give you the textbook, art book, fine-art reproduction, point-of- distance in feet. Visually you will find that the dots will purchase (POP) display, road sign, or billboard, among disappear at that distance. This is called ‘dot diffusion.’ thousands of different possibilities. Each of these items The image looks like a continuous tone. Some people requires varying amounts of image detail. For example, with very good eyesight or a good imagination say consumers have come to expect a great deal of image they can see the dots, but most people say they can’t” fidelity in catalogs they use to make purchasing (M. Ruff, RayHan PGF, email communication, May 23, decisions. On the other hand, resolution of images in 2005). invoices is not of much concern; the legibility of the amount due is more important than resolution. Based on the Rule of 240, the appropriate screen frequency for a given print job can be determined from Viewing Distance Table 1. One seldom considered end-use requirement is viewing distance. Printed photographs on billboards Distance LPI viewed from a distance can use very low LPI screens 20 feet 12 because the dots are too far away from the viewer 18 feet 13.33 to be discerned. POP displays in supermarkets, gas 16 feet 15 stations, and convenience stores, are viewed more 14 feet 17 closely than billboards. So, to minimize the appearance 12 feet 20 of the halftone dots, POPs must be printed with higher 10 feet 24 LPI screens than billboards. Very high resolution is 8 feet 30 6 feet 40 necessary for items such as art books. People look 4 feet 60 at photographs in such books closely—at perhaps a 2 feet 120 distance of six or fewer inches—to discern minute 1 foot 240 detail. Thus, these printed photographs require very 6 inches 480 high LPI screens to minimize the effect of the halftone dots on the image. Table 1. Viewing Distance vs. LPI using the Rule of 240.

The resolution of an image has an impact on its file An Empirical Study of LPI vs. Viewing Distance. size. Higher LPI image files are logarithmically larger Although the SGIA recommends the use of the Rule than smaller LPI files. Thus, to save hard drive space, of 240, they have never conducted a study to validate speed up file processing in image-processing and page it: “We’ve never conducted a project due to the layout programs, and to decrease RIP and output subjective nature of the topic” (J. Shell, SGIA, email time, it is important to constrain LPI to a reasonable communication, May 23, 2005). resolution. In a separate study, Waite and Oliver devised a test The Rule of 240. The Specialty Graphic Imaging to determine at what distance the dots in a printed Association (SGIA) recommends using the Rule halftone could be discerned by the average viewer of 240 when choosing the LPI for a given print (Waite & Oliver, in press). A color digital photograph application (Specialty Graphic Imaging Association, was converted to grayscale using Photoshop’s standard 2003). According to this rule, the optimal LPI for a 20% dot gain profile. The resultant file was duplicated given viewing distance is found by dividing 240 by the 11 times. Each of the duplicates was assigned one distance in question. Thus, if a halftone is to be viewed of the following LPIs: 5, 10, 18.75, 37.5, 50, 65, from a distance of two feet, the LPI would be: 85, 100, 110, 120, 133, or 150. The LPIs of 65 and 240 ÷ 2 feet = 120 LPI above are those commonly used in offset printing applications. Screen rulings of 18.75 and 37.5 LPI relate mathematically to the non-dithered output capabilities SGIA bases the Rule of 240 on a reference 175 LPI of 600 and 300 DPI marking engines. The 5 and 10 LPI screen that is often used in magazine production.

• THE TECHNOLOGY TEACHERe • October 2006 samples were included so that the test proctor could this way, the results in Table 2 are somewhat consistent explain to the subjects how to take the test. The dots with the Rule of 240 as disseminated by SGIA. in the 5 LPI sample were clearly visible at 20 feet, so However, Table 2 is generally more conservative than the participants could be shown what to look for when the Rule of 240 and its use will decrease the chance recording their observations. that viewers will be able to see the dots in a printed halftone. Subjects were then asked to view the print from varying distances that were clearly marked on the floor. The Printing Process These distances were: 20’, 18’, 16’, 14’, 12’, 10’, 8’, 6’, 4’, 2’, Today, an image can be reproduced in many ways. For 1’, and 6”. Participants then recorded the distance at example, a few copies of an image can be printed on a which they could first discern the individual dots in desktop or workgroup black-and-white or color laser each sample on a record sheet prepared for the study printer. Larger quantities can be made on analog or (see Figure 3 on page 10). The results of the study are digital black-and-white or color duplicators. If many presented in Table 2. copies are needed, or if the end use requires the copies to look better than those made on a laser printer or LPI Mean Mode Median duplicator, digital files can be reproduced on high-level 150 0.08 0 0.00 printing machines by a professional printing company. 133 0.32 0 0.00 The printing firm will use the file to print copies using 120 0.67 0 0.00 various printing processes, such as offset lithography, 110 0.85 0 0.00 gravure, screen-printing, or flexography. 100 1.17 1 0.50 85 1.65 1 1.00 Printing departments or firms accept digital files, 65 2.56 2 2.00 including photographic files, and subject them to a test 50 3.86 2 2.00 known as “preflighting.” This examination will reveal 37.5 5.84 4 4.00 defects in the files, such as improperly set LPI. If errors 18.75 11.10 10 10.00 are found, the printing company or the originating 10 17.14 20 18.00 graphic artist must perform “file intervention” and 5 19.90 20 20.00 repair the job before it goes to press. Table 2. Distances at which viewers can discern varying LPI screens Digital Printing Technologies New printing technologies are decreasing the The tested LPIs are presented in the first column number of steps between the creation of a file of Table 2. The second column provides the mean and its reproduction. For example, the Heidelberg distance at which the subjects could discern the dots. Quickmaster DI offset lithographic presses prepare Column three indicates the mode, or most often the printing plates directly from a digital file while occurring distance at which an observer could see the the press is running. The technology employed by dots in each sample. Finally, column four indicates the these machines eliminates most traditional prepress median viewing distance. activities. Therefore, there are few opportunities to correct errors. Simply stated, the file must be correct According to Table 2, the majority of people (the mode) the first time! Consequently, the person who creates could discern 5 and 10 LPI halftone dots at 20 feet. One a file to be reproduced using one of these machines hundred LPI screen dots could be seen by most people must possess a high degree of skill and knowledge to at a viewing distance of one foot. Screen frequencies patiently build into the file all the attributes it needs to of 110 or higher could not generally be seen by viewers print well. even at a distance as little as six inches. Contemporary “on-demand” printing technologies The results of the study indicate at what distance merge the convenience of copy machines with nearly dots can be discerned. Therefore, it is important to the quality of conventional printing processes. Digital use a higher LPI screen (smaller dots) than the one duplicating machines, with names such as Xerox indicated in the chart so that dot diffusion will occur. Docutech, Xerox Docucolor, HP Indigo, and Xeikon, Consequently, if the reading distance is one foot or less, can print beautiful images “on demand”—that is, as it is important to use an LPI higher than 100. If used in many or as few copies as needed whenever they are

• THE TECHNOLOGY TEACHERe • October 2006 needed. The Docutech prints only black and Process Characteristics Products white, but the Docucolor, Indigo, and Xeikon Offset Provides excellent quality Most printed products: print in full color. These machines can also Lithography/ and high resolution for newsletters, annual produce variable-data press sheets: i.e., each Waterless photographs and text. reports, books, many printed sheet can contain different text and/or Lithography Most offset lithography is magazines, brochures, images. Again, because there are few steps currently done using the and so on. between the creation of the digital file and conventional lithographic subsequent output on one of these digital process that requires the devices, it is critical that files be properly use of water as well as ink. prepared. Proper preparation includes the LPI Waterless lithography setting. uses no water and can Relationship Between LPI and Printing print better-looking images than conventional Process lithography. However, its Before beginning to prepare any photograph use is limited. for reproduction, the printing process that Flexography Every image has a slight Most packaging, will be used to reproduce it must be known. halo around its perimeter from cereal boxes to Table 3, Common Printing Processes, presents due to the soft flexible candy wrappers and a summary of different printing processes and plates used in the process. frozen food wrappers, their uses. Each printing process puts ink on The reproduction quality is printed using paper using different methods. The way the of photographs is not as flexography. ink is applied to the paper influences how high as offset-lithography. small the halftone dots can be. For example, Gravure Every image, including Extremely long-run offset lithography uses a soft and smooth text, is created using editions (millions of rubber surface—called a blanket—to press small dots. The process copies). Direct mail the ink onto the paper. The blanket is capable can produce very good catalogs, such as those of transferring very small dots, so high LPI quality photographic distributed by Sears screens can be used. On the other hand, reproductions if high- and JC Penney, are screen-printing uses a stencil mounted to a quality paper is used. often printed using fabric screen to print images. Ink is forced gravure. National through the stencil onto the substrate using Geographic is printed a squeegee. Halftone dots in a stencil must with the gravure be large enough for the ink to pass through process. them. Consequently, screen-printing cannot Screen printing Capable of applying the Short runs, difficult print as small dots as offset lithography can thickest layer of ink of to print materials print. Other printing processes are capable any printing process. It (such as textiles), of reproducing various LPI dots. Refer to can print on odd-shaped T-shirts, billboards, Table 4 for examples of LPIs that various objects and materials that convenience store processes can accommodate. It is important are difficult to print using signs, point-of- to remember that the LPIs in Table 4 are other processes, such as purchase displays, and examples only. Graphic artists should always glass bottles and T-shirts. printed circuit boards. check with the printing firm or department On-demand Any printing process that Short runs of many that will print the final job. printing makes only as many copies types of documents, as needed when they are including business In many cases, the same photograph will be needed. Examples include forms, personalized reproduced using several different printing laser and ink-jet printers, letters, brochures, and processes. For example, a fast-food restaurant copy machines, Docutech, memos. may have a photograph of a sandwich to be HP Indigo, and Xeikon reproduced in a national magazine printed presses. Machines can by gravure, on placemats printed by offset often produce variable- lithography, and on point-of-purchase displays data jobs. printed by screen printing. The halftone must Table 3. Common Printing Processes (based on Waite and Willis, 1998)

• THE TECHNOLOGY TEACHERe • October 2006 be prepared with a different LPI screen for each of paper than screen-printing can produce on coated these products. paper. Furthermore, screen-printing can print higher LPI screens on smooth materials than it can on textiles. The Substrate Halftones can be printed on a variety of surfaces. For Table 4 provides guidelines to be used to help choose example, a photograph of a fast-food sandwich could LPIs for jobs to be printed on various substrates using be reproduced on smooth, shiny-coated paper in a a range of printing processes. However, Table 4 is only national magazine, on rough absorbent uncoated paper a guideline. Printing presses not only vary by process for an in-restaurant place mat, and on plastic for point- but also by individual printing machine; no two presses of-purchase displays. The absorbency and roughness of print exactly the same. Professional printing firms test the substrate affect the LPI screen that should be used. each of their presses and commonly used substrates Generally speaking, rougher papers must use lower LPI using a process called fingerprinting (Waite, 1997, screens than smoother papers because printing presses 2003). This test will indicate the best LPI for a given have a difficult time pressing tiny dots into the crevices process, press, and substrate. The graphic artist should of rough papers. In addition, ink has a tendency to ideally check with the printing firm or department to blot when printed on absorbent (uncoated) paper. In ascertain this information rather than rely on generic particular, 50 percent and larger dots may become guidelines. enlarged by this blotting action—so much that they create a darker-looking image than they should. To Output Device Resolution—DPI keep dots in dark areas of photographs printed on In order to be printed, files containing halftones must absorbent paper from overlapping, lower LPI halftone be output to some type of device. Output devices can dots are generally used. Conversely, coated papers include laser printers, imagesetters (which expose film absorb very little and cause little blotting to occur. that will be used to image printing plates), platesetters Therefore, higher LPI screens can be used on coated (which image plates directly from digital files and papers. Most materials other than paper (plastic, bypass the use of film), direct-imaging (DI) presses metal, glass, and so on) tend not to be absorbent, so (which contain both a platesetter and a printing press higher LPI screens can usually be used. in one integrated unit), and digital duplicators (which image and print on the fly). Paper—and other substrate—vendors often provide printed samples of the lines of material they sell. These The resolution of an output device is rated according samples can be used to visually evaluate the suitability to the number of “printer” dots per linear inch (DPI) of varying LPI screens to different grades of paper. that a laser printer, imagesetter, platesetter, DI press, Printing firms or departments are also good sources of or variable-data duplicator can produce. Most laser information regarding the use of different LPI screens printers, DI presses, and digital duplicators in common on given paper stocks. use have output resolutions of 300 to 1,200 DPI. High- end imagesetters and platesetters can image at 1,200 When both the process and substrate variables DPI, 2,400 DPI, or higher. are considered, offset lithography is capable of reproducing much higher LPI screens on uncoated There is an important distinction between halftone

Process/Substrate LPI Screen Printing on Textiles 50 Xerographic copy machines 50 to 75 Xerox Docutech 35 to 75 Xeikon 35 to 75 Indigo 50 to 133 Screen printing—smooth surfaces 85 to 110 Flexography—uncoated paper up to 133 Flexography—coated or label paper 133 to 150 Flexography—pharmaceutical printing on coated paper up to 175 Conventional offset lithography—uncoated paper 120 to 133 Conventional offset lithography—coated glossy paper 150 to 250 Waterless offset lithography—coated glossy paper 200 to 800 Gravure—all substrates 150 to 200 Table 4. Screen rulings for process and substrate combinations (Waite and Willis, 1998)

• THE TECHNOLOGY TEACHERe • October 2006 dots and printer dots. Whereas conventional AM x 16 cell can produce 257 gray levels (256 sizes of dots halftone dots vary in size (light areas are reproduced plus blank paper). using small dots while dark areas are composed of large dots), every printer dot is the same fixed size. Laser If the number of gray levels is increased, the maximum printers, imagesetters, platesetters, DI presses, and LPI an output device can produce will decrease digital duplicators can either image a printer dot or because more of the device’s printer dots are being leave it blank. Printer dots cannot be partially imaged, used to make each halftone dot. If, on the other hand, so varying sizes of printer dots are not possible. the number of gray levels is decreased, the maximum LPI an output device can produce will increase because To create various sizes of halftone dots, the output fewer of the printer’s dots are needed to make each device must use groups of printer dots—known as halftone dot. halftone cells. Each halftone cell is composed of individual printer dots. Halftone cells with few black With a given output device, either LPI or levels of printer dots appear white to medium gray. Halftone gray can be increased, but not both. Higher resolution cells with more black printer dots appear dark gray or output devices can produce higher LPI halftones. The black. In Figure 2, the halftone cell on the left contains relation between printer dots and halftone dot cells is 256 printer dots and can produce 256 different size illustrated in Figure 2. dots plus white for a total of 257 gray levels. The halftone cells on the right each contain 64 printer dots, A halftone should contain enough gray levels to making them capable of representing 65 different gray faithfully reproduce the original. If a photograph levels. contains a lot of detail and is to be reproduced on high- quality paper, 257 gray levels is appropriate. However, for a lower-quality reproduction, such as a photograph for a newspaper, 65 gray levels may be enough. Relationship Between Gray Levels and an Output Device’s Maximum LPI To determine the maximum LPI an output device can produce, divide its DPI resolution by the square root of the desired number of gray levels minus one.

For example, to calculate the maximum LPI of a 2400 dpi output device at 257 gray levels, divide 2400 by 16 (the square root of 256). The result is 150. Thus a 2400 Figure 2. DPI vs. LPI DPI output device can successfully produce 257 gray levels when imaging a 150 LPI halftone. Gray Levels As previously mentioned, an output device produces Output devices can routinely image LPIs less than halftone dots using cells, each containing a group or equal to the maximum LPI calculated as above. of printer dots. A halftone cell containing two rows Manufacturers of some devices claim their machines and two columns of printer dots can produce 22 or can successfully image higher LPI screens than the four different sizes of halftone dots. Similarly, if a cell math would suggest. These devices utilize dithering contains eight rows and eight columns of printer dots, methods to create the illusion of additional shades. 82 or 64 sizes of halftone dots can be created. Most It is best to test the performance of such machines to photographs can be adequately reproduced using 256 determine if LPI settings above that calculated with the sizes of halftone dots. This corresponds to 16 rows and above formula provide satisfactory results. 2 16 columns of printer dots (16 = 256). Digital Photograph Resolution—PPI In addition to the number of sizes of halftone dots that The image resolution of a photograph impacts the can be produced, an output device can also leave the maximum LPI that can be used to print the photograph paper blank. Thus, a 2 x 2 cell can produce five gray as well as the quality of the printed image. Low levels (four sizes of dots plus blank paper), while a 16 resolution photographs should only be printed with

• THE TECHNOLOGY TEACHERe • October 2006 low LPI screens. If a high screen frequency is used on a low image resolution photograph, image quality will To determine how wide a scanned photograph can be suffer. printed same-size at a given LPI, divide the number of pixels across the image by either 1.5 times the LPI Photographic resolution is defined in terms of pixels (Photoshop calls this “good quality”) or 2 times the per inch (PPI). Scanners capture the grey or color levels LPI (Photoshop calls this “best quality”). For example, of small components of an original photograph. Each if a given digital photograph is 1,330 pixels wide and of these components is called a picture element (pixel). it is to be reproduced at the same size using a 133 LPI Scanned pixels are displayed using corresponding screen with best quality, divide 1,330 by 266 (twice the pixels on a computer monitor. Digital cameras also LPI). This photograph can be reproduced a maximum capture image data in discrete pixels. of five inches wide. If, however, the specified LPI is 150, the same halftone can only be 4.4 inches wide (1,330 The PPI resolution of a photograph must be adjusted divided by 300). to match the LPI of the screen chosen for the halftone reproduction. This is done using the Nylquist Theorem, If the PPI of a digital photograph does not support the which states that a photograph’s resolution should chosen halftone LPI, there are three things that can be be: a quality control factor (2) times the LPI times the done: rescan the photograph, decrease LPI, or decrease magnification. Thus, a photograph to be enlarged to the QC factor. Of these, the best choice is to rescan. If 200% of its original size and printed with a 150 LPI the original photograph is not available, decreasing LPI screen should be scanned at 600 pixels per inch. or the QC factor may work. Upsampling digital files should only be done as a last resort. In a perfect world, scanners would only need to scan one pixel per halftone dot. However, scanners are Making the LPI Choice imperfect, so Photoshop needs more than one pixel Once all the variables—aesthetics, end-use to make a high-quality halftone dot. Experts disagree considerations, printing process, substrate, output on the quality control factor, but most agree that the device resolution, and photograph resolution—have absolute minimum is 1.5 and the maximum is 2.0. been considered, the actual process of choosing the Thus, for a 100 LPI halftone to be reproduced the same appropriate LPI is quite simple. Simply choose the LPI size as the original (100%), the scanned photograph that is the lowest! For example, consider the following should contain a minimum of 150 PPI and a maximum table in which all the LPI variables for a given job have of 200 PPI. In the above formula, the more common been considered: QC factor of two is employed. However, a QC factor of 1.5 can appropriately be used when calculating Variable Appropriate LPI photograph resolution with the Nylquist Theorem. Aesthetics 150 LPI When Photoshop transforms a photograph into a End-Use considerations 150 LPI halftone, it takes the 1.5 to 2 pixels that are available Printing process 110–150 LPI for each halftone dot and averages the color or tone of Substrate 120 LPI those pixels to determine the optimum halftone dot Output device resolution 150 LPI size. Photograph resolution 150 LPI Overall LPI choice 120 LPI The Image Size dialog box in the Image menu in Photoshop can be used to set the PPI resolution to Table 5. LPI values for the attributes of a hypothetical print job. match the chosen LPI. If the image contains more pixels than are actually necessary, it is completely Even though the majority of the variables will accept acceptable to resample the image down (decrease its a 150 LPI screen, the appropriate choice is 120 LPI size). However, if the image does not contain sufficient because the substrate cannot handle such a fine screen. data to support the chosen LPI, upsampling (increasing Of course, the client is likely to be less than ecstatic the size) of an image can lead to unsightly pixelization. about this development. In such a case, the human- Thus, if an image contains too little data for the chosen relations skills and thorough knowledge base of the LPI, it is better to rescan the original photograph, being printing firm’s or department’s customer service careful to set the scanner according to the Nylquist representative will be of utmost importance. Theorem.

• THE TECHNOLOGY TEACHERe • October 2006 A Step-by-Step Process for Determining LPI References The following table can be used as a checklist when Microsoft. (1999). EncartaÆ World English Dictionary. determining the correct LPI to use for a given job. Redmond, WA: Author. Specialty Graphic Imaging Association. (2003). Step Process Datasheet: Choosing the right linescreen. 1  Discuss the job with the client and/or graphic Retrieved May 31, 2005 from www.sgia.org/db/ designer to ascertain the required paper, member/datashts/downloads/dsht1.pdf printing process, and desired LPI that will meet Romano, F. (Ed). (2003). Pocket Pal. Memphis, TN: the job’s aesthetic needs. Make a note of the International Paper Company. LPI. Waite, J. (1997). Altering grayscale images to 2  Consider the end use of the printed product. compensate for press fingerprints.Visual Items viewed from a great distance do not need Communications Journal. high LPI resolution, whereas items viewed Waite, J. (2003). Conventional offset lithography and close-up need fine screens. Use Table 1 and/or direct digital printing devices: How do they Table 2 as a guideline. Make a note of your compete on print characteristics? Visual answer. Communications Journal.  Consider the printing process. Contact the 3 Waite, J., & Oliver, G. (in press). Viewing distance as printing firm or department to find out which LPI is best for the particular printing process, a variable in discerning grayscale halftone press, and substrate to be used for the job. Make dots at varying screen frequencies. Journal of a note of the answer. Industrial Technology. 4  Consider the substrate. The printing firm or Waite, J., and Willis, C. (1998). Electronic Prepress department is the best source for information Using Photoshop 4. Albany, NY: Delmar about the suitability of various LPIs to Publishers. substrates. Paper vendors often provide printed samples of their merchandise so that visual comparisons can be made of varying screens. Jerry Waite has been involved in the Make a note of your answer. printing and publishing business since high 5  Determine the highest LPI resolution the school. He currently teaches graphics com- output device (laser printer, imagesetter, munications technology at the University platesetter, DI press, or digital duplicator) can of Houston. He can be reached via email at handle. Divide the device’s DPI resolution by [email protected]. 16 (if 257 gray levels are desired) or 8 (if 65 gray levels are sufficient). Make a note of the answer.  Determine the highest LPI the digital 6 Cheryl Willis, Ph.D. is an associate photograph file can support by using the professor of Information Systems Technol- Nylquist Theorem. Make a note of the answer.  ogy at the University of Houston. She has 7 Compare the LPIs calculated using Steps 1–6. developed curriculum for business educa- Whichever answer is lowest is the correct LPI tion and information technology at the for the job. secondary, post-secondary, undergraduate, Table 6. A checklist for determining LPI (adapted from Waite and and graduate levels. She can be reached Willis, 1997). via email at [email protected]. Garth Oliver is a high school assistant principal in Houston, TX. He can be Conclusion reached via email at GOliver@sheldon. Even though the variables involved in choosing the k12.tx.us. correct LPI screen for a given print job may appear intimidating, the beasts of LPI, DPI, and PPI can be tamed if the methodical process provided in this paper are followed. This is a refereed article.

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