WORKING WITH COLOR In order to work consistently with color printing, you need to calibrate both your monitor and your printer. The basic steps for doing so are listed below. This is really a minimum approach; if you want to understand color, light, resolution, and related issues more fully, there is a wealth of technical information on the web. Monitor Placement Calibrating your monitor will be of limited utility if you can t see your monitor clearly and without artefacts. For best viewing of images on a monitor, follow these guidelines: Place the monitor at roughly right angles to a window. If you sit facing the window, your pupils will contract because of the light coming in the window, and you will not be able to see the monitor display well. If you sit with your back to the window, you will very likely have glare spots and window reflections on the monitor surface that will make all your work a bit more difficult. Place the monitor at least several feet from any window as the fact that it is at right angles to the window means the light will fall more strongly on the window end of the monitor, causing that part of the image to appear comparatively washed out. The further your monitor is from the window, the more this effect is reduced. Note that laptop screens are not ideal for color work because of the narrow angle of viewing, which particularly affects the perception of gamma (brightness). If you get deeply into work that requires professional color matching and control, you should always use a large external display. Monitor Calibration On a Macintosh, this is normally done from System Preferences > Displays > Color. Calibrate your monitor twice, once in the daytime (when ambient daylight will be the dominant illumination) and once in the evening (when artificial lighting will dominate). If you have several lights in a room that are turned on and off in different combinations, calibrate the monitor under your normal working conditions. Then, when doing fine color corrections, be sure to have the correct monitor calibration for the time of day loaded. Use the recommended settings of gamma 1.8 and color temperature of 6500K/D65. Check your monitor calibration using a reference image. This is probably the single most important step you can take in setting up your computer system to work with color in a reliable way. Begin by opening a digital reference image such as the Getty image and check the whites and grays closely for color shifts. Since this image was professionally prepared, you can trust that it is properly color balanced. Pay special attentions to the grays (neutrals). If the image shows color shifts, recalibrate your monitor from System Preferences > Displays > Color until the reference image looks ok. Do not alter the image itself in Photoshop.
The next thing to do is to make a test using a reference image that from outside the computer: a piece of original art or a photograph with a good range of color values. Scan this into the computer and check how well it matches. If the digital reference image looks ok but your scan looks off, you probably have a problem with your scanner rather than with your monitor. Play with the scanner s own settings (gamma, color balance) until your scans also look right. Color Space and Color Profiles So you ve calibrated your monitor and your image looks great on the monitor but it s too magenta when it comes out of the printer. What s going on, and how can you fix it? First let s talk about color space. Each digital device a camera, a monitor, a printer can only represent a limited subset (termed the gamut) of all the colors there are. Plus, ink-based color is very different perceptually from light-based color. So a given pixel in your image that has a numerical value of 70R-200G-10B will be made visible as a set of varying bright greens on different devices. The rule sets used to determine these greens are called color spaces and color profiles. To begin with, images usually begin in a device-independent color space, called the working or reference space. This is normally generated when the image is created or captured (scanned, snapped). Such spaces tend to be what is termed well-behaved, meaning especially that grays are neutral and don t skew towards color. Examples: Adobe RGB is a color space that was designed to contain the entire color gamut available to most CYMK printers. By contrast, srgb is an HP/Microsoft-defined color space that describes the colors visible on a low-end monitor. It s also often typically used as the default profile for scanners and digital cameras. However, these reference color spaces don t function terribly well when it comes to outputting an image to specific devices such as monitors or printers. At the point of output, device-dependent color profiles come into play that accurately describe the unique color characteristics of the device. The most generally used are ICC profiles. For example, there is a color profile for my home printer, the wonderful Epson 2200 it has a gamut wider than many other inkjet printers, plus it prints with 7 inks rather than 4 (black, light black, cyan, light cyan, magenta, light magenta, and yellow). Its profile is used, among other things, to translate the 3 RGB colors of my 70R-200G-10B bright green into those 7 inks. Practically speaking what is happening is that the bright green is being translated into the nearest possible (but still duller) green that can actually be achieved with a mix of cyan, light cyan, magenta, light magenta, yellow, black, and light black inks. The printer s color profile also handles such matters as changing the amount of ink sprayed on matte paper stock vs glossy paper stock. You may have noticed in working with images and Photoshop that you can choose Printer manages colors or Photoshop manages colors. Essentially, what you re doing there is deciding which color profile (color space) dominates the generic reference
space (device-independent) or the specific space (device-dependent). By default, the printer profile dominates when going to print output. Tip: When opening an image in Photoshop, always check use srgb or use embedded profile unless you know what you are doing. Steps to take in calibrating your printer. During printer calibration, you ll be working with your reference image. It s critical to get the system to work without adjusting the reference image in Photoshop. That is, the base pixel information of the image should remain unchanged; what is changed is how that information is translated in the final print. For example, if your reference image has in it a medium gray say a 128R-128G-128B gray that is a true gray, in that the three values of RGB are exactly equal. If it s printing magenta, the wrong thing to do is to adjust it in Photoshop all you are doing is moving it away from gray. It may then print right on your printer, but the odds are it will print wrong on every other printer in the world! It s the printer that s going to get adjusted, as follows: Load the right printer driver, and use the correct settings for your paper stock. Using your reference image, make a test print at high-quality (not speed ) settings on heavy stock. (You can do the same calibration with regular paper stock or even light inkjet paper, but it s much more difficult to get good matches because of the way these papers absorb ink they will nearly always look too dark and desaturated.) If the test print is not excellent (that magenta shift!), adjust the printer s color control settings until you get a good match. It may take quite a few tries. Once you get a good match, save the settings with a highly descriptive name for example: epson2200-heavy-stock-hq-1440. Use those settings in future. You can have number of specialized printing profiles saved on your computer. Your desktop printer should now reliably print to match your onscreen images. Now that you ve done all your calibration with a reference (perfect) image, you can open one of your own images in Photoshop. Does it look too magenta? Now you can color correct it in Photoshop since you ve calibrated the monitor, you can trust that your monitor is showing you a true representation. And when you print it using your saved settings, it should come out right. And when you send it to a professional printer, it should also come out right. Checking the Image in Photoshop If you don t have a desktop printer of your own, or if you can t print to a printer where you can reset all the controls as in the steps above, use these quick-and-dirty tests to get some key information about how your image will print. The Info tool: use this tool to check the brightest whites and darkest blacks in your image. In most cases the whites should not be 100% (i.e. 255/255/255), but a few points off, or they will look blown out. (An exception is areas that you want to be entirely
white, such as the paper background of a line drawings). Similarly, the darks should be slightly above 0/0/0 say, 3/6/5 or 4/7/9. Most presses print slightly darker than your image shows a phenomenon called dot gain so a 100% black may end up being overinked, leading to a blotchy effect. Dot gain tends to fall in the 2-4% range, so a 94-97% black will end up very close to 100% black once the piece comes off the press. FYI, in color work, you never want a black that is 100% C, 100% M, 100% Y and 100% K so much ink will end up on the page that it is likely to smear and blotch before drying. You want what is called a rich black that is more like 63C, 52M, 51Y 100K. If any one of the three RGB colors is more than 10 points off the others (e.g. 15/3/6), your image probably has a slight color cast and should be corrected for that color using the Curves tool. Out-of-gamut colors: Monitors can display a much greater range of colors than printers can reproduce using standard 4-color (CYMK) printing. In particular, very bright greens, greeny-blues, and pinks are likely to print much more subdued than they appear on the monitor. Use the out-of-gamut color feature of Photoshop to check for colors in your image that will print quite differently than they appear on the monitor. (Another way to check is to temporarily shift your image from RGB to TIFF.) First go to Preferences > Transparency & Gamut and set the Gamut Warning color to some color that is not in your image (or only minimally in it). Then go to View > Gamut Warning and any pixels in your image that are out of gamut for the image s color space will show up in the warning color. Understanding Resolution, DPI, PPI, and LPI pixels per inch (ppi): is a measure of scan, or INPUT resolution (often confusingly called dpi instead). dots per inch (dpi): resolution of OUTPUT device (e.g. printer). standard dpi settings include: 72-75 dpi: monitors (monitor rez is often confusingly referred to as ppi) 600, 1200 dpi: laser printers 720, 1440, 2880 dpi: current inkjet printers 1270, 2540 dpi: old-style linos and negatives for commercial printing lines per inch (lpi): measure of rulings for SCREENS used in commercial halftone printing and desktop (inkjet) printers. The human eye cannot easily resolve more than 500 lpi, which is why art books tend to be printed in the 300-600 lpi range. Standard line screens include: 100 lpi: quick offset shops; often use cardboard master and have limited run (under 500) 150 lpi: good offset printing; eg. most magazines, catalogs 300, 600 lpi: art books
relationship between dpi and lpi: Computers and printers are binary, so their individual dots (dpi) are either on or off. These little square dots are clustered into larger dots of differing sizes to reproduce the full tonal range of the piece. In commercial printing, these larger dots are organized in a halftone grid pattern whose size and spacing is determined by lines per inch (lpi). LPI dots of different colors (e.g. CYMK) are usually placed at an angle to each other to avoid moire effects and banding effects. In inkjet printers and some new commercial printing methods, stochastic screening is used. The difference is that halftone screens use different sized dots in a fixed (lpi) grid, while stochastic screening uses much smaller, equal-sized dots whose distance from each other varies. That is, in one the dot size or amplitude varies and in the other the dot spacing or frequency or varies; thus the first is also called AM or ampitude modification printing and the second FM or frequency modulation printing. An advantage of stochastic methods is that you can get better-looking images at lower resolution. Grayscale Relationships High line rulings mean sharper images but fewer grays. # grays = 1 + [printer res in dpi / line screen in lpi]squared Example: 300 dpi printer, 60 lpi screen= 26 grays Example: 300 dpi printer, 100 lpi screen= 10 grays Example: 1270 dpi printer, 100 lpi screen= 145 grays 1.5 Rule for Scanning Rule of thumb: scan resolution in dpi = 2x desired lpi Example: if print using 150 lpi screen, scan at 300 dpi best rule: lpi x dimension x 1.5 = scan resolution Example: 100 lpi screen x 4.5 inches high (original/repro) x 1.5 = 675 pixels high 100 lpi screen x 6 inches high x 1.5 = 900 pixels high Scaling Images Up When size is set to 100%, the image is scanned at actual size in whatever resolution is pre-set. Example: a 1-inch square image scanned at 200 ppi = 200 pixels x 200 pixels = 40,000 total pixels When size is set to 200%, the image is enlarged FOUR times. Example: a 1-inch square image scanned at 200 ppi and 200% = 400 x 400 = 160,000 total pixels. Note that this is the same as a 1-inch-square image scanned at 400 ppi and 100% = 400 pixels x 400 pixels. Always scale the image up as you scan, rather than afterwards in Photoshop, if at all possible. Scaling up in the scanner results in a sharp image; scaling up in Photoshop results in fuzziness. If unsure of final dimension of work, try to scan larger as it usually
works better to downscale later, since downscaling normally sharpens up an image. A good rule of thumb is to guess the largest size image you will possibly need for your project say 8 x 10 @ 300 ppi and double that. COLOR & BIT DEPTH BITS GRAYS COLORS 1 2 8 2 4 64 3 8 512 4 16 4k 5 32 32k 6 64 262k 7 128 2M 8 256 16M