Colour Fundamentals in Print
How do we measure color and color difference?
Using a device called a spectrophotometer we can measure a color’s LCh and assign the color a unique numerical value on the Color Space graph. We can then measure the LCh of another color and plot its location on the graph to determine the direction and degree of color variation between the two colors. The linear distance from one plotted color value to another is called the “total color difference.” This value is labeled Delta-E (∆E) and is the most commonly used measure of color difference in industrial color applications.
Color Space – color by the numbers
Color can be described verbally or numerically using the same characteristics of Lightness, Chroma and Hue
CieLAB (or Cie L*a*b*, Cie Lab)
Color space in which values l*, a*, and b* are plotted at right angles to one another to form a three dimensional coordinate system. equal distances in the space approximately represent equal color differences. value l* represents lightness; value a* represents the redness/Greenness axis; and value b* represents the yellowness/blueness axis. Cielab is a popular color space for use in measuring reflective and transmissive objects.
Evaluating Color Difference
With a spectrophotometer, measure the color difference of the colors below from the red target color (circled color). Do the LCh and ∆E numbers correlate with what you see? Repeat this with a densitometer and compare the results.
What is the effect of light on color?
Since color can not exist without light, the lighting conditions under which color is evaluated have a considerable influence on color perception. Theoretical, standardized lighting conditions called “illuminants” are used in color measurement to simulate lighting conditions, such as fluorescent for a business or store and incandescent for a home. The graphic arts industry has standardized on an illuminant called D 50, which is based on natural daylight. Colors can appear differently under various lighting conditions because of their chemistry. In fact, two
similar colors may be perceived or measured as an exact match in one lighting condition, yet not match in another. This phenomenon is called “metamerism” (see sidebar), and has important implications on the evaluation of color matches in the graphic arts.
How can you best communicate color?
Color measurement data is a universal language. New software solutions are available that create a color data file, which can be shared electronically. These files are an efficient and accurate way to quickly communicate color. Color measurement data should include the illuminant (typically D50 in graphic arts), aperture size, observer angle (typically 2° in graphic arts) and the make and model of your measurement instrument.
Density—the primary control element used on press—is a measurement of light reflected off the press sheet. A densitometer measures ink density on a color bar, telling the press operator how to adjust the ink level. Proper density values are checked in each ink zone using a color bar or other areas of solid single-color ink.
Status refers to the kind of filters used in the densitometer. There are several different status filter sets, but Status T and Status E are the most common in the graphic arts. The difference between these two is in the filter used to measure yellow.
Backing what you measure against, can affect measurements. ISO standards specify black backing be used for consistent results. The tear off BLACK cover flap of this brochure can be used for this purpose. Always include status, backing and aperture size when communicating density data.
Apertures determine the size of the area measured, so always use a common aperture when comparing measurements. ANSI and ISO standards recommend the following minimum aperture sizes be used when measuring tone values with the corresponding screen ruling. If you are measuring tone values (dot area, for instance), refer to this chart to ensure your aperture is large enough for your printed screen ruling. recommended aperture sizes:
- 1.5 mm instrument, for 2 mm patches, for screening of 175 l/inch, 69 l/cm or above
- 2 mm instrument, for 3 to 5 mm patches, for screening of 133 l/inch, 52 l/cm or above
- 4 mm instrument, for 5 to 6 mm patches, for screening of 65 l/inch, 26 l/cm or above
- 6 mm instrument, for large patches (e.g. in packaging)
Tone value (dot area)
Most of the detail in printed images is located in the tonal areas. To control the highlight, mid-tone, and shadow areas for each color, a color bar typically has 25%, 50% (40%) and 75% (80%) patches. Monitoring throughout the press run gives a quick indication of halftone reproduction. Equal values for each color are critical.
Trap indicates how well a printed ink will accept a second ink printed over it compared to how well paper accepts that same second ink. Poor trapping will result in a color shift and may make it difficult to achieve proper color in different areas of the same image. Trap is measured from a color bar using patches representing the two-color over-print, the two individual inks, and paper.
Print Contrast is a comparison of the solid ink density and the shadow tint density (75% or 80%). This makes it a good indicator of print quality because shadow detail often carries important information. Print Contrast values correlate well to the subjective print quality evaluations of “flat” (low contrast) vs. “jumps off the page” (high contrast). Print Contrast is also used as a guide for maintaining proper ink and water balance.
Hue Error / Grayness
Hue Error describes the variation of a printed color from a theoretical pure color. Grayness indicates the presence of gray in a color that makes it appear less saturated. Hue Error and Grayness are used to check for color consistency throughout a press run. Today, colorimetric measurements are often substituted for Hue Error and Grayness because they correlate better to visual evaluation.
Gray balance is a simple concept that can be used to evaluate and control ink levels in order to reproduce pleasing color results. Combining nearly equal* tint values of cyan, magenta, and yellow (CMY) ink results in a neutral gray color. This fact can be used as the basis for a simple method of evaluating color reproduction. Because a gray balance patch includes all three of the colored inks, maintaining equal CMY densities will ensure that both the neutral and the colored areas of a reproduction have the right proportion of inks. This means that red, green, and blue, referred to as memory colors, will be printed with the right hue, ensuring pleasing color results.Using the density function you can measure a CMY gray patch and compare the CMY density levels to see if they are equal. If the density of one color is higher or lower than the others, an adjustment is required. Usually the press ink level is adjusted when gray is not in balance, although if dot gain is suspected to be the issue,ink and water balance may be the cause and the water level may be adjusted. In some cases, ink trapping or registration will affect gray balance, but often only density and water adjustments are used to keep the colors in balance.
The main disadvantage of using only gray balance to control color is the inability to troubleshoot and find the root cause of unbalanced color. Sometimes two or three types of gray balance patches are used to evaluate the highlight, mid-tone, and shadow.
Color bars (a.k.a. color control bars, color control strips, or proofing bars) are essentially test targets that are used to measure print and/or proof attributes. Normally, but not always, it is printed in the trim area of the press sheet.