Viewing conditions - The eye adapts to the ambient light source and sees it a neutral white. If the spectral power distribution of the source is uneven and weak in some wavelengths, it is impossible to judge colors accurately. Thus, it is important to standardize viewing conditions. Standards defined by the American National Standards Institute (ANSI PH2.30-1989), BS 950 and ISO 3664 are in use and are functionally identical to one another.

Standard 5000 oK. viewing stations are setup in the press room, prepress lab and the pilot plant. The specifications for viewing conditions include color coordinates (device independent, will return later), color temperature, spectral power distribution and light intensity. Standard viewing conditions should always be used for comparisons between originals, proofs and prints.

Surface Properties - A surface does not have a precise color but rather an ability to absorb certain wavelengths and reflect others. A leaf appears green because it absorbs red and blue and reflects only green. It always appears green so long as the light source doesn’t change. The reflection of light from a colored surface has two components. Some light is spectrally reflected from the first layer of the surface.

The remainder enters the substrate and undergoes scattering and multiple reflections before reemerging from the material as a diffuse reflection. When the light meets a pigment particle some wavelengths are absorbed, while others are reflected. The emerging light is perceived to have a color corresponding to the unabsorbed wavelengths. Since the light reflected from the first layer has not been absorbed, it makes the surface appear lighter. Thus, a computer monitor loses contrast because dark areas appear lighter. This is called flare.

The surface reflection characteristics are essential to the performance of printing inks and process inks that rely on the subtractive color system. Process colors are generally transparent so that overprints reflect based on all the colors present, while spot colors such and Pantone are opaque.

Process colors are formulated to have levels of absorbence close to the spectral response of the three types of cones in the retina of the human eye.

Overlaps of the cone responses and slight deficiencies in the pigments complicate the situation. The actual ink percentages used to reproduce a neutral gray are can vary with different pigments. For SWOP standard inks 50%C, 40%M and 40%Y are taken to be neutral gray. For such inks the 50% of each yields a decidedly red color cast, although perfect CMYs would reproduce a middle gray.

If the absorbencies of the CMY were perfectly matched to the spectral response of the eye, then overprinting the three colors would reproduce black. Instead, they generally produce an dark brown. Thus, to achieve a better black and to be able to reproduce a wider range of colors, black ink is generally used for process colors. Thus CMYK is used with the K denoting black which saves ink as well as enabling a wider color gamut (coverage of visible colors). The letter K is used by printers because it is the Key to good color printing.

Sometimes you will hear Cyan and Magenta referred to as blue and red respectively but you should remember that this can’t strictly be true. The overlap of true red and blue inks would appear black instead of blue. Other properties such as gloss, texture, absorbency and fluorescence determine how a surface reflects light. Some of these properties cannot be reproduced on a computer monitor.

Gloss is determined by the way that light is reflected from it. The greater the proportion that is spectrally reflected form the surface, the glossier it appears. A surface that scatters light randomly has a diffuse or matte appearance. Paper gloss depends on the smoothness of the paper surface and of any coating applied. It can be altered by finishes such as varnishing and laminating.