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Graphic Communications
Copyright Goodheart-Willcox Co., Inc.
Color Measurement
Precise standardized measurement is also part
of color evaluation. Measuring instruments vary in
both design and function. Some measure density,
some measure color values, and others measure
light waves. The most common color measuring
instruments are spectrophotometers, densitometers,
colorimeters, and spectrodensitometers.
Spectrophotometers
A photometer is used to measure light intensity.
Adding the prefix spectro- indicates that the
instrument is capable of measuring light of different
colors or wavelengths. When used for measurement
in the graphic communications industry, a
spectrophotometer converts this data to CIE color
specifications. A spectrophotometer is the most
accurate type of color measurement device.
The readout system of a spectrophotometer
either plots spectrophotometric curves or provides
the data from which the curves can be plotted.
Spectrophotometric curves provide a contour
or envelope that describes the reflection or
transmission characteristics of a sample. The
sample may be the image area on printed media, a
color negative, or a positive color transparency.
Spectrophotometric curves describe the physical
characteristics of viewing samples. They do not
describe the colors as perceived by a person viewing
the samples. For example, the curves for skin colors
show that human skin has the lowest reflectance in
the blue region and the highest in the red region of the
spectrum. See Figure 11-25. Light skin has the highest
reflectance at all wavelengths, but the curve is not very
smooth. The reflectance for any specific wavelength
is easily obtained from the curves. In the example at
700 nm, light skin has a reflectance of 70%, dark skin
45%, and very dark skin 15%.
Most spectrophotometers used in the graphic arts
industry are limited to reading light waves in the visible
spectrum. When reading light waves in the visible
spectrum, the samples must be placed as close to
the detector as possible to reduce the problem of light
scattered by the system, Figure 11-26.
topseller/Shutterstock.com
Figure 11-24. The
Channels
menu in an image manipulation application shows all the data on each color channel. You can
show them individually or in any combination.