Color Space Fundamentals
monitors emit color as RGB (red, green, blue) light. Although
all colors of the visible spectrum can be produced by merging
red, green and blue light, monitors are capable of displaying
only a limited gamut (i.e., range) of the visible spectrum.
Whereas monitors emit
light, inked paper absorbs or reflects specific wavelengths.
Cyan, magenta and yellow pigments serve as filters, subtracting
varying degrees of red, green and blue from white light to produce
a selective gamut of spectral colors. Like monitors, printing
inks also produce a color gamut that is only a subset of the
visible spectrum, although the range is not the same for both.
Consequently, the same art displayed on a computer monitor may
not match to that printed in a publication. Also, because printing processes such as offset lithography use CMYK (cyan, magenta, yellow, black) inks, digital art must be converted to CMYK color for print. Many printers now prefer digital art files be supplied in the RGB color space with ICC profiles attached. Images can then be converted to the CMYK color space by the printer using color management methods that honor profiles if present; this helps preserve the best possible detail and vibrancy.
Refer to the Instructions for Authors for your journal to determine if files should be supplied as RGB or CMYK. Some printers may prefer your files be delivered in RGB with ICC profiles attached, as this allows the printer to use color management methods when converting to CMYK. Other printers may prefer your files in the CMYK (Cyan/Magenta/Yellow/Black) mode, as this is the mode required for the printing process. If an RGB (Red/Green/Blue)
file is submitted, it must be converted to CMYK for print. When the conversion
takes place, color shifts can occur and TSG will do our best
to reproduce as close of a match to your printed output as possible.
can sometimes be difficult to visualize
the reason for color shift in color space
conversion. The best way to see the color
differences between the CMYK and RGB color
spaces is to look at a color gamut comparison
chart. The chart to the left plots the
visible color spectrum as the large "horse
shoe" area, and within this is a plot
of the CMYK colors, and the RGB colors.
You can see that in some areas the RGB
color space is "outside" that
of the CMYK space. It is these colors that
will be affected by a conversion from RGB
Most desktop scanners, digital cameras, and video capture systems save files
as RGB and the conversion of RGB files to CMYK can be done in many ways
(see how to convert RGB
to CMYK). RGB
converts to only CMY directly. However, when printing, we must add black
ink and in doing so must cut back on some color. The Undercolor Removal
(UCR) setup will help control this ratio so that a maximum ink density
for the four colors will be 300% when printing on a coated paper stock.
Digital art that is
comprised of spot colors (e.g., special colors: any colors that
are not CMYK process colors), generally require conversion to
the CMYK color space to enable file use. Because color gamut's
for spot color libraries, such as those associated with the PANTONE
MATCHING SYSTEM, usually extend beyond the ranges of the CMYK
color gamut, some spot colors may not be represented effectively
using CMYK process inks.
lithography, the density of CMYK inks can not be varied in continuous
fashion across an image, so a range is produced by means of halftoning.
In halftoning, translucent CMYK ink dots of variable size are
printed in overlapping grids. Grids are placed at different angles
for each of the ink colors. Smaller halftone dots absorb less
light; thus, as a result of an increase in the amount of reflected
light, apparent density is decreased and the object appears lighter.
screen angles (133lpi 40% screen enlarged)
information on Halftones
Color Demo 1
(386 KB) A collection
of 7 Photoshop files that you can open in Photoshop and use to
better understand color.
Color Demo 2
(122 KB) A second collection of 7 Photoshop files that you
can open in Photoshop and use to better understand color.
(2.3 MB) A guide for color in computer graphics, San Diego
Supercomputer Center, v 1.1, 1992. (Macintosh only)
(2.1 MB) A2B1 data exported
from an ICC profile using DuPont Color Scientist, 3D
rendered in JMP, and saved as a QuickTime movie.