block when it passes over the correct spot on the printed line. As the printhead passes over
the exact spot on the print line that calls for a certain color, either the nozzles for that color
heat up or the Piezo crystals fire for precisely the right amount of time and place precisely
the amount of ink called for in the image file sent from the application program. Remem-
ber that thisprocessinvolvessprayingdropsofinkthatareaboutonemilliontimessmaller
than a small dro pof water out of nozzles thinner than a strand of human hair.
Color Conversion
The challenge with printing an application-generated or scanned color image on a color
inkjet printer is in the conversion of the colors from the RGB (red, green, blue) color scheme
used inside the computer to the CMYK color scheme of the printer. Colors are actually
combined wavelengths of light. As color wavelengths are added or subtracted, different
colors are created.
The page or image displayed on the monitor in RGB color must be converted to
CMYK colors before it can be printed. The computer’s internal color schemes are de-
signed for the monitor, which is a radiant device. The RGB color scheme, which uses up
to 24 bits to specify each pixel of an image, depends on the radiance (the wavelengths) of
the RGB dots within each pixel to blend and create its colors. This additive approach to color
createsaspectrumthatrangesfromblack(theabsenceofcolorwavelengths)towhite(the
presence of all color wavelengths).
Paper is a reflective device that produces color through a subtractive process. An
unprinted sheet of paper appears white because it includes light from all color wave-
lengths. To print a color on a sheet of paper, wavelengths are absorbed from the paper to
create colors. The colors of the CMYK model absorb different color wavelengths. When
used in combination, they absorb enough color wavelengths to create a wide array of col-
ors. For example, cyan (blue) ink absorbs red wavelengths and produces greenish-blue
colors. Magenta (red) ink absorbs green wavelengths and creates reddish-blue colors.
Yellow absorbs blue wavelengths to create yellowish-red colors. Black ink absorbs all
color wavelengths.
To convert the color scheme, a color looku ptable (CLUT) is used. The binary RGB
code for each pixel is looked up in the CLUT, and its corresponding CMYK binary code is
recorded for the corresponding dot on the page. While this sounds straightforward, it is not.
It is impossible to exactly match RGB colors with CMYK colors. This is why a printed ver-
sion of a color document may often look slightly different than it did on the monitor.
Color Halftoning
After RGB colors are converted to CMYK colors, the printer’s driver software generates in-
structions for each of the printhead nozzles. The printhead’s nozzles are either on or off, so
binary data is added to the color instructions that indicates when the nozzles of a particular
color are to be turned on and off. Remember that colors are mixed to form other colors, so
the nozzles of one, two, three, or all four colors may be on or off for any given dot in the im-
age.Leavingacolor’snozzlesopenforalongertimeresultsinmoreofthatcolorbeingused
in producing the color of the dot on the page. The on and off commands to the nozzles
Chapter 17: Printers^419