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How many colors can an ink-jet printer can produce?
"I use a HP680C in the office, and it
have two cartridges, one for black and one for color (yellow/cian/magenta?).
If the printer fire one drop of each ink at a given point, we can
have only 6 different colors (ignoring white and black). If it can
fire two or more drops at a given point, maybe we can have more
colors, but I suspect that the printer use this to control quality
of the presentation, not the number of colors. Anybody knows for
sure? With dithering it can make more colors, with reduced resolution."
Like most print processes you only have a limited
selection of inks to use. Full colour can be derived from three
primary colors, just like a monitor. For monitors, these are Red,
Green, and Blue because monitors emit light resulting in an additive
color process. Inks, on the other hand, absorb light so printing
is a subtractive process. The resulting inks should then be cyan
(blue+green or -red), magenta (red+blue or -green), and yellow (red+green
or -blue).
Therefore, the colors used in common ink-jet printers
are not really capable of producing true full spectrum photorealistic
quality results since they are red (not magenta), blue (not cyan),
and yellow. These are optimized for nice saturated primary colors
when used independently. Also see the section: Why are red, blue,
and yellow inkjet primaries?.
In addition, the combination of the three primary
colors should be capable of being combined to produce black but
due to misregistration and the pigments used, this black would be
somewhat muddy and brown. Therefore, a separate black ink cartridge
is normally used for black printing.
(From: Tony Hardman (AHED_CIJ@f54x19.demon.co.uk).)
With printing there are more problems than solutions
and I do not know which method HP use in their printing.
If you can vary the drop size, you can change
the drop spread on the paper. This can be done by firing bigger
slugs of ink, or multiples of the drop at the same position. As
you can figure the ink will either spread and make a bigger drop,
or stay the same size and become denser. Depending on the resolution
you want these could both improve colour density. This depends on
two key components.. The ink, and the paper.
The problems with laying down multiple drops on
paper is that if you do a large block the paper will curl up and
the overall image becomes worse. This is why you can pay 1$ a sheet
for 'quality' paper.
Another problem with this is speed. Firing two
drops in the exact same place is difficult... Unless the head is
stationary but that is not good either. You may notice that most
DOD printers in high resolution mode do a number of passes over
the same place. This does allow dithering and other techniques for
resolution / colour enhancement. They usually only print while going
in one direction for improved mechanical control.
In the 1600 printer there is a heater to assist
with the drying times and reduce the curling problem.
Inks are a problem too. They can dry at different
times because of the different dyes used, or they may not mix how
you expect if you place two colours on top of each other. Its only
ink ... but to get the best balance of surface tension, drying time,
viscosity, colour, stability.... and more is not as straight forward
as it might seam. I have noticed that the water based inks are improving,
and there are some that do not run if they get wet (after drying
on the paper).
I think the spec in your manual may suggest what
method they use.. The printer resolution (best) is 600dpi (I guess),
and I recon the best full colour resolution is lower. Also the print
head is only 300dpi so you must do two passes to get 600dpi black
(single black ink cartridge). This suggests a partial step of 1/600
inch between the passes. What happens when you print black using
the colour head? How many passes, how much slower? The resolutions
quoted may also be 600 * 300, or what ever. If they make blocks
of colour from a potential 600dpi machine, the resultant image is
probably only 75dpi (possibly less). This still might be called
600dpi, because the drop placement uses this resolution, but it
is not 600dpi at full colour. The resolution of quality picturers
/ poster is several thousand dpi, but not a variable image (not
ink jet).
In the Lyra publications they did publish the
real print head specifications for the machines they review. They
also include some of the methods of colour printing.
After all this I have noticed that I have not
answered the question of how do HP et all get their colour resolutions.
All I have mentioned is a few of the parameters that the designers
have to deal with.
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