Understanding Color Subtraction: Color LaserJet Printing
Updated: Nov 20, 2018
The trend in service for the past few years has been moving to color and this trend makes understanding the process of image formation in color printing more important than ever. The process of creating an image in color has many similarities to monochrome, but there are important differences as well. Paramount in these differences is the use of multiple color toners and the utilization of producing color via color subtraction.
Color subtraction is the way that we perceive colors in the ordinary world. The light in the world around us contains all the colors of the rainbow, but not all light contains all these colors in all circumstances.
The surface of object has color if it reflects some of the light, and the color is determined by the frequency or the combination of frequencies of the light reflected. These frequencies still do not possess color on their own; rather, the light must be received by some means and interpreted as a color. These steps must all be taken into consideration when attempting to produce a color image on the printed page.
Only pure white light contains all the colors available to be perceived, and if any of these colors are unavailable from the light source then those frequencies are not available to be reflected from an object being viewed.
Understanding what light is available is the first step in understanding how color images are perceived. Most situations do not have pure white light with which to judge any color image, so when diagnosing color defects, one should find a good source of light to use in the diagnosis of a color subtraction issue.
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Use in Troubleshooting
Next, once we have found a good source of light with which to troubleshoot a color, we must understand how a color is produced on that printed page. Unlike light that is emitted from a light source, which is made up of the three primary colors red, blue, and green, the primary reflective colors cyan, magenta, and yellow are used to take away frequencies of light and leave a specific set of frequencies to be received and interpreted by the eye.
The remaining toner “K” or black toner is used to create a surface that absorbs all light, and the addition of black toner to the mixture of the other three primary colors has two effects.
The first effect is to add depth to the color, as small amounts of light throughout the surface do not reflect any light. The second is a by product of the first, requiring the use of smaller amounts of the more expensive color toner.
One should use this information to understand that the color green, on a printed surface, is the placement of cyan, yellow, and black toner or more accurately, in varying degrees, the absence of magenta. This information is used to aid the diagnosis of color subtraction in the most basic form.
If one is trying to produce green on a printed page, then the goal is to eliminate the presence of magenta and properly balance the application of cyan, yellow, and black.
This point is often missed by technicians who may understand the balance of cyan and yellow but overlook the addition of black toner to achieve the proper tone of the color on the printed page.
Committing to memory the coverage of the primary subtraction colors required to create even the most popular printed colors is difficult at best, so the use of Pantone color charts (Pantone is a copyright of Pantone, Inc) is a necessity.
Many of the major hardware manufacturers use Pantone color matching to determine the calibration of the toner, so this is a good starting place. The best match would be a chart that was designed specifically for the model that one would be working on, but realistically that is not always possible.
As a part of my tool kit, I keep a color chart on the HP Color LaserJet 5500 and the Lexmark Optra C710 for generic use.
The End of Guessing
The intended color should be determined next. Find a printed page that the customer finds acceptable, or use the color as represented on the monitor before the page is printed and compare that color to the chart.
This information will give a baseline for the toner mixtures required for the print job. Next, use the same chart and locate the color produced on the page that is unacceptable. In the two examples below we will assume that the first color is the desired shade and the second is the unacceptable print.
The example above shows very similar applications of cyan, magenta, and yellow; however, the black toner is almost completely missing.
Simply looking at the two colors and interpreting the missing link in this situation seems counter intuitive.
We must diagnose which color is missing; however, the colors are, in point of fact, almost identical but with different depth. Being unprepared for this eventuality might lead one to spend fruitless hours diagnosing a problem that does not exist. This costly mistake can be avoided. Let’s look at another example:
In this example the troubleshooting will be more troublesome but using the previous technique one can determine first that the black toner required for this color is once again missing; however, this case also has what would appear to be too much magenta.
Even more complex is banding that can occur in solid colors but this too can be resolved using this technique. Identifying the separate colors present in the bands can reveal which toner is present or missing depending on the situation.
Color subtraction is a vital component to the engineering of a color printed page, and should be well understood by technicians that work on color printers of any type.
The ability to diagnose efficiently print defects in a color print is becoming much more critical. The lack of this skill will be an increasing burden in the changing service industry, and these increased burdens must be avoided in the quickly evolving service market.
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