Gaurav Lokhande

Gaurav Lokhande holds B.Tech in electronics & telecommunication engineering. Currently, he is working at Lesics Engineers Pvt.Ltd. as a product developer. His areas of interest are embedded systems, Internet of Things(IoT), and designing of Printed Circuit Board (PCB). To know more about the author check this link.

Why are CMYK colors used in inkjet printers?

In the last article you saw, how a printer works in detail. You have seen which components are used in the printer to build an Inkjet printer. In this article, I will explain to you why are C(Cyan), M(Magenta), and Y(Yellow) colors used instead of the RGB (Red, Green, and Blue) in inkjet printers. We have seen the basic black and white printing technique of printers in the previous article, how can we produce any color?

What if we use RGB?

The most obvious and intuitive answer seems to be to use RGB (Red, Green, and Blue) colors because these are the fundamental colors. But in an inkjet printer, it just won't work. To understand why I will explain it to you through an experiment. Consider two colored flashlights. One is red, and the other is green. When I flash both torches at the same point, the resulting light is yellow as shown in fig 1a. But when I try the same experiment using ink colors of green and blue, those two colors create a muddy yellow color as shown in fig 1b.

Fig 1a : Flash of red and green torch
Fig 1b : Mixing of red and green ink on paper

Why do these two experiments produce totally different output colors even though the input colors are the same? The first case in which I mixed lights was an example of the additive color mixing method, while the second case with the ink mixing method is subtractive. The additive mixing method is so simple. The combined light portion directly reaches your eye and you see that color. The subtractive method is a little tricky. The lights reflected from the ink. Some portion of the light is absorbed. To better understand how the subtractive method works, we need to examine the ink at a molecular level.

How do we see colors?

We know, we see a red color because the red molecule absorbs everything except red light as shown in fig 2. In short, what we see in ink color is the remaining color after the subtraction. The same is the case with the green color.

Fig 2 : Reflection of red color

Let's consider two layers of molecules as shown in fig 3. Here, I am assuming that two different colors (red and green) molecules are uniformly mixed. Let's start the analysis with the bottom green molecule. The green molecule will obviously reflect green light. However, this green light has to pass through the upper molecular layer. As shown in fig 3. The top green molecule will just allow this green light. However, the neighboring red will completely absorb this green light. In short, that area will produce a black color. Similarly, now let's consider the light coming from the bottom red molecules. When I did the analysis the same way as the final output. The black is present in a good portion between the red and green lights, which affects the final output color. The output color we get is not perfectly yellow, it appears muddy yellow.

Fig 3 : Black portion in two-layer molecules of red and green color

A good example of the understanding of the effect of black color in the final output is the gray hair example. Sometimes we see some people's hair as gray, but in fact, it is a blend of white and black hair. This is why the subtractive method is totally different from the additive method, and why we got horrible printouts when we used R G B colors in the printheads.

Why do we use cmy?

I explained to you that we can't use primary colors to reproduce colors, because of the subtractive method. The color we see in an ink print is in fact the inverted portion after the absorption. The solution to this issue is simple. Just invert the fundamental colors. The inversion colors of red, green, and blue are cyan, magenta, and yellow, respectively. Thus, these are the colors we must use in the printer. I will explain to you how to produce a green color using these C Y M colors. First, release a drop of cyan ink from the nozzle. Before this drop dries up, release a yellow droplet at the same location. Mixing these two colors will produce a perfect green color. After some time, the drop gets absorbed in the paper and we get a green dot as shown in fig 4a.

Fig 4a : Perfect green color

Similarly, we can produce any color perfectly using C, Y, and M droplets. I had explained to you earlier how to produce a perfect green color in an inkjet printer. But can I produce a lighter shade of green color? Yes, I can do it. To achieve this, just print the normal green color with different spacing as shown in fig 4b. Increased spacing tricks will show a lighter shade of the color.

Fig 4b : Lighter shade of green color

What is the role of K in CMYK?

So far, we’ve learned that we can't use primary colors to reproduce colors, and the subtractive method is the villain. The color we see in an ink print is in fact the inverted portion after the absorption. The solution to this issue is simple. Just invert the fundamental colors. The inversion colors of red, green, and blue are cyan, magenta, and yellow, respectively. Thus, these are the colors we must use in the printer.

Until now I have explained how to achieve a lighter shade, but another challenge is to produce a darker shade. Darker shades cannot be achieved just by using C M and Y, the printer has to use black ink as well. The black ink is referred to as K, where K stands for a key.

Now, let's see the mechanism behind dark shades of green color printing. Can I achieve a dark shade by mixing black ink with green color? Logically, it's correct but there is an issue with this method. You won’t be able to increase the darkness of the green color by a small degree since the droplet size of the black and green are the same. To achieve different shades of the colors accurately, I have to drop black ink droplets between green droplets according to the shade of green. It will show a darker shade of color as shown in fig 4c.

Fig 4c : Darker shade of green color

The print quality of modern printers is quite high due to the very small size of the ink drops. The smaller the drop, the higher quality of the print. For decent quality, we required more than 300 dots per inch of paper. To achieve even more incredible results, the modern printer has 2100 to 4200 nozzles per print head.

I hope you might have learned how a printer works and its a color mechanism. I also hope you found both article 1 and article 2 related to Inkjet printer technology interesting.

Thanks for reading!