How does a printer make colorful pictures?

1. When you press print, the computer sends the picture to the printer. A roller pulls in a sheet of paper and the picture appears, stripe by stripe.
2. Look through a magnifying glass and you'll see a secret: the picture is made of thousands of tiny colored dots.
3. A color printer uses just four inks: cyan, magenta, yellow and black. Printers call them CMYK. K stands for black, the key color.
4. When dots overlap or sit close together, the colors mix. Cyan and yellow make green, magenta and yellow make red, and cyan and magenta make blue.
5. The printhead has hundreds of tiny holes called nozzles. A heater makes a little bubble that pushes out one drop of ink, thousands of times a second.
6. The printhead zooms from side to side, spraying a stripe of dots. Then the paper moves forward a little, and it prints the next stripe, until the page is done.

The lid is open! Tap each part of the printer and watch it move.

• Paper tray. The paper tray holds a stack of blank sheets. The printer takes one sheet at a time.
• Rollers. Rubber rollers grip the paper and pull it through the printer, moving it forward a tiny step after each stripe.
• Ink cartridges. The cartridges hold cyan, magenta, yellow and black ink. They ride along with the printhead.
• Printhead. The printhead slides back and forth on a metal rail, spraying ink exactly where the picture needs it.
• Nozzles. Under the printhead are hundreds of nozzles, each thinner than a hair. Each one can fire thousands of drops every second.
• Belt and motor. A motor turns a rubber belt, and the belt drags the printhead from one side of the page to the other.
• Output tray. The printed page slides out onto the output tray, where the ink dries in a few seconds.
• Control board. A circuit board receives the picture from the computer and tells the motors and nozzles exactly when to work.

Turn inks off and on and print again. Drag the magnifier to see how dots of different colors build each part of the picture.

Experiment: What happens to the green hills if you switch off the yellow ink?

1. Look at the hills with the magnifier: which dots make the green?
2. Switch off yellow and print again.
3. Try switching off cyan and magenta too. What changes?

What we found: The green hills are made of cyan dots and yellow dots side by side. Take away the yellow and only the cyan is left, so the hills turn blue. Every color in the picture is built from a mix of the four inks.

Turn on JavaScript to try it yourself.

Mix the target color by choosing how much cyan, magenta and yellow ink to use. Print a sample and get as close as you can.

A screen makes colors by shining light, but paper doesn't glow. A printer makes colors by taking light away. White paper reflects all the colors of light. Each ink soaks up some colors and lets the others bounce back to your eyes.

Cyan ink soaks up red light, magenta soaks up green, and yellow soaks up blue. So where cyan and yellow overlap, red and blue are both soaked up, and only green bounces back. That's why cyan and yellow dots look green.

To make a light color, the printer uses small dots with lots of white paper between them. To make a strong color, it uses big dots that almost cover the paper. Your eyes blend the dots together, so you see smooth colors instead of dots.

An inkjet printer fires tiny drops through hundreds of nozzles while the printhead slides across the page. The paper moves up a little after each pass. It's like a very fast, very careful painter that only ever paints dots!

Key words

CMYK

Cyan, magenta, yellow and key (black): the four process inks.

Halftone

A picture made of dots of different sizes, which your eyes blend into smooth colors.

Nozzle

A tiny hole in the printhead that fires drops of ink.

Subtractive color

Making colors by soaking up (taking away) some colors of light, as inks and paints do.

Resolution

How many dots fit in a small space. More dots means finer detail.

Check your thinking

1. If you look at a printed picture with a magnifying glass, what do you see?

   Show the answer

   Tiny dots of a few colors. Printers build pictures from tiny dots of cyan, magenta, yellow and black.

2. Which two inks make green?

   Show the answer

   Cyan and yellow. Cyan and yellow mix to make green.

3. Why do printers have black ink, even though C, M and Y can make dark colors?

   Show the answer

   Black is darker and crisper, and saves the colored inks. Mixing C, M and Y makes a muddy brown and uses lots of ink. One black dot is darker and cheaper.

4. How does a printer make a light pink instead of a strong magenta?

   Show the answer

   Smaller magenta dots with more white paper between them. Small dots let lots of white paper show through, so the color looks lighter.

5. How many inks does a color inkjet printer usually use?

   Show the answer

   Four: cyan, magenta, yellow and black. Four inks, mixed as tiny dots, can make thousands of colors.