What's in the chopstick-case sized "CIS Unit"

--- There are two types of flatbed image scanners - fatter ones using CCD sensors, and the slim ones using CIS sensors - right? Kurokawa: Yes. By the way, since CIS stands for Contact Image Sensor, we don't call them "CIS Sensors".--- Ahh, OK. So I heard before that a CCD sensor uses a very superfine optical unit that reduces down an A4-wide image and captures it on the sensor, and all of this fits in a carriage only 5 centimeters high. It's got a cold cathode ray tube that's the light source, several mirrors, lenses, CCD... However, the CIS unit is much smaller and thinner, isn't it Takeuchi: Yes, it's about the same size as a chopstick-case.It covers the width of 216 mm, which is the width of a ltter size sheet, and it can read in at 2400 dpi. It's a one-for-one optical unit, so the sensor elements are also placed at a definition of 2400 dpi. In fact, about 20,000 sensor elements are tightly laid out in a line.
CIS   CCD

CIS

  

CCD

Difference in structure between CIS type and CCD type (when scanning a reflected document)

While both types direct light at the document from below a glass plate and read the light reflected back, the type of sensor used determines the optical design, the structure of the carriage (the scanning part that houses the optical unit) and the final size of the whole scanner.

--- So when you scan something, the sensor moves up and down, reading in the document huh? Then it moves smoothly up and down emitting white light, it seems kind of noble and touching to me. Kurokawa: Did you know that the white light is actually the LED (Light Emitting Diode) light of three colors mixed together?--- You know, that's what I thought! If I wobble my head while looking at the light, I get the feeling that the light has color in it!
Kurokawa

"The white light [emitted by the scanner] is actually the LED light of three colors mixed together."
(Kurokawa)
Kurokawa: In a CCD scanner, we use elements with color filters on them to read the signal, and from those we reconstitute full color. In our current models we have two lines each of Red, Green and Blue, for a total of six lines.--- So you're using a technique like that in digital cameras. Kurokawa: Yes, but in a CIS scanner, by changing the color of the light source, we can do a full-color scan with just one row of sensors. So we only need one row of sensors, but three colors - RGB - of LEDs--- So you emit red light, and read the reflected light, emit green light and read the reflected light, emit blue light..., red..., green..., blue... and keep emitting and reading the reflected light while moving up the document. So you don't need filters, and you can get away with fewer elements and less wiring. Then you end up with a smaller sensor unit. Kurokawa: Yes. The software has to do more work, but the total cost can be kept down.