How do Scanners work?

So it's 11:45 p.m. and you're laying alone on your dust ridden bed watching fancy television series (GoT, maybe) and suddenly your phone lines up with a message from a friend who reminds you of a deadline to submit a copy of a very important document, to the administration, that is sitting in your closet dust covered. So you're going to take a picture of it with your phone but you realize your crappy built-in phone camera and the poor lighting from the dingy bare bulb hanging from the ceiling just aren't helping. So decisively you dash to your PC and use your flatbed scanner and as you sit there listening to the whirling of the scanning mechanism and pondering how your life got to this point you find yourself asking how the heck do these things work anyway.

Great question, your typical scanner uses something called a charge-coupled device or CCD to determine what's on the page. The main idea here being that the CCD can convert light into an electrical charge which it then sends to your computer as digital data. So when you start scanning a document that bright light you see coming from inside typically from either a xenon lamp or a cold cathode which as you may know is similar to what people use in PC case lighting a few years back. It's the sheet of paper and the light reflected back to a series of mirrors underneath the glass surface and then focused by a lens onto an array of CCD sensors, since darker areas of the page containing things like text and clipart will reflect less light than the usually white areas. The CCD array will see these differences which will show up as an accurate image of your document on your computer screen.

 If you've ever had a scan of something bulky and had to leave the lid open a little bit you'll have noticed the scan is black in areas that aren't covered by anything this is because nothing's there to reflect the light, though modern software is often smart enough to crop these areas out for you. CCD rates are also what determinate scanners true resolution. The more sensors in the array the more points of light it can capture and the more pixels it can send to your computer. If you're in the market for a scanner make sure to check that the resolution on the spec sheet is the hardware resolution that reflects the true abilities of the CCD array not some fake number achieved through software trickery like interpolation that uses average values of nearby pixels to approximate a higher pixel count image.

 But what about color scanning this uses additional lenses and built-in color filters to separate the scan into red, blue and green versions which are then processed to determine what the actual colors of your original document are, although this is usually done with just one pass of the scanner some older models lack these additional lenses and so need multiple passes to complete a color scan which is why they were much slower in color mode but not every consumer level scanner works this way.

There are also flatbeds that use something called a contact image sensor or CIS instead of these CCDs. These are simpler and construction use an array of LEDs to shine light on the document so an image sensor can essentially take a snapshot of it like you were trying to do on the table with your cell phone but better. Although the scans from the CIS are typically lower quality than CCD scanners. CIS is a cheaper more lightweight and more efficient technology so you'll find it in a lot of small portable scanners that can be powered solely from a USB port instead of requiring a wall outlet. Of course there are other types of scanners out there such as expensive drum scanners with high dynamic range for professional applications as well as 3d scanners.