A piece of paper is placed under a lid on a glass pane. The lid prevents any light from outside interfering with the image.
A fluorescent light source moves over the image.
The light reflected back is reflected onto a mirror. This mirror reflects the light onto a CCD capture chip.
This converts the light photons into electrical signals. These pass to a ADC giving the voltage of each photosite a binary code, done for each colour. Some scanners use RGB sensors but older ones use BW.
These colour codes are sent to the computer where the software writes the colour codes into a bitmap. It then saves the file to the hard drive.
Practical: the scanner can be used for photocopying or saving (less space and you can back up, easier to transfer and secure, keep it safe (you can lose paper)).
It's used in high security applications like air traffic control rooms and automated passport gates.
The eye looks into an infrared camera.
The infrared light reflected back is captured by a CCD.
You must position yourself correctly since otherwise the image can't be taken. The camera can focus automatically to compensate for this.
The light photons are converted into electrical signals corresponding to how much colour has been captured. These are sent to a ADC so the computer can retrieve the binary codes for the colours of the image.
An algorithm is applied to an image locating the areas of the eye, such as the centre of the pupil, the edge of the pupil, the edge of the iris and the eyelids and eyelashes. The algorithm ensures that only the iris is used for validation. A code is created typically of 512 bytes. This is compared to a series of iris codes stored in a database.
Practical: the chance of mistaking an iris code for another is 10^78:1
It's also significantly fast, much more so than a retina scanner.
However, people can fake the scan by using contact lenses.
It uses a low-energy infrared light source directed onto the retina.
The light is reflected back onto a CCD, which converts the reflected photons of the blood vessels into electrical signals. These are passed to an ADC, and the retinal pattern (based on the variations of the vessels) is stored in the computer. The blood vessels in the retina absorb light more readily than the surrounding tissue, so they are easily identified with the light.
You must use infrared light since visible light would make the iris contract and it could blind you. You cannot see infrared light.
Practical: your retina remains almost unchanged from birth to death so it's more accurate than an iris scan. It's also unique for every person and cannot be faked, being at the back of the eye.
However, it is significantly slower than an iris scan.