2.972 How a Scanner Works

DIGITAL SCANNER
QUESTIONS OR COMMENTS

	AUTHOR:	Jennifer Blundo
	E-MAIL:	jblundo@mit.edu
	COURSE:	2
	CLASS/YEAR:	1

MAIN FUNCTIONAL REQUIREMENT: Convert an image/photo from analog to digital format.

DESIGN PARAMETER: Digital Scanner

GEOMETRY/STRUCTURE:

	Item	Purpose
1.	Stepper Motor	Powers the transmission belt
2.	Logic Board	Relays information to and from the scanner and the computer
3.	Scanning Unit/Carriage	Contains a light source and CCD sensors
4.	Transmission Belt	Moves the scanning unit across the image
5.	Flex Circuit	Carries the CCD information to the logic board
6.	CCD sensors	Converts measured reflected light into a voltage proportional to the light intensity
7.	Glass Plate	Allows light to pass from light below glass to image and supports image
8.	ADC	Analog digital converter unit that changes CCD measured voltage to digital

	View as VRML
Sketch of Main Scanner Components	Mechanism For Moving Light Source In Scanner

EXPLANATION OF HOW IT WORKS/ IS USED:

The image to be scanned is placed on top of the scanner’s glass plate
The computer sends instructions to the logic board about how far the motor is to run and how fast
The logic board instructions place the scanning unit into an appropriate position to begin scanning
The scanning unit moves across the image to be scanned at a speed designated by the logic board instruction
As the scanning unit moves across the image, a light source shines on the image
The light strikes the image, reflects, and is then reflected by a series of mirrors to the scanner lens
The light passes through the scanner lens and reaches the CCD sensors
CCD sensors measure the amount of light reflected through the image and converts the light to an analog voltage
The analog voltage is changed to digital values by an ADC – analog to digital converter
The digital signals from the CCDs are sent to the logic board and transmitted back to the computer

The information is stored in the computer as an electronic file made up of pixels. The group of pixels is changed into a picture by the TWAIN scanning software.

scanner_flow_chart.gif (6715 bytes)

A Flow Chart of How a Scanner Works

A Color Scanner vs. a Black/White Scanner

Black/white scanners only have one light source, and color scanners have three light sources, one for each primary color—red, green, blue.
Color scanning can be achieved by a one-pass scanner or a three-pass scanner.
A one-pass scanner scans the image once and records all three colors at the same time, while a three-pass scanner makes three passes over the image and records only one color each pass.

DOMINANT PHYSICS (drive system):

Variable	Description	Metric Units	English Units
P_in	Power input to electric motor	Watts	Horsepower
P_out	Power output from shaft	Watts	Horsepower
w	Shaft rotation speed	rad/s	RPM
h_m	Mechanical efficiency	---	---

The scanning unit uses reflection to pass light from the image to the CCD sensors.

The velocity of the shaft connected to the motor can be determined. The power in and the power out of the motor can be calculated:

P_in = I x V (DC motor)

P_out = T x w

h_m = efficiency = P_out / P_in

P_in = P_out / h_m

The efficiency of these types of motors is about 90%

LIMITING PHYSICS (drive system):

None submitted

PLOTS/GRAPHS/TABLES:

None submitted

APPLICATION OF SCANNERS:

Compact document scanners, hand held scanners, photo scanners, flatbed scanners, and drum scanners
CCD sensors are also found in video and digital cameras

REFERENCES/MORE INFORMATION:

Scanning Information Center: Glossary of Scanning Terms
http://www.coe.uh.edu/course/cuin/7317/scanning/glossary.html

Optical Scanner—Encartaâ Concise Encyclopedia Article
http://encarta.msm.com/index/conciseindex/63/063aa000.html

Scanning Information Center: How does a Scanner Work?
http://www.coe.uh.edu/course/cuin/7317/scanning/work/html

Scanners
http://www.dircon.co.uk/pctechguide/images/18scanners.html

Sylvain’s Home page: CCD
http://www.multimania.com/srondeau.ccd.html