Scanner system
The flatbed image scanner captures an electronic image of the document on the glass. The scanner
does this by illuminating the document with LEDs (red, green, and blue) and capturing the image in
the image sensor to create an electronic format of the document. The flatbed scanner consists of
three main elements
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CIS scanner. The CIS (contact image sensor) scanner captures an image using the device's
optical path. Red, green, and blue LEDs sequentially illuminate a small strip of the document
(often called a raster line), and the optical system captures each color in a single row of CCD
sensors that cover the entire page width. Because only one color is captured for each line per
exposure, the three colors are recombined electronically to create the full color image. For
monochromatic scans or copies, all three LEDs are illuminated to create a white light for the
scan so the raster line can be captured in one exposure.
●
Mechanical carriage drive. The carriage drive moves the CIS scan head along the document
length to create the image. In this device, a small DC motor with an optical encoder creates this
motion. The speed of the carriage drive is proportional to the scan resolution (300 ppi is much
faster than 1200 ppi) and also proportional to the type of scan (color scans are three-times
slower than monochromatic scans). A 1200 ppi color scan moves so slowly that the product
may appear to not be working, whereas a monochromatic copy scan moves at fifty–times that
speed and will be a little noisy.
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Image processing system (formatter). The formatter processes the scanner data into either a
copy or a scan to the computer. For copies, the image data is sent directly to the printer without
being transmitted to the computer. Depending on user selections for the copy settings, the
formatter enhances the scanner data significantly before sending it to the printer. Image data is
captured at 300 ppi for copies and is user selectable for scans to the computer. Each pixel is
represented by 8 bits for each of the three colors (256 levels for each color), for a total of 24 bits
per pixel (24-bit color).
Scanner power-on sequence of events
When the device is turned on, it performs the following tests:
●
Motor test. The device moves the motor left and right to confirm operation. It reports a scanner
error 12 if no motion is detected in the motor encoder system.
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Wall find. The scan carriage moves slowly to the right while watching an encoder on the
carriage motor to determine when the carriage has found the right side wall or stop. This
enables the device to identify the document origin (position of the original). It reports a scanner
error 13 if this process fails.
●
LED check. The device moves the carriage to the white calibration label under the right side of
the flatbed image scanner, and it verifies that the minimum and maximum response is
acceptable. It reports a scanner error 14 if the response is unacceptable.
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Home find. The scan carriage uses the optical scanner to find physical reference features that
relate to the document origin at the right side of the image glass. This process ensures accurate
location of the first document pixels so that the user documents will have an accurate placement
of the image on scans and copies. It reports a scanner error 6 message if the reference features
are not found.
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Calibration. This test, also known as scanner color calibration, enables the device to identify
the black and white on every pixel in the CCD. Calibration occurs in two major processes: a
broad (analog) adjustment of all pixels to bring them into the target output range, and a pixel-by-
56 Chapter 4 Operational theory ENWW
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