Id Gamma (Γ) Correction; Gradation Processing - Ricoh Aficio 220 Service Manual

20 September 1999
ID Gamma (γ γ γ γ ) Correction
The machine automatically selects the most appropriate ID gamma correction
based on the selected original type (and the user tool Image Adjustment setting)
and ID setting made at the operation panel.
When the user selects "Service Mode" for any original type, you can use SP4-940
to change ID correction in service mode. The types that can be selected with SP4-
940 are different for each original mode (Text, Text/Photo, or Photo).

Gradation Processing

Overview
There are four types of gradation processing. Refer to the "Default Image
Processing Mode for Each Original Type" section for more details on which
processes are used for each original mode.
• Grayscale processing: This has 4 output levels for each pixel.
• Binary picture processing: This has only two output levels (black and white).
• Error diffusion: This is used with either grayscale processing or binary picture
processing mode.
• Dithering: This is used with either grayscale processing or binary picture
processing mode.
Grayscale Processing
In this machine, the 8-bit image data is converted into 2-bit data. This produces up
to 4 image density levels for each pixel.
To realize this, this machine uses a form of pulse width modulation. In this
machine, pulse width modulation consists of the following processes:
• Laser diode pulse positioning
• Laser diode power/pulse width modulation
Laser diode power and pulse width modulation is done by the laser diode drive
board (LDD). Briefly, the width of the laser pulse for a pixel depends on the output
level (image density level: from 0 to 255) required for the pixel.
This machine can also change the laser pulse position (at the left side of the pixel,
at the center, or at the right side) automatically, depending on the location of the
image pixel so that the edges of characters and lines become clearer. There is no
SP mode adjustment for this, unlike in some earlier models.
Note that although the LDD can create 256 levels per pixel, the machine only uses
16 of these, and only four are used for any one job. A gamma table determines
which four output levels are used. The gamma table is different for each original
type setting.
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IMAGE PROCESSING