Minolta Di620, Manual, Page: 46 / 466

M-27
<Realtime Scanning>
•
Realtime scanning is carried out by scanning the image data while copying is in
progress. (This is the same type of scanning used with analog PPCs).
•
The data samples are plotted into a reflectivity histogram. The histogram is used
to determine the type of the original, and the background removal threshold is
calculated accordingly.
<Reflectivity Histogram>
•
The CPU of the Auto Density (AE) Control Section generates the histogram
(image data reflectivity distribution) from the data samples obtained by realtime
scanning.
•
The reflectivity distribution varies according to the type of original being copied.
The CPU recognizes five different original types, based on the position of the
peak (*1) within the histogram, and calculates the background removal threshold
accordingly. The five recognized types are: two-tone B&W; newspaper; maga-
zine; map; and photograph.
*1: The peak value represents the density of the original’s background. For a
two-tone B&W image (such as a word-processor document), where the back-
ground is white, the peak would appear all the way to the right, as shown by
the dotted line in the above illustration. For a newspaper, which is character-
ized by a somewhat darker background, the peak would appear at about the
position shown by the solid line in the above chart.
•
Photographs, unlike the other original types, generally do not have a clearly
defined peak, since the reflectivity distribution tends to vary irregularly across the
entire length of the histrogram. The CPU is therefore unable to calculate a reli-
able background removal threshold. To prevent damage to the image caused by
inappropriate background removal, therefore, the copier sets the threshold to the
manual density mode’s center setting (setting “5”). [Note: Copying in PHOTO
mode may be advised for documents that contain many photographic images.]
(Black)
100 200
255 (White)
Reflectivity Histogram
Position of peak varies
according to type of original.
Peak Value
Reflectivity Data
Frequency
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