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16 April '88
2. Automatic Image Density Control
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When automatic image density control is on, the exposure lamp voltage is set
to Vo + 5 volts. A change in the bias voltage only controls the image 'density.
The normal factory setting is the fourth column; however, this setting can be
changed for troubleshooting or to satisfy customer requirements by selecting
the Auto ID mode, holding down the Clear/Stop key, and pressing the image
density keys.
At the start of the copy cycle, the original sensing mechanism measures the
background density of the original image. It does this by measuring the
strength of the light reflected from a 30 mm strip near the leading edge of the
original. A fiber optic cable pipes the reflected light to a photodiode on the
lamp regulator board. The photodiode is the input element of the auto ID
circuit.
The CPU checks the voltage output by the auto ID circuit. This circuit has a
peak hold function. The peak hold voltage corresponds to the maximum
reflectivity of the originaL The CPU then determines the proper bias base
level, with reference to the peak hold voltage, and sends a 4-bit control signal
to the power pack. The power pack then applies the proper bias to the
development roller.
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Development Bias Voltage (V)
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2-28
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