Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-07-26
2003-01-07
Lee, Susan S. Y. (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S133000
Reexamination Certificate
active
06504558
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus for forming an image by means of an electrophotographic process of scanning a photosensitive member with a light spot. An image forming apparatus according to the invention can suitably be used for a laser beam printer (LBP) or a digital copying machine.
2. Related Background Art
Image forming apparatus including laser beam printers and digital copying machines are designed to use a light beam emitted from a light source such as a semiconductor laser with an intensity modulated according to the image signal applied thereto.
FIG. 1
of the accompanying drawings is a schematic cross sectional view of a principal portion of the optical scanning unit of a known image forming apparatus of the above identified type taken along the main-scanning direction. The parallel light beam emitted from a laser unit
61
containing a semiconductor laser is made to enter a cylindrical lens
62
showing a refractive power only in the sub-scanning direction. The parallel light beam entering the cylindrical lens
62
leaves it as parallel light beam in a plane intersecting the optical axis along the main-scanning direction (hereinafter referred to as “main-scanning section”) but is converged in a plane intersecting the optical axis along the sub-scanning direction (hereinafter referred to as “sub-scanning section”) to form a linear image extending in the main-scanning direction on deflection plane
63
a
of an optical deflector
63
, which is typically a polygon mirror. The light beam reflected/deflected by the deflection plane
63
a
of the optical deflector
63
is then led to the surface to be scanned of a photosensitive drum
66
by way of a focussing optical system (f&thgr; lens system)
71
having an f&thgr; feature to form a light spot on the surface. Then, the surface of the photosensitive drum
66
is repeatedly scanned by the light spot. The focussing optical system
71
comprises a spherical lens
64
and a toric lens
65
.
The optical scanning unit is provided with a BD (beam detector) sensor
69
, which is a photodetector for regulating the timing of the image forming operation to be conducted on the surface of the photosensitive drum
66
before the light spot starts scanning the surface of the photosensitive drum
66
. The BD sensor
69
receives the part of the light beam deflected by the optical deflector
63
that gets to a region outside the image forming region of the surface of the photosensitive drum
66
before scanning the image forming region. This part of the light beam is referred to as BD beam. It is reflected by a BD mirror
67
and then condensed by a BD lens (condenser lens)
68
before it enters the BD sensor
69
. A BD signal (synchronizing signal) is detected from the output of the BD sensor
69
and the timing of starting the image recording operation on the photosensitive drum
66
is regulated on the basis of the BD signal.
The photosensitive drum
66
is driven to rotate at a constant rate in synchronism with the drive signal from the semiconductor laser in the laser unit
61
, whereby the photosensitive surface of the photosensitive drum
66
is driven to move in the sub-scanning direction relative to the light spot scanning the surface. Then, an electrostatic latent image is formed on the surface of the photosensitive drum
66
, subsequently developed by means of a known electrophotographic process and thereafter transferred onto an image receiving member such as a sheet of paper to produce a finished image thereon.
Meanwhile, the optical scanning unit is provided with an automatic power control (APC) circuit for controlling the power of the light beam emitted from the semiconductor laser, although the APC circuit is not shown in FIG.
1
. The laser unit of
FIG. 1
is provided with the semiconductor laser and a photodetector for detecting part of the light beam emitted from the semiconductor laser. The APC circuit samples the output of the photodetector at predetermined timing and controls the output power of the semiconductor laser on the basis of the sampled output levels.
FIGS. 2A through 2C
are respective timing charts of operation of the semiconductor laser, the BD sensor and the APC circuit of a known image forming apparatus of the type under consideration for the period of a scanning cycle of the light spot that is made to scan the photosensitive drum repeatedly.
FIG. 2A
shows the operation of the semiconductor laser. It will be seen from
FIG. 2A
that the semiconductor laser is ON when the signal applied to it is at level High (H) and OFF when the signal is at level Low (L).
FIG. 2B
shows the operation of the BD sensor. It operates to detect the BD signal (synchronizing signal) when the signal applied to it is at level High (H).
FIG. 2C
shows the operation of the APC circuit. It operates to control the output power of the semiconductor laser when the signal applied to it is at level High (H).
The photosensitive drum used for the image forming apparatus has a image forming region (effective image region) and blank regions (non-image-forming regions) arranged at the opposite sides of the image forming region in the main-scanning direction. The toner image formed in the image forming region is ultimately transferred onto an image receiving member. Assuming that a light spot is made to scan the surface of the photosensitive drum in the main-scanning direction, the period during which it scans the image forming region is referred to as image forming period whereas the period during which it scans either of the blank regions is referred to as blank period. As seen from
FIG. 2A
, a blank period exists before and after an image forming period. Additionally, there exists a period during which the light spot scans not the surface of the photosensitive drum but outside the surface before and after each blank period. This period is referred to as off-drum scanning period.
As seen from
FIG. 2A
, the light beam emitted from the semiconductor laser during an image forming period is modulated by the image signal applied to it and the light spot scanning the surface of the photosensitive drum is turned on/off accordingly. The surface of the photosensitive drum is electrified in advance and, as the surface is irradiated with the light spot whose intensity is modulated, the electric potential of the surface of the photosensitive drum becomes differentiated as a function of the location on the surface to form an electrostatic latent image there. More specifically, any area on the surface of the photosensitive drum irradiated with light (exposed area) loses the electric charge whereas any area on the surface of the photosensitive drum not irradiated with light (unexposed area) retains the electric charge.
In known image forming apparatus, toner of the type showing the polarity same as the electric charge that electrifies the surface of the photosensitive drum in advance is used onto the electrostatic latent image formed thereon in a manner as described above for socalled reversal development. In other words, an exposed area becomes a printed area (area carrying the toner) because, if an unexposed area is made to become a printed area, the period for which the semiconductor laser is turned on for printing should be made relatively long.
The operation of automatic power control (APC) of controlling the output power of the semiconductor laser of the image forming apparatus is made to take place in the off-drum scanning period preceding an image forming period as shown in FIG.
2
C. Then, the operation of the BD sensor for detecting a BD signal (synchronizing signal) is made to take place after the automatic power control operation also in the same off-drum scanning period that also precedes the image forming period as shown in FIG.
2
B. While it is theoretically not necessary to energize the semiconductor laser in the off-drum scanning period, the semiconductor laser is in reality energized in the period during which the APC operation and the BD signal detecting operatio
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Lee Susan S. Y.
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