Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-10-27
2002-11-05
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S205000, C347S236000, C347S235000
Reexamination Certificate
active
06476370
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling turn-on of light source and an image forming apparatus, and particularly to a method of controlling turn-on of a light source for an image forming apparatus in which a laser beam emitted from the light source is scanned on an image carrier by rotating a rotational polygonal mirror to form an image on the image carrier, and an image forming apparatus using the control method.
2. Description of the Related Art
Image recording apparatuses for recording an image with a laser beam, such as a laser printer, an electrophotographic copying machine, etc. have propagated. In these image recording apparatuses, a scanning operation using a laser beam is carried out on a photosensitive medium by an optical scanning device.
In the optical scanning device, generally, a laser beam output from a semiconductor laser is modulated on the basis of image data, and then made incident through a collimator lens, etc. to a reflection surface of a rotational polygonal mirror (hereinafter referred to as “polygon mirror”) rotating at a predetermined speed. By the rotation of the polygon mirror, the laser beam is deflected while the incidence angle of the laser beam is continuously varied, thereby performing a fast scanning operation on the photosensitive medium with the laser beam. The laser beam reflected from the reflection surface of the polygon mirror is guided through an f&thgr; lens, a cylinder mirror (or cylinder lens) or the like to the photosensitive medium to scan the photosensitive medium at a constant speed and also to be focused onto the photosensitive medium, whereby an image is exposed and recorded on the photosensitive medium.
The optical scanning device is provided with a start-of-scan (SOS) sensor at the substantially equivalent position as the start-of-scan position of the photosensitive medium, and the laser beam at the start-of-scan position is guided to the start-of-scan sensor by a reflection mirror to obtain a start-of-scan (SOS) signal. An exposure recording (image writing) timing, a laser diode light amount control (APC: Auto Power Control) timing, etc. are determined on the basis of the start-of-scan signal.
Here, when the beam width of the scan direction of the laser beam incident on the polygon mirror is set to be sufficiently larger than the size of the reflection surface of the polygon mirror (in the case of a so-called over-field type optical scanning device), the scanning operation is carried out so that the laser beam incident on the polygonal mirror is cut out by the polygonal mirror, and thus the ratio of the width of an image area to the scan-permissible width that is, the effective scanning rate can be set to a sufficiently large value.
This enables the frequency of the video clock to be reduced when the same resolution is required to be obtained at the same process speed, and also the cost of ASIC of the image processing to be reduced. Further, the radiation noise and the heating can be reduced, so that the cost required for shield design, etc. can be reduced.
However, in the case of an image forming apparatus having a function of varying the magnification in the fast scanning direction of the image by varying the frequency of image clocks, there is a case where a sufficient processing time cannot be secured to perform processing such as light amount control processing for a laser diode or the like when an optical scanning device having a large effective scanning rate such as an over-field type is used.
Next, there will be described the output timing of each of an SOS signal, an image area signal (LS signal), an SOS pre-turn-on signal and an APC signal, that is, the output timing of each signal on the basis of the output timing of the SOS signal (i.e., the lapse time from the output time of the SOS signal to the output time of each signal) and the number of image clocks when the magnification in the fast scanning direction of the image is not changed (nominal state) are respectively set as shown in Table 1.
TABLE 1
IMAGE
SOS PRE-
AREA
IMAGE
APC
TURN-ON
START
AREA END
START
START
SOS
LAPSE TIME
10
330
335
345
350
FROM SOS
SIGNAL (&mgr;s)
NUMBER OF
236
7795
7913
8149
8267.7
IMAGE
CLOCKS
Table 1 relates to the case that the outputting timing of a next SOS signal on the basis of the detecting time of the SOS signal (i.e., the timing at which the next SOS signal is detected after the previous SOS signal is detected: SOS period) is set to 350 &mgr;s (microsecond).
Here, for example when the frequency of the image clocks is reduced by 2% in order to increase the magnification in the fast scanning direction of the image, the above values are set as shown in the following Table 2. Since the scanning speed is unvaried, the SOS period is kept to 350 &mgr;s.
TABLE 2
IMAGE
SOS PRE-
AREA
IMAGE
APC
TURN-ON
START
AREA END
START
START
SOS
LAPSE TIME
10.2
336.6
341.7
351.9
350
FROM SOS
SIGNAL (&mgr;s)
NUMBER OF
236
7795
7913
8149
8102.3
IMAGE
CLOCKS
According to this table, the output timing of the next SOS signal is set during the execution of the light amount control due to the output of the APS signal. Therefore, there may occur such a case that the light amount control of the laser beam is inaccurate or the detection of the laser beam by the SOS sensor cannot be accurately performed and thus no SOS signal is output.
Therefore, Japanese Laid-open Patent Application No. 268332/1999 discloses a technique of accurately performing the light amount control of the laser diode, etc. even when the frequency of the image clocks is made variable in order to vary the magnification in the fast scanning direction of the image.
This technique is characterized in that an image forming apparatus is provided with a controller for controlling the output timing of each of a turn-on indicating signal for indicating turn-on of a laser beam (SOS pre-turn-on signal) and a light amount control indicating signal for indicating execution of light amount control of the laser beam (APC signal) so that these signals are generated at a fixed timing irrespective of the frequency of the image clocks.
More specifically, when the output finish timing of the LS signal exceeds the output start timing of the APC signal under the nominal state, the output start timing of the APC signal is altered just after the SOS signal is output, and also the output start timing of the SOS pre-turn-on signal is calculated on the basis of the following equation (1):
Timing after alteration=timing before alteration×(100−magnification to be varied)/100 (1)
Accordingly, the SOS pre-turn-on signal is output at a fixed timing at all times, and the detection of the laser beam by the SOS sensor can be accurately performed. For example, when the frequency of the image clocks is reduced from that under the nominal state of Table 1 by 2% in order to increase the magnification in the fast scanning direction of the image, the output timing of each signal is set as shown in the following Table 3.
TABLE 3
SOS
PRE-
IMAGE
IMAGE
TURN-
AREA
AREA
APC
APC
ON
START
END
START
END
START
SOS
LAPSE TIME
10.2
336.6
0
10
344.9
350
FROM SOS
SIGNAL (&mgr;s)
NUMBER OF
236
7795
0
231
7986
8102.3
IMAGE
CLOCKS
As described above, even when the frequency of the image clocks is made variable in order to alter the magnification in the fast scanning direction of the image, the time required to perform the light amount control of the laser beam can be sufficiently secured, and the light amount control can be accurately performed. In addition, the timing of the SOS pre-turn-on signal can be sufficiently secured, and the detection of the laser beam by the SOS sensor can be accurately performed.
In connection with a recent coloring requirement, an image forming apparatus having a function of forming (printing) a color image has rapidly propagated. The formation of the color image can be implemented by superposing four colors of cyan (C), magenta (M), yellow (Y) serving as three primary colors and black (K
Shioya Kohei
Suzuki Takayoshi
Fuji 'Xerox Co., Ltd.
Kim Robert H.
Morgan & Lewis & Bockius, LLP
Thomas Courtney
LandOfFree
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