Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-03-09
2001-08-21
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S237000
Reexamination Certificate
active
06278476
ABSTRACT:
BACKGROUND OF THE INVENTION
In an image forming apparatus such as an electronic copy machine, a facsimile, a printer, etc., an electrostatic latent image is formed on a photosensitive drum by scanning the surface of the drum with a laser beam emitted from a semiconductor laser such as a laser diode, thereby developing the latent image using a developer (toner) and transferring it onto a sheet of paper.
The diameter of a laser beam emitted from the laser diode is determined in accordance with the wavelength of the beam. The shorter the wavelength of the laser beam, the smaller the diameter of the laser beam. A laser beam of a short wavelength and a small diameter is suitable for high-speed, high-density image forming. A red laser beam is an example of a laser beam of a short wavelength and a small diameter.
The laser diode for emitting a red laser beam, however, has a high radiation efficiency. Accordingly, when a driving current in the form of a pulse wave has fallen, the temperature of the diode soon falls, whereas when the driving current has risen, the temperature does not promptly increase. A period of several microseconds to several tens of microseconds is necessary until a sufficient light output is obtained after the driving current rises. This delay in the rise of output light adversely affects image forming.
BRIEF SUMMARY OF THE INVENTION
It is the object of the invention to provide a highly reliable image forming apparatus capable of high-speed, high-density image forming, using a semiconductor laser for emitting a laser beam of a short wavelength, and also capable of always forming a high quality image free from the influence of the characteristics of the laser, and to provide a method for controlling the apparatus.
The invention provides an image forming apparatus for scanning an image carrier by a laser beam emitted from a semiconductor laser, comprising:
a driving circuit having a resistor for limiting a current, the driving circuit supplying the semiconductor laser with a driving current via the resistor;
a control circuit for controlling an output of the driving circuit such that an optical output of the semiconductor laser has a predetermined reference value;
a constant current supply connected parallel to the semiconductor laser for subtracting a current of a constant level from the driving current supplied to the semiconductor laser; and
a switch circuit for intermittently operating the constant current supply on the basis of an image to be formed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantage of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
REFERENCES:
patent: 4819241 (1989-04-01), Nagano
patent: 5651017 (1997-07-01), Genovese
patent: 5708673 (1998-01-01), Ikeuchi
patent: 10-76705 (1998-03-01), None
Foley & Lardner
Le N.
Pham Hai C.
Toshiba Tec Kabushiki Kaisha
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