Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2003-03-03
2004-12-28
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S258000
Reexamination Certificate
active
06836281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning optical system and an image forming apparatus using the same. Specifically, the present invention relates to a scanning optical system that is adapted to reflect and deflect a light flux emitted from a light source by means of a polygon mirror serving as a light deflector and have an f&thgr; characteristic, which is preferable for use in an image forming apparatus, such as a laser beam printer including an electrophotography process, a digital copying machine or a multi-function printer, which records image information by optically scanning a surface to be scanned through scanning optical means including an optical element provided with a fine structure grating.
2. Related Background Art
In conventional scanning optical systems of laser beam printers etc., a light flux emitted from light source means that has been modulated in accordance with an image signal is cyclically deflected by a deflector composed, for example, of a polygon mirror and converged into a spot on a surface of a photosensitive recording medium by scanning optical means having an f&thgr; characteristic, so that an image is recorded.
FIG. 16
is a cross sectional view taken in the main scanning direction (main scanning cross sectional view) showing the principal portion of a conventional scanning optical system (i.e. optical scanning apparatus).
In
FIG. 16
, reference numeral
91
designates light source means, which is composed, for example, of a semiconductor laser or the like. Reference numeral
92
designates a collimator lens, which converts a divergent light flux emitted by the light source means into a parallel light flux. Reference numeral
93
designates an aperture stop, which restricts a light flux passing through it to shape the beam. Reference numeral
94
designates a cylindrical lens, which has a certain power only in the sub-scanning direction, to image the light flux having passed through the aperture stop
93
onto a deflection surface (or reflection surface)
95
a
of a light deflector
95
(which will be described below) as a substantially linear image in a cross section in the sub-scanning direction.
Reference numeral
95
designates a light deflector serving as deflecting means, which is composed, for example, of a polygon mirror (i.e. a rotatory multi-surface mirror) having four faces. The light deflector
95
is rotated in the direction indicated by arrow A in
FIG. 16
at a constant speed by driving means (not shown) such as a motor etc.
Reference numeral
96
designates a scanning lens system serving as scanning optical means having a light collecting function and an f&thgr; characteristic, which is composed of first and second scanning lenses (two lenses in all)
96
a
and
96
b
. The scanning lens system
96
images a light flux corresponding to image information that has been reflected and deflected by the light deflector
95
onto a surface to be scanned, that is, a surface
97
of a photosensitive drum, while realizing a conjugate relationship between the deflection surface
95
a
of the light deflector
95
and the photosensitive drum surface
97
in the sub-scanning cross section, to perform a field tilt correcting function in order to correct the surface inclination of the deflection surface
95
a.
In
FIG. 16
, a divergent light flux emitted from the semiconductor laser
91
is converted by the collimator lens
92
into a substantially parallel light flux, and then the light flux is restricted (in light quantity) by the aperture stop
93
and incident on the cylindrical lens
94
. The substantially parallel light flux incident on the cylindrical lens
94
emerges from it without any modification with respect to the main scanning cross section. On the other hand, in the sub-scanning cross section, the light flux is converged so as to be imaged onto the reflecting surface
95
a
of the light deflector
95
as a substantially linear image (namely, a linear image that is longitudinal in the main scanning direction). The light flux reflected and deflected by the reflecting surface
95
a
of the light deflector
95
is imaged by means of the first and second scanning lenses
96
a
and
96
b
onto the photosensitive drum surface
97
as a spot, whereby the imaged light spot scans the photosensitive drum surface
97
in the direction indicated by arrow B (i.e. the main scanning direction) at a constant speed, as the light deflector
95
is rotated in the direction of arrow A. Thus, an image is recorded on the surface
97
of the photosensitive drum as a recording medium.
However, the conventional scanning optical system as described above suffers from the problems as described in the following.
Recently, it has become a general practice to manufacture scanning optical means (i.e. scanning lens systems) of a scanning optical system using a plastic material, which is easy to process into an aspheric shape and with which the manufacturing is easy. However, it is difficult to apply anti-reflection coatings on plastic lenses for technical and economical reasons. So, plastic lenses suffer from Fresnel reflection generated at their optical surfaces.
FIG. 17
is a graph showing incident angle dependency of reflectance and transmittance of an example of an optical element made of a resin, having a refractive index n=1.524, under a condition in which P-polarized light is incident on that element. As will be seen from the graph, the surface reflection at each surface becomes large, as the angle of incidence increases.
In connection with this, since in the scanning optical means, the angle of incidence generally varies as the position of incidence changes away from the on-axis position toward an off-axis position, the Fresnel reflection at each optical surface also varies largely. As a result, there is a difference between the light quantity at the on-axis position and the light quantity at the off-axis position. As the angle of incidence increases from 0 degree to the Brewster's angle, the reflectance decreases (i.e. the transmittance increases), and therefore the transmittance of the whole system increases as the position changes from the on-axis position to an off-axis position. Therefore, the illuminance distribution on the surface to be scanned also increases toward the off-axis position. It will be seen from
FIG. 17
that the light quantity at the outermost off-axis position is larger than the light quantity at the on-axis position by 5%. As a result, a density difference would be created in an image output by the image forming apparatus between the central portion and the peripheral portion thereof, which is a problem.
As a solution for the above-mentioned problem, Japanese Patent Application Laid-Open No. 2000-206445 proposes adjusting the diffraction efficiency of a diffraction grating surface provided in scanning optical means appropriately in order to eliminate that problem. Specifically, it proposes adjusting the depths of cuts of diffraction grating surface, on which grating is cut at a predetermined pitch that realizes a desired power distribution for the purpose of correcting chromatic aberration of magnification or correcting focus, to vary the diffraction efficiency of the diffracted light (i.e. the first-order diffracted light) so as to cancel the variation in transmittance created by other refracting surfaces.
However, the diffraction grating proposed by Japanese Patent Application Laid-Open No. 2000-206445 suffers from a problem as described below.
It is known that when the pitch of a grating becomes as small as or smaller than the wavelength of light (i.e. a fine structure grating), it shows a structural birefringence.
In “KOUGAKU-NO-GENRI Vol. III” (a Japanese translation of “Principle of Optics” by Max Born and Emil Wolf), Tokai University Press, p1030, it is describes that regularly arranged particles each of which is made of optically isotropic material and having a size sufficiently larger than its molecule size and smaller than the wavelength of light show
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pham Hai
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