Electric photograph system

Details Electric photograph system Electric photograph system

2001-06-19

2003-12-02

Nghiem, Michael (Department: 2863)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

Reexamination Certificate

active

06657653

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an electric photograph system for performing analog printing and more particularly, to a technique for use in a laser printer of a light intensity modulation type wherein a light output power versus current characteristic of a light beam emitted from a semiconductor laser at a focusing position onto a photo-conductor drum is made to be linear so that the quantity of light can be changed to one of a plurality of power levels accurately and therefore printing dots arithmetically changing in diameter or size are generated, thus realizing high-resolution multiple density printing.
As a printer for analog printing, there is known an electric photograph system (electric photograph recording system). One of the electric photograph systems is a laser printer (laser beam printer) which has two types, that is, pulse width modulation and light intensity modulation.
The laser printer of the light intensity modulation type controls a light output power of a semiconductor laser element according to image information to change the spot size of a light beam focused on a surface of a photo-conductor drum (photo conductor) and to control the size of a printing dot for multiple density printing.
An image quality adjusting device for the laser printer is disclosed, for example, in JP-A-3-269456. This literature discloses a technique for solving the problem of non-linear light output power versus characteristics at the time of multiple density printing by increasing the sensitivity level of a photo sensitive material to use only a good linear area thereof.
Also disclosed in “SID 9 DIGEST”, pp. 278-299 is a technique for modifying a dot size or printing position. Further disclosed in IEEE Journal of Quantum Electronics, Vol. QE-21, No. 8, Aug.1985, pp. 1264-1270 is mode-hopping noise in a semiconductor laser.
The inventors of the present application suggest, for the purpose of improving the performance of multiple density printing in a laser printer, a technique for controlling the small spot size of a laser beam by setting a light intensity distribution of the laser spot to be triangular (refer to JP-A-9-74251). In this literature, in order to set the light intensity distribution of the laser spot to be triangular, a non current injected area having a length of about 70 &mgr;m is provided at halfway (at a position about 30 &mgr;m away from its end) of a stripe-shaped waveguide so that the phase difference between fundamental and higher modes is &pgr;/2 at a laser facet.
A technique for providing a non current injected window area at both facets for the purpose of preventing destruction of the facets from which a light beam of a semiconductor laser is emitted, is disclosed, for example, in JP-A-62-65391 and JP-A-62-179193. However, these techniques fail to disclose an application example of improving the linearity of a light output power versus current characteristic of a semiconductor laser.
The electric photograph system (electric photograph recording system) has a function of scanningly directing the spot of a light beam emitted from a semiconductor laser element according to image information onto a surface of uniformly charged photo conductor (such as a photo-conductor drum) for exposure thereof, emitting electric charges therefrom in such a manner that the potential of the surface of the photo-conductor drum is reduced to zero to thereby form an electric potential image, and changing the quantity of a light beam to a plurality of levels at the time of the scanning exposure to change the size of the electric potential image for multiple density printing.
In the prior art laser printer, multiple density printing is carried out by controlling the light output power of a semiconductor laser as a light source to a plurality of intensities to control the size of printing dot.
Explanation will now be made as to an exposing optical system of a general laser printer (laser beam printer), by referring to FIG.
21
. In the printing of the laser printer, as shown in
FIG. 21
, a laser beam
2
emitted from a semiconductor laser
1
is made parallel or collimated by a collimate lens
3
, and the collimated beam is once focused on a polygon mirror
5
by a cylindrical lens
4
. The laser beam
2
reflected by the polygon mirror
5
is focused through a non-spherical lens system
6
on a drum coated with a photo conductor
7
, that is, on a photo-conductor drum
8
, so that the photo-conductor drum
8
is scanned with the beam at a constant speed along the axial direction of the drum. The surface of the photo-conductor drum
8
is previously charged uniformly, so that, when the drum is scanned with the laser beam, electric charges on the surface are discharged therefrom and thus the surface potential of the photo-conductor drum
8
is reduced to zero.
When toner particles are electrically adsorbed on an electric potential image thus formed, a toner image is formed and then printed. Since the toner electric adsorption takes place on the surface of the photo conductor subjected to beam exposure with a constant light intensity or more, a change in the light output power of the semiconductor laser enables a change of the size (printing dot size) of a dot to be printed, thus realizing multiple density printing.
The inventors of the present application have analyzed and studied an exposing optical system for the purpose of obtaining high-resolution multiple density printing, and have found that, with respect to the size of a printing dot formed by a beam spot focused on the photo-conductor drum of a laser printer, it is difficult to obtain accurate levels of multiple densities, i.e., high-resolution multiple density printing in an area having small printing dot sizes.
That is, the prior art laser printer is arranged so that the light beam to be focused on the photo-conductor drum is obtained by collimating or converging a laser beam emitted from the semiconductor laser element with use of the aforementioned optical system and by changing the optical path of the beam for laser printing. Accordingly the light intensity of the light beam irradiated onto the photo-conductor drum is the light output power itself of the semiconductor laser (semiconductor laser element), which largely depends on the characteristic of the semiconductor laser element.
In general, with regard to the light output power of a semiconductor laser element, it is already known that the linearity of a light output power versus current characteristic is deteriorated in its low optical power range, but it is not recognized that the fact adversely affects high-resolution multiple density printing of the laser printer.
In other words, in the laser printer, as the number of power levels increases, the range of light output power of a laser beam used is required to be broad and correspondingly a low optical power range is also required to be inevitably used.
In a high-resolution laser printer, it is demanded that printing be carried out with a printing dot finely changing arithmetically in size, but irregular change in the printing dot size in the low optical power range makes it difficult to obtain high-resolution laser printing.
FIG.
22
(
a
) shows a graph of a light output power versus current characteristic of a semiconductor laser element, and FIG.
22
(
b
) shows, in a model form, an example of a printing dot changing arithmetically in size. In FIG.
22
(
a
), positions denoted by white and black small circles are current positions at which the printing dot is to be formed, and the current value of each position is arithmetically selected.
In FIG.
22
(
a
), a characteristic line A denotes an actual characteristic and a characteristic line B is an ideal characteristic desirable for multiple density printing. On the characteristic line B as the ideal characteristic, there is a clear inflection point in a low optical power part. In a large current area subsequent to the inflection point, the characteristic line A is linear (exhibits a linearity). Thus when the current value is arithmeti

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