JP2588880B2 - Image density correction device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image recording apparatus that forms an image on a photosensitive member by scanning a laser beam modulated according to an image signal, and particularly relates to an image recording apparatus. The present invention relates to an apparatus for correcting unevenness in image density in accordance with sensitivity unevenness in the axial direction of a photoconductor.

[Conventional technology]

Conventionally, in this type of apparatus, a reader (original reading apparatus), a computer, or the like scans and divides the image in the main scanning direction, modulates the read image signal into an on / off signal of a semiconductor laser, and converts the signal into a cylindrical rotation signal. A rotating polyhedral mirror (polygon mirror) with mirror-finished surfaces in the axial direction of the photoconductor
Scans at a constant speed via the controller to form an electrostatic latent image on the photoreceptor rotating at a constant speed. Next, development, transfer, and fixing are performed by a known electrophotographic process, and the recording medium is conveyed.
For example, it is configured such that a developed image can be fixed on recording paper.

The above-mentioned photoreceptor has a slight sensitivity unevenness compared to the conventional one.Especially, it is difficult to suppress the above-mentioned sensitivity unevenness within 20 V, particularly in an amorphous silicon photosensitive body formed by plasma CVD or the like, and furthermore, In an electrophotographic apparatus having a laser optical system, for example, a laser beam printer, it has been difficult to obtain a uniform potential in combination with non-uniform incident light quantity due to interference.

For this reason, in an image recording apparatus using a halogen lamp or a fluorescent lamp as a light source, a single or a plurality of slits are provided in the optical path from the light source to the surface of the photosensitive member with respect to the sensitivity variation in the axial direction of the photosensitive member. Is provided, and by adjusting the amount of irradiation, the potential of the latent image is made uniform and the image unevenness is corrected.

[Problems to be solved by the invention]

However, in an image recording apparatus having a laser scanning optical system, it is technically difficult to adjust the amount of light by adjusting the exposure width, which inevitably causes unevenness in sensitivity of the amorphous silicon photosensitive member and unevenness in incident light amount due to interference. And other manufacturing problems.

In the case of a laser scanning optical system, normally, when exposing a document, exposing a black background portion or a character portion of the document can shorten the laser emission time, and a latent image resolution due to a bleeding or fluctuation of a laser spot. Reversal development has often been used because it is effective in preventing image degradation. In this case, since the bright portion potential is a black background portion of the image, there is a problem that the sensitivity unevenness of the amorphous silicon photoreceptor becomes image density unevenness and the image quality is deteriorated.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and the unit dot emission time of laser light is determined based on the sensitivity unevenness characteristic in the axial direction of the amorphous silicon photoreceptor and an input digital value per pixel. The present invention provides an image density correction device that eliminates uneven density in the axial direction of an amorphous silicon photoreceptor, can reproduce halftones, and can obtain a uniform image density for a uniform halftone input. With the goal.

[Means for solving the problem]

An image density correction apparatus according to the present invention is a digital image signal indicating an image density per pixel in an image recording apparatus for forming an image by scanning a laser beam modulated according to an image signal on an amorphous silicon photosensitive member. Input means for inputting the data, a storage means for storing correction data obtained in accordance with the sensitivity unevenness characteristic of the amorphous silicon photoreceptor in the axial direction, a digital value of the digital image signal input by the input means, and the storage Means for obtaining an exposure time of a laser beam for equalizing the sensitivity unevenness characteristic in the axial direction of the amorphous silicon photoreceptor based on the correction data stored in the means.

[Operation] In the present invention, according to the digital value of the digital pixel signal indicating the level per pixel input from the input means and the correction data corresponding to the axial unevenness characteristic of the amorphous silicon photosensitive member in the storage means. Since the exposure time of the laser beam is obtained by using the above method, it is possible to accurately correct the sensitivity unevenness of the amorphous silicon photoreceptor for each pixel up to the halftone as well as the bright portion potential of the amorphous silicon photoreceptor.

〔Example〕

FIG. 1 is a block diagram for explaining a configuration of a laser beam printer according to an embodiment of the present invention. Reference numeral 1 denotes a reader unit, which comprises an image reading unit 1a, an amplifier 1b, and an A / D converter 1c. The image reading section 1a is composed of an image sensor such as a CCD. A printer unit 2 includes a PWM converter 2a, a laser driver 2b, a laser unit 2c, a correction circuit 2d, and the like. The PWM converter 2a performs pulse width modulation on / off of the laser light based on the image information sent from the A / D converter 1c. The laser unit 2c emits a laser beam LB1 based on correction data 3a described later and a laser beam LB2 based on correction data 3b. The correction circuit 2d corrects the pulse width of the modulation signal output from the PWM converter 2a by a predetermined amount based on correction data (lighting time data) 3a according to the sensitivity unevenness characteristic of the photosensitive member input from the input device 3, The corrected laser exposure signal is output to the laser driver 2b also serving as the exposure means of the present invention. Further, the correction circuit 2d sends a laser exposure signal for emitting a predetermined amount of laser light to the laser driver 2b based on correction data (lighting time data) 3b corresponding to the charging unevenness characteristic input from the input device 3. In addition,
The input device 3 has a function corresponding to sensitivity unevenness characteristics and charging unevenness characteristics in the generatrix direction of the photosensitive member recorded on paper such as a data sheet.
It is composed of a digitizer for inputting lighting time data of an image signal in pixel units, a magnetic card or a ROM chip in which the lighting time data is written in advance.

FIG. 2 is a view for explaining a laser exposure operation of a laser beam printer according to one embodiment of the present invention.
The same components as those in the drawings are denoted by the same reference numerals.

In this figure, reference numeral 11 denotes a DC controller, which has a BD synchronization signal generation circuit 11a and sends out a video signal to a laser driver 2b. A scanner driver circuit 12 drives the scanner motor 13 at a constant speed based on a drive signal sent from the DC controller 11. Reference numeral 14 denotes a rotating polyhedral mirror (polygon mirror), which is composed of, for example, ten mirror surfaces, is rotated at a constant speed by a scanner motor 13 in the direction of the arrow, deflects laser light emitted from the laser unit 2c, and forms an imaging lens 15 , The laser beam deflected via the scanning mirror 16
For example, horizontal scanning is performed on a photosensitive drum 17 formed of an amorphous silicon photosensitive member. A scanning mirror 18 guides the deflected laser light to a fiber cable 19. The fiber cable 19 sends the deflected laser light, that is, a beam detect signal (BD signal) serving as an image writing start signal, to the BD synchronization signal generating circuit 11a.

FIG. 3 is a characteristic diagram for explaining the correction data 3a and 3b input from the input means 3 shown in FIG. 1, in which the vertical axis indicates the surface potential (V), and the horizontal axis indicates the axial direction of the photoconductor. Indicates the length (mm). Note that the average potential in the dark portion is obtained when the surface potential is 400 (V).

In this figure, 21 is a bright portion potential characteristic (sensitivity unevenness characteristic).
The point A indicates the bright portion highest potential, and the point B indicates the bright portion lowest potential. _VL1 indicates the bright portion maximum potential value, and _VL2 indicates the bright portion minimum potential value. Reference numeral 22 denotes dark portion potential characteristics (charging unevenness characteristics), point C indicates a sampling potential, point D indicates a dark portion highest potential, and point E indicates a dark portion lowest potential. V _D1 indicates the dark portion highest potential value, and V _D2 indicates the dark portion lowest potential value.

Next, the correction circuit 2d shown in FIG. 1 while referring to FIG.
Will be described.

First, the light-part potential characteristic 21 of the photosensitive drum 17 is input from the input device 3.
When correction data 3a relating to (see FIG. 3) is input to the correction circuit 2d, the correction circuit 2d emits a predetermined amount of laser light based on the following equations (1) and (2). Calculate TB and TC.

TB = (V _LB −V _L2 ) * K1 (1) TC = (V _LC −V _L2 ) * K1 (2) where K1 is 4.0, for example, when the amount of laser light is 4.0 pixels. When μJ / cm ² , K = 64/400 = 0.16
(Emission unit / V).

Therefore, the potential unevenness at point B (V _LB −V _L2 ) is 30 (V), and the image signal emission time of this dot is 61/64 (3D /
3F (hexadecimal), the exposure time after correction is 57 (61-
| 0.16 * 30 | = 57), and the light intensity of 3.8 μJ / cm ² is 3.6 μJ / cm ²
Is corrected to Thereby, the surface potential at point B rises to the surface potential at point A. By performing the same processing for point C, the surface potential can be similarly increased to the surface potential at point A, and the surface potentials of all the pixels in the generatrix direction of the photosensitive drum 17, that is, the bright portion potentials are made uniform. it can.

 Next, the uniformization of the dark portion potential will be described.

FIG. 4 is a diagram for explaining the EV characteristic of the photosensitive drum 17 shown in FIG. 2, in which the vertical axis indicates the surface potential (V) of the photosensitive drum 17, and the horizontal axis indicates the exposure amount of the laser beam. (ΜJ / cm ² )
Is shown.

In this figure, 31 indicates the E / V characteristic, and K2 indicates the E-V characteristic.
The slope of the V characteristic 31 (ΔE / ΔV (where ΔE indicates a light quantity variation and ΔV indicates a surface potential difference))
However, it is known that, for example, when an amorphous silicon photoreceptor is used, the EV characteristic 31 becomes a linear function, that is, a linear function. Since the EV characteristic 31 has the best linearity, the sensitivity correction control is simplified. In addition, in this embodiment, the photosensitive drum 17 is made of an amorphous silicon photosensitive member because of its excellent strength and durability. First, when the lighting time data 3b relating to the dark area potential characteristic 22 (see FIG. 3) of the photosensitive drum 17 is input to the correction circuit 2d from the input device 3, the correction circuit 2b obtains the following equations (3) and (4). Based on the calculated values, laser exposure times TD and TF for emitting a predetermined amount of laser light are calculated.

_{TD = (V D -V E)} * K2 ...... (3) sent to the _{TF = (V F -V E)} * K2 ...... (4) calculating the laser exposure time TD, TF laser driver 2b than the correction circuit 2d When the laser unit 2c irradiates the photosensitive drum 17 with the laser beam LB2 before the image writing start position based on the laser exposure times TD and TF, the charged potential of the photosensitive drum 17 at the points D and F is changed to, for example, the dark area average potential. 400
(V). By performing this operation on all pixels in the generatrix direction of the photosensitive drum 17, the photosensitive drum
The charging potential of all pixels in the 17 bus direction can be made uniform.

In the above-described embodiment, the case where the laser beams LB1 and LB2 emitted from the laser unit 2c composed of the dual laser element composed of one chip are exposed on the photosensitive drum 17 by the single rotating polygon mirror 14 has been described. Since the focus accuracy of the laser beam LB2 for correcting the dark portion potential is not required as compared with the focus accuracy of the laser beam LB1 for correcting the bright portion potential, the laser beam LB2 may be swung from another polygon mirror. Further, the number of laser elements may be appropriately increased.

Further, since the exposure amount of the laser beam LB2 for correcting the dark portion potential is only a little, it is preferable that the laser beam LB2 be dimmed by an ND filter or the wavelength of the laser beam LB2 be shifted from the peak sensitivity wavelength of the photosensitive drum 17.

When the photosensitive drum 17 is made of an amorphous silicon photoreceptor, the peak sensitivity is 680 to 700 nm, so if a normal semiconductor laser having a wavelength of 778 to 788 nm is used, the sensitivity range is considerably lower than the peak sensitivity. Causes the semiconductor laser to oscillate. For this reason, by using a semiconductor laser having a wavelength around 800 nm, the sensitivity becomes about half that of a normal semiconductor laser and control becomes possible without extremely lowering the laser power. However, if the wavelength is 820 nm or more, it affects charging, so that it is desirable to use it at 800 nm or more and 820 nm or less.

〔The invention's effect〕

As described above, according to the present invention, according to the digital value of the digital image signal indicating the image density per pixel input from the input means and the charging unevenness characteristic in the axial direction of the amorphous silicon photosensitive member in the storage means. Since the exposure time of the laser beam is determined according to the correction data, the sensitivity unevenness in the axial direction of the amorphous silicon photoconductor is accurately corrected on a pixel-by-pixel basis, not only to the light portion potential of the amorphous silicon photoconductor but also to a halftone. be able to.

Therefore, in laser recording, there is an effect that the unevenness of the density in the axial direction of the amorphous silicon photoreceptor is eliminated and a uniform pixel density can be obtained for a uniform halftone input.

[Brief description of the drawings]

FIG. 1 is a block diagram for explaining a configuration of a laser beam printer showing one embodiment of the present invention, and FIG. 2 is a diagram for explaining a laser exposure operation of the laser beam printer showing one embodiment of the present invention. FIG. 3 is a characteristic diagram for explaining correction data input from the input means shown in FIG. 2,
FIG. 4 is a diagram for explaining the EV characteristic of the photosensitive member according to the present invention. In the figure, 1 is a reader unit, 2 is a printer unit, 2d is a correction circuit,
3 is an input device.

 ────────────────────────────────────────────────── ─── Continued from the front page (72) Inventor Toshiyuki Ehara 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (56) References JP-A-60-64869 (JP, A) −26854 (JP, U) Actually open 58-86648 (JP, U)

Claims (1)

(57) [Claims] 1. An image recording apparatus for forming an image by scanning a laser beam modulated according to an image signal on an amorphous silicon photoreceptor, wherein an input means for inputting a digital image signal indicating an image density per pixel. Storage means for storing correction data obtained according to the sensitivity unevenness characteristic in the axial direction of the amorphous silicon photoreceptor; digital values of the digital image signal input by the input means and stored in the storage means Means for calculating an exposure time of a laser beam for equalizing the sensitivity unevenness characteristic of the amorphous silicon photoreceptor in the axial direction based on the correction data.