Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-11-30
2002-10-29
Lee, Susan S. Y. (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
Reexamination Certificate
active
06473106
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an LED head suitably applicable to the formation of an image in combination with the electrophotography and, more particularly, to an LED head with high resolving power and an image forming apparatus such as an LED printer or the like using the LED head. Further, the invention concerns a method of measuring an amount of light from the LED array, for measuring emission characteristics of the LED array used in LED heads and LED printers.
2. Related Background Art
(Prior Art 1)
It is common practice heretofore to use the LED printers with relatively low resolution, e.g., 300 dpi in combination with a bright array of two lines of rod lenses having the nominal angular aperture of 20° and the nominal line size of 0.9 mm or 1.1 mm. Using this rod lens array, a photosensitive body is exposed to an emission pattern of LEDs whereby an electrostatic image is formed on the photosensitive body. This electrostatic image is developed with toner and this toner image is transferred onto transfer sheet and then fixed. After that, the transfer sheet is discharged out of the LED printer.
AlGaAs-base materials and the like are generally known as materials for the LEDs of radiative regions for use in combination with this rod lens array.
It is, however, a recent tendency that the resolving power required of the printers is the high resolving power of 600 to 1200 dpi. Under such circumstances, there is such an increasing common tendency that as to a rod lens array employed, a stack of two lines of rod lenses of high resolution type having the nominal angular aperture of 12° and the nominal line size of 0.6 mm is used in combination with the LED array.
On the other hand, however, the AlGaAs-base LEDs demonstrate the phenomenon that there often exists a subsidiary (sub) emission band: near 870 nm in addition to a principal (main) emission band near 780 nm, as shown in the spectrum of FIG.
3
.
FIG. 3
is a diagram in which the axis of abscissa indicates the wavelength and the axis of ordinate the photosensitive intensity, i.e., how the photosensitive body used can be sensitive to each spectral region by emission intensity of the LEDs.
In the conventional printer heads of low resolution, the dot-to-dot pitch of the rod lens array was sufficiently larger than blur amounts, and thus interference rarely occurred between blurs of dots. Accordingly, the influence of emission of this sub emission band posed no serious problem.
In recent years, however, this sub emission band is coming to affect the image with increases in the resolving power of printers. It is thus extremely difficult to achieve high resolution and high image quality of the printer heads using the AlGaAs-base LED array exhibiting the sub emission band at random.
FIG. 4
shows the imaging relation of an LED radiative point
1
of LED chip
2
with the sub emission band, including LEDs arrayed at the pitch P, through the high-resolution rod lens array
3
of currently well-known type with the nominal angular aperture of 12° and with relatively suppressed chromatic aberration. This figure illustrates that the main emission band and the sub emission band demonstrate a small difference D in TC length between TCmain and TCsub, the F-number is also large, and thus the light of the sub emission band is not so blurred on the photosensitive body
4
.
FIG. 5
schematically shows how the dots are resolved where wafers with different intensities of the sub emission band are adjacent to each other.
In
FIG. 5
, the upper part shows a state in which the luminance B of the sub emission band, which varies wafer by wafer across the chip boundary indicated by a dotted line at the center, is superimposed on the luminance A of the main emission band of the constant light intensity, and the middle part schematically shows how a spot image of each LED chip is formed. Consequently,
FIG. 5
shows a case in which the sub emission band B affects the spot luminance distribution more on the right side than on the left side. Since the blur of the left sub emission band is small, the sub emission band appears as a light amount unevenness component randomly overlaid on a predetermined development level and thus developed spot sizes vary chip by chip, as seen in the lower part of FIG.
5
. As a consequence, the density difference occurs in chip units and it appears as degradation of image quality. Particularly, in the case wherein a wafer chip with different sub emission band characteristics is inserted in a repair step of a chip after die bonding, there appear uneven stripes in the range of several millimeters in a halftone image. This was the drawback of degrading the image quality, particularly, in the case of pictorial imagery.
In addition, it is very difficult to manage the height of the peak of this sub emission band for every wafer in the fabrication process. Further, a method of managing each of these wavelength distributions and carrying out works of the die bonding of chips could greatly affect cost and was not so practical.
An object of the present invention is, therefore, to decrease the influence of the sub emission band, based on the construction of the rod lens array in the LED printer head, provide a configuration in which the light of the sub emission band does not reach the development level, and realize the high image quality.
(Prior Art 2)
In recent years, color office documents are rapidly increasing with spread of personal computers and along therewith the LED printers are drawing attention as printing heads for color printers capable of printing such color documents at high speed. With the conventional LED printers, however, the principal emphasis was on the quality of letters, but emphasis was not laid so much on pictures, halftone images, and so on. In addition, correction for light amounts was also in such a level that variation among chips was corrected by chip resistance.
Therefore, this coming era requires techniques of precisely controlling light amounts while precisely measuring variation of light amounts themselves associated with the imagery, in order to output pictorial color documents.
Meanwhile, for development of high speed printers, the AlGaAs-base materials and the like are generally known as materials for the LEDs enabling highly efficient emission.
The AlGaAs-base LEDs involve the phenomenon that the sub emission band B considered to originate in a GaAs substrate appears in addition to the main emission band A, as illustrated by the solid line in FIG.
10
. The wavelength of the main emission band A is approximately 780 nm and the wavelength of the sub emission band B is approximately 870 nm.
It is also common practice to use a silicon PIN photodiode with spectral sensitivity characteristics as indicated by a chain line C in
FIG. 10
, as a sensor used in measurement of light amounts.
FIG. 11
shows a typical configuration example of a conventional LED-array light-amount measuring device.
This configuration is a typical configuration of measuring apparatus, which is commonly employed by many LED light-amount measuring devices, for example as described in applications filed by the inventor, or in other applications, for example, Japanese Patent Application Laid-Open No. 10-185684.
In
FIG. 11
, first, an emission signal enough for emission of a light amount to be measured is supplied from a driver
21
of an emission signal generator to the LED array
22
as an object to be measured, to make a predetermined LED emit light. The light emitted travels through an imaging lens
23
to reach a PIN photodiode
26
with the spectral sensitivity indicated by the chain line C of
FIG. 10 and a
sensor part
24
thereof provides an electric output signal proportional to the light amount. The analog signal of this electric output signal is converted to a digital signal by an A/D converter
25
and a processing system
27
thereafter performs an operation to determine whether the emission amount of the predetermined LED is normal or not.
However, recent resear
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