Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1995-12-27
2003-10-21
Nguyen, Thinh (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
Reexamination Certificate
active
06636252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image exposure apparatus used with a copying machine, a printer and the like, and an image forming apparatus having such an image exposure apparatus, and more particularly, it relates to an image exposure apparatus in which an image is exposed by lightening a plurality of luminous elements such as LEDs.
2. Related Background Art
In conventional image forming apparatuses having an array of light emitting diodes (referred to as “LED” hereinafter) as an exposure source, an photosensitive drum is exposed by light emitted from the LED array and an image is formed on the photosensitive drum by an electrophotographic process.
FIG. 12
schematically shows such as LED array. As shown in
FIG. 12
, a plurality of LED chips
101
are disposed on a substrate
102
of an LED array
100
equidistantly along a direction parallel with a rotational axis of a photosensitive drum (not shown). A length of the LED array
100
is determined by a length of the photosensitive drum. As shown in
FIG. 13
, each LED chip
101
includes a plurality (normally, 64 to 128) of light emitting points.
FIGS. 14A and 14B
show sections of the LED chip
101
. The LED arrays are generally divided into two groups, i.e. GaAs group and AlGaAs group which have different features.
FIG. 14A
shows the LED array of GaAs type wherein GaAs
x
P
(1-x)
of n-type are formed on a GaAs substrate of n-type by a gas phase crystal growth method. In this case, as the rate of P is increased, a light emitting wavelength is lengthened to increase light emitting efficiency. A luminous junction is formed by forming a p-area in an n-GaAsP layer by thermal diffusion of zinc (Zn). An interface of the p-n junction acts as a light emitting diode. In general, in order to define the diffusion of zinc within a limited area of the light emitting portion, a film of SiO
2
is formed in an opening portion, and density of carrier is controlled through the film to effect the diffusion of zinc. P-electrodes for applying current are made of aluminium (Al) or Au—Se—Te alloy (gold/selenium/tellurium alloy) and an n-electrode is common to the arrays and is made of Au/Au—Ge—Ni (gold/gold-germanium-nickel).
The LED is an element for applying voltage to the p-n junction in a normal (positive) direction and for pouring small amount of carrier and for picking up natural light generated by re-binding of carrier. In order to improve the light emitting efficiency, it is important that internal quantum yield for converting the applied current into the light is maximized by utilizing direct transition to the re-binding process and that the emitted light is efficiently taken out to the exterior. The efficiency for taking out the light to the exterior (external quantum yield) is several percentage (%) or less since there are components entirely reflected into the interior of the semiconductor at a critical angle determined by refractive index of material or substance, and, thus, a major part of light is absorbed into the interior and consumed as heat. Accordingly, in the LED array, it is important that the efficiency of the internal quantum yield is improved by purifying the crystal and at the same time the efficiency of the external quantum yield is increased.
FIG. 14B
shows the LED array of AlGaAs type wherein AlGaAs is formed on a GaAs substrate of p-type by a liquid phase crystal growth method. In this LED array, a mixture ratio between gallium (Ga) and aluminium (Al) can be controlled within a wide range. First of all, a p-layer of Al
(1-x1)
Ga
x1
As on the p-substrate is grown, and then, an n-layer of Al
(1-x2)
Ga
x2
As is grown, thereby forming a p-n junction portion between the layers. By changing x at the interface of the junction, it is possible to form heterojunction and to make the current applying efficiency (i.e., the re-binding contributing to the light emission) more effective. Further, since the value of x
2
can be selected as a transparent layer having less light-absorbing feature with respect to a light taking-out direction, it is possible to take out a larger amount of external light emitting output. Incidentally, the common electrode of p-side is made of AuZn—Ni—Au (gold/zinc-nickel-gold) and the electrode of n-side is made of AuGe—Ni—Au (gold/germanium-nickel-gold) and these electrodes become ohmic electrodes.
The LED array is formed by arranging the plural LED chips as mentioned above side by side on the substrate
102
(die bonding). All of the light emitting elements (pixels) in the LED chip
101
are connected to corresponding wires (wire bonding). The LED (light emitting element) is illuminated by applying current to the corresponding wire. The light emitting points
103
are equidistantly disposed in the chip. Since the pixels are associated with the wires one by one, for example, when there are 128 light emitting points
103
in one LED chip
101
, the number of the wire bondings becomes 128.
FIG. 15
is a perspective view showing a condition that the LED chips
101
are mounted on the substrate in this way and the light emitting elements in the LED chips are connected to drivers by wire bondings.
Next, a method for driving the LED will be explained.
In order to drive the LED, generally, a driving method utilizing constant current driving elements is used. The constant current driving methods are generally grouped into two methods as shown in
FIGS. 18A and 18B
. In the first method, internal resistance is added to a P-channel open drain CMOS circuit or serial resistance as external resistance is added (serial resistance type). In the second method, a constant current circuit is provided by controlling a gate voltage of a driver IC. The second method having less current fluctuation in comparison with the first method is more preferable for voltage fluctuation. In
FIG. 18A
, the current is made constant by base current Q
1
of transistor Q
2
, thereby controlling the driving current of the LED. On the other hand, in
FIG. 18B
, false constant current is established by high resistance R.
Methods for inputting a signal are generally grouped into four, as shown in
FIGS. 19A
to
19
D. In the methods shown in
FIGS. 19A and 19B
, signals are successively supplied to shift registor(s) and are latched upon illumination, and an output signal is time-controlled by an enable signal, thereby determining a time period for illuminating the LED. The difference between
FIGS. 19A and 19B
is that the entire head is constituted by a single serial shift register (FIG.
19
A), whereas, the signals are supplied, in parallel, to a plurality of input terminals of plural shift registers.
In the method shown in
FIG. 19D
, the division is effected every one dot, and this method apparently bears resemblance to a parallel input method shown in FIG.
19
C.
FIG. 19C
shows the complete parallel input fashion, in which the data are always inputted to the head in parallel and the light emitting position is determined by the timing of the latches. Now, the method shown in
FIG. 19C
will be further fully explained. Eight-bit parallel signals are inputted to n-th (n=0 to 7; eight in total) ports in accordance with the latch signal of the data, and the 8-th to 15-th data are read by the next clock input. After all of the data are latched, the data are transferred to another latch portion, where light emitting time period for illuminating the LED is determined.
Regarding the characteristics shown in
FIGS. 19A
,
19
B and
19
C, in
FIG. 19A
, the maximum speed is limited by a transmitting speed of the shift register, and in
FIGS. 19B and 19C
, since the time period is reduced to 1
(n is the number of the input ports), the high speed operation can be expected. Particularly, the circuit shown in
FIG. 19C
is suitable for the highest speed operation since there is no data transmission.
In any cases, in the final output stage, in the case where the light emitting dots in the LED is great, particularly, when all of the light emitting elements are illuminated simultane
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