A raster output scanning system is disclosed which utilizes two light beams to double scan two scan lines on a photoreceptor in order to generate three exposure levels required for a tri-level printing system. In the raster output scanner of this invention, the two light beams scan two separate scan lines. At the completion of the simultaneous scanning of two light beams when the first light beam starts to scan a new scan line, the second light beam start to scan the scan line that the first light beam just completed scanning. In this invention the first light beam generates two exposure levels and the second light beam adds additional exposure to the second exposure level in order to generate a third exposure level. The same method can be utilized to generate additional levels for printing systems with more xerographic levels than three.

BACKGROUND OF THE INVENTION 
This invention relates to a multi-level xerographic system such as high 
light color systems. More particularly, this invention relates to a 
xerographic system which utilizes multi-beams to overscan each line to 
generate multiple levels of xerographic exposures. 
A conventional raster output scanner utilizes either a light source, a 
modulator and a multi-faceted rotating polygon mirror as the scanning 
element or a light source, which serves as both a light source and a 
modulator, along with a multi-faceted rotating polygon mirror. In a raster 
output scanner with a light source and a separate modulator, the light 
source, which can be a laser source, generates a light beam and sends it 
to the modulator. The modulator receives pixel information and modulates 
the pixel information onto the light beam. However, in the raster output 
scanner without a separate modulator, the light source, which can be a 
laser diode, both generates and modulates the light beam. Then, the 
modulated light beam is directed onto a facet of a rotating polygon 
mirror. The rotating polygon mirror reflects the light beam and also 
causes the reflected light to revolve about an axis near the center of 
reflection of the rotating polygon and scan a straight line. This 
reflected light beam can be utilized to scan a document at the input of an 
imaging system or can be used to impinge upon a photographic film or a 
photosensitive medium, such as a xerographic drum at the output of the 
imaging system. 
A tri-level printing system is a system which uses two color inks. A 
typical tri-level system utilizes a single light beam which will be 
modulated to have two different pixel informations, one for the first ink 
and the second for the second ink. The single light beam, modulated by two 
different trains of pixel informations, will expose the photoreceptor 
plane at three different exposure levels: one level for color ink, one 
level for black ink and the third level for no printing. It should be 
noted that for each pixel the photoreceptor will be exposed by only one of 
these three levels. 
Referring to FIG. 1, there is shown a train 10 of different exposure levels 
on the photoreceptor corresponding to different pixels of a tri-level 
system. Usually in a tri-level system, level 12, which is the lowest level 
and usually is kept at 0 volts (ground level), represents black ink and is 
called black level. Level 14 represents no printing and is called white 
level. White is a term used for no printing since when there is no 
printing the color of the paper which usually is white will be shown. Of 
course, if a different color paper is used, white level represents the 
color of the paper. Finally level 16 represents a second ink which can 
have any color other than black and the color of paper. 
The modulation of a single light beam with two different pixel informations 
can be achieved through various methods such as amplitude modulation or 
pulse width modulation. 
In tri-level systems, the amplitude modulation is based on three levels. 
Typically in a black and white printing system, the light beam will be 
modulated to be either On or Off. With amplitude modulation for tri-level, 
the light beam will be turned On or Off, but when it is turned On, it will 
have either full intensity for color or it will have a lesser intensity 
for white. The full intensity creates the highest level 16 of exposure on 
the photoreceptor, the lesser intensity creates the exposure level 14 and 
when the light beam is turned Off, it will create the lowest level of 
exposure 12. 
The same result can be achieved by utilizing a pulse width modulation. In 
Pulse width modulation the width of each pulse determines the amount of 
exposure. Depending on the width of the pulse for each pixel the 
photoreceptor will be exposed less or more. For color level 16 the width 
of the pulse is more than the width of the pulse for the white level 14 
and for the black level there is no pulse. Therefore, if the pulse has a 
shorter pulse width the photoreceptor will be exposed less (white level) 
and if the pulse has a longer pulse width, the photoreceptor will be 
exposed for a longer time and therefore it will reach to a higher exposure 
level 16 (color level). 
The problem with amplitude modulation is controlling the color level and 
the white level. A slight variation in the color level causes the color to 
become either lighter or darker. However, the problem with variation of 
the white level is more severe than the variation of the color level. If 
the white level varies, instead of no printing, a pale color or a pale 
gray will be printed on the paper. Therefore, keeping the white level at a 
precise level is more critical. Typically, to control the white level the 
power of the laser diode will be divided into small steps which will be 
used to adjust the white level. The more the number of the steps, the more 
the control over the white level. 
Also, for the color level, the power of the laser diode is divided into 
steps. However, the number of the steps for the color level is less than 
the number of the steps for the white level. Typically, a single channel 
laser diode is utilized to produce a light beam for both the white level 
and the color level. This requires the laser diode to have a high power 
adjustment (high number of steps) for the white level and also a 
reasonable power adjustment for the color level which usually is a 
difficult requirement to be placed on a single channel. Also, since the 
single channel has to produce the light beam for both levels, it has to 
work in a power range which covers both levels. This also adds to the 
complexity of the power adjustment for both levels. 
The problem with pulse width modulation is the required high frequency. In 
pulse width modulation, for every change of level (color change) a pulse 
should be generated. Therefore, for high resolution printing systems which 
have higher number of pixels per inch (higher number of color changes), if 
a pulse width modulation is used, the frequency will be very high. 
SUMMARY OF THE INVENTION 
This invention suggests a different approach which can be expanded to 
multi-color systems with more exposure levels than three. In this 
invention two different light beams are used to scan one scan line to 
create the different exposure levels necessary for a tri-level printing. 
By utilizing two light beams, one of the light beams scans a scan line 
once and when the first light beam starts to scan a different scan line, 
the second light beam starts to scan over the scan line which is already 
scanned by the first light beam. With this approach each scan line will be 
scanned twice. 
This approach solves the aformentioned problems. By utilizing two channels 
of a multi-channel laser diode, one channel can be dedicated to produce 
the white level and the second channel can be dedicated to produce the 
color level. This allows each channel to be controlled for a different 
requirement and also each channel has a lower power range to cover which 
improves the power adjustment. 
Also, by utilizing two channels of a multi-channel laser diode, since each 
channel is responsible to produce one level, the number of the level 
changes for each channel will be less than the number of level changes for 
a single channel laser diode. Therefore, the modulation frequency of each 
channel will be much less than the frequency of a single channel laser 
diode.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 2 and 3, there are shown two different trains 20 and 22 
of pixel information which are used to modulate the two light beams 
suggested by this invention in order to achieve the exposure levels shown 
in FIG. 1. The train of pulses 22 shown in FIG. 2 will be used to modulate 
the first light beam and the train of pulses 22 will be used to modulate 
the second light beam. 
Referring to FIGS. 2 and 3, when the first light beam scans the 
photoreceptor, it creates the black exposure level 12 and the white 
exposure level 14. When the second light beam overscans the same line on 
the photoreceptor plane, it exposes the photoreceptor for a second time 
and therefore the exposure from the second light beam adds to the exposure 
level 14 created by the first light beam to generate the color level 16 
(FIG. 1). For example, to create the color level 16, when the first light 
beam scans the photoreceptor, it creates the white level 14 and when the 
second light beam scans the photoreceptor, an extra level 24 (FIG. 3) from 
the second light beam will be added to the white level 14 which generates 
the color level 16. As it can be observed, by adding the two trains of 
pulses 20 and 22 of FIGS. 2 and 3, the train of exposure levels shown in 
FIG. 1 can be created. 
Double scanning can be done with different scan separations between the two 
light beams. Referring to FIG. 4, there is shown a double scanning with 
two light beams 1 and 2 with one scan line separation between them. It 
should be noted that the scan line separation is the space between the two 
centers of the two light beams The two light beams 1 and 2 scan two 
adjacent scan lines. Considering all scan lines above the scan line L1 to 
be outside of the printing boundary, when the light beam 1 scans the scan 
line L1, the light beam 2 scans along a line which is outside of the 
printing boundary. When the light beam 1 completes scanning the line L1, 
the photoreceptor moves in such a way that the light beam 1 starts 
scanning the scan line L2. At this time, the light beam 2 starts scanning 
the scan line L1. Therefore, while the light beam 1 scans the scan line 
L2, light beam 2 scans over line L1 which is already scanned by the light 
beam 1. 
It should be noted that the laser light source is stationary and the 
photoreceptor moves in a direction perpendicular to the direction of the 
scan. However, for the purpose of simplicity of describing the invention, 
at the end of each scan, when the photoreceptor moves in such a manner 
that the light beams will start scanning different scan lines, hereinafter 
will be referred to as the relative movement of the light beams. 
As the light beam 1 moves onto the start of the next line to be scanned, 
light beam 2 follows the light beam 1 onto the start of the line which the 
light beam 1 just completed scanning. 
It should be noted that in FIG. 4 and also in the following FIGS. 5, 6, 7 
and 8, for the purpose of clarity, the light beams on different scan lines 
are shown on different locations. However, the light beams start scanning 
each line from line 100 which is the start of scan for all the scan lines. 
Referring to FIG. 5, there is shown a double scanning with two light beams 
1 and 2 which have two scan line separation between them. With one scan 
line separation, the light beam 1 starts scanning line L1, while the light 
beam L2 scans a line outside of the printing boundary. Then the light beam 
1 moves onto line L2 and light beam 2 moves onto a line still outside of 
the printing boundary. When the light beam 1 moves onto line L3, then 
light beam 2 moves onto line L1 which is already scanned by light beam 1. 
As the light beam 1 moves onto the next line to be scanned, the light beam 
2 moves onto a line which is one line apart from the line that the light 
beam 1 is on. With this approach, the light beams can be apart by as many 
scan lines as desired. 
Double scanning can also be achieved through four light beams. With this 
approach, a pair of light beams will be assigned to double scan certain 
scan lines and the other pair will be assigned to double scan the 
remaining scan lines. 
Referring to FIG. 6, there is shown an example of double scanning with four 
light beams 1, 2, 3 and 4. In this example, light beams 1 and 3 are 
assigned to double scan the scan lines L1, L3, L5 and L7 and the light 
beams 2 and 4 are assigned to double scan lines L2, L4, L6 and L8. 
To double scan the scan lines with four light beams, an interlace format 
should be used. When light beam 1 starts scanning the scan line L1, the 
light beams 2, 3 and 4 scan some lines outside of the printing border. 
When the light beam 1 reaches the end of the scan line L1 it moves onto 
the scan line L3. At the same time, light beams 2 and 3 move onto scan 
lines L2 and L1 respectively. At the end of scan line L3, light beam 1 
moves onto line L5 and the light beams 2, 3 and 4 move onto the scan lines 
L4, L3, L2 respectively. In this fashion, all the scan lines will be 
double scanned. As it can be observed, scan lines L1, L3, L5 and L7 will 
be scanned by the light beams 1 and 3 and the scan lines L2, L4, L6 and L8 
will be double scanned by the light beams 2 and 4. 
As in the case of double scanning with two light beams in which the two 
light beams can have multi-scan line separation between the two light 
beams, with four light beams double scanning, the light beams can also 
have multi-scan line separation. 
Referring to FIG. 7, there is shown an example of double scanning with four 
light beams which have multi-scan line separation. In this example, there 
are two scan line separation between each two light beams. However, the 
light beams scan the scan lines with an interlace format. 
When light beam 1 starts to scan line L1, the light beams 2, 3 and 4 scan 
lines which are outside of the printing boundary. When the light beam 1 
moves onto the scan line L3, the light beams 2, 3 and 4 are still outside 
of the printing boundary. When light beam 2 moves onto the scan line L5, 
light beam 2 moves onto the scan line L2 and when light beam 1 moves onto 
the scan line L7, light beams 2 and 3 move onto the scan line L4 and L1 
respectively. In this fashion, scan lines L1, L3, L5, L7, L9 and L11 will 
be double scanned by light beams 1 and 3 and the scan lines L2, L4, L6, L8 
and will be double scanned by light beams 2 and 4. 
In all the above examples there is a method in selecting the number of the 
light beams and the number of the scan lines which the light beams should 
move while moving from one scan line onto the next line to be scanned. For 
double scanning with two light beams with or without separation between 
the two light beams, the light beams should scan the scan lines 
consecutively. However, for double scanning with four light beams with or 
without separation between the two light beams, the light beams should 
move by two scan lines while moving from one scan line onto the next line 
to be scanned. Therefore, depending on the number of the light beams used, 
which should be an even number, the light beams should move by a number of 
scan lines equal to half the number of the light beams. 
This concept can be applied to printing systems with more xerographic 
levels than three. In general, the number of the light beams and the 
number of the scan lines that the light beams should move while moving 
from one scan line to the next with respect to the number of the 
xerographic levels should agree with the following equation: 
EQU N=n(K-1). 
Where N is the number of the light beams, n is an integer equal to the 
number of the scan lines that the light beams should move while moving 
from one scan line onto the next line to be scanned and K is the number of 
the xerographic levels including the zero exposure level. 
It should be noted that for n.gtoreq.2 the number of scan line separation 
between light beams (the space between the centers of the two light beams) 
should be at opposite parity of n, wherein parity is defined as even or 
odd. 
For example, in a tri-level system which has three xerographic levels, K is 
equal to 3. If n is selected to be 1, then the number of the light beams 
should be 2; 
EQU N=1(3-1)=2. 
In this case, the light beams have to move by one scan line in order to 
move from one scan line to the next line to be scanned. 
However, if n is selected to be 2, then the number of the light beams 
should be 4; 
EQU N=2(3-1)=4. 
In this case, the light beams have to move by two scan lines in order to 
move from one scan line to the next line to be scanned and the number of 
scan line separation between the two light beams could be 1 (such as FIG. 
6), 3 (such as FIG. 7), 5, 7, etc. 
Yet, if n is selected to be 3, then the number of the light beams should be 
6; 
EQU N=3(3-1)=6. 
Referring To FIG. 8, there is shown an example of double scanning with 6 
light beams. In this example, the light beams have to move by three scan 
lines in order to move from one scan line to the next line to be scanned 
and the number of scan line separation between the light beams should be 
2. It should be noted that the line separation between the light beams can 
also be 4, 6, 8, etc. Also, in this case, since the system is a tri-level 
system, the 6 light beams should double scan the scan lines. Therefore, 
the intensity of the light beams should be adjusted in such a manner that 
three light beams should have one intensity (I1=I2=I3, I being the 
intensity of a light beam) to provide the white and black levels. Also, 
the other three light beams should also have one intensity (I4=I5=I6) to 
provide the color level. However, the intensity of the first three light 
beams should be different than the intensity of the second three light 
beams (I1=I2=I3.noteq.I4=I5=I6). With this arrangement, any two light 
beams each being selected from a different group will provide the same 
white and color levels. 
As it was previously mentioned, the same equation applies to xerographic 
systems with more exposure levels than three. For example, if a system has 
four exposure levels and if n is selected to be 2, then the number of the 
light beams should be equal to 6 and the light beams should move by two 
scan lines in order to move from one scan line to the next line to be 
scanned and the number of scan line separation between the light beams can 
be equal to 3. 
By using equation N=n (K-1), different combinations of light beams for 
different systems with different exposure levels can be designed.