Dot corrected laser printer

To provide a smooth representation in half-tones of a record, the area of a dot recorded by a laser printer is controlled. Binary image information which is applied to the laser printer is examined to determine if it is isolated when it is to be recorded "black". When the laser printer is operated in the background exposure scheme, the off time of the laser which records an isolated "black" is increased whereas when the laser printer is operated in the image exposure scheme, the on time of the laser which records an isolated "black" is reduced. The control of the recorded area per dot is disclosed for one dimension array (in the direction of main scan X) and two dimension array (in the directions of main scan X and subscan Y). Also, the recorded area per dot can be controlled by controlling the transmission of laser beam through an AO modulator in a manner corresponding to the detection of the isolation.

TECHNICAL FIELD 
The invention relates to a laser printer, in particular, to the control of 
a recording density in a laser printer. 
PRIOR ART 
The arrangement of a laser printer which is typical in the prior art is 
schematically shown in FIG. 7. In this Figure, a laser 5 emits a laser 
beam which passes through a beam compressor 6, A0 modulator 7, a beam 
expander 8 and a cylindrical lens 9 to impinge upon a rotating multi-facet 
mirror 12. The multi-facet mirror 12 is driven for rotation at a uniform 
rate by means of a motor 11. Laser beam reflected by the multi-facet 
mirror passes through a toroidal lens 13 and an f-.theta. lens 16 to 
irradiate a drum 17 formed of a photosensitive material. As the 
multi-facet mirror 12 rotates, the laser beam repeatedly traverses along 
the axis of the drum 17 to scan the surface of the drum 17 (main scan X). 
It will be appreciated that the drum 17 is driven for rotation at a 
uniform rate (subscan Y). 
While not shown, the surface of the drum 17 is uniformly charged by means 
of a charger, and the charged surface is scanned by the laser beam in the 
manner mentioned above. During one line scanning in which the laser beam 
traverses from one end to the other end of the drum 17, the laser beam is 
modulated spotwise by means of the A0 modulator in an on- and off-manner 
corresponding to a binary signal to be recorded. As a consequence, an 
electrostatic latent image is formed on the surface of the drum 17 in a 
manner corresponding to binary information, and the latent image is 
developed by a developing unit to provide a visual toner image, which is 
then transferred onto a record paper. 
Binary information has a correspondence to a position in the directions of 
the main scan X and the subscan Y (or a point in the two dimensional 
coordinate system or a picture element). In a background exposure scheme 
where a white background is exposed to laser beam while areas representing 
an image are not exposed so that a toner may be applied to the image 
subsequently, dots recorded will be generally configured as indicated by 
hatching lines in FIGS. 9a to 9e. In actuality, dots formed by continuous 
exposure and not recorded (shown in solid line) do not assume a circular 
configuration, but represent a line having a width corresponding to the 
diameter of a circle and which is continuous in the direction of main scan 
X. Consequently, recorded dots (shown hatched) in FIGS. 9a to 9e have a 
side which adjoins with a white dot (shown by a solid line circle) which 
is represented by a straight line parallel to the direction X. In other 
words, the dot will be slightly less in area than that shown by hatched 
lines. 
Generally, when a single dot is recorded in an isolated manner, its four 
sides will be surrounded by white dots, as shown in FIG. 9a, and hence the 
area Sd of the recorded single dot is very small. 
When two dots are recorded in succession, an overlapping area Sf between 
black dots are recorded as black, whereby the recorded area of the two 
dots will be equal to 2 Sd+Sf. Accordingly, the recorded area per dot will 
be Sd+1/2 Sf, thus slightly greater than that mentioned initially. 
Similarly, when three dots are recorded as indicated in FIG. 9c, the total 
area of the three dots will be 3 Sd+2 Sf, and thus the area per dot will 
be Sd+2/3 Sf, representing a further greater recorded area. When four dots 
are recorded as indicated in FIG. 9d, the total area of the four dots will 
be equal to 4 Sd+3 Sf, and the area per dot will be Sd+3/4 Sf, producing 
an increased recorded area. When five dots are recorded as indicated in 
FIG. 9e, the total area of the five dots will be 5 Sd+4 Sf, and hence the 
area per dot will be Sd+4/5 Sf, which has been increased furthermore. 
It will be seen from the foregoing description that considering a single 
recorded dot, the area of the recorded dot in question will vary depending 
on whether adjacent dots are or are not black. 
Representing the single dot area Sd which is produced when the single dot 
is recorded in an isolated manner as shown in FIG. 9a, as one unit, it 
will be seen that the recorded area should ideally increase as indicated 
by a phantom line curve shown in FIG. 9f as the number of adjacent black 
dots increases. However, as mentioned previously, the area per dot 
increases depending on the number of adjacent black dots, so that the 
recorded area as a function of the number of adjaceant black dots will be 
as indicated by a solid line curve in FIG. 9f where such recorded area 
increases beyond the proportional relationship with respect to the number 
of adjacent black dots. In other words, the recorded density will 
increase. Accordingly, a print which is defined by an isolated single dot 
tends to disappear, and an oblique line or diagonal defined by the single 
dot width tend to fade out, thus degrading the print quality. When a 
halftone is to be represented in terms of the number of recorded picture 
elements for a given area which corresponds to a given number of picture 
elements, it is difficult to produce a smooth halftone. 
In an image exposure scheme where the white background of an image remains 
unexposed while an image area is exposed to laser radiation and the image 
is then developed with a toner, the dots recorded will be generally 
configured as indicated by hatching lines shown in FIGS. 10a to 10e. 
Actually, continuously exposed or recorded dots (shown in solid line 
circles, as hatched) do not appear as circular, but represent a line 
having a width corresponding to the diameter of the circle and which is 
continuous in the direction of main scan X. Thus, they will be slightly 
greater than the area shown. 
Generally, when a single dot is recorded in an isolated manner, it is 
configured substantially circular as shown in FIG. 10a, and has a maximum 
area Sd per dot recorded. When only two dots are recorded in succession, 
an area of overlap Sf between black dots are shared by the two dots, so 
that the total area of the two dots recorded will be equal to 2 Sd-Sf, and 
thus the area per dot recorded will be Sd-1/2 Sf, thus representing a 
slight reduction in the recorded area. Similarly, when three dots are 
recorded as indicated in FIG. 10c, the total area of the three dots will 
be 3 Sd-2 Sf, and the area per dot will be Sd-2/3 Sf, representing a 
further reduction in the area recorded. When four dots are recorded as 
shown in FIG. 10d, the total area of the four dots will be equal to 4 Sd-3 
Sf, and thus the area per dot will be Sd-3/4 Sf, representing an even 
greater reduction in the area recorded. Where five dots are recorded as 
shown in FIG. 10e, the total area of the five dots will be equal to 5 Sd-4 
Sf, and the area per dot will be Sd-4/5 Sf, representing a further 
reduction in the area recorded. It will be seen that considering the area 
of a single dot recorded, such recorded dot area depends on whether 
adjacent dots are or are not black. 
Assuming that the area Sd per dot which is obtained when a single dot is 
recorded in an isolated manner as shown in FIG. 10a represents one unit, 
it will be seen that the recorded area should ideally increase with the 
number of adjacent black dots as indicated by phantom lines shown in FIG. 
10f. However, the area per dot reduces as a function of the number of 
adjacent black dots in the manner mentioned above. The recorded area 
actually reduces from its proportional relationship with respect to the 
number of black dots, as the number of adjacent black dots increases, in a 
manner indicated by a solid line curve in FIG. 10f. It will thus be seen 
that the recording density changes. Again there arises a problem that it 
is difficult to achieve a smooth halftone representation. 
SUMMARY OF THE INVENTION 
It is an object of the invention to reduce a change in a recording density 
per picture element recorded which would result from the correlation 
between a picture element being recorded and adjacent picture elements 
which may or may not be recorded. 
To achieve the above object, in accordance with the invention, there is 
provided means for correcting the amount of laser beam to which a picture 
element in question is exposed, in a manner depending on the correlation 
between the picture element in question or being recorded and adjacent 
picture elements which may or may not be recorded. To explain this for the 
background exposure scheme mentioned above, when a picture element in 
question is surrounded by a reduced number of adjacent black picture 
elements recorded or by an increased number of adjacent white picture 
elements, the dot for that picture element is recorded to a greater extent 
or the amount of laser beam which forms the adjacent white picture 
elements is reduced. On the contrary, in the image exposure scheme, when a 
picture element in question is surrounded by a reduced number of adjacent 
black picture elements or by an increased number of adjacent white picture 
elements, that dot is recorded to a lesser extent or the amount of laser 
beam which forms the black picture elements is reduced. In one embodiment, 
this can be accomplished by controlling the amount of laser beam in terms 
of a time interval allocated to a picture element (during which it remains 
non-irradiated in the background exposure scheme and is irradiated in the 
image exposure scheme). In another embodiment, the amount of laser beam is 
controlled by the level of emission from AO modulation. With this 
arrangement, the dot which is highly isolated has a greater recorded area 
or a greater recording density in the background exposure scheme. 
Alternatively, in the image exposure scheme, a dot which is less isolated 
has a greater recorded area or a recording density. In this manner, there 
is obtained a record with a high reproducibility of an image and with a 
smooth halftone representation. 
Other objects and features of the invention will become apparent from the 
following description with reference to the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1a, there is shown one embodiment of the invention. A 
laser 10 emits laser beam which is reflected by a multi-facet mirror 12 
which is driven for rotation at a uniform rate. The laser beam is then 
passed through an f-.theta. lens 16 to impinge upon a mirror 14, which 
reflects the beam for reflection by another mirror 15. After being 
reflected by the mirror 15, the laser beam passes through a cylindrical 
lens 13 to irradiate a drum 17 which is formed from a photosensitive 
material. As the multi-facet mirror rotates, the laser beam repeatedly 
scans the surface of the drum in a direction parallel to the axis thereof 
(main scan X). It will be appreciated that the drum 17 is driven for 
rotation at a uniform rate (subscan Y). 
While not shown, the surface of the drum 17 is uniformly charged by a 
charger, and the charged surface is scanned by the laser beam in the 
manner mentioned above. It will be noted that a laser driver 20 is 
connected to the laser 10, and during the scan by the laser beam, a beam 
sensor 18 disposed adjacent to one end of the drum 17 detects the presence 
of laser beam. A binary signal having a high or H logic level, 
representing a recording operation and a low or L logic level representing 
a non-record operation is applied to the laser driver 20 at a given time 
lag after the detection of the laser beam by the sensor 18 or when the 
scanning point of the laser beam reaches the beginning of a recording 
region. The laser driver 20 controls the emission of laser beam from the 
laser 10 in accordance with the recording signal. In the background 
exposure scheme, the high level H causes the emission of laser beam to be 
turned off while the low recording signal level L causes the emission of 
laser beam to be turned on. 
The binary recording signal or data 1 which represents an image data is 
applied to a recorded dot width correction circuit 30 which is effective 
to correct the amount of exposure caused by the laser beam. It is to be 
understood that the circuit 30 operates to compare a binary signal 
associated to a picture element to be recorded or a picture element in 
question with binary signals associated with adjacent picture elements, 
and develops a corrected binary signal or data 2 depending on the 
correlation therebetween which is applied to the laser driver 20. 
In this embodiment, the circuit 30 comprises a monostable multivibrator 31 
having an input B to which data 1 is applied. When data 1 rises from its L 
(non-record) to its H (record) level, the multivibrator 31 is triggered to 
produce a signal STc having an H level during a time interval Tm and which 
then returns to its L level (non-record) level. In addition, the circuit 
30 comprises an OR gate 33 which produces a logical sum of data 1 and the 
output STc from the multivibrator 31. The time interval Tm during which 
the multivibrator 31 develops the signal STc is determined by a resistor 
32, which is shown as a variable resistor, thus permitting the length of 
Tm to be adjusted. 
FIG. 1b graphically shows various signals appearing within the correction 
circuit 30. In this example, Tm has a length slightly longer than the time 
period Td of the binary signal or data 1 which is allocated to a single 
picture element. Suppose that data 1 associated with a picture element 
which immediately precedes the picture element in question has an L 
level(non-record) and data 1 associated with the picture element in 
question has an H level (record) while data 1 associated with a picture 
element which immediately follows the picture element in question in the 
direction of the main scan X has an L level (non-record) or that the 
picture element in question is black and is isolated from preceding and 
following picture elements which are white's as viewed in the direction of 
the main scan X. When data 1 changes from its L (for preceding picture 
element) to its H (picture element in question) level, the multivibrator 
31 is triggered to develop an H output. During the dot period Td for the 
picture element in question, both data 1 and the output STc from the 
multivibrator 31 assume their H level, whereby OR gate 33 develops an H 
output as data 2, which indicates a record operation. After the dot period 
Td associated with the picture element in question has passed, the next 
dot period Td will be associated with the following picture element. 
However, while data 1 has an L level which is associated with the 
following picture element, the output STc from the multivibrator 31 
remains to be at its H level until Tm passes, and accordingly the gate 33 
continues to deliver the H output (indicating a record operation) as data 
2. After Tm has passed, both data 1 and STc are L levels, and hence the 
gate 33 provides an L output (non-record operation) as data 2. 
Alternatively, when the binary signal associated with the picture element 
in question and the binary signal associated with the immediately 
following picture element both have an H level (record), after the time 
period (Tm) allocated to the picture element in question has passed, there 
remains the H level associated with the following picture element, so that 
the extension time (Tm-Td) during which the record for the picture element 
in question is extended will be shared by the following picture element as 
well. It will be seen that when black's appear in succession in the 
direction of the main scan, there is no change from the L to the H level 
in data 1, and hence the multivibrator 31 cannot be triggered. This means 
that there is no extension time (Tm-Td) for the last one of the black 
picture elements. 
It will be seen that the recorded dot width correction circuit 30 operates 
to increase the recording time for the picture element in question from Td 
to Tm only when the picture element in question has an H level (record) 
and is preceded by a picture element having an L level (non-record) and 
followed by another picture element having an L level (non-record) 
considering the distribution of blacks and white among binary information 
or data 1 associated with successive picture elements. It will also be 
noted that the extension of the recording time for the picture element in 
question substantially does not occur when the picture element in question 
and its preceding picture element both have an H level or when the picture 
element in question and a following picture element both have an H level. 
In this manner, the correction circuit 30 considers the correlation 
between recording information of picture elements which are adjacent to 
the picture element in question in the direction of the main scan X, by 
changing the recording time for the picture element in question to Tm when 
the picture element in question is to be recorded black and is isolated. 
It will be understood that when the recording time is changed to Tm, the 
recording time has been increased from Td to Tm, thus resulting in an 
increase in the width of the recorded dot by an amount corresponding to 
(Tm-Td). 
It should be understood that the monostable multivibrator 31 serving as 
time limit means may be replaced by a delay circuit, for example, a shift 
register which is clocked by a clock having a period which is 
substantially reduced than Td. A time lag provided by the shift register 
is chosen to be equal to (Tm-Td). In this instance, the correlative 
comparison is changed. Specifically, in the above embodiment, the 
multivibrator 31 is triggered in response to a change from an L to an H 
level between the preceding picture element and the picture element in 
question. However, when the shift register is used, the recording time for 
the picture element in question is changed by the correlative comparison 
between the binary value of the picture element in question and that of 
the following picture element. Specifically, when the picture element in 
question has an H level while the following picture element has an L 
level, the recording time for the picture element in question is increased 
from Td to Tm, with the time delay (Tm-Td) being provided by the delay 
circuit. It is to be understood that the extension of the recording time 
occurs when the picture element in question which is to be recorded black 
is spaced from a subsequent black picture element. 
FIG. 2 shows essential parts of another embodiment of the invention where 
the recorded dot width correction circuit 30 comprises an isolation 
detection circuit 39 and a time limit circuit 38. The isolation detection 
circuit 39 comprises a shift register 34 in which binary recording signal 
or data 1 is latched, NOR gate 35 and AND gate 36. Representing a picture 
element in question by B, and a preceding picture element and a following 
picture element as viewed in the direction of the main scan X by A and C, 
respectively, the binary signals associated with these picture elements A 
and C are applied to the inputs of the gate 35. It will be seen that the 
gate 35 produces an H level output only when the both input signals are at 
L levels, indicating that the picture element in, question, is preceded 
and followed by a white element . The gate 36 develops an H level output 
(isolation detection signal) for application to the time limit circuit 38 
when the gate 35 produces an H level output and the picture element in 
question B has an H level, or when the picture element in question of 
black level is isolated from the preceding and the following picture 
element. 
The time limit circuit 38 comprises a count N counter 37, and OR gate 33. 
The isolation detection signal of H level enables the counter 37 to begin 
counting clock pulses CLK having a period which is shorter than Td, and to 
change its output STc from its L to its H level. When the count in the 
counter 37 reaches a value corresponding to Tm, it changes the output STc 
from its H level to its L level. The output STc from the counter 37 is 
applied to the gate 33 together with the binary signal associated with the 
picture element B in question, and the gate 33 provides an output which 
represents a logical sum of the both inputs. 
With this arrangement, it will be understood that the recorded dot width 
correction circuit 30 shown in FIG. 2 operates to change the recording 
time for the picture element in question from Td to Tm only when the 
picture element in question is black and is isolated from preceding and 
following picture elements as viewed in the direction of the main scan X. 
The counter 37 shown in FIG. 2 may be replaced by a delay circuit such as a 
shift register, for example, which provides a time lag of (Tm-Td). The 
same extension of recording time occurs with this modification. 
FIG. 3 shows essential parts of a further embodiment of the invention where 
the recorded dot width correction circuit 30 comprises an isolation 
detection circuit 39 and a time limit circuit 38. In this embodiment, the 
isolation detection circuit 39 comprises three line shift registers 
34.sub.0, 34 and 34.sub.1, each of which has the capacity to store binary 
recording information for one full line in the direction of the main scan 
X, in order to enable the correlation in the direction of subscan to be 
determined. In addition, the detection circuit 39 includes NOR gata 35 and 
NAND gate 36. Representing a picture element in question by A, the binary 
recording signals associated with picture elements A.sub.0 and A.sub.1 
which are adjacent to the picture element A in question in the direction 
of subscan Y are applied to the gate 35, which therefore develops an H 
level output only when the both signals from the picture elements A.sub.0 
and A.sub.1 have L levels, indicating that the picture element A is 
preceded and followed by white's in the direction of subscan. The binary 
signal associated with the picture element A and the output of the gate 35 
are applied to the inputs of the gate 35, which therefore develops an L 
level signal only when the binary signal associated with the picture 
element A has an H level and the gate 35 delivers an H level output 
(indicating the picture element A is isolated in the direction of 
subscan). The L level signal from the gate 36 represents a detection 
signal that the picture element in question is isolated in the direction 
of the subscan Y. 
The time limit circuit 38 comprises a monostable multivibrator 31 and OR 
gate 33. The multivibrator 31 has an input A which is connected to the 
output of the gate 36. The multivibrator 31 is triggered when the input A 
has an L level and when another input B changes from its L to its H level. 
The multivibrator 31 is not triggered when the input A assumes its H 
level. The input B is supplied with the binary signal associated with the 
picture element A in question. Accordingly, the multivibrator 31 is 
triggered when the output from the gate 36 is at its L level, indicating 
that the picture element in question is isolated in the direction of the 
subscan Y and when in the direction of the main scan X the binary 
recording signal of the immediately preceding picture element has an L 
level and the picture element A in question has a binary signal of H 
level. In other words, the multivibrator 31 is triggered only when the 
picture element A in question is isolated from its immediately adjacent 
picture elements A.sub.0, A.sub.1 in the direction of the subscan Y and is 
also isolated from the immediately preceding picture element in the 
direction of the main scan X. The binary signal associated with the 
picture element A is also applied to OR gate 33 together with the Q output 
of the multivibrator 31. Thus, the gate 33 delivers a binary recording 
signal or data 2 which increases the recording time for the picture 
element A in question from Td to Tm. 
It will be readily understood that an isolation detection circuit which 
detects the isolation in the direction of main scan X may be combined with 
another isolation detection circuit which detects the isolation in the 
direction of subscan Y to provide a combined detection of isolation in the 
both directions X and Y. As mentioned previously, the recorded area per 
dot varies with the distribution of adjacent black picture elements around 
the picture element in question. Accordingly, it may be desirable in some 
instances to control the amount of laser radiation at multiple levels 
depending on the distribution of adjacent black picture elements in a more 
refined manner. 
FIG. 4a shows essential parts of still another embodiment of the invention 
in which the recording time for the picture element in question is 
controlled at multiple levels in accordance with the distribution of 
adjacent black picture elements in both X and Y directions, and FIG. 4b 
shows various signals which appear at selected points within the 
arrangement. More specifically, the recording time for a picture element B 
in question, which is assumed to be recorded black, is controlled in a 
manner indicated in Table 1 below, in accordance with the distribution of 
adjacent picture elements A, B.sub.0, C and B.sub.1 (see FIG. 9a) which 
surround the picture element B. 
TABLE 1 
______________________________________ 
adjacent picture recording time 
elements A B.sub.0 C B.sub.1 
of pixel B 
______________________________________ 
information L L L L Td + t.sub. + t.sub.3 
of pixel H L L L Td + t.sub.2 + t.sub.3 
L H L L Td + t.sub.2 + t.sub.3 
L L H L Td + t.sub.1 + t.sub.4 
L L L H Td + t.sub.1 + t.sub.4 
H H L L Td + t.sub.3 
H L H L Td + t.sub.2 + t.sub.4 
H L L H Td + t.sub.2 + t.sub.4 
L H H L Td + t.sub.2 + t.sub.4 
L H L H Td + t.sub.2 + t.sub.4 
L L H H Td + t.sub.1 
H H H L Td + t.sub. 4 
L H H H Td + t.sub.2 
H L H H Td + t.sub.2 
H H L H Td + t.sub.4 
H H H H Td 
______________________________________ 
In this table, H represents a record operation while L represents a 
non-record operation, and t.sub.1, t.sub.3 &gt;t.sub.2, t.sub.4 ; t.sub.1 
+t.sub.3.ltoreq.Tm. The extension of the recording time as indicated by 
t.sub.1 and t.sub.2 takes place by reducing the recording time Td for the 
picture elements which procede the picture element in question. The 
extension of the recording time as indicated by t.sub.2 and t.sub.4 takes 
place by extending the recording time Td for the picture element in 
question. 
Referring to FIG. 4a, a recorded dot width correction circuit 30 comprises 
an isolation detection circuit 39 which detects the isolation in two 
dimensions X and Y and a time limit circuit 38 which controls the 
recording time for the picture element in question (as indicated in Table 
1) in accordance with the result of detection of isolation by the circuit 
39. 
The isolation detection circuit 39 comprises three line shift registers 
34.sub.0, 34, 34.sub.1, each of which stores binary recording information 
or data 1 for one full line, NOR gates 40 and 43, exclusive OR gates 41 
and 44, and a pair of latches 46 and 47 which provide a time delay of Td 
(one bit). In the description to follow, it is to be noted that a picture 
element in question is represented by B. The gate 40 operates to determine 
whether or not an adjacent picture element B.sub.0, which precedes the 
picture element B by one line in the direction of subscan, and an adjacent 
picture element A which precedes the picture element B in the direction of 
main scan X, are both white's (L). It produces a signal a of H level only 
when both of these adjacent picture elements are white's. The exclusive OR 
gate 41 produces an output signal b of H level only when only one of these 
adjacent picture elements is white (L). Similarly, NOR gate 43 produces an 
output signal c of H level only when the picture element B is black (H) 
and when the following picture element C in the direction of main scan X 
and the following picture element B.sub.1 in the direction of subscan Y 
are both white's (L). The signal c is delayed by Td by the delay circuit 
46, which provides a signal e. Similarly, the exclusive OR gate 44 
produces an output signal d of H level when the picture element B is black 
(H) and when only one of the following picture element C in the direction 
of main scan X and the following picture element B.sub.1 in the direction 
of subscan Y is white (L). The signal d is delayed by Td by the delay 
circuit 47, which provides a signal f. 
The time limit circuit 38 comprises four monostable multivibrators 48 to 
51, a pair of AND gates 52 and 53 and OR gate 54. The multivibrator 48 is 
triggered when the output a from NOR gate 40 changes from its L to its H 
level, or when the binary signals associated with the picture elements A 
and B.sub.0 have both changed to L levels (white's) when they were 
initially both H levels or only one of them was H level, thus changing its 
output from its H level to its L level. After a time interval of Tm1 
(=Td-t.sub.1), the output reverts to its H level. Accordingly, AND gate 52 
produces an output signal g of H level only during a time interval t.sub.1 
which immediately precedes the initiation of a recording of the picture 
element B when the picture element B has an H level and both picture 
elements A and B.sub.0 have an L level. 
The multivibrator 49 changes its output from its H to its L level by being 
triggered when the output b from the exclusive OR gate 41 changes from its 
L to its H level, or when the binary signals associated with the picture 
elements A and B.sub.0 they had initially an H level both and then only 
one of them has changed to its L level (white). After a time interval Tm2 
(=Td-t.sub.2). the output reverts to its H level. Accordingly, AND gate 53 
produces an output signal h of H level only during a time interval t.sub.2 
which immediately precedes the initiation of a recording of the picture 
element B when the picture element B has an H level and only one of the 
picture elements A and B.sub.0 has an L level. 
Since the signal g or the signal h which assumes an H level only during the 
time interval t.sub.1 or t.sub.2 resides in a recording time associated 
with the picture element A which precedes the picture element B in 
question and is contiguous with the initiation of a recording of the 
picture element B, it will be appreciated that the recorded width of the 
picture element B is extended by extending the recording time into the 
recording time associated with the preceding picture element A, inasmuch 
as either signal g or h is developed only when the picture element A has 
an L level. 
The multivibrator 50 is triggered to change its output from its L to its H 
level when the signal e changes from its L to its H level or during the 
recording time of the picture element B when it is being delivered to the 
OR gate 54 and when the binary signals associated with the picture 
elements B.sub.1 and C have both changed to L levels (white's) when they 
were initially both H levels or only one H level. Subsequently it delivers 
a signal i of H level during a time interval Tm3 (=Td+t.sub.3) 
The multivibrator 51 is triggered when the signal f changes form its L to 
its H level, or when the picture element B is being delivered to OR gate 
54 and when the binary signals associated with the picture elements 
B.sub.1 and C have changed in a manner such that only one of them has an L 
level (white) when both of them initially had an H or an L level. The 
multivibrator 51 then delivers an output signal j of H level during a time 
interval Tm4 (=Td+t.sub.4) 
Since the signal i or the signal j which remains at its H level only during 
the time interval t.sub.3 or t.sub.4 resides in a recording time 
associated with the picture element C which is one picture element after 
the picture element B in question and is consecutive to the termination of 
a recording of the picture element B, and since the signal i or j at its H 
level is produced only when the picture element B has an H level, the 
recording width for the picture element B in question is increased by 
increasing it toward the picture element C which follows it. 
The outputs g to j from the multivibrators 48 to 51 are passed through OR 
gate 54 to be applied to the laser driver 20 as the binary recording 
signal or data 2. 
In this embodiment, when both picture elements B.sub.0 and A which are 
adjacent to the picture element B in question are white's (L level), the 
recording time for the picture elemet B is extended by t.sub.1 toward the 
picture element A. When only one of them is white (L level), the recording 
time for the picture element B is extended by t.sub.2 toward the picture 
element A. In addition, when both picture elements C and B.sub.1 which are 
adjacent to the picture element B in question are white's (L level), the 
recording time for the picture element B is extended by t.sub.3 toward the 
picture element C, and when only one of them is white (L level), the 
recording time for the picture element B is extended by t.sub.4 toward the 
picture element C. 
Generally speaking, a square shown in FIG. 8a is chosen as the reference 
for the recorded area per dot in this embodiment. It is to be understood 
that the square has one side which is defined by the dot pitch P shown in 
FIGS. 9a and other Figures. In addition, t.sub.1 corresponds to a 
recording time which increases the dot area by an amount (Sf.sub.1 
+Sf.sub.2) shown in FIG. 8a, t.sub.2 corresponds to a recording time which 
increases the dot area by an amount Sf.sub.1 (=Sf.sub.2), t.sub.3 
corresponds to a recording time which increases the dot area by an amount 
(Sf.sub.3 +Sf.sub.4) shown in FIG. 8a, and t.sub.4 corresponds to the 
recording time which increases the dot area by an amount Sf.sub.3 
(=Sf.sub.4). 
The described embodiments are used with a laser printer of the background 
exposure scheme. A modification which is necessary when applied to a laser 
printer of the image exposure scheme will now be described. As mentioned 
previously, in the background exposure scheme, the recorded area of an 
isolated dot is reduced (see FIG. 8a) in contrast to the image exposure 
scheme in which the recorded area of an isolated dot increases (see FIG. 
8b). Accordingly, in the image exposure scheme, it is necessary that the 
recording time be reduced in proportion to the degree of isolation, in a 
manner contrary to that used in the background exposure scheme. 
At this end, when the arrangement shown in FIG. 1a is modified for use in 
the image exposure scheme, the time limit Tm of the monostable 
multivibrator 31 is chosen less than Td, and OR gate 33 is replaced by AND 
gate. As a result, the recording time for a single black dot which is 
isolated in the direction of the main scan X will be equal to Tm. 
The monostable multivibrator may be replaced by a delay circuit. Such an 
example is shown in FIG. 5a, and its input and output are shown in FIG. 
5b. A shift register 55 is clocked by a clock CLK which has a period less 
than Td, providing a time delay of Tm to data 1. A logical product of a 
delayed output SHO from the shift register 55 and data 1 is output from 
AND gate 56. This output is applied to the laser driver 20 which is used 
in the laser printer of the image exposure scheme. As shown in FIG. 5b, 
only when the preceding picture element in the direction of the main scan 
X is white, the recording time for the picture element in question which 
follows is reduced, the reduction in time being equal to Tm. 
FIG. 6 shows a modification of the isolation detection circuit shown in 
FIG. 2 when it is applied to the image exposure scheme. In this instance, 
in the isolation detection circuit 39, if the picture element B in 
question is isolated from its preceding picture element A and its 
following picture element C in the direction of the main scan X, an H 
level representing the isolation is applied to a delay circuit 57 together 
with the data (H level) of the picture element B. This enables AND gate 
56, and a logical product of an output from a shift register 55 which 
provides a time delay of Tm and the H level representing the isolation of 
the picture element B is passed through OR gate 59 to be fed to the laser 
driver 20. When the H level representing the isolation is absent, AND gate 
56 is disabled while the AND gate 58 is enabled, passing the signal 
associated with the picture element B directly as an output. Accordingly, 
when the picture element in question is isolated, its associated recording 
time will be (Td-Tm) while when it is not isolated, the recording time 
will be Td. 
The determination of the isolation and the correction of the recording time 
can be effected in the image exposure scheme in the similar manner as 
illustrated in connection with the embodiment shown in FIG. 4a. 
Specifically, choosing the recorded area per dot to be a square shown in 
solid line in FIG. 8b having a side equal to a dot pitch P, and referring 
to FIG. 10a, a recording time for the picture element B in question is 
reduced by (Sf.sub.1 +Sf.sub.2) when both adjacent picture elements 
B.sub.0 and A are white's (L level), reduced by Sf.sub.1 when only one of 
the adjacent picture elements is white, reduced by (Sf.sub.3 +Sf.sub.4) 
when both adjacent picture elements C and B.sub.1 are white's (L level), 
and reduced by Sf.sub.3 when only one of these adjacent picture elements 
is white (L level). To achieve such modification, a pair of latches 
similar to the one bit delay latches 46 and 47 shown in FIG. 4a may be 
connected to the output of NOR gate 40 and to the output of the exclusive 
OR gate 41, respectively. Alternatively, the latches 46 and 47 may be 
omitted, and the time limit Tm3 of the multivibrator 50 is chosen equal to 
(Td-t.sub.3), the time limit Tm4 of the multivibrator 51 is chosen equal 
to (Td-t.sub.4) and the logical product of the output from the 
multivibrator 48 and the signal associated with the picture element B in 
question, the logical product of the output from the multivibrator 49 and 
the signal associated with the picture element B, the logical product of 
the output from the multivibrator 50 and the signal associated with the 
picture element B and the logical product of the output from the 
multivibrator 51 and the signal associated with the picture element B are 
applied to AND gate which is used in place of OR gate 54, the output of 
the AND gate driving the laser driver. 
In the described embodiments, the recording time or recorded area per dot 
is corrected in accordance with the degree of isolation in either the 
background or image exposure scheme so that the recorded area ratio 
(recorded area/number of recorded dots) can be maintained nearly constant. 
However, the invention can be carried out by regulating the recording 
density in terms of the emission level from the laser being controlled. 
This embodiment is schematically shown in FIG. 7 where an isolation 
detection circuit 39 supplies isolation information (either two-valued or 
multi-valued) and a binary recording signal (data 1) to a modulator 4. The 
modulator develops AO modulating voltage which corresponds to the 
isolation information for application to the deflecting electrode of the 
AO modulator 7. Thereupon, the transmission of the modulator 7 to laser 
beam changes in accordance with the isolation information to vary the 
emission level from the modulator 7 to provide a correction in the amount 
of exposure applied to a single dot. In this manner, a variation in the 
recording density per dot is minimized. By way of example, the isolation 
detection circuit shown in FIG. 4a may be used to develop modulation 
information, as indicated in Table 2 below for application to the 
modulator 4, in accordance with the binary recording signal associated 
with the picture element B is question and related signals a, b, c and d. 
TABLE 2 
______________________________________ 
output from circuit 39 
modulating voltage 
recording 
information modulating 
image background 
(pixel B) 
a b c d information 
exposure 
exposure 
______________________________________ 
H H L H L 4 V.sub.1 
0 
H L L H 3 V.sub.2 
V.sub.1 
H L L L 2 V.sub.3 
V.sub.2 
L H H L 3 V.sub.2 
V.sub.1 
L H L H 2 V.sub.3 
V.sub.2 
L H L L 1 V.sub.4 
V.sub.3 
L L H L 2 V.sub.3 
V.sub.2 
L L L H 1 V.sub.4 
V.sub.3 
L L L L 0 V.sub.5 
V.sub.4 
L 0 V.sub.5 
______________________________________ 
Note: V.sub.4 &gt; V.sub.3 &gt; V.sub.2 &gt; V.sub.1 &gt; 0. The higher the voltage, 
the higher the emission level of laser beam. It is to be noted that the 
value of Vi differs depending on whether the background or the image 
exposure scheme is employed. 
It will be appreciated that when the isolation detection circuit shown in 
FIGS. 2, 3 or 6 is used to provide a determination of whether or not the 
isolation occurs in terms of a two value, the corresponding modulating 
voltage will also be two-valued, or three-valued if the non-record 
operation 0 is included. 
It will be understood from the foregoing that in accordance with the 
invention, when a picture element in question is to be recorded, the 
amount of exposure to laser beam which is applied to the picture element 
is corrected depending on whether adjacent picture elements are recorded 
or not recorded so as to bring the recorded area ratio (recorded area/the 
number of dots recorded) of the picture element in question close to a 
constant value. In this manner, the image quality of the record is 
improved, and a more smooth representation in halftones is enabled.