GB2138162A

GB2138162A - Scanning optical system

Info

Publication number: GB2138162A
Application number: GB08327259A
Authority: GB
Inventors: Kazuo Minoura
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1980-02-06
Filing date: 1983-10-12
Publication date: 1984-10-17
Also published as: GB2069176A; DE3104236C2; GB2069176B; DE3104236A1; GB2138162B; GB8327259D0

Abstract

A scanning optical system comprises two or more light emitting sources which can be photomodulated independently of each other, an optical system through which the beams from the light emitting sources are focused on a surface to be scanned and a deflector for deflecting the beams in a determined direction. The optical system is anamorphic and is designed such that the magnification beta d in a plane parallel to the beam deflection plane and the magnification ???s in a plane perpendicular to the latter and containing the optical axis of the system, satisfies: ¦ beta d¦>¦ beta s¦

Description

1

SPECIFICATION

Scanning optical system employing two or more beams Background of the Invention

Field of the Invention

The present invention relates to a scanning optical system employing two or more light emitting sources for scanning a surface to be scanned with two or more scanning lies at the same time the number of lo scanning lines being equal to that of the light emitting sources. The scanning optical system to which the 10 present invention relates is useful, for example, for such type of recording appratus in which recording of information is effected by scanning a surface to be scanned with a plural number of scanning lines from a light source part such as a semiconductor array laser at the same time.

GB 2 138 162 A 1 Description of the PriorArt

Such scanning system according to which a surface to be scanned is scanned by a plural number of independently modulatable beams all at once is known in the art as multibearn scanning system. In this type of the known scanning system, adjacent scanning lines on the scanned surface are scanned with a plural number of beams all at once. Therefore, with this scanning system, it is required to focus the plural number of beam spots on the scanned surface closely in the direction intersecting the direction of scanning. To meet 20 the requirement, U.S. Pat. No. 4,019,186 proposed to use a light modulator for separating the plural number of beams. However, use of such light modulator involves the disadvantage of increasing the cost. With the rapid development of semiconductor laser array and light emitting diode array in these years, it has also been proposed to use such array as a light source in the scanning system. However, the use of such light source also involves another problem. To attain the very small space between the adjacent beams spots on 25 the scanned surface as required, the space between the adjacent light emitting elements has to be reduced accordingly. This puts a limitation on the manufacture of semiconductor laser array or diode array useful for the above mentioned scanning system.

A solution to the problem already proposed is shown in Figure 1. In Figure 1, the arrow A-A' indicates the scanning direction and M' is the direction in which a light source array SA is disposed. The scanning direction is determined by a deflector. As seen in Figure 1, according to the proposed solution, the direction of the array SA is inclined relative to the scanning direction so as to form a small angle 0 therebetween for scanning adjacent scanning lines on a scanned surface with beam spots all at once. However, this solution needs a very fine adjustment for setting the inclination which is practically very difficult to carry out. When the inter-array distance is Po and the focus magnification of light source on the scanned surface is P, then the 35 space P between two neighbouring scanning lines on the surface will be given by the following equation:

Po = 101. Po sinO If the set angle 0 has a small error of AO, then there is produced in the scanning line space an error of AP which is:

AP= 1P1 - PO - AO As an example, assuming that Po is 0.1 mm, P is 30 and AP is wished to be less than 0.01 mm, then it is required to reduce the angle error A0 up to a value less than about 1 V. This example demonstrates the difficulty in correctly adjusting the disposition of the light source array.

The scanning optical system employing a plural number of light sources has another problem that the quantity of light coming to the scanned surface from the light sources can be taken up only with a great 50 difficulty. This problem will be described hereinafter with reference to Figure 2.

In Figure 2, two light emitting sources are dsignated by 1 a and 1 b. 2 is a beam focusing objective lens, 3 is a deflector such as a rotary polygon mirror and 4 is a lens for focusing the deflected beams. Designated by 5 is a rotary cylinder on which a photosensitive medium 6, that is, a surface to be scanned is placed.

Designated by 1 a' and 1 b' are scanning fines corresponding to the light emitting sources 1 a and 1 b respectively.

In the shown scanning optical system, each of the lenses 2 and 4 is formed of a revolution symmetric surface. Let the focus magnification of the composite system resulting from these lenses be P. Then the space between scanning lines 1 a' and 1 b', that is, P and the space between light emitting sources 1 a and 1 b, that is, Po have the following relation:

P = lpj - PO On the other hand, let F'denote the F number of beams focused on the photosensitive medium surface 65 2 GB 2 138 162 A 2 and F and of beams emitted from the sources and received by the objective lens 2. Then, F and F' have the following relation:

F' = 1P1 - F Using above two equations, 5 P= F. PO Fromthe above it is seen that the scanning iinedistance Pcan be reduced onlybyusing a largerFvalueof 10 theobjective lens2, namely decreasing the beams of lightreceived bytheobjective lens.Therebythe beams of light which can reach the surface of the photosensitive medium are also decreased as a matter of course.

This will result in shortage of light energy for performing high speed recording. Such shortage of light energy is, of course, undesirable.

In the art there is known and used also such type of image forming method according to which a latent 15 image is formed by scanning a photosensitive medium using a plural number of light beams at the same time while subjecting the photosensitive medium to corona discharge. For this type of image forming method, it is essential to make the plural number of beams scan such areas of the photosensitive medium on which charges by the corona discharge are nearly equal in amountto each other. Otherwise there may be produced those images which are irregular in contrast. This means thatthe beams on the photosensitive 20 medium surface should be space from each other as small as possible. In other words, the space P is wished to be as small as possible.

Accordingly, it is a general aim of the invention to alleviate the above mentioned disadvantages involved in the prior art scanning systems.

In one aspect the invention provides such a scanning optical system in which the optical arrangement has 25 a sufficiently large allowance.

In another aspect the invention provides a recording apparatus provided with a scanning optical system suitable for a cylindrical photosensitive drum with which the whole process of electrostatic copying can be completed every one revolution of the medium to be scanned.

In a further aspect the invention provides a scanning optical system which can be manufactured at a reduced cost.

In a still further aspect the invention provides a scanning optical system which enables to transmit a sufficient amount of fight effectively to the scanned surface from the light source part while scanning the surface by a plural number of beams at the same time.

In yet another aspect the invention provides a scanning optical system in which a semiconductor laser 35 array can be advantageously and suitably used as its light source part.

In another aspect the invention provides a scan type recording apparatus in which the amount of light from the light source part can be effectively and sufficiently taken up while keeping the space between adjacent beam spots on the scanned surface as small as possible.

In a still further aspectthe invention provides a scan recording apparatus with which it is possible to 40 reproduce an image with an ordinary space between adjacent scanning lines even when the scanning beam spots are not so closely arranged on the photo-sensitive medium to be scanned.

Other and further objects, features and advantages of the invention will appear more fullyfrom the following description.

Brief Description of the Drawing Figure 1 is an illustrative view of a multi-beam scanning system according to the prior art; Figure2 is a schematic view of an optical system relevantto the subject of the invention; Figures 3through 5 illustrate the manner of scanning by an optical system in accordance with the invention; Figure6showsan embodimentof laserbeam printer to which the present invention wasapplied; Figure 7shows another embodiment of optical system accordingtothe invention; Figure 8 illustrate the light emitting characteristics of a semiconductor laser; Figure 9 shows a further embodiment of optical system according to the invention; Figures 10A and 108 show a concrete example of the anamorphic lens system shown in Figure 7; 55 Figures 1 1A and 118 show a concrete example of the scanning lens system shown in Figure 7; and Figure 12 is a schematic view of an embodiment of such type of image forming process in which writing with scanning beams is carried out simultaneously with corona charging.

Description of the Preferred Embodiments

In the scanning optical system of the invention, the severe requirementfor precision in arrangement of the light source part is moderated to a great extent as compared with that in the prior art system. Atfirst this aspect of the invention is described with reference to Figure 2.

As previously described, according to the prior art, a surface is scanned with a plural number of beam spots in such manner that adjacent scan lines can be scanned by adjacent scanning beam spots. In contrast 65 3 GB 2 138 162 A 3 with the prior art, according to the invention, adjacent beam spots do not scan adjacent lines but scan such lines spaced from each other by a predetermined distance. In addition, the optical system is so disposed as not to make the same line scanned twice or more by these beam spots.

In Figure 2, it is assumed that the light emitting sources 1 a, 1 b. are arranged at intervals of Po, the reflector 3 has N in number of reflecting surfaces with its revolution number being X, the peripheral speed of the rotary cylinder 5 is V and the focus magnification of the light emitting source by the composite system resulting the lenses 2 and 4 is p. Under the conditions, the requirement for avoiding overlap of scanning lines and making the line space always equal between every two adjacent recorded scanning lines (1'91, 02) is only that (NX-1) Po. 1pl/V should be a value as obtained by multiplying the reciprocal of M (the number of light 10 emitting sources) by a whole number. Namely, it is required only to satisfy the following relation:

(NX - 1) Po. ipi/V = (mM + 1)/M wherein, M -- 2 and m is a whole number more than 0.

Assuming that p = WM(NX - 1) and substituting this into (1) gives:

p = 0 - PO mm+1 (1) wherein p is the space between adjacent scanning lines recorded an the photosensitive medium.

When the above equation (1)' is satisfied, the scanning lines never overlap each other but are equally spaced from each other as clearly seen from the following description with reference to Figure 3.

In Figure 3, 1 a'- 1 and 1 b'- 1 are lines scanned at the same time for the first scanning time using the light emitting sources 1 a and 1 b respectively. For the second scanning time 1 a'-2 and 1 b'-2 are scanned at the same time as the deflector 3 as well as the cylinder 5 rotates. Similarly, 1 a'- 3, 1 b'- 3. are scanned. When such scanning is repeated many times more than K -- m + 1 + k(k = 0, 1, 2. ), the space p between the 25 scanning line 1 a'-k scanned at Kth scanning and the scanning line 1 b' - (k + 1) at (k + 1)th scanning has a certain relation relative to the space P between the scanning lines 1 a' - k and 1 b' - k scanned at the same time at Kth scanning. So long as p and P hold the following relation:

p = P/(mM + 1) the scanning lines never overlap each other but are equally spaced from each other by p.

Since P is determined depending upon the distance Pc between light emitting sources and the focus magnification P of the composite optical system resulting from the objective 2 and scanning lens 4 as follows:

P = 101 - PO 111 (2) 30 then substituting this in (2) givesthe aboveformula (V.

From the above it is concluded that so long as the above relation (1) is held in an arrangement as shown in Figure 2 it is made possible to carry out high speed recording using a plural number of light emitting source 40 without making the rotation of deflector unstable and also to realize the recording of equally spaced scanning lines without any overlap thereof.

As an example illustrating the invention, the following factors are selected for the arrangement shown in Figure 2:

Pc (space between light emitting sources 1 a and 1 b) = 0.1 mm; N (number of reflecting surfaces of deflector 3) = 10; V (peripheral speed of cylinder 5) = 6,000/min; p (focus magnification of composite system 2,4) -20; M (number of light emitting sources) = 2 and m 20.

Under these conditions, the revolution number of the reflector 3, X = 6, 150.1 rpm. Therefore, from the above equation (1)'the scanning line space is obtained. Namely, p = 0. 04878mm. In this example, scanning lines are recorded with this space p between every two adjacent lines regularly. Data signals introduced into the light emitting sources 1 a and 1 b at the same appear, in this example, two points spaced by 2mrn from each other on the recording surface.

Figure 4 illustrates the manner how signals are produced at Kth scanning. As seen from Figure 4, signals in 55 pairs such as (Akl, Bkl), (M2, Bk2), (M3, Bk3) are produced simultaneously on the scanning lines 1 a' - k and 1 b' - k respectively.

Figure 5 illustrates the manner how signals of Japanese character "T" are produced. In the manner shown in Figure 5, signals Mj, B,j are applied to the light emitting sources la and 1 b. Suffixe indicates scanning line number and suffix j indicates deflection position.

As shown in the above, in the apparatus according to the invention, the light source array extends in the direction intersecting the scanning direction at right or nearly right angle. If there is produced an angle error of AO in the light source array, it brings about an error AP in scanning line space in the following relation:

Ap=lpl PO. (A0)2 4 GB 2 138 162 A 4 This means that in the case of the apparatus according to the invention, angle error in light source array less affects the error in scanning line space than in the case of the prior art ones. For example, in case that Po,

P, AP have the same values as those previously given in connection with Figure 1, the apparatus according to the invention has a far larger allowance of angle error AO, that is, about 3.3 degrees. In this manner, the severe precision in setting angle hitherto required is moderated to a great extent in accordance with the 5 invention.

While in the above embodiment the deflector has been shown and described to be a rotary polygon mirror, other types of deflector such as oscillating mirror (for example a galvano mirror), acousto-optical modulator and electro-optical modulator also may be used for the same purpose.

In the above embodiment, the direction in which the light source array is arranged has been shown to be 10 such direction which intersects, at right angle, the plane in which the beams of light are deflected. However, it is not always necessary for carrying out the present invention although it is preferable because of the largest allowance of angle error obtained thereby. The arrangement of the invention has an effect even when the direction in which the light source array is arranged does not intersects the beam deflection plane at right angle.

Now, description will be made of an electro-photographic apparatus in which the present invention is embodied.

Figure 6 schematically shows the basic arrangement of a computer output printer in which the scanning optical system according to the invention is used.

In Figure 6, reference numeral 21 designates a semiconductor array laserwhich generates beams of light. 20 The beams are collimated by a collimator lens system 22 and the collimated beams enter an afocal lens system 23 by which the cross-section of the individual beams is shaped into a desired form. After passing through the afocal lens system, the laser beam enters a rotary polygon mirror 25 which is driven into rotation by driving mechanism 24.

Designated by 26 is a control circuitfor controlling the state of light emission from the respective light 25 source parts in the semiconductor array laser 21 in response to writing signals coming from a computer (not shown).

The rotary polygon mirror 25 sweeps the incident laser beam to direct it to a focusing lens 27. The focusing lens 27 is of f - 0 characteristic and focuses the beam on a photosensitive drum 28 as a spot.

The reason why a f - 0 lens has to be used as the focusing lens 27 is as follows:

For a common focusing lens, the incident angle 0 of beam and the position r of the beam focused on the image plane hold the following relation:

Z r = f - tanO wherein f is focal length of the focusing lens.

In the shown embodiment in which the polygon mirror 26 rotates at a uniform speed, the incident angle of the reflected laser beam to the focusing lens 27 changes linearly with time. Therefore, the moving speed of the spot position focused on the drum 28, that is, the image plane is not constant but variable non-linearly.

The moving speed of the spot position increases up at such point where the incident angle becomes larger. 40 When man turns on the laser beam at constant time intervals to describe an array of spots on the photosensitive drum 28, it will be found that the space between adjacent spots in the described spot array is larger in the both edge parts of the array than in the middle part thereof. This phenomenon is unfavourable and therefore it must be avoided. To eliminate such unfavourable phenomenon the focusing lens 27 is so designed as to have the following characteristic:

r = f. 0 Such focusing lens is generally called f - 0 lens.

When a collimated light beam is focused as a spot by a focusing lens, the minimum diameter dmin of the 50 focused spot is given by the following equation:

dmin = f X A wherein, f is the focal length of the used focusing lens, X is the wavelength of the used light; and A isthe entrance aperture of the focusing lens.

It is seen from the above that a smaller spot diameter dmin can be obtained using a larger value of A providing that f and X are constant. In the above embodiment, the afocal optical system 23 has been used to 60 obtain the effect.

The laser beam deflected and modulated in the manner described above in then incident upon the photosensitive drum 28 to form a latent image thereon. The latent image is visualized by an electrophotographic processing process known per se and the developed image is transferred onto a sheet of common paper. After fixing the image, the paper sheet is put out as a hard copy. One example of such electrophotographic process is disclosed in our Japanese Patent Application Publication No. 23,910/1967. 65 p GB 2 138 162 A 5 According to the known process, the photosensitive drum 28 comprises essentially an electrically conductive supporting member, a photoconductive layer and an insulating layer. The top surface of the insulating layer is precharged uniformly with positive or negative charge using a first corona charger 29. Charges of opposite polarity to that of charged insulating layer are captured in the inter surface between the insulating layer and the underlying photoconductive layer or within the latter. Subsequent to the primary charging step, the charged insulating layer surface is exposed to the laser beam mentioned above and simultaneously subjected to AC corona discharge or discharge of opposite polarity to that of the above primary charging by a second corona charger 30. Thereby there is formed on the insulating layer surface a pattern of surface potential difference corresponding the pattern of dark and bright of the laser beam. Then, the whole surface of the insulating layer is illuminated by a whole surface exposure lamp 31 uniformly so as 10 to form a high contrast electrostatic latent image on the insulating layer. The electrostatic latent image is visualized by a developing device 32 using a developer mainly composed of charged and colored particles. The developed image is transferred onto a web of paper 33 by means of a transfer charger 34, The paper 33 is being held fixed on the drum by holding means as later described during the step of transferring the image. The transferred image is fixed by fixing means. Thus, an electrophotographically printed image is 15 obtained. On the other hand, after transferring the insulating layer surface is cleaned from residual charged particles by a cleaning device 35. Thus, the photosensitive drum 28 is prepared for its reuse.

As another embodiment, an electrostatic image forming process for electrophotography is also applicable.

An example of such electrostatic image forming process is disclosed in our Japanese Patent Application Publication No. 19,748/1967. The photosensitive drum used in the process also comprises an electrically 20 conductive supporting member, a photoconductive layer and an insulating layer as its essential elements.

The insulating layer surface is uniformly precharged with positive or negative charge by a first corona discharge so that charges of opposite polarity to that of above charges are captured in the interface between the insulating layer and the underlying photoconductive layer or within the latter. In addition, the charged surface of the insulating layer is subjected to the action of AC corona discharge so as to make the charge on the insulating layer decayed. Thereafter, the surface is exposed to the laser beam as data signal. As a result, on the insulating layer surface there is formed an electrostatic latent image corresponding to the dark and bright pattern of the laser beam. The electrostatic latent image is developed and further processed in the same manner as in the above first embodiment.

In Figure 6, designated by 36 is a charger for predischarging and 37 is a pre-exposure lamp. The predischarging charger 36 serves to keep constant and uniform the surface potential on the drum 28. The pre-exposure lamp 37 serves to keep constant and uniform the characteristics of the photosensitive layer.

Even after passing through the cleaning device 35, the photosensitive drum 28 has various hysteresis matters still remained thereon such as residual potential. The charger for predischarging 36 and pre-exposure lamp 37 cooperate to erase these hysteresis matters. This has an effect for producing always 35 good and stable images.

We, the applicant of the present application have already proposed a method for stabilizing electrostatic latent images (British Patent Application No. 38136/77) which is useful as means for producing always good and stable images employing the above described electrophotographic process to which the present invention is applicable. Designated by 38 is a meter for measuring electrostatic potential provided for 40 realizing such stabilizing means. The meter 38 measures the electrostatic potentials on the bright part and dark part particularly provided on the photosensitive drum 28. The bright part is a part of the drum scanned by and exposed to the laser beam whereas the dark part is another part of the drum not exposed to the laser beam. 39 is a carrier removing device. As well known in the art, the developer within the developing device 32 contains an amount of carrier admixed therewith. The carrier removing device serves to prevent such carrier from adhering onto the paper 33 or from entering the cleaning device 35. The drum 28 is often accompanied with carrier.

The paper 33 is an unprinted paper as usually used for outputfrom a computer or the like. The paper 33 has perforations provided at both side edges thereof for feeding the paper. To aid the paper in moving smoothly there is provided a holding rod 40. A light source such as light emitting diode 41 and a photo detector such as photo diode constitute together an end detector for the paper 33. Designated by 43 is a tractor having pins arranged thereon for engaging in the above mentioned paper feed perforations in a manner known per se. With the rotation of a tractor shaft (not shown) the pins rotate to convey the paper. 44 is a guide roller for transporting the paper and 45 is a stripper pawl for separating the paper from the drum 28. 46 and 47 are a pair of transfer rollers which, when actuated, press the paper against the drum surface. 55 Therefore, during the operation of the transferring charger 34, the paper is in close contact with the drum surface 28. After completing the transfer of image onto the paper, the operation of transfer charger 34 is stopped and the pair of transfer rollers 46 and 47 are retracted from the drum surface. The paper still sticking on the drum surface is separated from the latter by the stripper pawl 45.

Designated by 48 is a guide roller which serves also as a tension absorber. When the tension applied to the 60 paper during the paper transportation is deviated from a certain normal value to a great extent, the guide roller 48 absorbs the tension. 49 is a preheating roller. The preheating roller is in the form of hollow cylinder and has a heating source such as heater provided within the hollow cylinder. 50 is a fixing roller for fixing the toner image on the paper and 51 is a back-up roller. These rollers 49, 50 and 51 together constitute a fixing device. The fixing roller 50 is also in the form of a hollow cylinder and has a heat source such as a heater 65 6 GB 2 138 162 A 6 provided within the cylinder. The back-roller 51 presses the paperwith the transferred toner image against thefixing roller 50so as to apply a high pressureto thetoner as well asto promote heat transmission from the fixing roller to the sheet and the toner thereon. 52 and 53 are a pair of discharge rollers for discharging the paper from the apparatus after fixing. 54 designates the completely printed paper.

As previously noted, according to the present invention, the quantity of light emitted from the light source parts can be guided to the scanned surface very effectively with reduced of light. Hereinafter this aspect of the invention will be described in detail.

In the scanning optical system according to the invention functioning as an optical system forfocusing the beams of light coming from light source parts on a surface to be scanned, the light beam take-up rate in the beam deflection plane and the light beam take-up rate in a plane intersecting the beam deflection plane at 10 right angle and containing the optical axis of the optical system are considered independently of each other.

The latter mentioned beam take-up rate has a close relation with the space between adjacent beam spots. If the spot space represents the scanning line space directly, the component of the light beam in this direction will be lost to some extent. On the contrary, the component of the light beam in the deflection plane can be taken up without any substantial loss of light. Therefore, the quantity of light of the beam spot on the scanned surface can be increased by taking up the quantity of light of the component in the deflection plane as much as possible.

From the above consideration, according to the invention there is used an anamorphic optical system as the optical system for focusing the beams of light from the light source parts on a surface to be scanned. The anamorphic optical system used in the invention has different focus magnifications in the beam deflection 20 plane and in the plane intersecting the deflection plane at right angle. In addition, in the apparatus according to the invention there is used a semiconductor laser array as light source parts most suitable for such optical system. Figure 7 shows an embodiment of such arrangement according to the invention.

In Figure 7, reference numerals 60a and 60b designate light emitting sources such as semiconductor laser array. 61 is a rotation symmetricalobjective lens and 62 is an anamorphic lens which has different refractive 25 powers in different directions intersecting each other at right angle. 63 is a deflector which may be a rotary polygon mirror or oscillating mirror. 64 is a rotation symmetrical lens. 65 is an anamorphic lens having different refractive powers in different directions intersecting each other at right angle. 67 is a rotary cylindrical member and 68 is a photosensitive medium mounted on the cylinder 67. 60a'and 60b' indicate scanning lines.

In the shown optical system, let the focus magnification of the lens composite system within the deflection plane be Pd and the focus magnification of the composite system within the plane normal to said deflection plane and containing the optical axis be ps. Then, the space P between the scanning lines 60a' and 60b'and the space Po between the light emitting sources 60a and 60b hold the following relation:

P = 1PS1 - PO (4) On the other hand, let the F number of the beam of light focused on the photosensitive medium within the deflection plane be F'd, that of the objective lens 61 within the same plane be Fd, that of the beam of light focused on the photosensitive medium within the plane normal to the deflection plane be F's and that of the 40 objective lens 61 within the same plane be Fs. Then, F'd = ipdi. Fd F's = ipsi. Fs From the above equations (4) and (6) (5) (6) 45 p = F's PO (7) Fs 50 To reduce the scanning line space P, it is required to darken F number in the direction normal to the beam deflection plane. However, it has been found that in the apparatus according to the invention, use of a light emitting source which has a particular light distribution characteristic has an effect to reduce the space P.

Namely it is preferred to use such light emitting source which has different cross-sectional intensity distributions of divergent beam in two different directions (,,q) intersecting each other at right angle as shown in Figure 8. Semiconductor laser is a preferable example of such light emitting source.

In Figure 8, 0 indicates the light distribution angle in the direction and 0,q that in another direction T1 in an equiintensity distribution of light. In case that O < O'n requirements for attaining the above effect are that the direction q should be selected to be parallel to the deflection plane, that the direction t should be selected to be coincident with the direction normal to the deflection plane and that the light emitting sources should be arranged in the direction g.

As easily understood from the above equations (5) and (7), when the above requirements are satisfied, it is 65 k; 1 7 GB 2 138 162 A 7 made possible to determine the factor for receiving the beam of light in the direction normal to the deflection plane independently of the factor for receiving the beam of light in the direction parallel to the beam deflection plane.

Consequently, according to the above mentioned of selecting arrangement and orientation, it is possible to reduce the scanning line space independently of the received quantity of light in the directionq. At this time, it is possible to keep the reduction in the received quanitity of light in the direction t relatively small. This is because the light distribution angle ot of the light emitting source in the direction is small.

Again, let the focus magnification of the above lens composite system in the direction normal to the beam deflection plane be Ps, that of the same composite system in the direction parallel to the deflection plane be 0d and F number of the beam of light focused on the photosensitive medium (in the direction of deflection as 10 well as in the two directions intersecting the deflection direction) be F'. Then, the divergent angle 0, q of the beam of light received by the objective lens 2 in the deflection direction (-q) is represented by:

o,n = sin-' Pcl (8) CF 15 Similarly, the divergent angle 0 in the direction intersecting the above direction, namely in the direction is represented by:

0 = sin PS (9) 15-9F-I-1 20 With a smaller value of the focus magnification lpsI in the direction intersecting the deflection plane, a larger space between adjacent light emitting sources is allowed in the array of light emitting sources which in turn makes easy the manufacture of a light source having a plural number of light emitting sources such as semiconductor laser.

On the other hand, as for the beam deflection plane, it is desired to increase the value of focus magnification ipdl in the direction parallel to the deflection plane as much as possible so as to receive the light from the light source as much as possible. Therefore, it is desirable to satisfy the following condition:

When this condition is satisfied, the manufacture of light source is made easy (in other words, the scanning line space on the photosensitive medium can be made smaller) and also the beam of light emitted from the light source can be received more effectively.

As seen from the above, a closer scanning line space can be attained by employing an optical system having different refractive powers in the beam deflection plane and in another plane intersecting said deflection plane at right angle. This effect can be obtained while keeping the light reception rate at a high level. Therefore, the object of high speed recording is attained thereby.

It is known in the art that irregularity of scanning line pitch is often caused by fall-down of the rotation axis of deflector 63 or fall-down of deflecting surfaces due to error in machining and that such pitch irregularity brings forth poor quality of recorded images.

Optical systems for correcting such pitch irregularity are known and disclosed, for example, in U.S. Pat.

No. 3,750,189; 3,946,150; 3,865,465; 3,877,777 and 4,054,360.

Such line pitch irregularity may be corrected also by the line image focusing system 62 shown in Figure 7.

To this end, a line image is formed in the vicinity of the deflecting surface of the deflector 63 and a conjugated relation is established between the line image and the point on the photosensitive medium. In 45 this case, the optical system becomes also a correction optical system for correcting the irregularity of scanning line pitch.

All of the embodiments containing anamorphic lenses described above are useful for realizing the principle of the present invention. In these embodiments the light emitting sources are arranged in row in the direction intersecting above said line image at right angle. By doing so, the irregular scanning line pitch 50 can be corrected without losing the effect of the invention.

However, the present invention is applicable not only the line pitch correcting system but also to a system as shown in Figure 9 which is not a pitch correcting system.

In Figure 9, reference numeral 71 designates an anamorphic lens system which has a structure as shown in Figure 1 OA or 1 OB in detail. Namely, the lens system 71 is an afocal anamorphic lens which has a cylindrical 55 surface (72, 74) with negative refractive power and another cylindrical surface (73, 75) with positive refractive power. It exhibits the refractive powers within a plane containing the axis of light emitting source array 76 and the optical axis of the objective lens 2. In the embodiments shown in Figures 1 OA and 10B, the incident beam lo which is a collimated beam by the objective lens 2 enters the anamorphic lens system at an incident angle of 0 relative to the optical axis. After passing through the anamorphic lens system, the exit beam lo' 60 emerges from the system at an exit angle of 0'. In this case, the relation between the incident and exit angles is characterized by; 01 < 0 8 GB 2 138 162 A 8 When this relation is satisfied, the condition lpdl > ipsi mentioned above is also satisfied so that the effect of the invention can be obtained.

Hereinafter, a concrete example of the scanning lens system 66 shown in Figure 7 will be described with reference to Figure 11.

Figure 1 1A is a cross-sectional view of an embodiment of the scanning lens system 66 taken along a plane parallel to the beam deflection plane and Figure 11 B is another cross-sectional view of the same taken along a plane normal to the beam deflection plane. The embodiment comprises a spherical surface single lens 64 and a toric surface single lens 65. Elements of the shown embodiment of scanning lens system 66 are given in the following table, Table 1.

TABLE 1 ri -20.102 r,' -20.102 d, 5.359 n, 1.70916 15 r2 -24.047 r2 1 -24.047 d2 14.877 r3 0C r3 1 -30.798 d3 4.697 n2 1.63398 r4 -58.485 r4 1 -10.614 In Table 1, r, - r4 are radii of curvature of the lenses 64 and 65 measured in the plane parallel to the deflection plane and r,' - r4' are those in the plane normal to the deflection plane (therefore, r, = rl', r2 = r2' for the spherical surface single lens 64). d, is lens thickness of the lens 64 on the axis and c12 is air distance between the surface r2 of the lens 64 and the surface r3 of the lens 65 on the axis (this air distance is equal to that between the surfaces r2' and r3' on the axis). d3 is the lens thickness of the toric surface single lens 65 on the axis. n, is the refractive index of the lens 64 and n2 is that of the lens 65.

As an application form, the scanning optical system described above has been applied to a scan recording apparatus using a plural number of beams of light for writing data. Figure 12 schematically shows an embodiment of image forming process useful for such scan recording apparatus in which scanning is carried out with a plural number of beams simultaneously with corona charging.

In Figure 12, a drum 82 has a photosensitive medium 81 placed thereon. The photosensitive medium 81 comprises an electrically conductive base layer, a photoconductive layer on the base layer and a transparent top insulating layer. The drum is supported rotatably within the apparatus and is driven into rotation in the 35 direction of arrow at a uniform speed by an electric motor not shown. The rotational speed of the drum remains constant for different copy magnifications. At first, the surface of the photosensitive medium 81 is charged uniformly by DC corona discharger 83.

At the next step, the photosensitive medium is exposed to the respective light images of the light emitting sources through the above described optical system and also at the same time the photosensitive medium is 40 subjected to AC corona discharge or corona discharge by DC corona discharger 84 with the opposite polarity to that of the above discharger 83.

The discharger 84 has a slit-like opening through which the beam focused can pass. After image-wise exposure, the whole surface of the photosensitive medium 81 is uniformly illuminated by a lamp 85 so that a high contrast electrostatic latent image of the original is formed on the medium 81. The latent image is then 45 developed with toner applied to the photosensitive medium 81 from a developing device 86 which may be, for example, of magnet brush type. The visualized toner image is transferred onto a transfer sheet 87 fed from a paper cassette not shown at a speed equal to the peripheral speed of the drum 82. At the step of transferring the image to increase the efficiency of transfer, a corona discharger 88 gives the backside of the image charges with the opposite polarity to the charge on the toner. Feed of the transfer sheet from the cassette is performed one by one in timing with the rotation of the drum. The sheet comes into contact with the photosensitive medium 81 with the aid of a guide 89. After completing the transfer of toner image, the transfer sheet is separated from the photosensitive medium by a pawl 90. Mechanism for conveying the transfer sheet is well known in the art and therefore need not be further described. The toner image on the transfer sheet is then fixed by a fixing device 91 which may be, for example, of heating roller type.

On the other hand, after transferring the drum enters a cleaning station where toner remains on the photosensitive medium 81 is removed by a cleaning device 92 which may be, for example, a rubber blade in pressure-contact with the photosensitive medium. Thus, the photosensitive medium gets cleaned and is prepared for reuse in the next cycle of above image forming process.

In the above process, the distance between the light images of light emitting sources 60a'and 60b'to which the photosensitive medium 81 is exposed should be kept as small as possible during the exposure with simultaneous corona discharge for example, by DC corona discharger 84. Otherwise, the difference in charge distribution will become too large to obtain good images. Namely, reduction in quality of electrostatic latent image and also of the corresponding final image may be caused by a larger distance between the light images 60a'and 60b'.

9 GB 2 138 162 A 9 The surface of the photosensitive medium shown in Figure 12 is a surface to be scanned. In this case, the light emitted from the light source has to be taken up into the photosensitive medium as much as possible with the minimum loss of light. To attain the object it is necessary to use an optical as shown in Figure 7.

Since the component of light in the direction intersecting the beam deflection plane is taken up to some extent in term of quantity of light, the space between adjacent beam spots on the photosensitive medium is 5 made larger than that in ordinary scanning. For this reason, the scanning line space must be reduced as much as possible by employing the interlaced scanning method previously described with reference to Figures 2 and 3. Setting of the optical system required in connection of interlaced scanning is entirely the same as in the above described case and need not be further described.

Also, by using an anamorphic optical system as described above the necessary close distance between the lo beam spots on the photosensitive drum can be attained without decreasing the quantity of light received so much. Therefore, it is made possible to scan such area where corona charge is distributed uniformly by a plural number of beam spots at the same time. This is because the use of anamorphic optical system enables to determine the factors for receiving the beams of light in the direction normal to the beam deflection plane independently of the factors for receiving the beams of light in the direction parallel to the beam deflection 15 plane.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the invention.

Reference is directed to our co-pending application 8103627 from which this application is divided. 20

Claims

1. A scanning optical system for scanning with a plural number of beams of light, said scanning optical system comprising:

a light source part enable to supply a plural number of beams which can be photomodulated independently of each other; an optical system for focusing said beams on a surface to be scanned; and a deflector disposed between said light source part and said scanned surface for deflecting said beams in determined directions, wherein said focusing optical system is an anamorphic optical system which has 30 different focus magnifications in the plane within which said beams are deflected and in a plane intersecting said deflection plane at right angle and containing the optical axis of said focusing optical system.

2. A scanning optical system asset forth in Claim 1 wherein the focus magnification Pcl in said beam deflection plane and the focus magnification ps in the plane intersecting said deflection plane at right angle and containing the optical axis of said focusing optical system satisfy the following condition:

lpdi > ipsi A scanning optical system asset forth in Claim 1 wherein said light emitting source is formed of a p-n junction type semiconductor laser and a plural number of light emitting sources are arranged in the direction 40 parallel to the p-n junction surface.

4. A scanning optical system for scanning with a plural number of beams of light, said scanning optical system comprising:

a light source part enable to supply a plural number of beams which can be photomodulated independently of each other; an optical system for focusing said beams on a surface to be scanned; and a deflector disposed between said light source part and said scanned surface for deflecting said beams in determined directions, wherein said light source part comprises two or light emitting sources arranged regularly with a space of Po therebetween and said space Po and the space p between adjacent scanning lines on said scanned surface hold the following relation:

p = 1P1 - PO/(mM + 1) wherein, p is focus magnification of said focusing optical system, 55 M is number of said light emitting sources and m is a whole number more than 0.

5. A scanning optical system as set forth in Claim 1 wherein said deflector is a rotary polygon mirror having N in number of surfaces and revolutions per minute of X and wherein said surface to be scanned is a photosensitive medium moving in the direction intersecting the direction of said beams being deflected at right angle and at a speed of V satisfying the following relation:

(NX - 1). Po. ipl/V = (mM + 1)/M GB 2 138 162 A 6. Image forming apparatus comprising image forming means for forming an image on a photosensitive medium by an image forming process including scanning said medium with a plurality of beams of light, said image forming means including a scanning optical system according to any preceding claim.

7. A scanning optical system for scanning with a plurality of beams of light, substantially as hereinbefore 5 described with reference to the accompanying drawings.

8. Image forming apparatus in the form of a laser beam printer, substantially as herein before described with reference to the accompanying drawings.

Printed in the UK for HMSO, D8818935, 8184, 7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

1 c i z