GB2185653A

GB2185653A - Image recording method

Info

Publication number: GB2185653A
Application number: GB08700616A
Authority: GB
Inventors: Satoru Tomita
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 1984-01-13
Filing date: 1985-01-03
Publication date: 1987-07-22
Also published as: GB8700614D0; GB2185652B; GB2184914B; GB8700616D0; GB2185652A; GB2184914A; GB2185653B; GB8700615D0

Description

GB 2 185 653 A 1

SPECIFICATION

Image Recording Method Background of the Invention

5 The present invention relates to an image 70 recording method for an optical scan type electrophotographic recording apparatus and, further, to an image recording method for an optical scan type electrophotographic recording apparatus 10 of the kind which uses minute light emitting 75 segments as a light source.

Generally, in a laser printer or like optical scan type electrophotographic recording apparatus, a light beam modulated by binary video data is 15 manipulated to sequentially expose a photoconductive element to form a latent image electrostatically thereon. The problem encountered with this kind of image recording is that because the pulse width of video data per pixel and, therefore, 20 the ratio of a light beam exposing time to a one pixel scanning time is fixed, the latent image potential in an image portion which borders a non image portion is lowered due to building and failing of the latent image potential. Such causes the image 25 to be developed to have different pixel diameters in the main scan and subscan directions and, thereby, considerably lowers the resolution. Especially, when it comes to document images, characters are prevented from appearing clear-cut.

Japanese Patent Laid-Open Publication No. 568112/1981 discloses an implementation for an optical scan type electrophotographic recording apparatus which modulates the pulse width of video data in order to eliminate thinning of images which 35 is apt to occur during positive-to-positive recording, 100 which develops unexposed portions. However, it fails to improve the quality of recorded images when a countermeasure is provided against thinning in the main scan direction only. That is, it 40 cannot offer a desirable image quality unless a 105 countermeasure covering both the main scan and subscan directions is provided in due consideration of gitter occurring on a photoconductive element, developing method, developing characteristics, etc.

45 In that case, reproducibility on a one-dot line basis is 110 very important.

Meanwhile, the countermeasure against thinning and a measure for an improvement in resolution, which the present invention contemplates, are 50 contradictory to each other; the anti-thinning measure is not always advantageous in enhancing or stabilizing the image quality. That is, whetherthe recording be negative-to-positive which develops exposed portions or positive-to-negative which 55 develops unexposed portions, attaching importance 120 to the resolution rather than anti-thinning is advantageous from the viewpoint of improvement or stabilization of the image quality.

An optical scan type electrophotographic 60 recording apparatus has been proposed which is 125 elaborated to free a laser printer from the intricacy of construction of its optical system (Japanese Patent Laid-Open Publication No. 58-108864) and other problems. In the proposed apparatus, to 65 simplify alight source and an optical system, alight 130 source comprises a phosphor dot array tube having an array of phosphor elements arranged in the main scan direction in correspondence with pixels. Light issuing from the phosphor dot tube and modulated by binary video data is passed through an imaging system toward a photoconductive element, which is fed in the subscan direction, to provide a latent image thereon, the latent image being developed to record data associated with the video data.

However, due to the use of minute, light emitting phosphor elements as a light source, the abovedescribed prior art method limits the available potential of latent images and, thereby, electrostatic contrast. This is apt to cause the resolution in the

80 subscan direction to fluctuate due to gitter on the surface of the photoconductive element, greatly effecting the quality of the entire recorded images.

In addition, the dot arraytube type scheme cannot accomplish a sufficient resolution, particularly 85 sufficient reproducibility of hairlines such as onedot lines. Specifically, because with respectto the main scan direction the intensity distribution of the light issuing from the regularly arranged light emitting elements has great influence and because, 90 with respectto the subscan direction, the light emitting elements emit light at a predetermined timing associated with the movement of the photoconductive element in the subscan direction, in the case of recording hairlines such as one-dot 95 lines, the width of the lines to be developed differs from the main scan direction to the subscan direction thereby deteriorating the resolution.

Summary of the Invention

It is therefore a first object of the present invention to provide an image recording method which allows an optical scan type electrophotographic recording apparatus to record images with a high. resolution.

It is a second object of the present invention to provide an image recording method which allows an optical scan type electrophotographic recording apparatus of the type using minute light emitting segments as a light source to provide latent images with considerable electrostatic contrast.

It is a third object of the present invention to provide an image recording method which allows an optical scan type electrophotographic recording apparatus to record high resolution images which have the same pixel diameter both in the main scan 115 and subscan directions.

It is a fourth object of the present invention to provide an image recording method which allows an optical scan type eiectrophotographic recording apparatus of the type using minute light emitting elements as a light source to record high resolution images which have the same pixel diameter in both the main scan and subscan directions.

It is a fifth object of the present invention to provide an image recording method which allows an optical scan type electrophotographic recording apparatus to record images with desirable reproducibility on a one-dot line basis by controlling the formation of a latent image during exposure.

It is a sixth object of the present invention to provide an image recording method which allows GB 2 185 653 A 2 an optical scan type electrophotographic recording apparatus of the type using minute light emitting elements as a light source to record high resolution images excellent in reproducibility on a one-dot line record data associated with the video data, there is provided the improvement wherein a pulse width of video data is varied such that a ratio pp. where p, indicates a ratio of a beam diameter in a main scan 5 basis by controlling the formation of a latent image 70 direction to a pixel pitch in the main scan direction during exposure.

It is another object of the present invention to provide a generally improved image recording method.

In one aspect of the present invention, in an image 75 recording method using an optical scan type electrophotographic recording apparatus which includes a device for varying a pulse width of video data which modulate a light beam, there is provided and py indicates a ratio of a beam diameter in a subscan direction to a pixel pitch in the subscan direction satisfies a condition 0.6:spVp,:sl.O, and that a product of the ratio p, and a ratio Tp of an exposing time to a one-pixel scanning time at a boundary between an image portion and a non 15 the improvement wherein alight beam scans such 80 image portion satisfies a condition that at a boundary between an image portion and a non-image portion a ratio of a light beam exposing time to a one-pixel scanning time satisfies a condition 0.2:sTd:51.1.

In another aspect of the present invention, in an image recording method using an optical scan type 25 electrophotographic recording apparatus which modulates by binary video data light issuing from minute light emitting segments which are associated with pixels, passes the modulated light through an imaging system to a surface of a 30 photoconductive element to form an electrostatic 95 latent image, and develops the latent image to record data associated with the video data, there is provided the improvement wherein a pulse width of video data for modulating the light issuing from the 35 light emitting segments is varied.

In another aspect of the present invention, in an image recording method for an optical scan type electrophotographic recording apparatus, there is provided the improvement wherein a light beam 40 scans such that a ratio pVp., where p, indicates a ratio of a beam diameter in a main scan direction to a pixel pitch in the main scan direction and p, indicates a ratio of a beam diameter in a subscan direction to a pixel pitch in the subscan direction satisfies a condition 1.0:5pVp':51.5' and that a product of the ratio py and a ratio Td of an 50 exposing time by the light beam to a one-pixel scanning time at a boundary between an image portion and a non-image portion satisfies a condition I 0.5,--p, - Td<1.5.

In another aspect of the present invention, in an image recording method using an optical scan type electrophotographic recording apparatus which 60 modulates by binary video data light issuing from minute light emitting segments which are associated with pixels, passes the modulated light through an imaging system to a surface of a photoconductive element to form an electrostatic 65 latent image, and develops the latent image to 0.5:5p, - Tp:51.5.

In another aspect of the present invention, in an 85 image recording method using an optical scan type electrophotographic recording apparatus, there is provided the improvement wherein a light beam scans such that a ratio 1611p where lp indicates a width of a latent image line substantially parallel to 90 a developing direction and k indicates a width of a latent image line substantially perpendicularto the developing direction satisfies a condition 1.0!-_-/c//P:51.2.

In another aspect of the present invention, in an image recording method using an optical scan type electrophotographic recording apparatus which modulates by binary video data light issuing from 100 minute light emitting segments which are associated with pixels, passes the modulated light through an imaging system to a surface of a photoconductive element to form an electrostatic latent image, and develops the latent image to 105 record data associated with the video data, there is provided the improvement wherei a light beam scans such that a ratio IcIlp where lp indicates a width of a latent image line substantially parallel to a developing direction and Ic indicates a width of a 110 latent image line substantially perpendicular to the developing direction satisfies a condition 1.0<10p:51.3.

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings.

120 Brief Description of the Drawings

Fig. 1 is a schematic view of an ordinary optical scan type electrophotographic recording apparatus to which the first, third and fifth embodiments of the present invention are applied; Fig. 2 is a graph showing a relationship between a relative potential and a relative distance associated with a pattern which was exposed at every two pixels in the main scan direction in the first and second embodiments; 130 Fig. 3 is a graph showing a relative potential and a 1.

GB 2 185 653 A 3 Y.

relative beam diameter provided by fully exposing lines in the horizontal scan direction; Fig. 4 is a graph showing a relationship between a relative potential difference and a pixel frequency 5 duty associated with a pattern which was exposed at every two pixels in the main scan direction; Fig. 5 is a graph showing a relationship between a potential contrast and a pixel frequency duty; Fig. 6 is a schematic view of an optical scan type 10 electrophotographic recording apparatus using a phosphor dot array tube as a light source and to which the second, fourth and sixth embodiments of the present invention are applied; Fig. 7 is a perspective view of an example of the 15 phosphor dot array tube shown in Fig. 6; Figs. 8A and 8B are graphs showing relative potentials with respect to a relative distance in the subscan direction and that in the main scan direction provided by p, of about 0.94 in accordance 20 with the third embodiment; Figs. 9A and 9B are graphs showing relative potentials with respect to a relative distance in the subscan direction and that in the main scan direction provided by p, of about 1.18; Figs. 10A and 1 OB are graphs showing relative potentials with respect to a relative distance in the subscan direction and that in the main scan direction provided by p,, of about 1.42; Fig. 11 is a two-dimensional view of exposure 30 energy distribution resulted when a pixel-by-pixel grid pattern was drawn; Fig. 12 is a view of surface potential distribution on a photoconductive element associated with Fig. 11; Figs. 13A and 13B are graphs showing relative potentials with respect to a relative distance in the main scan direction and a relative distance in the subscan direction provided by py of about 0.94; Figs. 14A and 14B are graphs showing relative 40 potentials with respect to a relative distance in the main scan direction and that in the subscan direction provided by py of about 1.18; Figs. 15A and 15B are graphs showing relative potentials with respect to a relative distande in the 45 main scan direction and that in the subscan direction provided by py of about 1.42; Fig. 16 is a two-dimensional view of exposure energy distribution resulted when a pixel-by-pixelgrid pattern was drawn; 50 Fig. 17 is a view of surface potential distribution on a photoconductive element associated with Fig.

16; Figs. 18A and 18B are graphs showing relative potentials with respect to a relative distance in the 55 subscan direction and that in the main scan direction provided by p., of about 0.94 in accordance with the fifth embodiment of the present invention; Figs. 19A and 19B are graphs showing relative potentials with respect to a relative distance in the 4 60 subscan direction and that in the main scan 125 direction provided by p, of about 1.18; Figs. 20A and 208 are graphs showing relative potentials with respect to a relative distance in the subscan direction and that in the main scan direction provided by p. of about 1.42; Figs. 21A-21C show respectively a distribution of relative exposure energy Q as viewed in the main scan direction, a distribution of relative potentials as viewed in the main scan direction, and a distribution 70 of relative potentials as viewed in the subscan direction, each being associated with a case wherein a potential distribution provided by drawing a pixelby-pixel grid pattern is substantially equal in both the main and subscan directions; Figs. 22A-22C show respectively a distribution of relative exposure energy Q as viewed in the main scan direction, a distribution of relative potentials as viewed in the main scan direction, and a distribution of relative potentials as viewed in the subscan 80 direction, each being associated with a case wherein a ratio between latent image line widths provided by drawing a pixel-by-pixel grid pattern satisfies a condition in accordance with the present invention; Figs. 23A and 23B are graphs showing relative 85 potentials with respect to relative distances in the subscan direction and those in the main scan direction provided by py of about 0.94 in accordance with the sixth embodiment of the present invention; Figs. 24A and 24B are graphs showing relative 90 potentials with respect to relative distances in the subscan direction and those in the main scan direction provided by py of about 1.18; Figs. 25A and 25B are graphs showing relative potentials with respect to relative distances in the 95 subscan direction and those in the main scan direction provided by p,, of about 1.42, Figs. 26A and 26B show respectively a distribution of relative exposure energy Q as viewed in the main scan direction and a distribution of relative potentials as viewed in the main scan direction, each being associated with a case wherein a distribution of potentials provided by drawing a pixel-by-pixel grid pattern is substantially equal in both the main and subscan directions; and 105 Figs. 27A and 27B show respectively a distribution of relative exposure energy Q as viewed in the main scan direction and a distribution of relative potentials as viewed in the main scan direction, each being associated with a case wherein a ratio 110 between latent image line widths provided by drawing a pixel-by-pixel grid pattern satisfies a condition in accordance with the present invention.

Description of the Preferred Embodiments

While the image recording method of the present invention is susceptible of numerous physical embodiments, depending upon the environment and requirements of use, substantial numbers of the herein shown and described embodiments have 120 been made, tested and used, and all have performed in an eminently satisfactory manner.

A first embodiment of the present invention elaborated to achieve the first object will be described first.

An image recording method in accordance with the first embodiment is implemented by means which is capable of varying the pulse width of video data which modulate a light beam in an optical scan type electrophotographic recording apparatus. Light 130 beam scanning is effected such that the ratio of a GB 2 185 653 A 4 light beam exposing time to a one-pixel scanning time satisfies a condition which will be described at boundaries between image portions and non-image portions, so that the latent image potential in the 5 image portions maybe increased at the boundaries. 70 In this particular embodiment, in recording images by such an optical scan type electrophotographic recording apparatus as one 10 shown in Fig. 1, an optimum light beam scanning 10 condition is provided which is such that the pulse width of binary image data is controlled to suitably vary the power of an exposing beam to enhance the latent image potential difference at a border between image and non-image portions and, 15 thereby, the electrostatic contrast, thereby providing recording images with high resolution.

In the recording apparatus 10 shown in Fig. 1, a light source 14 which comprises a laser diode is turned on and off by binary data 12 to produce a 20 directly modulated laser beam 16. The laser beam 16 is routed through an optical scanning system and compensating optical system 18 to sequentially illuminate in a main scan direction a drum-like photoconductive element 22 which isfed in a 25 subscan direction and deposited with a uniform charge by a charger 20. The resulting latent image formed on the drum 22 is developed by a developing unit 24 and, then, transferred to a paper 26 in a transfer station. In this construction, images 30 are recorded in a predetermined density which depends upon the scanning rate of the laser beam in thu main scan direction and the linear velocity of the drum 22 in the subscan direction. Alternatively, a beam emanating from a gas laser may be 35 modulated by means of an acoustooptical 100 modulator in response to binary data.

Referring to Fig. 2, there is shown a characteristic associated with a pattern provided by exposing a photoconductive element at every two pixels in the 40 main scan direction (i.e. a stripe pattern repeating at every one of black and white lines which extend in the subscan direction). In Fig. 2, the abscissa shows a relative distance which is a ratio of each exposure distance to a pixel pitch in the main scan direction, 45 while the ordinate shows a relative potential which is a ratio of each exposed surface potential to a surface potential of the uniformly charged drum 22.

A parameter in the plot of Fig. 2 is a pixel frequency duty, i.e., a ratio of an exposure time to a one-pixel 50 exposure time, which is varied over a range of 10-150%.

It will be seen from Fig. 2 that as the pixel frequency duty increases, the bottoms of the surface potential become shallower and the peaks, lower.

55 This implies that the pixel frequency duty constitutes one of various conditions which allow the boundaries between black and white in recorded images, or image and non-image portions, to appear clear-cut and set up adequate black and 60 white line widths, and that an adequate pixel frequency duty exists in the above-discussed range.

For the selection of an adequate pixel frequency duty, it is a primary requisite to determine an adequate range of the ratio of the beam diameter to of ratios between the peak of surface potential on the drum 22 resulted from the full exposure of lines in the main scan direction and the surface potential on the drum 22 resulted from zero exposure energy. In Fig. 3, the abscissa indicates a relative beam diameter which is the ratio of a beam diameter to a pixel pitch in the main scan direction and the ordinate, a relative potential. It is known by experience that a relative potential lower than about 75 0.2 is desirable in which case, as shown in Fig. 3, the beam diameter to pixel pitch ratio is larger than about 1.0.

As shown in Fig. 4, the ratios shown in Fig. 2 may be represented in relation to the pixel frequency 80 duties employing the relative beam diameter as a parameter. In this instance, it is known by experience that a relative potential higher than about 0.6 is desirable. This, coupled with the previously mentioned beam diameter to pixel pitch 85 ratio which is larger than about 1.0, provides an adequate pixel frequency range as indicated by hatching in Fig. 4.

In Fig. 5, there is shown a relationship between a potential contrast and a pixel frequency duty with 90 respect to the various relative beam diameters. In this case, experience teaches that the potential contrast is desirable if higher than about 60%. Such a desirable range of potential contrast and the previously stated relative beam diameters provide 95 an adequate pixel frequency duty range as indicated by hatching in Fig. 5.

Therefore, considering conditions essential for recording, i.e., that the potential contrast and potential difference of latent images be large and, in addition, the exposure potential be kept low and stable, the relationships shown in Figs. 4 and 5 teach that the optimum range of pixel frequency duty is 20-110%.

Thus, in this particular embodiment, scanning by 105 the light beam is effected such that, assuming that the ratio of the light beam exposure time to the onepixel scanning time is Td, it satisfies a predetermined condition at, in particular, the boundaries between image and non-image 110 portions, as shown below:

0.25Td:51.1.

If the pulse width of binary video data is selected within the adequate pixel frequency range as described above, images will be recorded with high contrast and resolution, whether the recording mode be positive-to-positive or negative-topositive. In addition, the areas of image and non- 120 image portions become similar to those of video data so that various factors detrimental to sharpness, such as thickening and thinning of lines, are effectively reduced. Experimentarily, in positiveto-positive recording, the best images were 125 achieved at the developing level of 150V and in the pixel frequency duty range of 60-70%.

In the foregoing description, attention has been paid exclusively to pixel frequency duty which greatly effects recording quality. However, because 65 the pixel pitch. Fig. 3 shows a curve representative 130 various other factors such as scanning rate, beam

GB 2 185 653 A 5 power and developing level have influence on recording quality, they also have to be taken into consideration in selecting an optimum pixel frequency duty so that the quality of recorded 5 images may be further enhanced.

The method in accordance with the illustrative embodiment may be practiced with any of various means which will not be shown or described. In any case, it can readily be practiced by modifying the 10 ratio of an exposure time to a one-pixel scanning time at a boundary between image and non-image portions, or pulse width of binary video data, in response to binary video data.

As described above, the image recording method 15 in accordance with the first embodiment varies the 80 pulse width of binary video data within an adequate pixel frequency duty range at a boundary between image and non-image portions, thereby increasing the latent image potential in the image portion. This 20 allows an optical scan type electrophotographic recording apparatus to record images with a high resolution.

Hereinafter will be described a second embodiment of the present invention elaborated to 25 achieve the second object.

The method in accordance with the second embodiment is applicable to an optical scan type electrophotographic recording apparatus which uses minute light emitting segments such as 30 phosphors or light emitting diodes (LEDs), modifies 95 light emanating therefrom by binary video data, scans a photoconductive element with the modulated light to form an electrostatic latent image thereon, and turns the latent image to a 35 visible image. The method is implemented by means which is capable of varying the pulse width of video data which modulate light emanating from the light-emitting elements. The pulse width of video data is varied such that the ratio of an 40 exposing time to a one-pixel scanning time satisfies 105 a given condition which will be described at boundaries between image and non-image portions, thereby increasing the latent image potential in image portions adjacent to non-image 45 portions.

In this particular embodiment, in recording images by such an optical scan type electrophotographic recording apparatus as one shown in Fig. 6, generally 30, an optimum exposure condition is provided which is such that the pulse 115 width of binary video data for modulating light output from minute light emitting segments is varied to enhance the latent image potential difference at a border between image and non- image portions and, thereby, electrostatic contrast, 120 thereby recording images with a high resolution.

In the recording apparatus 30 shown in Fig. 6, phosphor elements arranged in lines in a phosphor dot array 34 pixel by pixel in the main scan direction 60 are turned on and off by binary data 32 to produce 125 directly modulated fine beams. The beams are routed through an optical imaging system 36 to illuminate in the main scan direction a drum-like photoconductive element 40 which is fed in a subscan direction and deposited with a uniform charge by a charger 38, thereby sequentially exposing the drum surface line by line. A latent image resulting from the exposure is turned to a visible image by a developing unit 42 and, then, 70 transferred to a paper 46 by a transfer unit 44. The reference numeral 48 in Fig. 6 designates a cleaner for removing residual toner particles from the drum surface after the transfer.

Referring to Fig. 7, a specific construction of the 75 phosphor dot array 34 is shown which comprises an array of phosphor elements 52 arranged in a face glass 50 pixel by pixel, and drive integrated circuits (ICs) 54 built in a single substrate 58 integrally with terminals 56. The phosphor dot array 34 serving as a light source may be replaced by an array of LEDs each constituting a pixel.

The various plots shown in Figs. 2-4 and discussed in relation with the first embodiment are directly applicable to the second embodiment as 85 well.

Therefore, considering conditions essential for recording, i.e., that the potential contrast and potential difference of latent images be large and, in addition, the exposure potential be kept low and 90 stable, the relationships shown in Figs. 4 and 5 teach that the optimum range of pixel frequency duty is 20-110%.

Thus, in accordance with this particular embodiment, the pulse width of video data is varied such that, assuming that the ratio of an exposure time to a one-pixel scanning time is Td, it satisfies a predetermined condition at, in particular, the boundaries between image and non-image portions, as shown below:02!---Td:!5 1. 1.

As described abov% the image recording method in accordance with the second embodiment varies the pulse width of binary video data within an adequate pixel frequency duty range at a boundary between image and nonimage portions, thereby increasing the latent image potential in the image portion and, thereby, the electrostatic contrast. This 110 allows an optical scan type electrophotographic recording apparatus, especially one which uses miniature light-emitting segments as a light source, to record images with a high resolution.

A third embodiment contemplated to achieve the third object of the present invention will be described.

Where an optical scan type electrophotographic recording apparatus such as one shown in Fig. 1 is constructed to electrostatically form a latent image on a photoconductive element by exposing it to a light beam modulated by binary video data, the method in accordance with the third embodiment sets up a light beam condition and a light beam scanning condition which makes the pixels in an image to be developed have the same diameter both in the main scan and subscan directions.

Referring to Figs. SA, 813, 9A, 98, 1 OA and 10B, there are shown examples of relative potentials V and relative exposure energy Q in relation to relative 130 distances X and Y in the main scan and subscan GB 2 185 653 A 6 directions in an exposure pattern which comprises one line in each of the main scan and subscan directions, provided by varying the light beam condition and the light beam scanning condition in 5 various ways. The relative distance X or Y represents a ratio of distance to each pixel pitch in the main scan or subscan direction, relative potential V a ratio of a surface potential on the drum 22 after exposure to a surface potential (uniform) 10 associated with zero exposure energy, and the relative exposure energy Q the ratio of actual exposure energy to maximum exposure energy. For illustration, Figs. 813, 9B and 1 OB share the same data.

15 The curves shown in Figs. 8A and 8B were 80 provided by a ratio p., of a beam diameter in the and main scan direction to a pixel pitch in the main scan direction which was about 0.94. In Fig. 8A, the frequency duty Td which is the ratio of an optical 20 beam exposure time to a one-pixel scanning time is varied while, in Fig. 813, a ratio p, of a beam diameter in the subscan direction to a pixel pitch in the subscan direction. The characteristics indicated by bold lines are associated with a condition wherein 25 the potential distribution is analogous in both the main scan and subscan directions and, in such a condition, there are provided p,=0.94, p,= 1.18 and Td=0.6 and, thereby, a light beam condition p p.=1.255 and a light beam scanning condition p, - Td=0.564.

The "beam diameter" referred to is defined by a sectional shape in a position which is e-' (about 35 13.5%) of a peak of abeam intensity distribution having a Gaussian distribution.

The curves shown in Figs. 9A and 9B resulted from a ratio p. of the beam diameter in the main scan direction to the pixel pitch in thu main scan 40 direction which was about 1.18. In Fig. 9A, the frequency duty Td which is the ratio of a light beam exposing time to a one-pixel scanning time is varied while, in Fig. 9B, the ratio py of a beam diameter in the subscan direction to a pixel pitch in the subscan 45 direction is varied. The characteristics indicated by bold lines are associated with a condition wherein the potential distribution in the main scan direction is analogous to that in the subscan direction and, in such a condition, there are provided p,=1.18, 50 p,=1.42 and Td=0.7 and, thereby, alight beam condition pVp,=1.203 and a light beam scanning condition p., - Td=0.826. The curVes shown in Pigs. 10A and 10B resulted from a ratio px of the beam diameter in the main 55 scan direction to the pixel pitch in the main scan direction which was about 1.42. In Fig. 10A, the frequency duty Td which is the ratio of a light beam exposing time to a one-pixel scanning time is varied while, in Fig. 1 OB, the ratio p, of a beam diameter in 60 the subscan direction to a pixel pitch in the subscan direction is varied. The characteristics indicated by bold lines are associated with a condition wherein the potential distribution in the main scan direction is analogous to that in the subscan direction and, in 65 such a condition, there are provided p,=1.42, 130 py=1.65 and Td=0.8 and, thereby, a light beam conditi6n pVp.,=1.162 and a light beam scanning condition p, - Td=1.136.

By selecting other suitable values of p., to provide other various parameters py and Td, potential distributions which are analogous in the main scan 75 and subscan directions will be obtained.

It will be understood from the above analysis and by experience that if 1.0<pp,<1.5 0.5<p, - Td<1.5 85 are satisfied, a potential distribution substantially analogous in the main scan and subscan directions in practice is achievable.

Referring to Figs. 11 and 12, a two-dimensional distribution is shown which is associated with one 90 of the various conditions discussed hereinabove. Fig. 11 represents a distribution of exposure energy 0 provided when a grid pattern is drawn pixel by pixel underthe conditions p,=1.18, py=1.42 and Td=0.7. Fig. 12 shows a surface potential 95 distribution on a photoconductive element associated with the exposure energy distribution of Fig. 11. Although the graphs of Figs. 11 and 12 are the results of computer simulation, it has been proved by experiments that when a latent image is 100 formed on a photoconductive element under the above conditions and then turned to a visible image, lines of the resulting grid are substantially identical in width in the main scan and subscan directions.

As described above, in accordance with the 105 method of the third embodiment, where an optical scan type electrophotographic recording apparatus is operated to record an image, a latent image associated with binary video data is formed electrostatically on a photoconductive element 110 under a particular light beam condition and a particular light beam scanning condition which provide a potential distribution analogous in the main scan and subscan directions. The method, therefore, allows images to be recorded always with 115 a high resolution and with the same pixel diameter in the main scan and subscan directions.

As described above, in accordance with the method of the third embodiment, when an optical scan type electrophotographic recording apparatus 120 is operated to record an image, a latent image associated with binary video data is formed electrostatically on a photoconductive element under a particular light beam condition and a particular light beam scanning condition which 125 provide a potential distribution analogous in the main scan and subscan directions. The method, therefore, allows images to be recorded always with a desirable resolution and with an equal diameter both in the main scan and subscan directions.

Basically, the pixel diameters in the main scan and srl GB 2 185 653 A 7 subscan directions can be controlled if p,, py and Td are determined at the step of forming a latent image on a photoconductive element. This particular embodiment, which provides a potential 5 distribution analogous in the main scan and 70 subscan directions, is effectively applicable to both positive-to-positive recording and negative-to negative recording.

The third embodiment described has 10 concentrated to a light beam condition and a light beam scanning condition. However, because the quality of recorded images also depends upon other various factors such as scanning rate, beam power and developing level, such various factors need also 15 betaken into account in the selection of pVp., and p,, - Td if higher quality images are desired. The control over Td is readily practicable by modifying the pulse width of binary video data at boundaries between image and non-image portions.

As described above, the method in accordance with the third embodiment sets up a particular light beam condition and a particular light beam scanning condition which make each pixel recorded by an optical scan type electrophotographic 25 recording apparatus identical in diameter in the main scan and subscan directions, thereby allowing images to be recorded with an excellent resolution.

A fourth embodiment of the present invention directed to achieving the fourth object will be 30 described.

The method in accordance with the fourth embodiment is applicable to, for example, the optical scan type electrophotographic recording apparatus 30 shown in Figs. 6 and 7 which is of the 35 type using, as alight source, the phosphor dot array 100 tube 34 having phosphor elements arranged in an array in the main scan direction on a pixel basis.

When a light beam modulated by binary video data 32 and output from the dot array tube 34 is to be 40 focused onto the surface of the photoconductive 105 element 40, which is fed in the subscan direction, the method of the fourth embodiment provides a specific light beam condition and a specific light beam scanning condition which make the pixels in 45 the resulting image equal in diameter in the main scan and subscan directions.

Referring to Figs. 13A, 13B, 14A, 14B, 15A and 15B, there are shown examples of relative potentials V and relative exposure energy Q in relation to 50 relative distances X and Y in the main scan and subscan directions in an exposure pattern which comprises one line in each of the main scan and subscan directions, provided by varying the light beam condition and the light beam scanning 55 condition in various ways. The relative distance X or 120 Y represents a ratio of a distance to each pixel pitch in the main scan or subscan direction, the relative potential V a ratio of a surface potential on the drum after exposure to a surface potential (uniform) 60 associated with zero exposure energy, and the relative exposure energy Q a ratio of actual exposure energy to maximum exposure energy. Here, Figs. 13A, 14A and 15A share the same data for illustration purpose.

65 The curves shown in Figs. 13A and 13B were provided by a ratio p, of a beam diameter in the subscan direction to a pixel pitch in the main scan direction which was about 0.94. In Fig. 13A, the ratio p., of a beam diameter in the main scan direction to a pixel pitch in the main scan direction is varied while, in Fig. 13B, the ratio Tp of a light beam exposing time to a one-pixel scanning time is varied. The characteristics indicated by bold lines are associated with a condition wherein the potential distribution is 75 analogous in the main scan and subscan directions and, in such a condition, there are provided p,=0.94, p,=1.18 and Tp=0.6 and, thereby, a light beam condition pVp.=0.797 and a light beam scanning condition py - Tp=0.564. The "beam diameter" 80 referred to is defined by a sectional shape in a position which is e-' (about 13.5%) of a peak of a beam intensity distribution having a Gaussian distribution.

The curves shown in Figs. 14A and 14B resulted 85 from a ratio p, of a beam diameter in the subscan direction to a pixel pitch in the subscan direction which was about 1.18. In Fig. 14A, the raticy p,, of a beam diameter in the main scan direction to a pixel pitch in the main scan direction is varied while, in 90 Fig. 14B, the ratio Tp of a light beam exposing time to a one-pixel scanning time is varied. The characteristics indicated by bold lines are associated with a condition wherein the potential distribution is analogous in the main scan and subscan directions 95 and, in such a condition, there are provided py=1.1 8, p,=1.42 and Tp=0.7 and, thereby, a light beam condition pVp,,=0.831 and a light beam scanning condition py - Tp=0.826.

The curves shown in Figs. 15A and 15B resulted from a ratio py of a beam diameter in the subscan direction to a pixel pitch in the subscan direction which was about 1.42. In Fig. 15A, the ratio p. of a beam diameter in the main scan direction to a pixel pitch in the main scan direction is varied while, in Fig. 15B, the ratio Tp of a light beam scanning time to a one-pixel scanning time is varied. The characteristics indicated by bold lines are associated with a condition wherein the potential distribution is analogous in the main and subscan directions and, 110 in such a condition, there are provided py=1.42, p,=1.65 and Tp=0.8 and, thereby, a light beam condition pVp.-O.861 and a light beam scanning condition p, - Tp=1.136.

By selecting other suitable values of p, to provide 115 other various parameters p. and Tp, potential distributions which are analogous in the main scan and subscan directions will be obtained.

It will be understood from the above analysis and by experience that if and 0.6:!-_-pp,:51.0 0.5:5py - Tp:51.5 125 are satisfied, a potential distribution substantially analogous in the main scan and subscan directions in practice is achievable.

Referring to Figs. 16 and 17, a two-dimensional distribution is shown which is associated with one 130 of the various conditions discussed hereinabove.

GB 2 185 653 A 8 Fig. 16 represents a distribution of exposure energy Q provided when a grid pattern is drawn pixel by pixel under the conditions p,1.18, p,,=1.42 and Tp=0.7. Fig. 17 shows a surface potential 5 distribution on a photoconductive element associated with the exposure energy distribution of Fig. 16. Although the graphs of Figs. 16 and 17 are the results of computer simulation, it has been proved by experiments that when a latent image is 10 formed on a photoconductive element under the 75 above conditions and then turned to a visible image, lines of the resulting grid are substantially identical in width in the main scan and subscan directions.

As described above, in accordance with the 15 method of the fourth embodiment, when an optical scan type electrophotographic recording apparatus is operated to record an image, a latent image associated with binary video data is formed electrostatically on a photoconductive element 20 under a particular light beam condition and a particular light beam scanning condition which provide a potential distribution analogous in the main scan and subscan directions. The method, therefore, allows images to be recorded always with 25 a desirable resolution and with the same pixel diameter in the main scan and subscan directions.

Basically, the pixel diameters in the main scan and subscan directions can be controlled if p,, p, and Tp are determined at the step of forming a latent image 30 on a photoconductive element. This particular embodiment, which provides a potential distribution analogous in the main scan and subscan directions, is effectively applicable to both positive-to-positive recording and negative-to 35 negative recording.

The third embodiment described has concentrated to a light beam condition and a light beam scanning condition. However, because the quality of recorded images also depends upon other 40 various factors such as scanning rate, beam power 105 and developing level, such various factors need also be taken into account in the selection of pp, and py, Tp if higher quality images are desired.

The control over Td is readily practicable by 45 modifying the pulse width of binary video data at 110 boundaries between image and non-image portions.

As described above, the method in accordance with the third embodiment sets up a particular light 50 beam condition and a particular light beam scanning condition which make each pixel recorded by an optical scan type electrophotographic recording apparatus, particularly one which uses minute light emitting segments as a light source, 55 identical in diameter in the main scan and subscan 120 directions, thereby allowing imagesto be recorded with an excellent resolution.

A fifth embodiment of the present invention elaborated to achieve the fifth object will be 60 described.

Where an optical scan type electrophotographic recording apparatus such as one 10 shown in Fig. 1 is constructed to electrostatically form a latent image on the drum 22 by exposing it to a light beam modulated by binary video data, the method in 130 accordance with the fifth embodiment sets up a particular light beam condition which confines the ratio between a width lp of a latent image line substantially parallel to the developing direction 70 and a width k of the same substantially perpendicular to the developing direction to an optimum range of 1.0:5/c//P:51.2.

Referring to Figs. 18A, 18B, 19A, 19B, 20A and 20B, there are shown examples of relative potentials V and relative exposure energy Q in relation to relative distances X and Y in the main scan and 80 subscan directions in an exposure pattern which comprises one line in each of the main scan and subscan directions, provided by varying the light beam condition and the light beam scanning condition in positive-to- positive recording in various 85 ways. The relative distance X or Y represents a ratio of a distance to each pixel pitch in the main scan or subscan direction, the relative potential V a ratio of a surface potential on the drum 22 after exposure to a surface potential (uniform) associated with zero 90 exposure energy, and the relative exposure energy Q a ratio of actual exposure energy to maximum exposure energy.

The curves shown in Figs. 18A and 18B were provided by a ratio p. of a beam diameter in the main scan direction to a pixel pitch in the main scan direction which was about 0.94. In Fig. 18A, the pixel frequency duty Td which is the ratio of an optical beam exposing time to a one- pixel scanning time is varied while, in Fig. 18B, the ratio p, of a beam 100 diameter in the subscan direction to a pixel pitch in the subscan direction is varied. The characteristic indicated by a dotted line in Fig. 18A and one indicated by a bold line in Fig. 18B share substantially the same potential distribution and, in such a condition, there are provided p,=0.94, py=1.1 8 and Td=0.6. In this case, by changing the pixel frequency duty Td from 0.6 to 0.7 as indicated by a bold line in Fig. 18A, the latent image line width Ic in the subscan direction can be made greater than one in the main scan direction and the ratio IcIlp can be confined to the previously mentioned range.

The curves shown in Figs. 19A and 19B resulted from a ratio p,, of a beam diameter in the main scan direction to a pixel pitch in the main scan direction 115 which was about 1.18. In Fig. 19A, the ratio Td of a light beam exposing time to a one-pixel scanning time is varied while, in Fig. 19B, the ratio py of a beam diameter in the subscan direction to a pixel pitch in the subscan direction is varied. The characteristic indicated by a bold line in Fig. 19A and one indicated by a doffed line in Fig. 19B share substantially the same potential distribution and, in such a condition, there are provided p,=1.18, py=1.42 and Td=0.7. In this case, by changing the 125 ratio py of a beam diameter in the subscan direction to a pixel pitch in the subscan direction from 1.42 to 1.30 as represented by a bold line in Fig. 19B, the latent image line width Ic in the subscan direction can be made greater than one in the main scan direction and the ratio IcIlp can be confined to the tR GB 2 185 653 A 9 A previously mentioned range.

The curves shown in Figs. 20A and 20B resulted from a ratio p, of a beam diameter in the main scan direction to a pixel pitch in the main scan direction 5 which was about 1.42. In Fig. 20A,the ratioTd of a light beam exposing time to a one-pixel scanning time is varied while, in Fig. 20B, the ratio p, of a beam diameter in the subscan direction to a pixel pitch in the subscan direction is varied. The 10 characteristics indicated by dotted lines in Figs. 20A and 20B share substantially the same potential distribution and, in such a condition, there are provided p.=1.42, p,=1.65 and Td0.8. In this case, by changing the ratio p, of a beam diameter in the 15 subscan direction to a pixel pitch in the subscan direction from 1.65 to 1.42 as indicated by a bold line in Fig. 20B and the pixel frequency duty Td from 0.8 to 0.7 as indicated by a bold line in Fig. 20A, the latent image line width k in the subscan direction 20 can be made greater than one in the main scan direction and the ratio IcIlp can satisfy the condition concerned.

Further, if p, is suitably varied so that, in the manner described, the ratio p, of a beam diameter in 25 the subscan direction to a pixel pitch in the subscan 90 direction and/or the pixel frequency duty Td associated with a potential distribution which is substantially identical in the main scan and subscan directions is varied, the ratio IcIlp will satisfy the 30 same condition.

Taking one of the various conditions described so far for example, a twodimensional distribution will be analyzed. When a pixel-by-pixel grid pattern is drawn under the conditions p,=1.1 8, p,=1.42 and 35 Td=0.7 so that one dot line in each of the main and subscan directions may be evaluated under the same conditions, the distributions shown in Figs. 21 A-21 C hold if the potential distribution is substantially identical in the main and subscan 40 directions, and the distributions shown in Figs. 22A-22C if the line width ratio satisfies the condition 1.0:5/c//P:51.2.

Figs. 21A and 22A show distributions of relative exposure energy Q each viewed in the main scan direction, Figs. 21 B and 22B distributions of relative potentials each viewed in the main scan direction, 50 and Figs. 21 C and 22C distributions of relative potentials each viewed in the subscan direction.

it will be apparent from the drawings that the portions labelled A and B in Figs. 21 B, 21 C, 22B and 22C represent lines extending in the subscan 55 direction, the potential being slightly lower in the portions A than in the portions B. Needless to mention, the potential distribution width is wider in the portions B than in the portions A.

The above analysis, coupled with experience, 60 teaches that if the ratio of the latent image line 125 widths satisfy the condition 1.0:5/c//P<1.2, 65 images can be recorded with desirable reproducibility dot on a one-dot line basis.

Concerning negative-to-positive recording, as distinguished from the above-described positive-topositive recording, it is necessary in tendency that 70 the condition for setting up the relation between the line widths in the main scan and subscan directions be inverted. In such a case, too, the ratio 1611p associated with an image portion needs to satisfy the previously mentioned condition. While the 75 method in accordance with this particular embodiment is applicable to both the positive-topositive recording and negative-to-positive recording, the application to positive-to-positive recording will prove particularly effective in view of 80 the fact that the reproducibility of hairlines are inherently fair in the case of negative-to-positive recording. The increase in the line width in the direction substantially perpendicular to the developing direction will be greatly effected by 85 developing characteristics and velocity characteristics of a photoconductive element ard, therefore, the ratio IcIlp has to be selected taking such characteristics have into consideration.

As described above, the method in accordance with the fifth embodiment selects an exposing beam diameter and/or a pixel frequency duty in an optical scan type electrophotographic recording apparatus such thatthe latent image line width in a direction substantially perpendicular to the developing 95 direction becomes greater than one in a direction substantially parallel to the same. Hence, even if the image width is somewhat disturbed by fluctuations in developing characteristics, moving velocity of a photoconductive element and otherfactors, the 100 method allows even a hairline such as a one-dot line to be desirably reproduced only if the line width ratio is predetermined in consideration of the fluctuations.

A sixth embodiment directed to achieving the 105 sixth embodiment will be described.

The method in accordance with the sixth embodiment is applicable to, for example, the optical scan type electrophotographic recording apparatus 30 shown in Figs. 6 and 7 which is of the 110 type using, as a light source, the phosphor dot array tube 34 in which phosphor elements are arranged in an array in the main scan direction in a pixel configuration. When a light beam modulated by binary video data 32 and output from the dot array 115 tube 34 is to be focused onto the surface of the drum 40, which is fed in the subscan direction, the method in accordance with the sixth embodiment provides a light beam condition and a light beam scanning condition which confine the ratio between the width 120 Ic of a latent image line substantially parallel to the developing direction and the width Ic of a latent image line substantially parallel to the developing direction to an optimum range of 1.0:5/c1/P:51.3.

Referring to Figs. 23A, 23B, 24A, 24B, 25A and 25B, there are shown examples of relative potentials V and relative exposure energy Q in relation to relative distances X and Yin the main scan and GB 2 185 653 A 10 subscan directions in an exposure pattern which comprises one line in each of the main scan and subscan directions, provided by varying the light beam condition and the light beam scanning 5 condition in positive-to-positive recording in various 70 ways. The relative distance X or Y represents a ratio of a distance to each pixel pitch in the main scan or subscan direction, the relative potential V a ratio of a surface potential on the drum after exposure to a 10 surface potential (uniform) associated with zero exposure energy, and the relative exposure energy Q a ratio of actual exposure energy to maximum exposure energy.

The curves shown in Figs. 23A and 23B were 15 provided by a ratio py of a beam diameter in the main scan direction to a pixel pitch in the main scan direction which was about 0.94. In Fig. 23A, the ratio p,, of a beam diameter in the main scan direction to a pixel pitch in the main scan direction is varied while, 20 in Fig. 23B, the pixel frequency duty Tp which is the ratio of a light beam exposing time to a one-pixel scanning time is varied. The characteristic indicated a dotted line in Fig. 23A and one indicated by a bold line in Fig. 23B share substantially the same 25 potential distribution and, in such a condition, there are provided py=0.94, p,=1.18 and Tp=0.6. In this case, by changing the beam diameter in the main scan direction to the pixel pitch in the main scan direction from 1.18 to 1.30 as indicated by a solid 30 line in Figs. 23B, the line width Ic in the subscan direction can be made greater than one in the main scan direction and the ratio IcIlp can be confined to the previously mentioned range.

The curves shown in Figs. 24A and 24B were 35 provided by a ratio p, of a beam diameter in the subscan direction to a pixel pitch in the subscan direction which was about 1.18. In Fig. 24A, the ratio p, of a beam diameter in the main scan direction to a pixel pitch in the main scan direction is varied while, 40 in Fig. 24B, the pixel frequency duty Tp is varied.

The characteristic indicated by a bold line in Fig. 24A and one indicated by a dotted line in Fig. 24B share the substantially same potential distribution and, in such a condition, there are provided p,=1.1 8, 45 p,=1.42 and Tp=0.7. In this case, by changing the 110 pixel frequency duty Tp from 0.7 to 0.6 as indicated by a bold line in Figs. 24B, the line width Ic in the subscan direction can be made greater than one in the main scan direction and the ratio IcIlp can be 50 confined to the previously mentioned range.

The curves shown in Figs. 25A and 25B were provided by a ratio p, of the beam diameter in the subscan direction to the pixel pitch in the subscan direction which was about 1.42. In Fig. 25A, the ratio 55 p.of the beam diameter in the main scan direction to the pixel pitch in the main scan direction is varied while, in Fig. 25B, the pixel frequency duty Tp is varied. The characteristic indicated a dotted line in Fig. 25A and one indicated by a dotted line in Fig. 60 25B share a substantially identical potential 125 distribution and, in

such a condition, there are provided p,,=1.42, p.=1.65 and Tp=0.8. In this case, by changing the pikel frequency duty Tp from 0.8 to 0.7 as indicated by a bold line in Figs. 25B and the ratio p. of the beam diameter in the main scan 130 direction to the pixel pitch in the main scan direction as indicated by a bold line in Fig. 25Afrom 1.65 to 1.42, the latent image line width Ic in the subscan direction can be made greater than that in the main scan direction and the ratio IcIlp can be confined in the previously mentioned range.

Further, if p,, is suitably varied so that, in the manner described, the ratio p, of a beam diameter in the main scan direction to a pixel frequency duty Tp 75 and/or a pixel pitch in the main scan direction associated with a potential distribution which is substantially identical in the main scan and subscan directions is varied, the ratio 1611p will satisfy the previously presented condition.

Taking one of the various conditions described so far for example, a twodimensional distribution will be analyzed. When a pixel-by-pixel grid pattern is drawn underthe conditions py=1.18, p,=1.42 and Tp=0.7 so that one-dot line in each of the main and 85 subscan directions may be evaluated under the same conditions, the distributions shown in Figs. 26A and 26B hold if the potential distribution is substantially identical in the main scan and subscan directions, and the distributions shown in Figs. 27A 90 and 27B if the line width ratio satisfies the condition 1.05/cUP:51.3.

Figs. 26A and 27A show distributions of relative exposure energy Q each viewed in the main scan direction, Figs. 26B and 27B distributions of relative potentials each viewed in the main scan direction.

It will be apparent from the drawings that the portions labelled A and B in Figs. 26B and 27B 100 represent lines extending in the subscan direction, the potential being slightly lower in the portions A than in the portions B. Needless to mention, the potential distribution width is wider in the portions B than in the portions A.

The above analysis, coupled with experience, teaches that if the line width ratio satisfies the condition 1.05/cAPS1.3, images can be recorded with desirable reproducibility on a one-dot line basis.

Concerning negative-to-positive recording, as distinguished from the above-described positive-to- 115 positive recording, it is necessary in tendency that the condition for setting up the relation between the line widths in the main scan and subscan directions be inverted. In such a case, too, the ratio IcIlp associated with an image portion needs to satisfy 120 the previously mentioned condition. While the method in accordance with this particular embodiment is applicable to both the positive- topositive recording and negative-to-positive recording, the application to positive-to-positive recording will prove particularly effective in view of the fact that the reproducibility of hairlines are inherently fair in the case of negative-to-positive recording. The increase in line width in the direction substantially perpendicular to the developing direction will be greatly effected by developing -ft il GB 2 185 653 A 11 W il A characteristics and velocity characteristics of a photoconductive element and, therefore, the ratio IWO has to be selected taking them into account.

As described above, the method in accordance with the sixth embodiment selects an exposing beam diameter and/or a pixel frequency duty in an optical scan type electrophotographic recording apparatus, particularly one which uses miniature light emitting segments-as a light source, such that 10 the latent image line width in a direction substantially perpendicular to the developing direction becomes larger than one in a direction substantially parallel to the same. Hence, even if the image width is somewhat disturbed by fluctuations 40 15 of developing characteristics, moving velocity of a photoconductive element and other factors, the. method allows even a hairline such as a one-dot line to be desirably reproduced only if the line width ratio is predetermined in consideration of the 20 fluctuations.

Claims

1. In an image recording method using an optical scan type electrophotographic recording apparatus 50 25 which modulates by binary video data light issuing from minute light emitting segments which are associated with pixels, passes the modulated light through an imaging system to a surface of a photoconductive element to form an electrostatic 30 latent image, and develops the latent image to record data associated with the video data, the improvement wherein a light beam scans such that a ratio IcIlp where lp indicates a width of a latent image line substantially parallel to a developing 35 direction and /c indicates a width of a latent image line substantially perpendicular to the developing direction satisfies a condition 1.051cflp:551.3.

2. The improvement as claimed in claim 1, wherein the developing direction is substantially coincident with an intended direction of movement of a photoconductive element.

3. The improvement as claimed in claim 1, wherein a factor for determining the ratio IcIlp comprises at least one of an exposing beam diameter and a pixel frequency duty.

4. The improvement as claimed in claim 2, wherein a factor for determining the ratio 1611p comprises at least one of an exposing beam diameter and a pixel frequency duty.

Printed for Her Majesty's Stationery Office by Courier Press, Leamington Spa, 7/1987. Demand No. 8991685.

Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.