GB2144943A - Optical recording head for halftone picture - Google Patents

Abstract

In the production of a halftone picture, electrical signals obtained by photoelectrically scanning an original are processed to obtain picture signals which are used to reproduce the original as a halftone picture. The system includes a recording head 3 comprising 3 LED raws A,B,C, spaced in the main scanning direction. Each of the LED rows is formed of LEDs of the same width and arranged side by side with a prescribed pitch in the subscanning direction. The width of each LED in any row is different from that of each LED in any other row. The row with a LED spacing corresponding to a desired screen ruling can be selected and controlled for exposing a photolitho plate. This allows a system with a variable screen ruling which is nevertheless simple in structure and small in size. <IMAGE>

Description

SPECIFICATION Exposure system suitable for use in production of halftone picture This invention relates to an exposure system useful upon direct production of a halftone picture, which is suitable for use in the fabrication of a printing plate, from an original.

Ar+ ion lasers, He-Cd gas lasers, He-Ne gas lasers and the like have generally been used as light sources in conventional dot-generating exposure systems in which picture signals are output for the production of halftone pictures directly from scanned signals obtained by photoelectric scanning of originals. It is however impossible to modulate directly the luminous flux of a laser beam at a high speed. In an exposure system of the digital type, a single laser beam is hence branched out optically into a plurality of exposing beams and each of the resulting exposing beams is then modulated externally. It is therefore required to dispose an optical modulator and the like within the exposure system, leading to unavoidable dimensional enlargement and structural complexation.

In conventional exposure systems, the screen ruling may be changed by altering the width of an exposure beam by means of a zoom lens. If one tries to keep a projected image sharp over a wide zooming range, it is indispensable to increase the F number of the zoom lens. As a result, such conventional exposure systems are accompanied by such drawbacks that the thus-increased F number not only renders images darker but also makes the structure of a drive mechanism for the zoom lens complex.

With the foregoing circumstances in view, an object of this invention is to provide a highly-reliable exposure system suitable for use in the production of a halftone picture, in which exposure system the structure of the exposure system per se has been simplified and especially, the alteration of the screen ruling can be achieved readily without failure.

In one aspect of this invention, there is thus provided an exposure system suitable for use in the production of a haiftone picture, which exposure system is adapted to subject electrical signals obtained by photoelectric scanning of an original to a prescribed processing so as to convert the electrical signals into picture signals and then to record the original as a desired halftone picture on the basis of the thus-processed picture signals, characterized in that said exposure system is equipped with a light source unit formed by arranging LED arrays one after another in a main scanning direction, each of said LED arrays being formed of LEDs having light-emitting regions of the same width and arranged side by side with a prescribed pitch in a subscanning direction and the width of the light-emitting region of each LED in one of the arrays being different from that of each LED in another one of the LED arrays, and at least one condenser lens for gathering an exposing beam from the light source unit, and in accordance with a desired screen ruling, its corresponding LED array is selected and controlled for exposure.

The exposure system of this invention makes use of the LED arrays as its light source. Their LEDs can be modulated directly, thereby permitting the omission of an external modulator which is indispensable in a conventional laser beam exposure system. It is thus possible not only to make the exposure system per se smaller but also to simplify its structure.

The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which: Figure 1 is a schematic illustration of a unit dot structure of a halftone picture; Figure 2 is a schematic plan view of the recording unit of a color scanner, in which recording unit the present invention has been incorporated; Figure 3 is a schematic front elevation illustrating the outline of one embodiment of a light source unit; and Figure 4 is a schematic front elevation showing another embodiment of the light source unit schematically.

Referring first to Fig. 1, halftone dots are formed to 50% dot area percent within a unit dot region defined by four corners A, B, C, D.

Generally speaking, a screen ruling I is dependent on the size of a screen pattern, in other words, the number and width of exposing beams to be used upon exposure. Representing the size of a unit dot, the number of beams making up the unit dot and the beam width by Dp (cm), P and Bp (cm) respectively, the following relationship is established.

Dp = ss Bp (1) When the above screen pattern is used, the screen ruling I (line/cm) is expressed by the following equation: I1 /ss Bp (2) Therefore, the width (Bp) of each exposing beam was, as mentioned above, changed using a zoom lens in a conventional system in order to alter the screen ruling.

Reference will next be made to Fig. 2. An exposure system 1 is arranged in a face-toface relation with a drum 2 bearing a photosensitive film wrapped on the circumferential wall thereof. The' exposure system 1 is internally equipped with an exposing light source unit 3, to which are input picture signals S obtained by subjecting electrical signals, which have in turn been obtained by photoelectric scanning of an original, to a prescribed electric processing. The exposure system 1 is slidably mounted on a shaft 4 and is caused to move with a prescribed pitch in a subscanning direction indicated by an arrow.

Fig. 3 illustrates one example of the light source unit 3 depicted in Fig. 2. On a substrate 5, LED arrays A, B, C having three types of light-emitting regions respectively are provided side by side. Each of the arrays A, B, C is connected electrically to an array-selecting circuit 9. In Fig. 3, the vertical direction of the light source unit 3 corresponds to the main scanning direction of the color scanner illustrated in Fig. 2 while the horizontal direction of the light source unit 2 corresponds to the subscanning direction of the same color scanner.

The first LED array A is formed of lightemitting regions a,-a6 having the same width P1 and arranged side by side with a prescribed pitch in the subscanning direction.

The second LED array B has the same array length as the first LED array A and is formed of light-emitting regions b1-b8 arranged side by side with a prescribed pitch in the subscanning direction.

On the other hand, the third LED array C has the same length as the first and second LED arrays A, B. It is formed of light-emitting regions cl-c12 arranged arranged side by side with a prescribed pitch in the subscanning direction.

When altering the screen ruling to a desired screen ruling, the LED array A, B or C which corresponds to the desired screen ruling is selected by means of an array-selecting circuit 9. At the same time, by selecting each beam number with a multiplexer in such a dot generator circuit as disclosed for example in copending Japanese Patent Application No.

80639/1983, electrical signals obtained by scanning the original are input to a selected LED array so as to ON-OFF control desired LEDs. Accordingly, LED beams are projected through an optical lens 6 onto an exposurerecording surface 7 and are thus focused there as a halftone picture.

On the exposure-recording surface 7 of the drum 2, there is wrapped for example a lithographic film for halftone picture scanner.

Since the drum 2 is rotated in synchronization with an original drum (not illustrated) by means of drive shaft 8, beams given off from an LED array on which an image has been focused are allowed to scan the exposurerecording surface 7 to obtain a desired halftone picture.

Fig. 4 illustrates another example of the light source unit depicted in Fig. 3. The lightemitting regions of individual LED arrays are arranged with a prescribed interval in the main scanning direction into a staggered pattern as seen in the subscanning direction.

In the embodiment of Fig. 4, the individual light-emitting regions are arranged with the prescribed interval in the main scanning direction to ensure the electrical and optical isolation or separation of adjacent light-emitting regions. Accordingly, in the illustrated embodiment, each of LED arrays A', B', C' is formed by light-emitting regions arranged in two rows with the light-emitting regions in the upper row as seen in the main scanning direction being staggered with those in the lower row. In other words, the light-emitting regions are arranged in a zig-zag pattern as seen in the subscanning direction. Therefore, picture signals input to a light source unit 3' are delayed in accordance with the relative velocities of the recording surface 7 and exposing beams.In order to make this delay time constant, the vertical interval between each light-emitting region in one of the arrays A', B', C' and its correspondong light-emitting region in the adjacent array is designed to be constant.

By constructing the light source unit in the above-described manner, it is possible to project exposing beams ciosely and thus to obtain a halftone picture having excellent picture quality.

As apparent from the foregoing description, the LED arrays which are used as a light source unit in the present invention may be a hybrid LED microcircuit obtained by bonding separate LEDs accordance with the hybridizing technique or a monolithic LED microcircuit obtained by forming separate LEDs in arrays on a monocrystalline substrate.

In each of the above-described embodiments, three types of LED arrays are arranged one after another on the substrate 5. The number of such LED arrays may however be determined as needed. Furthermore, the length of each LED array and the width of each light-emitting region are not necessarily limited to those employed in the above embodiments. Neediess to say, they may be determined freely.

Owing to the above-described construction, the exposure system according to this invention has the following effects which are extremely advantageous from the practical viewpoint.

First of all, it is possible to alter with extreme ease the screen ruling to be scanned on the recording surface 7 to a desired screen ruling by selecting an LED array corresponding to the desired screen ruling to change the width and number of exposing beams upon formation of a halftone picture.

Secondly, the exposure system according to this invention can obtain beams with a desired beam width from a selected LED array by means of a single piece of lens when altering the screen ruling. Therefore, it is possible to simplify the structure of an exposure system and at the same time to reduce the overall dimensions of the exposure system per se without developing such inconvenience that resulting images would become darker or the lens system would be complexed.

Thirdly, it is possible to maintain the feeding pitch of the exposure system in the subscanning direction whichever LED array is selected, becausethe LED arrays arranged one after another in the exposure system have the same array length. Therefore, the present invention can simplify the drive mechanism for the exposure system. It is also possible to obtain a halftone picture of a desired screen ruling even where LED arrays have the same number of light-emitting regions but have different array lengths, provided that the feeding pitch in the subscanning direction is set in accordance with the array length of each selected LED array. Needless to say, such a case is also embraced in the scope of this invention.

As apparent from the above description, the present invention has provided an extremelyadvantageous exposure system suitable for use in the production of a halftone picture, which exposure system is extremely simple in structure and small in size but has solved various drawbacks of conventional exposure systems and at the same time improved the accuracy of exposure and the operability.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

Claims (8)

1. An exposure system suitable for use in the production of a halftone picture, which exposure system is adapted to subject electrical signals obtained by photoelectrically scanning an original to a prescribed processing so as to convert the electrical signal into picture signals and then to record the original as a desired halftone picture on the basis of the picture signals, characterized in that said exposure system is equipped with a light source unit formed by arranging LED arrays one after another in a main scanning direction, each of said LED arrays being formed of LEDs having light-ernitting regions of the same width and arranged side by side with a prescribed pitch in a subscanning direction and the width of the light emitting region of each LED in one of the LED arrays being different from that of each LED in another one of the LED arrays, and at least one condenser lens for gathering exposing beams from the light source unit, and in accordance with a desired screen ruling, its corresponding LED array is selected and controlled for exposure.

2. An exposure system as claimed in Claim 1, wherein the LED arrays have the same array length.

3. An exposure system as claimed in Claim 1, wherein the LED arrays have the same number of light-emitting regions, and the light-emitting regions are controlled in such a way that the light-emitting regions move in the subscanning direction of the exposure system in accordance with the width of the light-emitting region of each LED array.

4. An exposure system as claimed in Claim 1, wherein each LED array extends in a single row in the subscanning direction.

5. An exposure system as claimed in Claim 1, wherein the LED arrays are arranged one after another with the phases thereof shifted with a prescribed interval in the main scanning direction.

6. An exposure system as claimed in Claim 1, wherein the LED arrays are a hybrid LED microcircuit.

7. An exposure system as claimed in Claim 1, wherein the LED arrays are a monolithic LED microcircuit.

8. An exposure system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.