HALFTONE DOT IMAGE RECORDING APPARATUS EMPLOYING HIGH DENSITY SCREEN PATTERN SIGNAL

Abstract: In a screen pattern memory (23), screen pattern data provided with a value for each element area which is smaller than a light spot diameter of a light beam on a surface of a photosensitive material is stored. Level of a screen pattern signal obtained from the screen pattern data and that of an image signal obtained from a digital image data are compared in a comparator (22). An intensity of the light beam is modulated according to a result of comparison in the comparator (22). Because the screen pattern signal is given as a high density signal, the light beam is modulated in each element area which is smaller than a light spot diameter.

Description: The invention relates to a halftone dot image recording apparatus wherein density of a screen pattern signal is increased thereby to substantially enhance reproducibility of gradation. In a halftone dot image recording apparatus such as a process scanner of electronic control type, a level of an image signal is compared with a level of a prescribed screen pattern signal, and a photosensitive material is scanned by an exposure of light beam which is turned on and off according to a result of such comparison, to record a halftone dot image thereon. The screen pattern signal is generated based on the screen pattern data which are prepared previously. As well known in the art, the screen pattern data express a thershold level distribution in the halftone dot in which a small area A ij shown in Fig. 1 (hereinafter called as "unit area") is employed as a quantized space unit, and the unit area A ij is obtained by dividing one halftone dot HD into lattice or matrix. Fig. 2 shows a state in which the screen pattern data prepared for the halftone dot HD of Fig.1 is stored in a memory and numerals in Fig. 2 represent data value given to each unit area A ij . A shaded unit area shown in Fig. 1 represents an area which has a screen pattern data that is greater than "128" and a region on the photosensitive material corresponding to the shaded area is exposed by the light beam in recording a halftone dot with respect to an image signal having a level of "128". The size of the unit area A ij is determined according to light spot diameter d of the light beam to expose at a position on an image recording surface of the photosensitive material. More specifically, the size of the unit area A ij is determined so that the light spot diameter d will be a range of 0.75 to 1.5 times a length a of one side of the unit area A ji (The length of the side is hereafter called as "unit area size"). In Fig. 1, a diameter of a circle which inscribes the unit area A ij is employed as the light spot diameter d for convenience of illustration. Turning on and off of the light beam for exposure is judged every time a scanning is advanced by a distance according to the light spot diameter d, and the intensity of the light beam is subjected to turning on and off according to the results of the judgement. Therefore, in a conventional halftone dot image recording apparatus, a space interval capable of the beam intensity change is restricted to the intergral multiples of the unit area size a . As a result, in a square halftone dot having a screen pitch K where the following relation (1) holds, K = Na (N is an intergral number) (1) gradation number of recorded image has a maximum value M of:. M = (K/a)² = N² (2) Therefore, in order to increase the gradation number M, it is necessary to increase the value (K/a) in the formula (2). In a flat bed type scanner, however, since it is necessary to secure a certain degree of scanning length, long focal distance of an image forming lens of a recording optical system must be provided. Therefore, in the flat bed type scanner, the light spot diameter of the light beam for exposure at a position of the photosensitive material cannot be made too small, and to realize the light spot diameter of about 15 to 16 µm for example involves extremely technical difficulties and increased costs. Consequentially, in the flat bed type scanner, the unit area size a is also relatively large and there exists a condition where the gradation number M is compelled to become small according to the formula (2). In a drum type scanner which is capable of making the light spot diameter d and the unit area size a to be relatively small as well, when the screen pitch K is small, the gradation number M also becomes low according to the formula (2). As stated above, in the conventional halftone dot image recording apparatus, there is a problem wherein the gradation number is low when the ratio of the screen pitch K to the unit area size a cannot be made large, and the gradation reproducibility of the recorded image becomes to be reduced. Furthermore, because there is a necessity to secure the spatial periodicity of the halftone dot in the image recording, the screen pitch K depends upon the size of the light spot diameter d (therefore the unit area size being a ). In an example of the halftone dot having a screen angle of 0° as shown in Fig. 1, the screen pitch K is restricted by the integer multiple of the unit area size a . Therefore, in a conventional apparatus, there is a problem of low degree of freedom in selection the screen pitch. The paper APPLIED OPTICS vol. 19, no. 15, August 1980, pages 2513-2519; J.P. ALLEBACH: "Binary display of images when spot size exceeds step size" discloses algorithms for binary representations of continuous tone images using output devices for which the light spot size exceeds the screen step size. However, these are not implementable by thresholding techniques. The present invention is directed to an apparatus for recording halftone dot images on photosensitive material on the basis of a digital image data. According to the present invention, the apparatus comprises (a) image signal generating means for generating image signals on the basis of a digital image data, (b) screen pattern signal generating means for generating periodic screen pattern signal, (c) comparing means for comparing a level of said image signal and a level of said screen pattern signal, (d) light beam generating means for generating a light beam to be irradiated on a photosensitive surface of said photosensitive material, said light beam having a prescribed light spot diameter on said photosensitive surface of said photosensitive material, (e) intensity modulating means for modulating an intensity of said light beam according to a result of a comparison in said comparing means, (f) scanning means for relatively moving said light beam and said photosensitive materials thereby to scan said photosensitive surface by said light beam. According to the present invention, said screen pattern signal generating means has (b-1) high density screen pattern signal generating means for generating a high density screen pattern signal whose levels are changed at time intervals shorter than a time interval in which a scanning by said scanning means advances by said light spot diameter on said photosensitive surface, and (b-2) a high density screen pattern signal transfer means for transferring said high density screen pattern signal to said comparing means as said screen pattern signal. According to a preferred embodiment of the present invention, the screen pattern data are stored in a memory means, said screen pattern data are provided with values for each element area on said photosensitive surface which is smaller than said light spot diameter, and said high density screen pattern signals are generated on the basis of said screen pattern data. According to another embodiment of the present invention, high density screen pattern signals are generated through interpolation process. According to further another embodiment of the present invention, the high density screen signal may be an analog screen pattern signal. The image signal may also be an analog image signal. The present invention is also directed to provide a method to record halftone dot images on the photosensitive material on the basis of the digital image data. This method has a step for generating a high screen pattern signal whose level changes at the time intervals which are shorter than the time interval in which the scanning advances by the light spot diameter of the light beam. When the analog screen pattern signal are used as the high density screen pattern signal, said time intervals at which the high density screen pattern signal varies is substantially zero. Accordingly, an object of the present invention is to provide a halftone dot recording apparatus which is capable to substantially increase the gradation reproducibility of a recorded image and has a high degree of freedom in selection of the screen pitch even when a ratio of a screen pitch to a light spot diameter of a light beam for exposure is not large. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjuction with the accompanying drawings. Fig. 1 is a diagram showing a screen pattern data in a conventional apparatus; Fig. 2 is an explanetory diagram showing an expression of gradation according to a conventional apparatus: Fig. 3 is a diagram showing overall structure common to all embodiments of the present invention applied to a flat bed type scanner; Fig. 4 is a diagram showing an internal structure of first embodiment; Fig. 5 and Fig. 6 are explanetory drawings showing the screen pattern data of the first embodiment: Fig. 7 is the timing chart showing operations of the first embodiment; Fig. 8 and Fig. 9 are explanetory diagrams wherein exposure condition according to the first embodiment is compared with that of the conventional apparatus; Fig. 10A through Fig. 10C are diagrams showing second embodiment; Fig. 11 is an explanetory diagram showing screen pattern data and screen pattern signal of the second embodiment; Fig. 12 through Fig. 14 are explanetory drawings showing operations of the second embodiment; Fig. 15 is a diagram showing an internal structure of third embodiment; Fig. 16 is a timing chart showing operations of the third embodiment: Fig. 17 and Fig. 18 are explanetory diagrams of the third embodiment: Fig. 19 is a diagram showing an internal structure of fourth embodiment: and Fig. 20 is an explanetory diagram showing operations of the fourth embodiment. A. Overall construction of embodiments Fig. 3 is a diagram showing an overall structure of an embodiment applied to the flat bed type process scanner according to the present invention. In this application, a plurality of embodiments is disclosed but the configuration shown in Fig. 3 is common to all embodiments. In Fig. 3, a digital image data S O which contains image information to be recorded is inputted into a halftone dot signal generating circuit 1 having a detailed structure described later. The halftone dot signal generating circuit 1 generates an exposure output signal S which gives a halftone dot image based on the digital image data S O inputted sequentially along scanning lines. The halftone dot signal generating circuit 1 is connected to a microcomputer 2 which is used for control of operations. This microcomputer has a CPU 3 and a memory 4 and is connected to a keybord 5 for inputting operation signal. A laser beam LB generated in a laser oscillator 7 as a light source for recording exposure is intensity-modulated in an acoustic optical modulator (AOM) 8 in response to the exposure output signal S and inputted into a beam expander 14. The beam expander 14 expands the inputted light beam in diameter. A laser beam L outputted from the beam expander 14 reaches a galvanomirror (or polygonal rotary mirror) 15 to be deflected for scannning, and is irradiated onto a photosensitive surface of a photosensitive material 17 through a fϑ lens 16. The laser beam L has a light spot diameter d on the photosensitive surface of the photosensitive material 17. A main scannning is accomplished by moving the laser beam L periodically to X direction by vibration of the galvanomirror 15 or the rotation of the polygonal rotary mirror provided instead of the galvanomirror 16. A subscanning is accomplished by moving the photosensitive material 17 to the direction perpendicular to a plane on which Fig. 3 is drawn. As a result, a scanning exposure of the photosensitive material 17 is accomplished thereby a halftone dot image is recorded on the photosensitive material 17 on the basis of the digital image data S O . B. Detail construction and operation of the first embodiment Fig. 4 is a diagram of an internal structure of the halftone dot signal generating circuit 1 according to the first embodiment. In Fig. 4, the inputted digital image data S O is latched periodically by a flip flop (hereafter called "F/F") 21 to be outputted from the same as an image signal S₁. The image signal S₁ given to an input A of a comparator 22. On the other hand, in the halftone dot signal generating circuit 1, a screen pattern memory 23 is provided in which a digital screen pattern data is stored previously. The pattern data to be stored in the screen pattern memory 23 has four times the spatial density (two times the spatial density when only one scanning direction is considered) of the conventional screen pattern data provided for the same screen pitch and the light spot diameter. In an example of a halftone dot HD shown in Fig. 5A, a conventional screen pattern data is prepared as shown in Fig. 1 which consists of 25 data (5 × 5 = 25), while a high density screen pattern data consisting of 100 data D ij (i, j = 1 ∼ 5), (5 × 2) × (5 × 2) = 25 × 4 = 100, is prepared and stored in the embodiment, as shown in Fig. 6. Namely, four data in a square G in Fig. 6 are prepared for each of a unit area corresponding to the light spot diameter d. In Fig. 6, the symbol X and Y represent a main scanning direction and a subscanning direction, respectively. For the covenience of explanation, a screen angle is set as 0° and the screen pitch is set as K. Values of the screen pattern data D ij is determined by a mathematical function expressing a screen pattern. In other words, when the main scanning coordinates is expressed as X and the subscanning coordinates as Y, and an element area B ij (i.j = 1 --10) obtained by halving vertically and horizontally each unit area A ij shown in Fig. 5A which has a size according to the light spot diameter d, is considered (refer to Fig. 5B), X and Y coordinates of the center position P ij of these element areas A ij are substituted into a prescribed two dimensional screen pattern function D (X, Y) to obtain value of each data D ij . This screen pattern function D (X, Y) may be determined according to a desired standard gradation reproduce character and may be determined by using an arbitrary function (a trigonometrical function, for example). The value of the high density screen pattern data D ij may be determined by interpolating the conventional density screen pattern data shown in Fig. 1 for example. The unit area A ij having a unit area size a is halved vertically and horizontally to form an element area B ij in this embodiment. In general, the unit area A ij may be divided into n columns aligned along the main scanning direction and further divided into m rows aligned along the subscanning direction thereby to obtain the (n × m) element areas B ij whose size is smaller than the light spot diameter d in the main scanning direction and/or subscanning direction. These numbers n and m are a set of positive interger at least one of which is equal to or greater than two. These numbers n and m may be an arbitrary positive number at least one of which is greater than one. Now back to Fig. 4, in an address input ADRS of the screen pattern memory 23 which stores the high density screen pattern data D ij , an output A d of address counter 24 is provided. The screen pattern data D O (= D ij ) which is outputted in time series from the screen pattern memory 23 is latched by F/F 25 to be a screen pattern signal D given to an input B of the comparator 22. The comparator 22 outputs a signal S₂ which becomes a level "H" only when the input B is greater than the input A, and this signal S₂ is latched by F/F 26 and becomes an exposure output signal S. An output synchronizing signal generator 27 is a circuit which inputs first clock signal  ₁ which is synchronized with the input timing of the digital image data S O and outputs a second clock signal (output synchronizing signal)  ₂ which has a pulse frequency of two times that of the first synchronizing signal  ₁. In general, the pulse frequency of the second clock signal  ₂ may be n times of that of the first clock signal  ₁, where the number n is a positive number greater than one. When a scanning speed is expressed as v, the first clock signal  ₁ is a pulse signal having repeat frequency f expressed by the following formula; f = (d/v)⁻¹ (3) whereas the repeated frequency F of the second clock signal  ₂ is expressed by the following formula as; F = 2f (4) The second clock signal  ₂ is used as an input pulse of the address counter 24 and is also used as the latching signal of F/F 21, 25, and 26. Operations of the circuit shown in Fig. 4 will now be described with reference to a timing chart shown in Fig. 7, in which time delay in latching operation or the like is ignored in Fig. 7. The first clock signal  ₁ (Fig. 7(a)) which is synchronized with the input timing of the digital image data S O is supplied to the output synchronizing generator 27, thereby the second clock signal  ₂ (Fig. 7(b)) is generated. In synchronism with the second clock signal  ₂, the digital image data S O is latched by F/F 21 to generate the image signal S₁ in time series. In this example, a case where the digital image data S O (Fig. 7(c)) is a constant level data is considered, thereby the image signal S₁ (Fig. 7 (e)) also becomes a constant level signal. The digital image data S O may be a variable data. Further, the digital image data S O is not necessarily generated in synchronism with the first clock signal  ₁ but may be generated in synchronism with a signal corresponding to more rough sampling pitch. On the other hand, by counting the second clock signal  ₂, the address counter 24 generates an address signal A d which sequencially access to the screen pattern data D ij of Fig. 6 in the memory 23, while repeating from D₁₁ to D 1A . By this operation, the pattern data D O (= D ij ) shown in Fig. 7 (d) is repeatedly read out from the screen pattern memory 23 with a period of one halftone dot to be latched by F/F 25 thereby to become a periodic screen pattern signal D (Fig. 7(e)). Respective levels of the image signal S₁ and the screen pattern signal D are transferred to a comparator 22 through transfer lines and compared by the comparator 22 for every repeated cycle of the second clock signal ₂ and the signal S₂ which becomes the level "H" only when D is greater than S₁ is outputted. For this reason, the exposure output signal S obtained by latching the signal S₂ by F/F 26 has a waveform shown in Fig. 7 (f). In the example shown in Fig. 7(f), an exposure output signal S which becomes "H" in a time range of t₁<t<t₂ is obtained. On the other hand. the scanning speed v of the laser beam L in the main scanning direction is so selected that the main scanning advances by the distance according to the light spot diameter d for every pulse period T₁ of the first clock signal ₁, as shown in Fig. 7(a). In other words, a time required for the scanning to advance by the light spot diameter d is T₁. The screen pattern signal D is a high density signal whose level changes for every pulse period T₂ (= T₁/2) of the second clock signal ₂ (that is, for every scanning distance of half the light spot diameter d). The level changing interval of the screen pattern signal D is shorter than the scanning distance according to the light spot diameter d and the high density screen pattern signal D is generated. The comparison by the comparator 22 and the intensity change control (modulation control) of the laser beam L based on the comparison are accomplished at the high density having a period of T₂. Therefore, as shown in Fig. 8 (a), the spot center of the laser beam L is located in positions P₁ and P₂ when the intensity of the light beam L is changed at the times t₁ and t₂, respectively, and the distance Δ R between the intensity changing positions P₁ and P₂ is integral multiples of half the light spot diameter d. In case of a conventional apparatus, as shown in Fig. 8 (b), the screen pattern data for every scanning distance (unit area size a ) are prepared according to the light spot diameter d as shown in Fig. 1 and respective signal processings are caused in synchronizm with the first clock signal ₁ only. Therefore, ON/OFF intensity change control of the laser beam is accomplished each time the scanning advances by the light spot diameter d, and the distance Δ R between intensity changing positions P₁ and P₂ becomes interger multiples of the light spot diameter d. In the present embodiment, the recording density of the main scanning direction is substantially two times the conventional recording density, when the same light spot diameter d is employed. Almost the same processing is accomplished with respect to the subscanning direction, but unlike the case of the main scanning direction, the scanning speed (that is, the moving speed of the photosensitive material 17 in (-Y) direction) is taken as a half the conventional moving speed (generaly, it is (1/m) times of the conventional moving speed where m is a positive number greater than one). In other words, the adjoining or mutually neighbouring scanning lines are overlapped with each other by the half thereof as shown in Fig. 9 (a), and each line D ij , D 2j , .... shown in Fig. 6 is correlated with each scanning line, so that exposures with high density are accomplished in the subscanning direction. The scanning lines are substantially defined on the photosensitive surface of the photosensitive material 17 by the main scanning, and the laser beam L is scanned along the scanning line. In this case, with respect to the digital image data S O of inserted scanning lines (ℓ₁₂, ℓ₂₃, .... of Fig. 9(a)) for example, the digital image data which is obtained by interpolating the digital image data for the scanning lines ℓ₁, ℓ₂, .... aligned with a interval of the light spot diameter d may be used. An interpolation for the inserted scanning lines is performed along the subscanning direction. In this embodiment, an interpolation circuit (not shown in Fig. 2) is provided in the preceding stage of the halftone dot signal generating circuit 1 for the interpolation of the digital image data. With respect to other embodiments discribed later, the interpolation circuit is provided as long as the improvements according to the present invention are employed in connection with the subscanning direction. The same image data for the scanning lines ℓ₁ and ℓ₂ may be repeatedly used without the interpolation. High resolution image data also be used and furthermore, it is possible to use the same image data for more than two scanning lines. In case the present invention is applied to the subscanning direction, the screen pattern signal D is increased in its density by causing its level to change along the subscanning direction for every distance (d/2) which is shorter than the subscanning distance defined by the light spot diameter d. In accordance with an increase in the density of scanning lines, intensity change control frequency (control density) of the laser beam L is also increased. Fig. 9(b) represents an example of the exposure pattern of a conventional apparatus, wherein exposures are accomplished by causing the levels of the screen pattern signal to change for every light spot diameter d in all scanning directions. An example of a halftone dot image exposed in above manner is shown schematically in Fig. 5C. As understood by the comparion with the case shown in Fig. 1, the exposure pattern of this embodiment is a fine pattern and that the reproducibility of gradation is substantially improved. Furthermore, an exposure pattern substantialy equivalent to the case where the exposure control is accomplished for each element are B ij is obtained, it is possible to use an arbitrary interger multiples (but equal to or greater two times) of half the light spot diameter d as the screen pitch, thereby improving the degree of feedom in screen pitch selection (thus the number of screen lines). In case that the number of division (n × m) of the unit area A ij is to be variable number depending upon a factor such as the screen size and the light spot diameter, screen patterns of different number of divisions may be prepared separately and used selectively. Alternatedly, only a screen pattern having the greatest number of divisions may be prepared, and in this case, a screen pattern of lower division number may be obtained by skipping the data in the screen pattern of the greatest number of divisions. In later case, there is an advantage that a necessary memory capacity required for a storage of screen pattern data is reduced. In the first embodiment, even if the number of dividions is increased, it is only necessary to increase the data for the screen pattern, which in turn provides a merit where the construction of the circuits is not complicated. C. Detail of construction and operation of the second embodiment Fig. 10A through Fig. 10°C show inner construction of the halftone dot signal generating circuit 1 which are used in the second embodiment of the present invention. In the following description, portions which are different form the first embodiment will be described mainly. In the second embodiment, the same screen pattern data whose valves are given for each unit area A ij similarily to the conventional apparatus (data shown in Fig. 2 for example) are stored in the screen pattern memory 23 or Fig. 10A. The address counter 31 counts the first clock signal ₁ so as to generate an address signal A d ′. Therefore, the screen pattern data D O is outputted from the screen pattern memory 23 in synchronism with the first clock signal ₁ and this data D O is outputted to a subscanning direction interpolation circuit 32. The data D O is interpolated along the subscanning direction by the operation described later to obtain an interpolated screen pattern data D₁. The data D₁ is supplied to a main scanning interpolation circuit 33 for every scanning line to be interpolated along the main scanning direction in the interpolation circuit 33, thereby to obtain a screen pattern signal D interpolated along the main scanning and the subscanning directions. The signal D is given to the comparator 22 in the same manner as the first embodiment. In the following description, an example in which a distance equivalent to the light spot diameter d is divided into n parts in both of the main scanning direction and the subscanning direction will be described. The second clock signal  ₂ has a pulse frequency of n times the pulse frequency of the first clock signal  ₁. The interpolation circuits 32, 33 and a selection timing generating circuit 34 and the like described later operate in synchronism with the second clock signal  ₂. In the subscanning interpolation circuit 32, a switching circuit 35 in Fig. 10B alternately selects line memory 36a and 36b in response to a selection signal SL generated in the selection timing generating circuit 34, and the screen pattern data D O for each one line is transferred from the screen pattern memory 23 sequentially along the subscanning direction to be alternately stored in the line memory 36a and 36b. Each time the screen pattern data D O for one line is stored in these line memory 36a or 36b, the screen pattern data D ij , D (i+1)j of the unit areas having the same main scanning coordinates are read out from the memory 36a and 36b, respectively, along the main scanning direction. The data read out from the memory 36a and 36b are expressed as data D a and D b , respectively. The data D a and D b are processed by a multiplier group 37 and an adder group 38 to obtain (n+1) number of data interpolating the data D ij , D i(i+1) through the following formula, which are supplied to the selector 39. [(n-k) D a + kD b ] / n (k = 0.1, ...., n) (5) The (n+1) number of input of the selector 38 are sequentially selected in response to the selection signal SL and the screen pattern data are outputted along the main scanning direction from the line memory 36a and 36b in each selected condition. The interpolated data of each interpolation line is sequentially outputted as the data D₁ from the selector 39. In the main scanning direction interpolation circuit 33 of the following stage (Fig. 10C), the data D₁ inputted along the main scanning direction is supplied to the delay circuit 41 and is delayed in this delay circuit 41 by one clock of the second clock signal  ₂ to obtain the data D₂. The data D₁ inputted next and the data D₂ are operated by an adder group 42 and a multiplier group 43 to obtain (n+1) number of data by interpolating the data D₁ and D₂ in the main scanning direction through the following formula: [(n-k) D₂ + kD₁] / n (k = 0, 1, ..., n) (6) These data are given to a selector 44 and are sequentially selected and outputted in response to a selection signal SL, thereby to obtain the screen pattern signal in time series corresponding to the screen pattern data which is interpolated in each scanning direction. Fig. 11(b) shows the interpolated data obtained by interpolating the data Fig. 11(a) with respect to the case n = 2. While repeating the above operations, the screen pattern signal generated periodically are processed similarily to the first embodiment. Levels of the screen pattern signal D change for each time interval which is one n-th of the pulse period T₁ of the first clock signal ₁. The feed speed of the photosensitive material 17 in the subscanning direction is taken as one n-th of the convertional feed speed where the interpolation is not performed. The level change interval of the screen pattern signal and the interval of the intensity change control of the laser beam L become one n-th of the light spot diameter d in each scanning direction and the frequencies (densities) of the level change and the control is increased. Fig. 12 shows an of exposed pattern in a case of n=4. In the second embodiment, there is an advantage where the memory capacity of the screen pattern memory 23 may be less than that of the first embodiment. When the subscanning direction interpolation circuit 32 is omitted in the circuit shown in Fig. 10, interpolation will be accomplished only in the main scanning direction and the screen pattern signal D which corresponds to the screen pattern data shown in Fig. 13(a) is obtained. If the main scanning direction interpolation circuit 33 is omitted, only the interpolation in the subscanning direction is accomplished and the screen pattern signal D which corresponds to the data shown in Fig. 13(b) is obtained. In these cases, only with respect to the scanning direction along which the interpolation is performed, an interval of the level change of the screen pattern signal D and an interval of the intensity change control of the laser beam L are shorter than the scanning interval corresponding to the light spot diameter d, and the density thereof are increased accordingly. Fig. 14 (a) shows an exemplary exposure pattern when the interpolation is accomplished only in the main scanning direction and Fig. 14 (b) shows an exemplary exposure pattern when the interpolation is accomplished only in the subscanning direction. Instead of the interpolation circuit 32 and 33 in the second embodiment, a digital filter which smoothes the time series digital signals may be employed. D. Detail of construction and operation of the third embodiment Fig. 15 shows an internal construction of the halftone dot signal generating circuit in the third embodiment. In this embodiment, a periodic triangular wave oscillator 50 which generates analog screen pattern signal D (Fig. 16 (d)) is provided. Digital image data S O (Fig. 16 (a)) is converted to analog image signal S₁ (Fig. 16 (d)) by D/A converter 51. Levels of these signals D and S₁ are compared with each other by the analog comparator 52 to obtain the exposure output signal S (Fig. 16 (e)). This exposure output signal S is a bi-level signal of "H" and "L" which does not synchronize with the clock signals. In the triangular wave oscillator 50 shown in Fig 15, line feed signal ₃ synchronized with the scanning operation is given from an encoder provided in the main scanning drive mechanism (not shown) to move the photosensitive material 17 in the main scanning direction. This line feed signal ₃ is given to PLL circuit 53. A signal which is changed in synchromism with the beam scanning is given to a pulse generating circuit 54 from PLL circuit 53. The pulse generating circuit 54 generates a bipolar pulse S₁₀ shown in Fig. 16(b) in response to the signal supplied from the PLL circuit 53. The repeating period T a of the bipolar pulse S₁₀ specify a main scanning time required for scanning by the length of one halftone dot and changes according to the pulse interval of the line feed signal ₃. The bipolar pulse S₁₀ is given to an integration circuit 55 and integrated in time. Therefore, when the positive section C P and the negative section C N shown in Fig. 16 (b) are symmetrical with each other, a triangular wave S T1 shown in Fig. 16 (c) can be obtained as the result of the integration. The triangular wave S T1 is amplified by the variable gain amplifier 56 to become the analog screen pattern data D shown in Fig. 16 (d). The analog screen pattern data D is a high density data whose level changes for each interval of substantial zero time (therefore, zero scanning distance) and has no quantization error. For this reason, there is no restriction to the level changing timing of the exposure output signal S outputted from the comparator 52 and the intensity change control of the laser beam L is performed continuously (that is, at substantially infinite density or at zero time interval). The exposure interval Δ R (Fig. 17) can have an arbitrary value and the gradation number can be substantially infinite. The screen pitch can also be arbitrary (but should be equal to or greater than d). In the circuit shown in Fig. 15, the level of the screen pattern data D is changed for each scanning line to obtain a two dimensional halftone dot structure. More specifically, a scanning line currently scanned is found by couting the line feed signal ₃ by a line counter 57. The line counter 57 outputs to the amplifier 56 a signal S c whose level changes periodically according to the value of counts. The amplifier 56 is so constructed that its gain is continuously variable in response to the level of the signal S c . For this reason, the number of scanning line required for scanning an area defined by one halftone dot is set as the preset value m O of the line counter 57, so that the screen pattern data D whose level changes periodically for each scanning line as shown in Fig. 18 is obtained, where the period of the level change expressed by a number of main scanning lines is m 0 . Instead of causing the level change shown in Fig. 18, the level change may be caused by uniformly enhancing or depressing the overall level of the screen pattern data D. E. Detail of construction and operation of the fourth embodiment Fig. 19 is a diagram showing fourth embodiment. In this embodiment, the screen pattern data D ij is stored in the screen pattern memory 23 for each unit area A ij corresponding to the light spot diameter d shown in Fig. 2. After the data D ij is periodically read out in synchronism with the clock signal ₁ and converted into an intermediate analog signal D I by D/A converter 61. High frequency components of the intermediate signal D I is removed by the analog low-pass filter 62 to accomplish low-pass filtering. The screen pattern signal D obtained by the above becomes the smooth data as shown in Fig. 20 (a). Therefore, by comparing the level of an analog image signal S₁ obtained through D/A converter 51 and the level of the screen pattern signal D, the exposure output signal S is obtained from an analog comparator 52 as shown in Fig. 20 (b). In this embodiment, both of an interval at which the level of the screen pattern signal D changes and an interval at which the intensity controls of the laser beam L are made shorter than the light spot diameter d to have high density, and the gradation reproducibility of the recorded image is enhanced. This is due to the same reason as that of the third embodiment and therefore the redundant description is omitted. F. Examples of other versions Although it is desirable to apply the present invention to the main scanning direction and the subscanning direction, the present invention may be applied to only one scanning direction. When applied to the main scanning direction only, because it is not necessary to change the feed speed of the subscanning operation, there is an effect where the recording speed does not decreased. In each of the above embodiment, a square halftone dot of 0° screen angle is considered, but the present invention can also be applied to the screen structure other than the above. Further, when the cross section of the light beam for exposure is not a circle, the diameter of the beam spot in the scanning direction to which the present invention is applied is considered to be the light spot diameter d. Therefore, when the present invention is applied to both of the main scanning direction and the subscanning direction, the light spot diameters for both directions may be not identical to each other. When the present invention is applied to the subscanning direction, a plurality of light beam for exposure may be prepared and multiple exposure may be accomplished by causing these light spots to overlap partially on the photosensitive material. In such a case, the scanning line ℓ₁ of Fig. 7 (a) can be exposed by the first beam and ℓ₁₂ by the second beam, respectively. In Fig. 7, the digital image data S 0 is in synchronism with the first clock signal ₁. However, the digital image data S O may be in synchronism with the second clock signal ₁, or a clock which has larger pulse period than that of the first clock signal ₁. The latter is employed in a rotary drum type scanner, because the data sampled by about 1/2.5 pitch of the halftone dot is used as the digital image data in the rotary drum type scanner. Therefore, the input timing of the digital image data may be arbitrary as long as it is synchronized with the record scanning and the number n of division in the main scanning direction can take an arbitrary positive number. In other words, the pulse repeat periods of the first and second clock signals ₁ and ₂ may be set arbitrarily, as long as their ratio is a positive number and they are synchronized with each other. The present invention can be applied not only to the flat bed type scanner but also to various other halftone dot image recording apparatus such as rotary drum type scanner or stationary drum type (inner surface scanning type) scanner. As have been described above, according to the present invention, because it is possible to change the intensity of the light beam for exposure in every pitch smaller than the light spot diameter of the light beam, the gradation reproducibility of the recorded image is increased even in case the ratio of the screen pitch and the light spot diameter of the light beam is not relatively large. Also, because it is possible to reduce the intensity changing pitch of the light beam, the degree of freedom in screen pitch selection is increased.

Claims: An apparatus for recording halftone dot images on a photosensitive material on the basis of a digital image data, said apparatus comprising: (a) image signal generating means for generating image signal on the basis of said digital image data, (b) screen pattern signal generating means for generating a periodic screen pattern signal, (c) comparing means for comparing a level of said image signal with a level of said screen pattern signal, (d) light beam generating means for generating a light beam to be irradiated on a photosensitive surface of said photosensitive material, said light beam having a prescribed light spot diameter on said photosensitive surface of said photosensitive material, (e) intensity modulating means for modulating an intensity of said light beam according to a result of comparison in said comparing means, and (f) scanning means for relatively moving said light beam and said photosensitive material thereby to scan said photosensitive surface by said light beam, said screen pattern signal generating means having; (b-1) high density screen pattern signal generating means for generating a high density screen pattern signal whose level is changed at predetermined time intervals shorter than a time interval in which scanning by said scanning means advances by said light spot diameter, and (b-2) high density screen pattern signal transfer means for transferring said high density screen pattern signal to said comparing means as said screen pattern signal. An apparatus according to claim 1, wherein said high density screen pattern signal generating means has; (b-11) memory means for storing a prescribed screen pattern data provided with a value for each element area on said photosensitive surface having a size smaller than said light spot diameter, and (b-12) reading means for reading out said screen pattern data from said memory means to generate said high density screen pattern signal on the basis of said screen pattern data. An apparatus according to claim 2, wherein relative movements of said light beam and said photosensitive material by said scanning means include a relative movement in first direction for main scanning and a movement in second direction for subscanning, a scanning line on said photosensitive surface along which said light beam is scanned is substantially defined by the relative movement in said first direction, a size of said element area in the main scanning direction is smaller than said light spot diameter, and said intensity of said light beam is modulated at time intervals shorter than a time interval in which the light beam is advanced by said light spot diameter along said scanning line. An apparatus according to claim 3, wherein a size of said element area in said subscanning direction is smaller than said light spot diameter, and the relative movement in said second direction is carried out so that mutually neighbouring scanning lines overlap partially with each other. An apparatus according to claim 1, wherein said high density screen pattern signal generating means comprises; (b-21) memory means for storing a prescribed screen pattern data provided with a value for each unit area of said photosensitive surface whose size corresponds to a size of said light spot diameter, and (b-22) interpolating means for reading out said screen pattern data from said memory means to interpolate said screen pattern data, thereby to generate said high density screen pattern signal. An apparatus according to claim 5, wherein relative movements of said light beam and said photosensitive material by said scanning means include a relative movement in first direction for main scanning and a relative movement in second direction for subscanning, a scanning line on said photosensitive surface along which said light beam is scanned is substantially difined by the relative movement in said first direction, said interpolating means comprises: a main scanning direction interpolating means for interpolating said screen pattern data with respect to the main scanning direction, and subscanning direction interpolating means for interpolating said screen pattern data with respect to the subscanning direction, said intensity of said light beam is modulated at time intervals shorter than a time interval in which the light beam is advanced by said light spot diameter along said scanning line, and the relative movement in said second direction is carried out so that mutually neighbouring scanning lines overlap partially with each other. An apparatus according to claim 1, wherein said image signal generating means comprises: (a-1) D/A conversion means for converting said digital image data into an analog image signal as said image signal, and said high density screen pattern signal generating means includes: (b-31) analog screen pattern signal generating means for generating analog screen pattern signal as said high density screen pattern signal. An apparatus according to claim 7, wherein relative movements of said light beam and said photosensitive material by means of said scanning means include movement in first direction for main scanning and a relative movement in second direction for subscanning, a scanning line on said photosensitive surface along which said light beam is scanned is substantially defined by the relative movement in said first direction, said analog screen pattern signal generating means comprises: analog oscillation means for generating a periodic analog signal, amplifying means for amplifying said periodic analog signal, and means for changing a gain of said amplifying means depending on a scanning line on which said main scanning is currently carried out by said light beam, said analog screen pattern signal being obtained from an output of said amplifying means. An apparatus according to claim 7, wherein said analog screen pattern signal generating means comprises: memory means for storing a prescribed digital screen pattern data provided with a value for each unit area on said photosensitive surface which corresponds to a size of said light spot diameter, D/A conversion means for sequentially reading out said digital screen pattern data from said memory means to convert said digital screen pattern data into an analog intermediate signal, and a low pass filter for carrying out low-pass filtering of said analog intermediate signal, said analog screen pattern signal being generated on the basis of an output of said low-pass filter. A method for recording halftone dot image on a photosensitive material on the basis of a digital image data, said method comprising the steps of: (a) generating a image signal on the basis of said digital image data, (b) generating a periodic screen pattern signal, (c) comparing a level of said image signal and a level of said screen pattern signal, (d) generating a light beam to be irradiated on a photosensitive surface of said photosensitive material, said light beam having a prescribed light spot diameter on said photosensitive surface of said photosensitive material, (e) modulating an intensity of said light beam according to a result of comparison in said step (c), and (f) relatively moving said light beam and said photosensitive material to scan said photosensitive surface by said light beam, said step (b) including the steps of: (b-1) generating a high density screen pattern signal whose level changes at predetermined time intervals shorter than a time interval in which scanning by said step (f) is advanced by said light spot diameter, and (b-2) defining said screen pattern signal for said step (c) by said high density screen pattern signal. A method according to claim 10 wherein the step of (b-1) comprises a step of: (b-11) generating said high density screen pattern signal on the basis of prescribed screen pattern data provided with a value for each element area on said photosensitive surface which is smaller than said light spot diameter. A method according to claim 11, wherein relative movements of said light beam and said photosensitive material by said scanning means include a relative movement in first direction for main scanning and a relative movement in second direction for subscanning, the scanning line on said photosensitive surface along which said light beam is scanned is substantially defined by the relative movement in said first direction, a size of the said element area in the main scanning direction is smaller than said light spot diameter, and said intensity of said light beam is modulated at time intervals shorter than a time interval in which the light beam is advanced by said light spot diameter along said scanning line. A method according to claim 12, wherein a size of said element area in said subscanning direction is also smaller than said light spot diameter, and the relative movement in said second direction is carried out so that mutually neighbouring scanning lines overlap partially with each other. A method according to claim 10, wherein said step of (b-1) comprises a step of: interpolating a prescribed screen pattern data provided with a value for each unit area on said photosensitive surface whose size corresponds to said light spot diameter thereby to generate said high density screen pattern signal. A method according to claim 10, wherein said step (a) comprises a step of: (a-1) converting said digital image data into an analog image signal, said image signal is said analog image signal, and the step (b-1) includes a step of: (b-31) generating an analog screen pattern signal as said high density screen pattern signal. A method according to claim 15, wherein relative movements of said light beam and said photosensitive material by said scanning means include a relative movement in first direction for main scanning and a relative movement in second direction for subscanning, a scanning line on said photosensitive surface along which said light beam is scanned is substantially defined by the relative movement in said first direction, said step (b-31) comprises the steps of: generating a periodic analog signal, amplifying said periodic analog signal by amplifying means, and changing a gain of said amplifying means depending on a scanning line currently scanned by said light beam, said analog screen pattern signal being obtained from said amplifying means. A method according to claim 15, wherein said step (b-31) includes the steps of: storing in memory means a prescribed digital screen pattern data provided with a value for each unit element on said photosensitive surface whose size corresponds to said light spot diameter, sequentially reading out said digital screen pattern data from said memory means to convert said digital screen pattern data into analog intermediate signal, and carrying out low-pass filtering of said analog intermediate signal thereby to obtain said analog screen pattern signal.

Inventors: SHIMANO, NORIYUKI DAINIPPON SCREEN MFG. CO. LTD.
Applicants: DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1988
Main classification: H04N1
IPC codes: H04N1, G06T5, G03F3