Abstract:
A digital image signal is read out at a predetermined picture element density by causing a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed while moving the recording medium in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, thereby two-dimensionally scanning the recording medium with the light beam, photoelectrically detecting signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, sampling the analog image signal at a predetermined intervals, and quantizing the sampled values. A digital image signal at a desired picture element density different from the predetermined picture element density is read out by changing the sub-scanning speed to m(&gt;0) times the predetermined sub-scanning speed, and changing the intervals at which the analog image signal is sampled to intervals n(&gt;0) times the predetermined intervals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of and a system for reading out image signal, and more particularly to a method of and a system for reading out image signal in which the density of picture elements of the read-out image signal can be changed. 
     2. Description of the Related Art 
     When certain kinds of phosphors are exposed to a radiation such as X-rays, α-rays, β-rays, γ-rays, cathode rays or ultraviolet rays, they store a part of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been known a radiation image recording and reproducing system in which a sheet provided with a layer of the stimulable phosphor (will be referred to as “a stimulable phosphor sheet”, hereinbelow) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet so that the stimulable phosphor sheet emits stimulated emission as signal light bearing thereon information on the radiation image, the stimulated emission is photoelectrically detected, thereby obtaining an analog image signal, the analog image signal is sampled at predetermined intervals and quantized, thereby obtaining a digital image signal at a predetermined picture element density, and the radiation image of the object is reproduced as a visible image on the basis of the digital image signal on a recording medium such as a photographic film or a display such as a CRT. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395 and 56(1981)-11397. 
     This system is advantageous over a conventional radiography system using silver salt film in that an image can be recorded over a very wide radiation exposure range. 
     As a system for reading out the stimulated emission, there has been proposed a radiation image read-out system in which a photoelectric read-out means is disposed on each side of the stimulable phosphor sheet, stimulating light is projected onto one side or both sides of the stimulable phosphor sheet and the stimulated emission from both sides of the stimulable phosphor sheet is detected by each of the photoelectric read-out means. See, for instance, Japanese Unexamined Patent Publication No. 55(1980)-87970. In such a radiation image read-out system, since a single radiation image is stored in a stimulable phosphor sheet and a pair of photoelectric read-out means are disposed to detect the stimulated emission from both sides of the stimulable phosphor sheet, light collecting efficiency is improved, and by adding the obtained two image signals at a predetermined ratio of addition, positions in which noise components are detected differ by the sides of the stimulable phosphor sheet and accordingly, an addition signal which is improved in S/N ratio as compared with an image signal obtained from only one side can be obtained. 
     Further there has been proposed a method of superposing images in which an addition image signal is obtained after carrying out filtering processing, using a filter having frequency response properties such as will increase the S/N ratio of an image signal (including an addition signal), on a single image signal obtained from one side of the stimulable phosphor sheet or two image signals obtained from opposite sides of the same. (Japanese Unexamined Patent Publication No. 7(1995)-287330) In accordance with this method, since the amount of exposure to the radiation to which the object is exposed upon taking the radiation image is obtained and the parameter (the coefficient of filter) which is used for carrying out filtering processing is obtained on the basis of the amount of exposure to the radiation, an image signal representing an image of optimal quality or an addition image signal representing a superposed image of optimal quality can be obtained according to the amount of exposure to the radiation. Further, since processing to change the frequency characteristics is carried out on the overall image signal, it becomes unnecessary to effect frequency transformation such as Fourier transformation and the amount of computation can be reduced. 
     There has been a demand toward changing the density of picture elements of a single image signal or an addition signal in the image read-out section of the aforesaid radiation image recording and reproducing system or in the radiation image read-out system. 
     For example, when a large number of radiation images are taken as in a group examination, there is a demand toward increasing the radiation image read-out speed while the quality of the images to be reproduced need not be so high provided that whether reexamination is necessary can be judged. In such a case, the images need not be read out at a high picture element density. Conversely, there is a case where the image should be read out at a high picture element density so that the image can be reproduced at a high quality even if the read-out speed is lowered. 
     To read out the image at a picture element density other than the preset picture element density may be realized by simply changing the main scanning speed and the sub-scanning speed of the stimulating light beam. The main scanning speed of the stimulating light beam can be changed by changing the driving speed of the scanning optical system for causing the stimulating light beam to scan the stimulable phosphor sheet (e.g., a polygonal mirror or a galvanometer mirror). 
     However, when the driving speed of the scanning optical system is changed, it takes a certain time for the driving to be stabilized due to inertia of the optical system, and the optical system cannot be constantly stably driven over the entire driving speed range. Accordingly, it is preferred that the picture element density be changed without changing the main scanning speed of the stimulating light beam. Further when the picture element density is to be changed, it is necessary to change the picture element density in the sub-scanning direction in the same proportion as the picture element density in the main scanning direction. 
     Further when the picture element density is to be changed, it is preferred that the picture element density changing processing be carried out at a speed as high as possible. 
     Further, when the picture element density is simply changed, there is a possibility that the following problems arise. 
     That is, 
     1) When the picture element density is changed, energy of signal light emitted from the stimulable phosphor sheet per one picture element differs from that for the original picture element density. Accordingly, when the difference between the changed picture element density and the original picture element density is large, the density (or brightness) of the overall image to be reproduced can be changed, which can adversely affect diagnostic performance of the image. 
     2) When shading correction is to be carried out on the analog image signal, properties of shading to be corrected varies before and after the picture element density change and the shading sometimes cannot be properly corrected. 
     3) When the analog image signal is to be logarithmically amplified, the frequency transfer properties can vary before and after the picture element density change. 
     4) When filtering for removing aliasing noise is to be carried out prior to sampling the analog image signal, aliasing noise sometimes cannot be properly cut since Nyquist frequency varies before and after the picture element density change. 
     These problems arise not only when an image signal representing a radiation image is read out from a stimulable phosphor sheet but also, for instance, when a medium on which an image is printed is scanned by a light beam and light reflected from the medium according to the image printed thereon is read out as signal light. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and system for reading out an image signal in which the picture element density can be changed without changing the main scanning speed of the read-out light beam. 
     Another object of the present invention is to provide a method of and system for reading out an image signal from signal light emitted from both sides of a recording medium in which the picture element density can be changed without changing the main scanning speed of the read-out light beam. 
     The method of and the system for reading out an image signal in accordance with the present invention are characterized in that a digital image signal having a desired picture element density is obtained without changing the main scanning speed by changing the sub-scanning speed and the sampling intervals. 
     That is, in accordance with a first aspect of the present invention, there is provided a method of obtaining, in a method of reading out a digital image signal at a predetermined picture element density by causing a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed while moving the recording medium in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, thereby two-dimensionally scanning the recording medium with the light beam, photoelectrically detecting signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, sampling the analog image signal at a predetermined intervals, and quantizing the sampled values, a digital image signal at a desired picture element density different from the predetermined picture element density, the method comprising the steps of 
     changing the sub-scanning speed to m(&gt;0) times said predetermined sub-scanning speed, and changing the intervals at which the analog image signal is sampled to intervals n(&gt;0) times said predetermined intervals. 
     In a preferred embodiment, the method further comprises a step of carrying out, on the digital image signal, picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a&gt;0) times and the number of the picture elements in the sub-scanning direction to a/n times. 
     In accordance with a second aspect of the present invention, there is provided a method of obtaining, in a method of obtaining an addition image signal at a predetermined picture element density by causing a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed while moving the recording medium in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, thereby two-dimensionally scanning the recording medium with the light beam, photoelectrically detecting signal light emitted from both sides of the recording medium upon exposure to the light beam to obtain two analog image signals, sampling the analog image signals at a predetermined intervals, quantizing the sampled values, and adding two digital image signals thus obtained, an addition image signal at a desired picture element density different from the predetermined picture element density, the method comprising the steps of 
     changing the sub-scanning speed to m(&gt;0) times said predetermined sub-scanning speed, and changing the intervals at which the analog image signal is sampled to intervals n(&gt;0) times said predetermined intervals, thereby obtaining two intermediate digital image signals, and carrying out, on the intermediate digital image signals, picture element density changing processing for changing the number of the picture elements in the main scanning direction to aim (a&gt;0) times and the number of the picture elements in the sub-scanning direction to a/n times. 
     In the methods of the first and second aspects of the present invention, the recording medium may be a stimulable phosphor sheet used in the aforesaid radiation image recording and reproducing system as well as a reflection original such as a photographic print and a transparent original such as a photographic film. Accordingly, the signal light emitted from the recording medium includes stimulated emission emitted from a stimulable phosphor sheet upon exposure to the light beam, reflected light reflected by a reflection original, and transmitted light from a transparent original. 
     As a method of changing the sampling intervals, a method in which a clock frequency-divided from a reference clock for determining said predetermined sampling timings are made and the analog image signal is sampled on the basis of the frequency-divided clock, a method in which a new clock different from the reference clock in cycles are made by use of a PLL and the analog image signal is sampled on the basis of the new clock, or a method in which a plurality of sampling clocks which are different from each other in cycles are prepared and the analog image signal is sampled on the basis of one of the clocks may be employed. 
     As a method of obtaining the addition image signal, as well as a method in which the image signal components of the two digital image signals for the corresponding picture elements are added together, a method in which the two digital image signals are added together after they are subjected to filtering processing by use of a filter having frequency response properties such as to increase the S/N ratio of the addition image signal as disclosed, for instance, in Japanese Unexamined Patent Publication No. 7(1995)-287330 may be employed. 
     The picture element density changing processing may be carried out, for instance, by effecting one-dimensional mask operation in the main scanning direction of the image signal, by thinning picture elements according to the desired picture element density, by high-order interpolation such as B-spline interpolation or cubic spline interpolation (disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 8(1996)-16767 and 9(1997)-321981), or by linear interpolation (disclosed, for instance, in Japanese Unexamined Patent Publication No. 9(1997)-50516). 
     It is preferred that a parameter for the one-dimensional mask operation, a parameter for thinning the picture elements or a parameter of an operation expression for the interpolation be changed according to the values of m and n. The parameter for the one-dimensional mask operation is a coefficient of mask, and the parameter for thinning the picture elements is the intervals at which the picture elements are thinned. The parameter of the operation expression for the interpolation represents the operation expression to be employed in the interpolation (e.g., B-spline interpolation, Cubic spline interpolation or linear interpolation). 
     In the method of the second aspect of the present invention, it is preferred that the picture element density changing processing be carried out on said two intermediate digital image signals before they are added together. 
     Further in the methods of the first and second aspects of the present invention, it is preferred that at least one of the following properties be changed according to the values of said m and n. 
     (1) The beam diameter of the light beam. 
     (2) The power of the light beam. 
     (3) The sensitivity of detecting the signal light. 
     (4) The preset data for shading correction when shading correction is to be carried out on the analog image signal. 
     (5) The timing at which the data for shading correction is output from a memory. 
     (6) The frequency transfer properties when the analog image signal is to be logarithmically amplified. 
     (7) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal. 
     (8) The parameter for filtering processing in the picture element density changing processing (including the parameter for thinning the picture elements when thinning processing is carried out as the filtering processing). 
     It is not necessary that all of the items (1) to (8) are changed according to the values of said m and n, but at least one of the items (1) to (8) may be changed. For example, when the shading correction need not be carried out, the items (4) and (5) need not be included. 
     The items (1) to (3) to be changed according to the values of said m and n are for overcoming the aforesaid problem 1), that is, when the difference between the changed picture element density and the original picture element density is large, the density (or brightness) of the overall image to be reproduced can be changed, which can adversely affect diagnostic performance of the image. Specifically, when the picture element density is increased, (1) the beam diameter of the light beam is reduced, (2) the power of the light beam is weakened, and/or (3) the sensitivity of detecting the signal light is increased. To the contrast, when the picture element density is reduced, (1) the beam diameter of the light beam is enlarged, (2) the power of the light beam is increased, and/or (3) the sensitivity of detecting the signal light is lowered. 
     Similarly when the picture element density is increased, (6) the frequency transfer properties are widened to a high-frequency band, and (7) the cut-off frequency is shifted toward the high-frequency side. When the picture element density is reduced, (6) the frequency transfer properties are narrowed toward the low-frequency side, and (7) the cut-off frequency is shifted toward the low-frequency side. 
     The item (8) may be changed, for instance, in the following manner. That is, the parameter for filtering processing (the coefficient of the mask operation) in the picture element density changing processing is changed so that the cut-off frequency for mask operation is shifted toward the low-frequency side as the picture element density is reduced. When the parameter for thinning processing is applied, the parameter is changed so that the cut-off frequency is shifted toward the high-frequency side as the picture element density is reduced. 
     The item (4) is not shifted qualitatively according to the picture element density. Accordingly, by preparing a plurality of sets of data for shading correction for a plurality of representative picture element densities, and data for shading correction corresponding the desired picture element density is selected from the sets of data, or when there is prepared no data for picture element density equal to the desired picture element density, the data for shading correction corresponding the desired picture element density is obtained by interpolation by use of two sets of data in the prepared sets of data. 
     The item (5) is applied when shading correction is carried out on the analog image signal in real time, and by outputting prepared data for shading correction at a timing according to the desired picture element density, shading correction is carried out on an obtained analog image signal. Specifically, when the picture element density is increased, the sampling speed is increased and accordingly, the data output timing is advanced according to the sampling speed. To the contrast, when the picture element density is reduced, the sampling speed is lowered and accordingly, the data output timing is retarded according to the sampling speed. 
     Shading means fluctuation in an analog image signal obtained from a photoelectric read-out means (local reduction in photo-detecting efficiency) due to unevenness in the intensity of the scanning light beam caused by unevenness in reflectance on the reflecting surface of the light deflector for deflecting the scanning light beam (e.g., polygonal mirror, galvanometer mirror or the like), fluctuation in the scanning speed caused by fluctuation in deflecting speed of the deflector, or unevenness in detection caused by unevenness in sensitivity in the main scanning direction of a photoelectric detector disposed to extend in the main scanning direction. Further the data for shading correction means shade properties which are obtained in advance, for instance, by use of a reference recording medium such as a stimulable phosphor sheet which has been uniformly exposed to a radiation. See, for instance, Japanese Unexamined Patent Publication Nos. 61(1986)-189763, 62(1987)-47259, 62(1987)-47261, 64(1989)-86759, and 2(1990)-58973. 
     As for the items (1) to (3) and (6) to (8) to be changed according to the desired picture element density, a plurality of beam diameters (1), a plurality of beam powers (2), a plurality of sensitivities (3), a plurality of frequency response properties (6), a plurality of frequency response properties different in cut-off frequencies (7), and a plurality of parameters for filtering processing (8) may be prepared by picture element densities and the items may be changed by selecting one of the respective properties according to the desired picture element density. 
     The data for shading correction (4) may be changed, as well as by the method described above where one of a plurality of sets of data is selected, by obtaining a set of data for shading for the desired picture element density by sampling a single set of data which has been prepared for said predetermined picture element density. 
     The picture element densities may be divided to a plurality of levels, e.g., high, standard and low, and the values of m and n may be related to the “high picture element density”, the “standard picture element density” and the “low picture element density” so that when the values of m and n give a picture element density within the high picture element density, data for shading correction prepared for the high picture element density is selected, when the values of m and n give a picture element density within the standard picture element density, data for shading correction prepared for the standard picture element density is selected, and when the values of m and n give a picture element density within the low picture element density, data for shading correction prepared for the low picture element density is selected. 
     The data for shading correction may be selected from a plurality of sets of data for shading correction according to the set values of m and n. In this case, it is preferred that the data for shading correction be selected in one of the following manners [I] and [II]. That is; 
     [I] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, data for shading correction corresponding to the selected values of m and n is transferred from the first memory to a second memory each time the values of m and n are selected, and the transferred data for shading correction is read out from the second memory as the selected data for shading correction. 
     [II] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, all the sets of data for shading correction are transferred from the first memory to a second memory at different addresses by the values of m and n at a desired time such as starting of the system, and data for shading correction corresponding to the selected values of m and n is read out from the address of the second memory corresponding to the selected values of m and n as the selected data for shading correction. 
     When the method [I] is employed, the second memory may be small in capacity and the hardware may be simple in structure. On the other hand, when the method [II] is employed, the software may be simple in structure and the data for shading correction can be read out from the second memory at a high speed. In accordance with a third aspect of the present invention, there is provided a method of obtaining, in a method of reading out a digital image signal at a predetermined picture element density by causing a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed while moving the recording medium in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, thereby two-dimensionally scanning the recording medium with the light beam, photoelectrically detecting signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, sampling the analog image signal at a predetermined intervals, and quantizing the sampled values, a digital image signal at a picture element density 1/m×n times the predetermined picture element density, the method comprising the steps of 
     changing the sub-scanning speed to m(&gt;0) times said predetermined sub-scanning speed, changing the intervals at which the analog image signal is sampled to intervals n(&gt;0) times said predetermined intervals, and changing at least one of the following properties according to the values of said m and n. 
     (1) The beam diameter of the light beam. 
     (2) The sensitivity of detecting the signal light. 
     (3) The preset data for shading correction when shading correction is to be carried out on the analog image signal. 
     (4) The frequency transfer properties when the analog image signal is to be logarithmically amplified. 
     (5) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal. 
     In accordance with a fourth aspect of the present invention, there is provided an image signal read-out system for carrying out the method in accordance with the first aspect of the present invention. That is, in accordance with the third aspect of the present invention, there is provided an image signal read-out system for reading out a digital image signal at a predetermined picture element density comprising a main scanning means which causes a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed, a sub-scanning means which moves the recording medium and/or the light beam relatively to each other in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, a photoelectric detector means which photoelectrically detects signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, and an A/D convertor means which samples the analog image signal at a predetermined intervals and quantizes the sampled values, thereby obtaining a digital image signal at a predetermined picture element density, wherein the improvement comprises 
     a sub-scanning speed changing means which causes the sub-scanning means to move the recording medium and/or the light beam relatively to each other in the sub-scanning direction at a speed m(&gt;0) times said predetermined sub-scanning speed, and a sampling interval changing means which causes the A/D convertor means to sample the analog image signal at intervals n(&gt;0) times said predetermined intervals. 
     It is preferred that the image signal read-out system be further provided with a picture element density changing processing means which carries out, on the digital image signal output from the A/D convertor means, picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a&gt;0) times and the number of the picture elements in the sub-scanning direction to a/n times. 
     In accordance with a fifth aspect of the present invention, there is provided an image signal read-out system for carrying out the method in accordance with the second aspect of the present invention. That is, in accordance with the fourth aspect of the present invention, there is provided an image signal read-out system for obtaining an addition image signal at a predetermined picture element density comprising a main scanning means which causes a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed, a sub-scanning means which moves the recording medium and/or the light beam relatively to each other in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, a photoelectric detector means which photoelectrically detects signal light emitted from both sides of the recording medium upon exposure to the light beam to obtain a pair of analog image signals, an A/D convertor means which samples the analog image signals at a predetermined intervals and quantizes the sampled values, thereby obtaining a pair of digital image signals, and an adder means which adds together the digital image signal and obtains an addition image signal at a predetermined picture element density, wherein the improvement comprises 
     a sub-scanning speed changing means which causes the sub-scanning means to move the recording medium and/or the light beam relatively to each other in the sub-scanning direction at a speed m(&gt;0) times said predetermined sub-scanning speed, 
     a sampling interval changing means which causes the A/D convertor means to sample the analog image signal at intervals n(&gt;0) times said predetermined intervals, and 
     picture element density changing processing means which carries out, on the digital image signals output from the A/D convertor means, picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a&gt;0) times and the number of the picture elements in the sub-scanning direction to a/n times. 
     In the system of the fifth aspect of the present invention, it is preferred that the picture element density changing processing means carries out said picture element density changing processing on said two digital image signals before they are added together. 
     Further preferably the systems of the fourth and fifth aspects of the present invention are provided with a characteristic changing means which changes at least one of the following properties according to the values of said m and n. 
     (1) The beam diameter of the light beam. 
     (2) The power of the light beam. 
     (3) The sensitivity of detecting the signal light. 
     (4) The preset data for shading correction when shading correction is to be carried out on the analog image signal. 
     (5) The timing at which the data for shading correction is output from a memory. 
     (6) The frequency transfer properties when the analog image signal is to be logarithmically amplified. 
     (7) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal. 
     (8) The parameter for filtering processing in the picture element density changing processing (including the parameter for thinning the picture elements when thinning processing is carried out as the filtering processing). 
     The sub-scanning speed changing means and the sampling interval changing means may form a part of the characteristic changing means. 
     When the characteristic changing means includes a means for changing the data for shading correction, the data for shading correction may be selected from a plurality of sets of data for shading correction according to the set values of m and n. In this case, it is preferred that the data for shading correction be selected in one of the following manners [I] and [II]. That is; 
     [I] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, data for shading correction corresponding to the selected values of m and n is transferred from the first memory to a second memory each time the values of m and n are selected, and the transferred data for shading correction is read out from the second memory as the selected data for shading correction. 
     [II] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, all the sets of data for shading correction are transferred from the first memory to a second memory at different addresses by the values of m and n at a desired time such as starting of the system, and data for shading correction corresponding to the selected values of m and n is read out from the address of the second memory corresponding to the selected values of m and n as the selected data for shading correction. 
     When the method [I] is employed, the second memory may be small in capacity and the hardware may be simple in structure. On the other hand, when the method [II] is employed, the software may be simple in structure and the data for shading correction can be read out from the second memory at a high speed. 
     In accordance with a sixth aspect of the present invention, there is provided an image signal read-out system for reading out a digital image signal at a predetermined picture element density comprising a main scanning means which causes a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed, a sub-scanning means which moves the recording medium and/or the light beam relatively to each other in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, a photoelectric detector means which photoelectrically detects signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, and an A/D convertor means which samples the analog image signal at a predetermined intervals and quantizes the sampled values, thereby obtaining a digital image signal at a predetermined picture element density, wherein the improvement comprises 
     picture element density input means which receives values of m (m&gt;0) and n (n&gt;0) which respectively represent that the picture element density in the main scanning direction is to be changed to 1/m times that of the predetermined picture element density and that the picture element density in the sub-scanning direction is to be changed to 1/n times that of the predetermined picture element density, 
     a sub-scanning speed changing means which causes the sub-scanning means to move the recording medium and/or the light beam relatively to each other in the sub-scanning direction at a speed m(&gt;0) times said predetermined sub-scanning speed, 
     a sampling interval changing means which causes the A/D convertor means to sample the analog image signal at intervals n(&gt;0) times said predetermined intervals, and 
     a characteristic changing means which changes at least one of the following properties according to the values of said m and n. 
     (1) The beam diameter of the light beam. 
     (2) The sensitivity of detecting the signal light. 
     (3) The preset data for shading correction when shading correction is to be carried out on the analog image signal. 
     (4) The frequency transfer properties when the analog image signal is to be logarithmically amplified. 
     (5) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal. 
     In the methods and the systems in accordance with the present invention, by changing the sub-scanning speed to m times (m&gt;0) the predetermined sub-scanning speed, the number of the scanning lines of the light beam is changed to 1/m times, whereby the picture element density in the sub-scanning direction is changed to 1/m times. Further by changing the sampling intervals to n times (n&gt;0) the predetermined sampling intervals, the picture element density in the main scanning direction is changed to 1/n times. Accordingly, the picture element density of the image signal finally obtained (the addition image signal in the case where signal light emitted from both sides of the recording medium upon exposure to the light beam is photoelectrically detected and an addition image signal is obtained by adding two digital image signals) is a/(m×n) 2  times said predetermined picture element density. Further since the picture element density can be changed without changing the main scanning speed, it is not necessary to change the driving speed of the scanning optical system for scanning the light beam (e.g., a polygonal mirror or a galvanometer mirror), whereby generation of a dead time (a time which corresponds to the time necessary for the driving speed of the optical system to be stabilized and for which read-out of an image signal is impossible) can be avoided. 
     Further when picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a&gt;0) times and the number of the picture elements in the sub-scanning direction to a/n times is carried out, the picture element densities in the main scanning direction and the sub-scanning direction of the image signal finally obtained (the addition image signal in the case where signal light emitted from both sides of the recording medium upon exposure to the light beam is photoelectrically detected and an addition image signal is obtained by adding two digital image signals) are both changed to a/(m×n) times, whereby the picture element densities in the sub-scanning direction and in the main scanning direction can be changed in the same proportion and an image signal at a picture element density of {a/(m×n) } 2  times the predetermined picture element density can be obtained with the aspect ratio of the image kept unchanged and without changing the main scanning speed. 
     Especially in the case where signal light emitted from both sides of the recording medium is to be detected (will be referred to as “the both-side reading”, hereinbelow) as in the method of the second aspect of the present invention or the system of the fifth aspect of the present invention, it is necessary to slow the scanning speed as compared with the case where signal light from one side of the recording medium is to be detected in order to give energy of the light beam sufficiently to the back side of the recording medium. However even in the case of the both-side reading, the scanning time can be shortened by increasing the sub-scanning speed so long as the picture element density can be lowered. 
     Further, in the case of the both-side reading, by carrying out the picture element density changing processing on the two digital image signals and obtaining the addition signal by adding the processed image signals, the amount of operation to be performed when the digital image signals are added can be reduced, whereby the time required to add the digital image signals can be shortened and the processing can be carried out in a shorter time. 
     Further, in accordance with the present invention, various problems which arise when the reading picture element density is changed can be overcome. 
     That is, the aforesaid problem 1), that when the picture element density is changed, energy of signal light emitted from the stimulable phosphor sheet per one picture element differs from that for the original picture element density, and, when the difference between the changed picture element density and the original picture element density is large, the density (or brightness) of the overall image to be reproduced can be changed to adversely affect diagnostic performance of the image, can be overcome by changing (1) the beam diameter of the light beam according to the picture element density so that the amount of energy to be applied to a unit area of the recording medium is kept unchanged, or by changing (2) the power of the light beam according to the picture element density so that the amount of energy to be applied to a unit area of the recording medium is kept unchanged, or by changing (3) the sensitivity of detecting the signal light according to the picture element density so that the level of the image signal is kept unchanged. 
     Further the aforesaid problem 2), that when shading correction is to be carried out on the analog image signal, properties of shading to be corrected varies before and after the picture element density change and the shading sometimes cannot be properly corrected, can be overcome by changing (4) the preset data for shading correction according to the picture element density so that shading correction can be carried out following change in shading properties or by changing (5) the timing at which the data for shading correction is output from a memory so that shading correction can be carried out following change in the sampling speed. 
     The aforesaid problem 3), that when the analog image signal is to be logarithmically amplified, the frequency transfer properties can vary before and after the picture element density change, can be overcome by changing (6) the frequency transfer properties when the analog image signal is logarithmically amplified according to the picture element density so that the frequency transfer properties of the digital image signal obtained is kept unchanged. 
     The aforesaid problem 4), that when filtering for removing aliasing noise is to be carried out prior to sampling the analog image signal, aliasing noise sometimes cannot be properly cut since Nyquist frequency varies before and after the picture element density change, can be overcome by changing (7) the cut-off frequency of filtering according to the picture element density so that the frequency properties of the digital image signal obtained is kept unchanged. 
     Further, by changing (8) the parameter for filtering processing (the coefficient of the mask operation) in the picture element density changing processing, the aliasing noise upon picture element density change can be suppressed lower than a certain level and at the same time, error generated by interpolation processing such as spline interpolation operation can be suppressed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing an image signal read-out system in accordance with a first embodiment of the present invention, 
     FIG. 2 is a view showing an example of a structure for changing the clock pulses, 
     FIG. 3 is a view showing the structure of the characteristic changing means in the image signal read-out system shown in FIG. 1, 
     FIG. 4 is a view showing in detail memory control in the image signal read-out system shown in FIG. 1, 
     FIG. 5 is a schematic view showing an image signal read-out system in accordance with a second embodiment of the present invention, 
     FIG. 6 is a view showing the structure of the characteristic changing means in the image signal read-out system shown in FIG. 5, 
     FIG. 7 is a view showing an example of a structure for changing the clock pulses, 
     FIG. 8 is a view for illustrating the filtering processing. and 
     FIG. 9 is a schematic view showing an image signal read-out system in accordance with a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, an image signal read-out system in accordance with a first embodiment of the present invention comprises an endless belt  9   a  which is driven by an electric motor  8  with a stimulable phosphor sheet  1  storing thereon a radiation image placed thereon. There are disposed above the stimulable phosphor sheet  1  a laser  10  emitting a laser beam  11  which stimulates the stimulable phosphor sheet  1 , a rotary polygonal mirror  12  which is rotated by an electric motor  20  to deflect the laser beam  11  at a speed corresponding to a main scanning frequency of 160 Hz, and a scanning lens  21  which converges the laser beam  11  deflected by the polygonal mirror  12  onto the surface of the stimulable phosphor sheet  1  and causes the laser beam  11  to scan the surface of the stimulable phosphor sheet  1  at a constant speed (main scanning). 
     Just above the stimulable phosphor sheet  1 , there is disposed close to the stimulable phosphor sheet  1  a light guide  14   a  which collects stimulated emission  13   a  which is emitted from the upper surface of the stimulable phosphor sheet  1  in proportion to the stored energy of radiation upon stimulation by the laser beam  11 . A photomultiplier  15   a  is connected to the light guide  14   a  to photoelectrically detect the stimulated emission  13   a  collected by the light guide  14   a  and convert it to an analog image signal QA. The photomultiplier  15   a  detects the stimulated emission  13   a  at a sensitivity which is determined by an electric voltage applied to the photomultiplier  15   a  by an electric voltage application means  39   a.    
     A logarithmic amplifier  16   a  is connected to the photomultiplier  15   a  to logarithmically amplify the analog image signal QA detected by the photomultiplier  15   a  according to predetermined frequency characteristic and to output a logarithmic image signal QA′. 
     A memory  41   a  stores data Di for shading correction according to sampling intervals which have been set in advance and a D/A convertor  42   a  is connected to the memory  41   a . The D/A convertor  42   a  converts the data D 1  for shading correction to an analog signal D 1  under the control of a reference clock. To the D/A convertor  42   a  is connected an adder  43   a  which adds the analog signal D 1 ′ for correction to the logarithmic image signal QA′ and outputs a shading-corrected image signal Q 1 . 
     Further to the adder  43   a  is connected an anti-aliasing filter  35   a  which removes aliasing noise (folded noise) generated by A/D conversion to be described later, and an A/D convertor  36   a  is connected to the anti-aliasing filter  35   a  to convert the filtered image signal Q 1 ′ to a digital image signal S 1  under the control of a reference clock which has been set in advance. The anti-aliasing filter  35   a  comprises a high-density filter and a low-density filter, and the high-density filter is initially selected and is switched to the low-density filter as required by an input signal from a characteristic changing means  60  to be described later. 
     A picture element density changing means  37   a  which changes the picture element density of the digital signal S 1 , thereby obtaining an image signal S 1 ′ is connected to the A/D converter  36   a.    
     As shown in FIG. 3, the image signal read-out system of this embodiment is further provided with an input means  70  which outputs values of parameters (m, n) as m=n=1 when the operator selects “high picture element density” out of “high picture element density” (10 pix/mm) and “low picture element density” (5 pix/mm) and outputs values of the parameters (m, n) as m=n=2 when the operator selects “low picture element density”. The image signal read-out system of this embodiment is further provided with a characteristic changing means  60  which, according to the values of the parameters (m, n) input from the input means  70 , changes the rotating speed of the electric motor  8  which drives the endless belt  9   a , changes the beam diameter of the laser beam  11  on the stimulable phosphor sheet  1  by shifting the scanning lens  21  in the direction of its optical axis, changes the sensitivity of the photomultiplier  15   a  by controlling the electric voltage application means  39   a , changes the data for shading correction output from the memory  41   a  by changing the sampling clock of the D/A convertor  42   a , changes the sampling clock of the A/D convertor  36   a , changes the frequency characteristics of the logarithmic amplifier  16   a , switches the high-density filter and the low-density filter of the anti-aliasing filter  35   a , changes parameters for changing the picture element density in the picture element density changing means  37   a.    
     The picture element density changing means  37   a  carries out according to the values of parameters (m, n) picture element density changing processing to multiply the picture element density of the digital image signal S 1  by 1/m in the main scanning direction and by 1/n in the sub-scanning direction, thereby obtaining an image signal S 1 ′ whose picture element density is 1/(m×n) 2  times that for values of the parameters of m=n=1. Specifically, the picture element density changing processing may be carried out, for instance, by effecting one-dimensional mask operation in both the main scanning direction and the sub-scanning direction of the digital image signal S 1 , by thinning picture elements according to the desired picture element density, by high-order interpolation operation such as B-spline interpolation or cubic spline interpolation (disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 8(1996)-16767 and 9(1997)-321981), or by linear interpolation (disclosed, for instance, in Japanese Unexamined Patent Publication No. 9(1997)-50516). At this time, the parameter of the picture element density changing is changed according to the values of the parameters (m, n). The parameter for the one-dimensional mask operation is a coefficient of mask, and the parameter for thinning the picture elements is the intervals at which the picture elements are thinned. When interpolation operation is employed, the kind of the interpolation operation to be carried out on the digital image signal S 1  is changed. 
     Operation of the radiation image signal read-out system of this embodiment will be described, hereinbelow. 
     A signal representing “high picture element density” (an initialization of the system) selected by the operator is input into the input means  70 . Then the input means  70  outputs values of the parameters (m, n) corresponding to “high picture element density”, that is, (m, n)=( 1 ,  1 ), to the characteristic changing means  60 . 
     The characteristic changing means  60  sets upon receipt of the values of the parameters, (m, n)=( 1 ,  1 ), the rotating speed of the electric motor  8  for driving the endless belt  9   a , the position of the scanning lens  21 , the control signal to the electric voltage application means  39   a , the sampling clock of the D/A convertor  42   a , the sampling clock of the A/D convertor  36   a , the frequency characteristic of the logarithmic amplifier  16   a , the anti-aliasing filter  35   a  and the picture element density changing means  37   a  to initial state, which correspond to “high picture element density”. 
     The endless belt  9   a  is moved in the direction of arrow Y (FIG. 1) at a speed for “high picture element density” with a stimulable phosphor sheet  1  set in a predetermined position on the endless belt  9   a , whereby the stimulable phosphor sheet  1  is conveyed in the direction of arrow Y (FIG. 1) at the speed for “high picture element density” (sub-scanning). 
     The laser beam  11  emitted from the laser  10  is deflected by the rotary polygonal mirror  12  rotated by the electric motor  20  at a high speed in the direction of the arrow, and the deflected laser beam  11  is converged on the surface of the stimulable phosphor sheet  1  conveyed by the endless belt  9   a  and caused to scan the stimulable phosphor sheet  1  at a constant speed in the direction of arrow X (main scanning) through the scanning lens  21 . 
     The parts of the stimulable phosphor sheet  1  exposed to the laser beam  11  emit stimulated emission  13   a  in proportion to the radiation energy stored thereon, and the stimulated emission  13   a  enters the light guide  14   a  through the light inlet end face  18   a  of the light guide  14   a  and is guided the inside of the light guide  14   a  through total reflection to the photomultiplier  15   a . A laser beam cut filter  17   a  is provided on the junction of the light guide  14   a  and the photomultiplier  15   a  to prevent the laser beam  11  entering the light guide  14   a  from impinging upon the photomultiplier  15   a  while permit the stimulated emission  13   a  to impinge upon the same. With this arrangement, the laser beam  11  which is scattered at, for instance, the surface of the stimulable phosphor sheet  1  and enters the light guide  14   a  is prevented from impinging upon the photomultiplier  15   a.    
     The photomultiplier  15   a  has been applied with a high voltage, which gives a sensitivity corresponding to “high picture element density”, by the electric voltage application means  39   a , detects the stimulated emission  13   a  at the sensitivity for “high picture element density”, and converts the stimulated emission  13   a  to an analog image signal QA. The analog image signal QA output from the photomultiplier  15   a  is input into the logarithmic amplifier  16   a . The analog image signal QA is converted by the logarithmic amplifier  16   a , which has been set to have a frequency characteristic for “high picture element density”, to a logarithmic image signal QA′. The logarithmic image signal QA′ output from the logarithmic amplifier  16   a  is input into the adder  43   a.    
     The data D 1  for shading correction for “high picture element density” stored in the memory  41   a  is converted to an analog signal D 1 ′ by the D/A convertor  42   a  at a sampling rate governed by the reference clock, which is a clock for “high picture element density”. 
     The clock input into the D/A convertor  42   a  here is the reference clock which is selectively output to the D/A convertor  42   a  and the A/D convertor  36   a  by a selector  62  provided in the characteristic changing means  60  as shown in FIG.  2 . 
     The analog signal D 1 ′ for shading correction is input into the adder  43   a  and added to the logarithmic image signal QA′ input from the logarithmic amplifier  16   a , whereby the logarithmic image signal QA′ is converted to a shading-corrected image signal Q 1 . The shading-corrected image signal Q 1  is input into the anti-aliasing filter  35   a.    
     The anti-aliasing filter  35   a  has been switched to the high-density filter by the characteristic changing means  60  and the image signal Q 1  input into the anti-aliasing filter  35   a  is properly removed with aliasing noise by the high-density filter. The image signal Q 1 ′ removed with aliasing noise is input into the A/D converter  36   a . The A/D converter  36   a  converts the image signal Q 1 ′ to a digital image signal S 1  at a sampling rate governed by the reference clock, which is input into the A/D converter  36   a  at this time by the selector  62  of the characteristic changing means  60 . The digital image signal S 1  is output to the picture element density changing means  37   a.    
     The picture element density changing means  37   a  carries out picture element density changing processing to multiply the picture element density of the digital image signal S 1  by 1/m in the main scanning direction and by 1/n in the sub-scanning direction. However, since m=n=1 here, the image signal S 1 ′ is obtained without carrying out picture element density changing processing, and the image signal S 1 ′ is output, for instance, to an image processing system. 
     Operation of the radiation image signal read-out system of this embodiment when the operator selects “low picture element density” will be described hereinbelow. 
     When a signal representing “low picture element density” is input into the input means  70  by the operator, the input means  70  outputs values of the parameters (m, n) corresponding to “low picture element density”, that is, (m, n)=( 2 ,  2 ), to the characteristic changing means  60 . 
     The characteristic changing means  60  sets upon receipt of the values of the parameters, (m, n)=( 2 ,  2 ), the rotating speed of the electric motor  8  for driving the endless belt  9   a , the position of the scanning lens  21 , the control signal to the electric voltage application means  39   a , the sampling clock of the D/A convertor  42   a , the sampling clock of the A/D convertor  36   a , the frequency characteristic of the logarithmic amplifier  16   a , the anti-aliasing filter  35   a  and the parameter of the picture element density changing means  37   a  to those which correspond to “low picture element density”. Specifically, the characteristic changing means  60  doubles the rotating speed of the motor  8 , shifts the scanning lens  21  to a position where the beam diameter of the laser beam  11  on the surface of the stimulable phosphor sheet  1  is substantially doubled, changes the control signal to the electric voltage application means  39   a  to that which lowers the sensitivity of the photomultiplier  15   a , switches the selector  62  (FIG. 2) so that the clock output from a frequency divider  61  (clock which is obtained by frequency-dividing the reference clock and is twice the reference clock in cycles) is selectively input into the D/A convertor  42   a  and the A/D converter  36   a , changes the frequency characteristic of the logarithmic amplifier  16   a  to that for “low picture element density”, switches the anti-aliasing filter  35   a  to the low-density filter and changes the parameter of the picture element density changing means  37   a.    
     With this condition, an image signal is read out in the same manner as in the “high picture element density read-out”. That is, the endless belt  9   a  is moved in the direction of arrow Y at double the speed for “high picture element density” with a stimulable phosphor sheet  1  set in a predetermined position on the endless belt  9   a , whereby the stimulable phosphor sheet  1  is conveyed in the direction of arrow Y at double the speed for “high picture element density” (sub-scanning). 
     The laser beam  11  emitted from the laser  10  is deflected by the rotary polygonal mirror  12  rotated by the electric motor  20  at the same speed as for “high picture element density”, and the deflected laser beam  11  is converged on the surface of the stimulable phosphor sheet  1  conveyed by the endless belt  9   a  and caused to scan the stimulable phosphor sheet  1  at a constant speed in the direction of arrow X (main scanning) by the scanning lens  21 . At this time, the diameter of the laser beam  11  converged on the surface of the stimulable phosphor sheet  1  is double the beam diameter in the “high picture element density read-out”. 
     The parts of the stimulable phosphor sheet  1  exposed to the laser beam  11  emit stimulated emission  13   a  in proportion to the radiation energy stored thereon, and the stimulated emission  13   a  is guided to the photomultiplier  15   a.    
     The photomultiplier  15   a  has been applied with a voltage, which gives a sensitivity lower than that for “high picture element density”, by the electric voltage application means  39   a , detects the stimulated emission  13   a  at the sensitivity for “low picture element density”, and converts the stimulated emission  13   a  to an analog image signal QA. The analog image signal QA output from the photomultiplier  15   a  is input into the logarithmic amplifier  16   a . The analog image signal QA is converted by the logarithmic amplifier  16   a , which has been set to have a frequency characteristic for “low picture element density”, to a logarithmic image signal QA′. The logarithmic image signal QA′ output from the logarithmic amplifier  16   a  is input into the adder  43   a.    
     The data D 1  for shading correction for “high picture element density” stored in the memory  41   a  is converted to an analog signal D 1 ′ by the D/A convertor  42   a  at a sampling rate governed by a clock for “low picture element density” which is half the reference clock in frequency. That is, a frequency divider  61  provided in the characteristic changing means  60  divides the frequency of the reference clock and generates a clock for “low picture element density” which is double the reference clock in cycle. The selector  62  has been switched to selectively input the clock for “low picture element density” output from the frequency divider  61  into the D/A convertor  42   a  and the A/D convertor  36   a . As a result, the analog signal D 1 ′ for “low picture element density” is half the analog signal D 1 ′ for “high picture element density” in the number of picture elements in the main scanning direction. 
     The analog signal D 1 ′ for shading correction is input into the adder  43   a  and added to the logarithmic image signal QA′ input from the logarithmic amplifier  16   a , whereby the logarithmic image signal QA′ is converted to a shading-corrected image signal Q 1 . The shading-corrected image signal Q 1  is input into the anti-aliasing filter  35   a.    
     The anti-aliasing filter  35   a  has been switched to the low-density filter by the characteristic changing means  60  and the image signal Q 1  input into the anti-aliasing filter  35   a  is properly removed with aliasing noise by the low-density filter. The image signal Q 1 ′ removed with aliasing noise is input into the A/D converter  36   a . The A/D converter  36   a  converts the image signal Q 1 ′ to a digital image signal S 1  at a sampling rate governed by the clock for “low picture element density”, which is input into the A/D converter  36   a  at this time by the selector  62  of the characteristic changing means  60 . 
     The picture element density changing means  37   a  carries out picture element density changing processing for changing the picture element density to 1/m times in the main scanning direction of the digital image signal S 1  and to 1/n times in the sub-scanning direction of the same. Since m=n=2 here, the picture element density of the digital image signal S 1  is reduced to ½ in both the main scanning direction and the sub-scanning direction. One-dimensional mask operation will be described hereinbelow as an example of the picture element density changing processing. An example of a one-dimensional filter used in the one-dimensional mask operation is as follows. 
     
       
           a ( x ,1)=(−8/105,−5/105,34/105, 63/105,34/105,−5/105,−8/105) 
       
     
     Using the one-dimensional filter a(x,  1 ), filtering processing is carried out at intervals of one picture element in the main scanning direction of the digital image signal S 1 , and then filtering processing is carried out at intervals of one picture element in the sub-scanning direction of the digital image signal S 1 . That is, when the values of picture elements of the digital image signal S 1  are represented by S 1 (x, y) and the values of picture elements obtained by picture element density changing processing in the main scanning direction are represented by S 1 A(x/2, y), the values of picture elements S 1 A(x/2, y) are calculated according to the following formula (1) 
     
       
           S   1   A ( k/ 2 ,l )= a ( 1 , 1 )* S   1 ( k −3 ,l )+ a ( 2 , 1 )* S   1 ( k −2 ,l )+ a ( 3 , 1 )* S   1 ( k −1 ,l )+ a ( 4 , 1 )* S   1 ( k,l )+ a ( 5 , 1 )* S   1 ( k +1 ,l )+ a ( 6 , 1 )* S   1 ( k +2 ,l )+ a ( 7 , 1 )* S   1 ( k +3 ,l )  (1) 
       
     
     wherein k=1 to N (N being the number or position of the picture element as numbered in the main scanning direction) and l=1 to M (M being the number or position of the picture element as numbered in the sub-scanning direction). Operation according to formula (1) is carried out also in the sub-scanning direction, whereby an image signal S 1 ′ changed with picture element density is obtained. 
     In such mask operation using a one-dimensional filter, data for mask operation becomes short at an edge of the digital image signal S 1 . In such a case, imaginary picture elements are set on the outer side of the picture element, and the imaginary picture elements are given proper values. Then the filtering processing is carried out by use of the imaginary picture elements. 
     The picture element density changing processing in the picture element density changing means  37   a  may be carried out by picture element thinning processing or interpolation operation without limited to mask operation. As the interpolation operation, high-order interpolation operation such as B-spline interpolation operation where weight is given to smoothness or Cubic spline interpolation operation where weight is given to sharpness may be applied as well as linear interpolation. 
     The picture element density of the image signal S 1 ′ obtained by the picture element density changing means  37   a  when “low picture element density” is selected becomes {fraction (1/16)} of that when “high picture element density” is selected. The image signal S 1 ′ is output, for instance, to an image processing system. 
     As can be understood from the description above, in the radiation image signal read-out system of this embodiment, a low picture element density image signal whose picture element density is ½ of that of the high picture element density in both the main scanning direction and the sub-scanning direction and is ¼ of that of the high picture element density in total is obtained without changing the main scanning speed of the laser beam  11  and an image signal whose picture element density is {fraction (1/16)} of that of the high picture element density is obtained by carrying out picture element density changing processing, which reduces the picture element density to ½ in both the main scanning direction and the sub-scanning direction. At the same time, since the characteristic changing means  60  changes, according to the parameters which determine the picture element density, the position of the scanning lens  21 , the control signal to the electric voltage application means  39   a , the sampling clock for the D/A convertor  42   a , the frequency characteristic of the logarithmic amplifier  16   a , the anti-aliasing filter  35   a  and the parameter of the picture element density changing means  37   a , a low picture element density image signal can be read out with the amount of energy applied to the stimulable phosphor sheet per one picture element, the sensitivity of the photomultiplier, the shading correction and suppression of aliasing noise set properly according to the read-out picture element density. 
     Though, in the embodiment described above, the system is set for the high picture element density reading in the initial state, the system may be arranged to be initially set for the standard picture element density reading and to be switched for the high picture element density reading and the low picture element density reading. 
     When the picture element density in the standard picture element density reading is 10 pix/mm and the main scanning speed (main scanning frequency) is 160 Hz, the sampling intervals in the main scanning direction, the pitches of picture elements in the sub-scanning direction, the sub-scanning speed and the cut-off frequency of the anti-aliasing filter may be, for instance, as follows. 
     sampling intervals in the main scanning direction: 100 μm 
     pitches of picture elements in the sub-scanning direction: 100 μm 
     sampling cycles in the main scanning direction: 1.0 μsec 
     sub-scanning speed: 16 mm/sec 
     cut-off frequency of the anti-aliasing filter: 500 kHz (Since being an analog filter, the cut-off frequency is preferably not higher than 500 kHz, e.g., 400 kHz) 
     In the case of high picture element density reading where the picture element density is 20 pix/mm, these factors are changed to as follows with the main scanning speed kept unchanged at 160 Hz. 
     sampling intervals in the main scanning direction: 50 μm 
     pitches of picture elements in the sub-scanning direction: 50 μm 
     sampling cycles in the main scanning direction: 0.5 μsec 
     sub-scanning speed: 8 mm/sec 
     cut-off frequency of the anti-aliasing filter: 1000 kHz 
     In the case of low picture element density reading where the picture element density is 5 pix/mm, these factors are changed to as follows with the main scanning speed kept unchanged at 160 Hz. 
     sampling intervals in the main scanning direction: 200 μm 
     pitches of picture elements in the sub-scanning direction: 200 μm 
     sampling cycles in the main scanning direction: 2.0 μsec 
     sub-scanning speed: 32 mm/sec 
     cut-off frequency of the anti-aliasing filter: 250 kHz 
     Though, in the first embodiment, the read-out picture element density is set stepwise like “high picture element density” and “low picture element density”, the system may be arranged so that the read-out picture element density can be freely set by selecting the parameters (m, n) which determine the read-out picture element density. 
     The method of changing the data for shading correction stored in the memory  41   a  will be described in detail with reference to FIG. 4, hereinbelow. 
     As shown in FIG. 4, the memory  41   a  comprises a large capacity hard disc  41   c  in which a plurality of sets of data for shading correction are stored by values of parameters (m, n) probable to be selected and a shading memory (SHD memory)  41   d  which is a temporary memory for transferring data for shading correction read out from the hard disc  41   c  to a shading circuit (SHD circuit) for shading correction. The characteristic changing means  60  controls the hard disc  41   c  and the SHD memory  41   d . As the method for the characteristic changing means  60  to control the hard disc  41   c  and the SHD memory  41   d , for instance, the following two methods can be employed. 
     [I] With a plurality of sets of data for shading correction which have been set by values of m and n stored in the hard disc  41   c , data for shading correction corresponding to the selected values of m and n is transferred from the hard disc  41   c  to the SHD memory  41   d  each time the values of m and n are selected, and the transferred data for shading correction is read out from the SHD memory  41   d  to the SHD circuit [II] With a plurality of sets of data for shading correction which have been set by values of m and n stored in the hard disc  41   c , all the sets of data for shading correction are transferred from the hard disc  41   c  to the SHD memory  41   d  at different addresses by the values of m and n at a desired time such as starting of the system, and data for shading correction corresponding to the selected values of m and n is read out from the address of the SHD memory  41   d  corresponding to the selected values of m and n to the SHD circuit. 
     When the method [I] where only the selected data for shading correction is transferred to the SHD memory  41   d  each time the shading correction is to be carried out is employed, the SHD memory  41   d  may be small in capacity and the hardware may be simple in structure. On the other hand, when the method [II] where all the sets of data for shading correction are transferred from the hard disc  41   c  to the SHD memory  41   d  is employed, the software may be simple in structure and the data for shading correction can be read out from the SHD memory  41   d  at a high speed. 
     Am image signal read-out system in accordance with a second embodiment of the present invention will be described with reference to FIG. 5, hereinbelow. As shown in FIG. 5, the image signal read-out system of the second embodiment is for obtaining a pair of image signals from both the sides of a stimulable phosphor sheet  1  storing thereon a radiation image of an object. In FIG. 5, an image signal read-out system in accordance with the second of the present invention comprises a pair of endless belts  9   a  and  9   b  which are driven by electric motors  8  with a stimulable phosphor sheet  1  storing thereon a radiation image placed thereon. There are disposed above the stimulable phosphor sheet  1  a laser  10  emitting a laser beam  11  which stimulates the stimulable phosphor sheet  1 , a rotary polygonal mirror  12  which is rotated by an electric motor  20  to deflect the laser beam  11 , and a scanning lens  21  which focuses the laser beam  11  deflected by the polygonal mirror  12  on the surface of the stimulable phosphor sheet  1  and causes the laser beam  11  to scan the surface of the stimulable phosphor sheet  1  at a constant speed (main scanning). 
     A first light guide  14   a  which collects, from above the stimulable phosphor sheet  1 , stimulated emission  13   a  emitted from the stimulable phosphor sheet  1  in proportion to the stored energy of radiation upon stimulation by the laser beam  11  is disposed close to the stimulable phosphor sheet  1  above the portion of the stimulable phosphor sheet  1  along which the laser beam  11  scan the stimulable phosphor sheet  1 . A second light guide  14   b  which collects, from below the stimulable phosphor sheet  1 , stimulated emission  13   a  emitted from the stimulable phosphor sheet  1  is disposed substantially in perpendicular to the stimulable phosphor sheet  1  below the portion of the stimulable phosphor sheet  1  along which the laser beam  11  scan the stimulable phosphor sheet  1 . First and second photomultipliers  15   a  and  15   b  are connected to the respective light guides  14   a  and  14   b  to photoelectrically detect the stimulated emission  13   a  collected by the light guides  14   a  and  14   b . First and second logarithmic amplifiers  16   a  and  16   b  are connected to the respective photomultipliers  15   a  and  15   b  to logarithmically amplify the analog image signals QA and QB respectively detected by the photomultipliers  15   a  and  15   b  according to predetermined frequency characteristic and to output logarithmic image signals QA′ and QB′. 
     First and second memories  41   a  and  41   b  store two sets of data D 1  for shading correction according to sampling intervals which have been set in advance. First and second D/A convertors  42   a  and  42   b  are respectively connected to the memories  41   a  and  41   b . The D/A convertor  42   a  converts the data D 1  for shading correction to an analog signal D 1 ′ under the control of a reference clock which has been set advance. The D/A convertor  42   b  converts the data D 2  for shading correction to an analog signal D 2 ′ under the control of the reference clock. To the D/A convertor  42   a  is connected, an adder  43   a  which adds the analog signal D 1 ′ for correction to the logarithmic image signal QA′ and outputs a shading-corrected image signal Q 1 . To the D/A convertor  42   b  is connected, an adder  43   b  which adds the analog signal D 2 ′ for correction to the logarithmic image signal QB′ and outputs a shading-corrected image signal Q 2 . 
     Further to the adders  43   a  and  43   b  are connected anti-aliasing filters  35   a  and  35   b  which remove aliasing noise (folded noise) generated by A/D conversion to be described later, and A/D convertors  36   a  and  36   b  are connected to the anti-aliasing filters  35   a  and  35   b  to convert the filtered image signals Q 1 ′ and Q 2 ′ to digital image signals S 1  and S 2  under the control of the reference clock. Each of the anti-aliasing filters  35   a  and  35   b  comprises a high-density filter and a low-density filter, and the high-density filter is initially selected and is switched to the low-density filter as required by an input signal from a characteristic changing means  60  to be described later. 
     A pair of picture element density changing means  37   a  and  37   b  are respectively connected to the A/D converters  36   a  and  36   b  to change the picture element densities of the digital image signals S 1  and S 2 , thereby obtaining image signal S 1 , and S 2 ′. The image signals S 1 ′ and S 2 ′ are added together by an adder  38 , whereby an addition signal S 3  is obtained. 
     As shown in FIG. 6, the image signal read-out system of this embodiment is further provided with an input means  70  which outputs values of parameters (m, n) as m=n=1 when the operator selects “high picture element density” out of “high picture element density” (10 pix/mm) and “low picture element density” (5 pix/mm) and outputs values of the parameters (m, n) as m=n=2 when the operator selects “low picture element density”. The image signal read-out system of this embodiment is further provided with a characteristic changing means  60  which, according to the values of the parameters (m, n) input from the input means  70 , changes the rotating speed of the electric motors  8  which drive the endless belts  9   a  and  9   b , changes the power of the laser beam  11  by changing the output of the laser  10 , changes the beam diameter of the laser beam  11  on the stimulable phosphor sheet  1  by shifting the scanning lens  21  in the direction of its optical axis, changes the sensitivities of the photomultipliers  15   a  and  15   b  by controlling the electric voltage application means  39   a  and  39   b , changes the data D 1  and D 2  for shading correction output from the memories  41   a  and  41   b  by changing the sampling clock of the D/A convertors  42   a  and  42   b , changes the timing at which the D/A convertors  42   a  and  42   b  read out the data for shading correction from the memories  41   a  and  41   b , changes the sampling clocks of the A/D converters  36   a  and  36   b , changes the frequency characteristics of the logarithmic amplifiers  16   a  and  16   b , switches the high-density filter and the low-density filter of the anti-aliasing filters  35   a  and  35   b , and changes the parameters which are used in picture element density change by the picture element density changing means  37   a  and  37   b.    
     The picture element density changing means  37   a  and  37   b  carry out according to the values of parameters (m, n) picture element density changing processing to multiply the picture element density of the digital image signals Si and S 2  by 1/m in the main scanning direction and by 1/n in the sub-scanning direction, thereby obtaining image signals S 1 , and S 2 ′ whose picture element densities are 1/(m×n) 2  times those for values of the parameters of m=n=1. Specifically, the picture element density changing processing may be carried out, for instance, by effecting one-dimensional mask operation in both the main scanning direction and the sub-scanning direction of the digital image signals S 1  and S 2 , by thinning picture elements according to the desired picture element density, by high-order interpolation operation such as B-spline interpolation or cubic spline interpolation, or by linear interpolation. At this time, the parameter of the picture element density changing is changed according to the values of the parameters (m, n). The parameter for the one-dimensional mask operation is a coefficient of mask, and the parameter for thinning the picture elements is the intervals at which the picture elements are thinned. When interpolation operation is employed, the kind of the interpolation operation to be carried out on the digital image signals S 1  and S 2  is changed. 
     In the adder  38 , it is preferred that an addition image signal S 3  be obtained after carrying out filtering processing, using a filter having frequency response properties such as will increase the S/N ratio of the addition image signal S 3 , on the image signals S 1 ′ and S 2 ′ as disclosed in Japanese Unexamined Patent Publication No. 7(1995)-287330) though the image signals S 1 ′ and S 2 ′ may be simply added together. The filtering processing may be carried out in the picture element density changing means  37   a  and  37   b.    
     Operation of the radiation image signal read-out system of this embodiment will be described, hereinbelow. 
     A signal representing “high picture element density” (an initialization of the system) selected by the operator is input into the input means  70 . Then the input means  70  outputs values of the parameters (m, n) corresponding to “high picture element density”, that is, (m, n)=( 1 ,  1 ), to the characteristic changing means  60 . 
     The characteristic changing means  60  sets upon receipt of the values of the parameters, (m, n)=( 1 ,  1 ), the rotating speed of the electric motors  8  for driving the endless belts  9   a  and  9   b , the output of the laser  10 , the position of the scanning lens  21 , the control signals to the electric voltage application means  39   a  and  39   b , the sampling clocks of the D/A convertors  42   a  and  42   b , the timings at which the D/A convertors  42   a  and  42   b  read out the data D 1  and D 2  for shading correction from the memories  41   a  and  41   b , the sampling clocks of the A/D convertors  36   a  and  36   b , the frequency characteristics of the logarithmic amplifiers  16   a  and  16   b , the anti-aliasing filters  35   a  and  35   b , and the picture element density changing means  37   a  and  37   b  to initial states, which correspond to “high picture element density”. 
     The endless belts  9   a  and  9   b  are moved in the direction of arrow Y (FIG. 5) with a stimulable phosphor sheet  1  set on the endless belts  9   a  and  9   b , whereby the stimulable phosphor sheet  1  is conveyed in the direction of arrow Y (sub-scanning). The laser beam  11  emitted from the laser  10  is deflected by the rotary polygonal mirror  12  rotated by the electric motor  20  at a high speed in the direction of the arrow, and the deflected laser beam  11  is converged on the surface of the stimulable phosphor sheet  1  conveyed by the endless belts  9   a  and  9   b  and caused to scan the stimulable phosphor sheet  1  in the direction of arrow X substantially perpendicular to the sub-scanning direction (main scanning) through the scanning lens  21 . 
     The parts of the stimulable phosphor sheet  1  exposed to the laser beam  11  emit stimulated emission  13   a  upward from the upper side of the sheet  1  and stimulated emission  13   b  downward from the lower side of the sheet  1  in proportion to the radiation energy stored thereon. The upward stimulated emission  13   a  enters the light guide  14   a  through the light inlet end face  18   a  of the light guide  14   a  and is guided the inside of the light guide  14   a  through total reflection to the photomultiplier  15   a . The photomultiplier  15   a  photoelectrically converts the upward stimulated emission  13   a  to an analog image signal QA. Similarly, the downward stimulated emission  13   b  enters the light guide  14   b  through the light inlet end face  18   b  of the light guide  14   b  and is guided the inside of the light guide  14   b  through total reflection to the photomultiplier  15   b . The photomultiplier  15   b  photoelectrically converts the downward stimulated emission  13   b  to an analog image signal QB. 
     The analog image signals QA and QB are input into the logarithmic amplifiers  16   a  and  16   b . The analog image signal QA is converted by the logarithmic amplifier  16   a , which has been set to have a frequency characteristic for “high picture element density”, to a logarithmic image signal QA′ and the analog image signal QB is converted by the logarithmic amplifier  16   b , which has been set to have a frequency characteristic for “high picture element density”, to a logarithmic image signal QB′. The logarithmic image signals QA′ and QB′ output from the logarithmic amplifiers  16   a  and  16   b  are respectively input into the adders  43   a  and  43   b.    
     The data D 1  and D 2  for shading correction for “high picture element density” stored in the memory  41   a  and  41   b  are read out by the D/A convertor  42   a  and  42   b  at a timing for “high picture element density” and converted to analog signals D 1 ′ and D 2 ′ at a sampling rate governed by the reference clock, which is a clock for “high picture element density”. 
     The clock input into the D/A convertors  42   a  and  42   b  here is the reference clock which is selectively output to the D/A convertors  42   a  and  42   b  and the A/D convertors  36   a  and  36   b  by a selector  62  provided in the characteristic changing means  60  as shown in FIG.  7 . 
     The analog signals D 1 ′ and D 2 ′ for shading correction are input into the adders  43   a  and  43   b  and added respectively to the logarithmic image signals QA′ and QB′ input from the logarithmic amplifiers  16   a  and  16   b , whereby the logarithmic image signals QA′ and QB′ are converted to shading-corrected image signals Q 1  and Q 2 . The shading-corrected image signals Q 1  and Q 2  are respectively input into the anti-aliasing filters  35   a  and  35   b.    
     The anti-aliasing filters  35   a  and  35   b  have been switched to the high-density filter by the characteristic changing means  60  and the image signals Q 1  and Q 2  input into the anti-aliasing filters  35   a  and  35   b  are properly removed with aliasing noise by the high-density filter. The image signals Q 1 ′ and Q 2 ′ removed with aliasing noise are input into the A/D converters  36   a  and  36   b.    
     The A/D converters  36   a  and  36   b  convert the image signals Q 1 ′ and Q 2 ′ to digital image signals S 1  and S 2  at a sampling rate governed by the reference clock, which is input into the A/D converters  36   a  and  36   b  at this time by the selector  62  of the characteristic changing means  60 . 
     The picture element density changing means  37   a  and  37   b  carry out picture element density changing processing to multiply the picture element density of the digital image signals S 1  and S 2  by 1/m in the main scanning direction and by 1/n in the sub-scanning direction. However, since m=n=1 here, the image signals S 1 ′ and S 2 ′ are obtained without carrying out picture element density changing processing. 
     The adder  38  carries out filtering processing, using a filter having frequency response properties such as will increase the S/N ratio of an addition image signal S 3 , on the image signals S 1 ′ and S 2 ′ as disclosed in Japanese Unexamined Patent Publication No. 7(1995)-287330) and adds together the filtered image signals S 1 ′ and S 2 ′ by picture elements corresponding to each other, thereby obtaining an addition image signal S 3 . The addition image signal S 3  is output to, for instance, an image processing system. 
     Operation of the radiation image signal read-out system of this embodiment when the operator selects “low picture element density” will be described hereinbelow. 
     When a signal representing “low picture element density” is input into the input means  70  by the operator, the input means  70  outputs values of the parameters (m, n) corresponding to “low picture element density”, that is, (m, n)=( 2 ,  2 ), to the characteristic changing means  60 . 
     The characteristic changing means  60  sets upon receipt of the values of the parameters, (m, n)=( 2 ,  2 ), the rotating speed of the electric motors  8  for driving the endless belts  9   a  and  9   b , the output of the laser  10 , the position of the scanning lens  21 , the control signals to the electric voltage application means  39   a  and  39   b , the sampling clocks of the D/A convertors  42   a  and  42   b , the timings at which the D/A convertors  42   a  and  42   b  read out the data D 1  and D 2  for shading correction from the memories  41   a  and  41   b , the sampling clocks of the A/D convertors  36   a  and  36   b , the frequency characteristics of the logarithmic amplifiers  16   a  and  16   b , the anti-aliasing filters  35   a  and  35   b , and the picture element density changing means  37   a  and  37   b  to those which correspond to “low picture element density”. 
     Specifically, the characteristic changing means  60  doubles the rotating speed of the motors  8 , doubles the output of the laser  10 , shifts the scanning lens  21  to a position where the beam diameter of the laser beam  11  on the surface of the stimulable phosphor sheet  1  is substantially doubled, changes the control signals to the electric voltage application means  39   a  and  39   b  to that which lowers the sensitivity of the photomultipliers  15   a  and  15   b , changes the timings at which the D/A convertors  42   a  and  42   b  read out the data D 1  and D 2  for shading correction from the memories  41   a  and  41   b , switches the selector  62  (FIG. 7) so that the clock output from a frequency divider  61  (clock which is obtained by frequency-dividing the reference clock and is twice the reference clock in cycles) is selectively input into the D/A convertors  42   a  and  42   b  and the A/D converters  36   a  and  36   b , changes the frequency characteristic of the logarithmic amplifiers  16   a  and  16   b  to that for “low picture element density”, switches the anti-aliasing filters  35   a  and  35   b  to the low-density filter, and changes the parameters for picture element density changing processing by the picture element density changing means  37   a  and  37   b.    
     With this condition, image signals are read out in the same manner as in the “high picture element density read-out” 
     That is, the endless belts  9   a  and  9   b  are moved in the direction of arrow Y at double the speed for “high picture element density” with a stimulable phosphor sheet  1  set on the endless belts  9   a  and  9   b , whereby the stimulable phosphor sheet  1  is conveyed in the direction of arrow Y at double the speed for “high picture element density” (sub-scanning). 
     The laser beam  11  emitted from the laser  10  which is double in power is deflected by the rotary polygonal mirror  12  rotated by the electric motor  20  at the same speed as for “high picture element density”, and the deflected laser beam  11  is converged on the surface of the stimulable phosphor sheet  1  conveyed by the endless belt  9   a  and caused to scan the stimulable phosphor sheet  1  at a constant speed in the direction of arrow X (main scanning) by the scanning lens  21 . At this time, the diameter of the laser beam  11  converged on the surface of the stimulable phosphor sheet  1  is double the beam diameter in the “high picture element density read-out”. 
     The parts of the stimulable phosphor sheet  1  exposed to the laser beam  11  emit stimulated emission  13   a  and  13   b  in proportion to the radiation energy stored thereon, and the stimulated emission  13   a  and  13   b  is guided to the photomultipliers  15   a  and  15   b.    
     The photomultipliers  15   a  and  15   b  have been applied with a voltage, which gives a sensitivity lower than that for “high picture element density”, by the electric voltage application means  39   a  and  39   b , detect the stimulated emission  13   a  and  13   b  at the sensitivity for “low picture element density”, and converts the stimulated emission  13   a  and  13   b  to analog image signals QA and QB. The analog image signals QA and QB output from the photomultipliers  15   a  and  15   b  are input into the logarithmic amplifiers  16   a  and  16   b . The analog image signal QA and QB are converted by the logarithmic amplifiers  16   a  and  16   b , which have been set to have a frequency characteristic for “low picture element density”, to logarithmic image signals QA′ and QB′. The logarithmic image signals QA′ and QB′ output from the logarithmic amplifiers  16   a  and  16   b  are respectively input into the adders  43   a  and  43   b.    
     The data D 1  and D 2  for shading correction for “high picture element density” stored in the memories  41   a  and  41   b  are read out at the timings for “low picture element density” by the D/A convertors  42   a  and  42   b  and converted to analog signals D 1 ′ and D 2 ′ at a sampling rate governed by a clock for “low picture element density” which is half the reference clock in frequency. That is, a frequency divider  61  provided in the characteristic changing means  60  divides the frequency of the reference clock and generates a clock for “low picture element density” which is double the reference clock in cycle. The selector  62  has been switched to selectively input the clock for “low picture element density” output from the frequency divider  61  into the D/A convertors  42   a  and  42   b  and the A/D convertors  36   a  and  36   b . As a result, the analog signals D 1 ′ and D 2 ′ for “low picture element density” are half the data D 1  and D 2  for shading correction for “high picture element density” in the number of picture elements in the main scanning direction. 
     The analog signals D 1 ′ and D 2 ′ for shading correction are input into the adders  43   a  and  43   b  and added to the logarithmic image signals QA′ and QB′ input from the logarithmic amplifiers  16   a  and  16   b , whereby the logarithmic image signals QA′ and QB′ are converted to shading-corrected image signals Q 1  and Q 2 . The shading-corrected image signals Q 1  and Q 2  are input into the anti-aliasing filters  35   a  and  35   b.    
     The anti-aliasing filters  35   a  and  35   b  have been switched to the low-density filter by the characteristic changing means  60  and the image signals Q 1  and Q 2  input into the anti-aliasing filters  35   a  and  35   b  are properly removed with aliasing noise by the low-density filter. The image signals Q 1 ′ and Q 2 ′ removed with aliasing noise are input into the A/D converters  36   a  and  36   b . The A/D converters  36   a  and  36   b  convert the image signals Q 1 ′ and Q 2 ′ to digital image signals S 1  and S 2  at a sampling rate governed by the clock for “low picture element density”, which is input into the A/D converter  36   a  and  36   b  at this time by the selector  62  of the characteristic changing means  60 . 
     The picture element density changing means  37   a  and  37   b  carry out picture element density changing processing for changing the picture element density to 1/m times in the main scanning direction of the digital image signals S 1  and S 2  and to 1/n times in the sub-scanning direction of the same. Since m=n=2 here, the picture element density of each of the digital image signals S 1  and S 2  is reduced to ½ in both the main scanning direction and the sub-scanning direction. One-dimensional mask operation will be described hereinbelow as an example of the picture element density changing processing. An example of a one-dimensional filter used in the one-dimensional mask operation is as follows. 
     
       
           a ( x , 1)=(−8/105, −5/105, 34/105, 63/105, 34/105, −5/105, 8/105) 
       
     
     Using the one-dimensional filter a(x,  1 ), filtering processing is carried out at intervals of one picture element in the main scanning direction of each of the digital image signals S 1  and S 2 , and then filtering processing is carried out at intervals of one picture element in the sub-scanning direction of each of the digital image signals S 1  and S 2 . This will be described in more specifically on only the digital image signal S 1 . When the values of picture elements of the digital image signal S 1  are represented by S 1 (x, y) and the values of picture elements obtained by picture element density changing processing in the main scanning direction are represented by S 1 A(x/2, y), the values of picture elements S 1 A(x/2, y) are calculated according to the following formula (1). 
     
       
           S   1   A ( k /2 ,l )= a ( 1 , 1 )* S   1 ( k −3 ,l )+ a ( 2 , 1 )* S   1 ( k −2 ,l )+ a ( 3 , 1 )* S   1 ( k −1 ,l )+ a ( 4 , 1 )* S   1 ( k,l )+ a ( 5 , 1 )* S   1 ( k +1 ,l )+ a ( 6 , 1 )* S   1 ( k +2 ,l )+ a ( 7 , 1 )* S   1 ( k +3 ,l )  (1) 
       
     
     wherein k=1 to N (N being the number or position of the picture element as numbered in the main scanning direction) and l=1 to M (M being the number or position of the picture element as numbered in the sub-scanning direction). Operation according to formula (1) is carried out also in the sub-scanning direction, whereby image signals S 1 ′ and S 2 ′ changed with picture element density are obtained. 
     In such mask operation using a one-dimensional filter, data for mask operation becomes short at an edge of the digital image signals S 1  and S 2 . For example, in such an image signal as shown in FIG. 8, when filtering is carried out on picture element S 1 ( 1 ,  1 ) or S 1 (N,  1 ), data becomes short by three picture elements. In such a case, imaginary picture elements S 1  (− 1 ,  1 ), S 1 (− 2 ,  1 ) and S 1 (− 3 ,  1 ) or S 1  (N+1,  1 ), S 1  (N+2,  1 ) and S 1  (N+3,  1 ) are set, and the values of picture element S 1 ( 1 ,  1 ) or S 1 (N,  1 ) are copied to the imaginary picture elements. Then the filtering processing is carried out by use of the imaginary picture elements. 
     The picture element density changing processing in the picture element density changing means  37   a  and  37   b  may be carried out by picture element thinning processing or interpolation operation without limited to mask operation. 
     As the interpolation operation, high-order interpolation operation such as B-spline interpolation operation where weight is given to smoothness or Cubic spline interpolation operation where weight is given to sharpness may be applied as well as linear interpolation. 
     The Cubic spline interpolation operation and the B-spline interpolation operation will be described, hereinbelow. It is assumed that the image signals S 1  and S 2  in this embodiment have signal values (S k−2 , S k−1 , S k , S k−1 , S k−2 , . . .) respectively corresponding to sampling points (picture elements) X k−2 , X k−1 , X k , X k+1 , X k+2 , . . . arranged in one direction at regular intervals. In the Cubic spline interpolation operation, coefficients of interpolation c k−1 , c k , c k+1  and c k+2  respectively corresponding to interpolation data Y k−1 , Y k , Y k+1  and Y k+2  in the following cubic spline interpolation operation expression (2) which represents interpolation data Y′ for an interpolating point X p  between original sampling points (picture elements) X k  and  Xk+1  are calculated according to the following formulae. 
     
       
         
           Y′=c 
           k−1 
           Y 
           k−1 
           +c 
           k 
           Y 
           k 
           +c 
           k+1 
           Y 
           k+1 
           +c 
           k+2 
           Y 
           k+2 
         
       
     
     
       
           c   k−1 =(− t   3 +2 t   2   −t )/2 
       
     
     
       
           c   k =(3 t   3 −5 t   2 +2)/2 
       
     
     
       
           c   k+1 =(−3 t   3 +4 t   2   +t )/2 
       
     
     
       
           c   k+2 =( t   3   −t   2 )/2  (2) 
       
     
     wherein t(0≦t≦1) represents the position of an interpolating point X p  toward a picture element X k+1  from a reference picture element X k  when the sampling intervals in the main scanning direction and the sub-scanning direction are assumed to be 1. 
     In the B-spline interpolation operation, coefficients of interpolation b k−1 , b k , b k+1  and b k+2  respectively corresponding to interpolation data Y k−1 , Y k , Y k+1  and Y k+2  in the following cubic B-spline interpolation operation expression (3) which represents interpolation data Y′ for an interpolating point X p  between original sampling points (picture elements) X k  and  xk+1  are calculated according to the following formulae. 
     
       
         
           Y′=b 
           k−1 
           Y 
           k−1 
           +b 
           k 
           Y 
           k+b 
           k+1 
           Y 
           k+1+b 
           k+2 
           Y 
           k+2 
         
       
     
     
       
           b   k−1 =(− t   3 +3 t   2 −3 t +1)/6 
       
     
     
       
           b   k =(3 t   3 −6 t   2 +4)/6 
       
     
     
       
           b   k+1 =(−3 t   3 +3 t   2 +3 t +1)/6 
       
     
     
       
           b   k+2   =t   3 /6  (2) 
       
     
     wherein t(0≦t≦1) represents the position of an interpolating point X p  toward a picture element X k+1  from a reference picture element X k  when the sampling intervals in the main scanning direction and the sub-scanning direction are assumed to be 1. 
     In this embodiment, the kind of the interpolation operation (including linear interpolation operation is selected according to the values of m and n. 
     The image signals S 1 ′ and S 2 ′ obtained by the picture element density changing means  37   a  and  27   b  are input into the adder  38 . The picture element density when “low picture element density” is selected becomes {fraction (1/16)} of that when “high picture element density” is selected. 
     The adder  38  carries out filtering processing, using a filter having frequency response properties such as will increase the S/N ratio of an addition image signal S 3 , on the image signals S 1 ′ and S 2 ′ as disclosed in Japanese Unexamined Patent Publication No. 7(1995)-287330) and adds together the filtered image signals S 1 ′ and S 2 ′ by picture elements corresponding to each other, thereby obtaining an addition image signal S 3 . The addition image signal S 3  is output to, for instance, an image processing system. 
     As can be understood from the description above, in the radiation image signal read-out system of this embodiment, a low picture element density image signal whose picture element density is ½ of that of the high picture element density in both the main scanning direction and the sub-scanning direction and is ¼ of that of the high picture element density in total is obtained without changing the main scanning speed of the laser beam  11  and an image signal whose picture element density is {fraction (1/16)} of that of the high picture element density is obtained by carrying out picture element density changing processing, which reduces the picture element density to ½ in both the main scanning direction and the sub-scanning direction. At the same time, since the characteristic changing means  60  changes, according to the parameters which determine the picture element density, the position of the scanning lens  21 , the control signal to the electric voltage application means  39   a  and  39   b , the sampling clock for the D/A convertors  42   a  and  42   b , the frequency characteristics of the logarithmic amplifiers  16   a  and  16   b , the filters of the anti-aliasing filters  35   a  and  35   b , and the parameters of the picture element density changing means  37   a  and  37   b , a low picture element density image signal can be read out with the amount of energy applied to the stimulable phosphor sheet per one picture element, the sensitivities of the photomultipliers, the shading correction and suppression of aliasing noise set properly according to the read-out picture element density. 
     Especially in the case of “the both-side reading” as in the second embodiment, it is necessary to slow the scanning speed as compared with the case where signal light from one side of the recording medium is to be detected in order to give energy of the light beam sufficiently to the back side of the recording medium. However even in the case of the both-side reading, the scanning time can be shortened by increasing the sub-scanning speed so long as the picture element density can be lowered, whereby the time required to scan a stimulable phosphor sheet can be shortened. 
     Further, by carrying out the picture element density changing processing on the two digital image signals S 1  and S 2  and obtaining the addition signal by adding the processed image signals, the amount of operation in addition mask processing to be performed when the digital image signals S 1  and S 2  are added can be reduced, whereby the time required to add the digital image signals can be shortened and the processing can be carried out in a shorter time. 
     Though, in the second embodiment described above, the system is set for the high picture element density reading in the initial state, the system may be arranged to be initially set for the standard picture element density reading and to be switched for the high picture element density reading and the low picture element density reading. 
     When the picture element density in the standard picture element density reading is 10 pix/mm and the main scanning speed (main scanning frequency) is 160 Hz, the sampling intervals in the main scanning direction, the pitches of picture elements in the sub-scanning direction, the sub-scanning speed and the cut-off frequency of the anti-aliasing filter may be, for instance, as follows. 
     sampling intervals in the main scanning direction: 100 μm 
     pitches of picture elements in the sub-scanning direction: 100 μm 
     sampling cycles in the main scanning direction: 1.0 μsec 
     sub-scanning speed: 16 mm/sec 
     cut-off frequency of the anti-aliasing filter: 500 kHz (Since being an analog filter, the cut-off frequency is preferably not higher than 500 kHz, e.g., 400 kHz) 
     In the case of high picture element density reading where the picture element density is 20 pix/mm, these factors are changed to as follows with the main scanning speed kept unchanged at 160 Hz. 
     sampling intervals in the main scanning direction: 50 μm 
     pitches of picture elements in the sub-scanning direction: 50 μm 
     sampling cycles in the main scanning direction: 0.5 μsec 
     sub-scanning speed: 8 mm/sec 
     cut-off frequency of the anti-aliasing filter: 1000 kHz 
     In the case of low picture element density reading where the picture element density is 5 pix/mm, these factors are changed to as follows with the main scanning speed kept unchanged at 160 Hz. 
     sampling intervals in the main scanning direction: 200 μm 
     pitches of picture elements in the sub-scanning direction: 200 μm 
     sampling cycles in the main scanning direction: 2.0 μsec 
     sub-scanning speed: 32 mm/sec 
     cut-off frequency of the anti-aliasing filter: 250 kHz 
     Though, in the second embodiment, the read-out picture element density is set stepwise like “high picture element density” and “low picture element density”, the system may be arranged so that the read-out picture element density can be freely set by selecting the parameters (m, n) which determine the read-out picture element density. 
     Though, in the second embodiment, picture element density changing processing is carried out on each of the digital image signals S 1  and S 2  before they are added, picture element density changing processing may be carried out on an addition image signal S 3  obtained by adding the digital image signals S 1  and S 2 . 
     Further, though in the second embodiment, picture element density changing processing for changing the number of the picture elements of the digital image signals S 1  and S 2  in the main scanning direction to 1/m times and the number of the picture elements in the sub-scanning direction to 1/n times is carried out in the picture element density changing means  37   a  and  37   b , picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a&gt;0) times and the number of the picture elements in the sub-scanning direction to a/n times may be carried out in the picture element density changing means  37   a  and  37   b . In this case, the picture element density of the addition image signal S 3  becomes (a/m×n) 2 . 
     FIG. 9 shows an image signal read-out system in accordance with a third embodiment of the present invention. In FIG. 9, the image signal read-out system of the third embodiment comprises an endless belt  9   a  which is driven by an electric motor  8  with a stimulable phosphor sheet  1  storing thereon a radiation image placed thereon. There are disposed above the stimulable phosphor sheet  1  a laser  10  emitting a laser beam  11  which stimulates the stimulable phosphor sheet  1 , a rotary polygonal mirror  12  which is rotated by an electric motor  20  to deflect the laser beam  11  at a speed corresponding to a main scanning frequency of 160 Hz, and a scanning lens  21  which converges the laser beam  11  deflected by the polygonal mirror  12  onto the surface of the stimulable phosphor sheet  1  and causes the laser beam  11  to scan the surface of the stimulable phosphor sheet  1  at a constant speed (main scanning). 
     Just above the stimulable phosphor sheet  1 , there is disposed close to the stimulable phosphor sheet  1  a light guide  14   a  which collects stimulated emission  13   a  which is emitted from the upper surface of the stimulable phosphor sheet  1  in proportion to the stored energy of radiation upon stimulation by the laser beam  11 . A photomultiplier  15   a  is connected to the light guide  14   a  to photoelectrically detect the stimulated emission  13   a  collected by the light guide  14   a  and convert it to an analog image signal QA. The photomultiplier  15   a  detects the stimulated emission  13   a  at a sensitivity which is determined by an electric voltage applied to the photomultiplier  15   a  by an electric voltage application means  39   a.    
     A logarithmic amplifier  16   a  is connected to the photomultiplier  15   a  to logarithmically amplify the analog image signal QA detected by the photomultiplier  15   a  according to predetermined frequency characteristic and to output a logarithmic image signal QA′. 
     A memory  41   a  stores data D 1  for shading correction according to sampling intervals which have been set in advance and a D/A convertor  42   a  is connected to the memory  41   a . The D/A convertor  42   a  converts the data D 1  for shading correction to an analog signal D 1 ′ under the control of a reference clock. To the D/A convertor  42   a  is connected an adder  43   a  which adds the analog signal D 1 ′ for correction to the logarithmic image signal QA′ and outputs a shading-corrected image signal Q 1 . 
     Further to the adder  43   a  is connected an anti-aliasing filter  35   a  which removes aliasing noise (folded noise) generated by A/D conversion to be described later, and an A/D convertor  36   a  is connected to the anti-aliasing filter  35   a  to convert the filtered image signal Q 1 ′ to a digital image signal S 1  under the control of a reference clock which has been set in advance. The anti-aliasing filter  35   a  comprises a high-density filter and a low-density filter, and the high-density filter is initially selected and is switched to the low-density filter as required by an input signal from a characteristic changing means  60  to be described later. 
     The image signal read-out system of this embodiment is further provided with an input means  70  which outputs values of parameters (m, n) as m=n=1 when the operator selects “high picture element density”, out of “high picture element density” (10 pix/mm) and “low picture element density” (5 pix/mm) and outputs values of the parameters (m, n) as m=n=2 when the operator selects “low picture element density”. The image signal read-out system of this embodiment is further provided with a characteristic changing means  60  which, according to the values of the parameters (m, n) input from the input means  70 , changes the rotating speed of the electric motor  8  which drives the endless belt  9   a , changes the beam diameter of the laser beam  11  on the stimulable phosphor sheet  1  by shifting the scanning lens  21  in the direction of its optical axis, changes the sensitivity of the photomultiplier  15   a  by controlling the electric voltage application means  39   a , changes the data for shading correction output from the memory  41   a  by changing the sampling clock of the D/A convertor  42   a , changes the sampling clock of the A/D convertor  36   a , changes the frequency characteristics of the logarithmic amplifier  16   a , and switches the high-density filter and the low-density filter of the anti-aliasing filter  35   a.    
     Operation of the radiation image read-out system of this embodiment will be described, hereinbelow. 
     A signal representing “high picture element density” (an initialization of the system) selected by the operator is input into the input means  70 . Then the input means  70  outputs values of the parameters (m, n) corresponding to “high picture element density”, that is, (m, n)=( 1 ,  1 ), to the characteristic changing means  60 . 
     The characteristic changing means  60  sets upon receipt of the values of the parameters, (m, n)=( 1 ,  1 ), the rotating speed of the electric motor  8  for driving the endless belt  9   a , the position of the scanning lens  21 , the control signal to the electric voltage application means  39   a , the sampling clock of the D/A convertor  42   a , the sampling clock of the A/D convertor  36   a , the frequency characteristic of the logarithmic amplifier  16   a , and the.anti-aliasing filter  35   a  to initial state, which correspond to “high picture element density”. 
     The endless belt  9   a  is moved in the direction of arrow Y (FIG. 9) at a speed for “high picture element density” with a stimulable phosphor sheet  1  set in a predetermined position on the endless belt  9   a , whereby the stimulable phosphor sheet  1  is conveyed in the direction of arrow Y at the speed for “high picture element density” (sub-scanning). 
     The laser beam  11  emitted from the laser  10  is deflected by the rotary polygonal mirror  12  rotated by the electric motor  20  at a high speed in the direction of the arrow, and the deflected laser beam  11  is converged on the surface of the stimulable phosphor sheet  1  conveyed by the endless belt  9   a  and caused to scan the stimulable phosphor sheet  1  at a constant speed in the direction of arrow X (main scanning) through the scanning lens  21 . 
     The parts of the stimulable phosphor sheet  1  exposed to the laser beam  11  emit stimulated emission  13   a  in proportion to the radiation energy stored thereon, and the stimulated emission  13   a  enters the light guide  14   a  through the light inlet end face  18   a  of the light guide  14   a  and is guided the inside of the light guide  14   a  through total reflection to the photomultiplier  15   a.    
     The photomultiplier  15   a  has been applied with a high voltage, which gives a sensitivity corresponding to “high picture element density”, by the electric voltage application means  39   a , detects the stimulated emission  13   a  at the sensitivity for “high picture element density”, and converts the stimulated emission  13   a  to an analog image signal QA. The analog image signal QA output from the photomultiplier  15   a  is input into the logarithmic amplifier  16   a . The analog image signal QA is converted by the logarithmic amplifier  16   a , which has been set to have a frequency characteristic for “high picture element density”, to a logarithmic image signal QA′. The logarithmic image signal QA′ output from the logarithmic amplifier  16   a  is input into the adder  43   a.    
     The data D 1  for shading correction for “high picture element density” stored in the memory  41   a  is converted to an analog signal D 1 ′ by the D/A convertor  42   a  at a sampling rate governed by the reference clock, which is a clock for “high picture element density”. 
     The clock input into the D/A convertor  42   a  here is the reference clock which is selectively output to the D/A convertor  42   a  and the A/D convertor  36   a  by a selector  62  provided in the characteristic changing means  60  as shown in FIG.  2 . 
     The analog signal D 1 ′ for shading correction is input into the adder  43   a  and added to the logarithmic image signal QA′ input from the logarithmic amplifier  16   a , whereby the logarithmic image signal QA′ is converted to a shading-corrected image signal Q 1 . The shading-corrected image signal Q 1  is input into the anti-aliasing filter  35   a.    
     The anti-aliasing filter  35   a  has been switched to the high-density filter by the characteristic changing means  60  and the image signal Q 1  input into the anti-aliasing filter  35   a  is properly removed with aliasing noise by the high-density filter. The image signal Q 1 ′ removed with aliasing noise is input into the A/D converter  36   a . The A/D converter  36   a  converts the image signal Q 1 ′ to a digital image signal S 1  at a sampling rate governed by the reference clock, which is input into the A/D converter  36   a  at this time by the selector  62  of the characteristic changing means  60 . The digital image signal S 1  is output, for instance, to an image processing system. 
     Operation of the radiation image read-out system of this embodiment when the operator selects “low picture element density” will be described hereinbelow. 
     When a signal representing “low picture element density” is input into the input means  70  by the operator, the input means  70  outputs values of the parameters (m, n) corresponding to “low picture element density”, that is, (m, n)=( 2 ,  2 ), to the characteristic changing means  60 . 
     The characteristic changing means  60  sets upon receipt of the values of the parameters, (m, n)=( 2 ,  2 ), the rotating speed of the electric motor  8  for driving the endless belt  9   a , the position of the scanning lens  21 , the control signal to the electric voltage application means  39   a , the sampling clock of the D/A convertor  42   a , the sampling clock of the A/D convertor  36   a , the frequency characteristic of the logarithmic amplifier  16   a , and the anti-aliasing filter  35   a  to those which correspond to “low picture element density”. Specifically, the characteristic changing means  60  doubles the rotating speed of the motor  8 , shifts the scanning lens  21  to a position where the beam diameter of the laser beam  11  on the surface of the stimulable phosphor sheet  1  is substantially doubled, changes the control signal to the electric voltage application means  39   a  to that which lowers the sensitivity of the photomultiplier  15   a , switches the selector  62  (FIG. 2) so that the clock output from a frequency divider  61  (clock which is obtained by frequency-dividing the reference clock and is twice the reference clock in cycles) is selectively input into the D/A convertor  42   a  and the A/D converter  36   a , changes the frequency characteristic of the logarithmic amplifier  16   a  to that for “low picture element density”, and switches the anti-aliasing filter  35   a  to the low-density filter. 
     With this condition, an image signal is read out in the same manner as in the “high picture element density read-out”. 
     That is, the endless belt  9   a  is moved in the direction of arrow Y at double the speed for “high picture element density” with a stimulable phosphor sheet  1  set in a predetermined position on the endless belt  9   a , whereby the stimulable phosphor sheet  1  is conveyed in the direction of arrow Y at double the speed for “high picture element density” (sub-scanning). 
     The laser beam  11  emitted from the laser  10  is deflected by the rotary polygonal mirror  12  rotated by the electric motor  20  at the same speed as for “high picture element density”, and the deflected laser beam  11  is converged on the surface of the stimulable phosphor sheet  1  conveyed by the endless belt  9   a  and caused to scan the stimulable phosphor sheet  1  at a constant speed in the direction of arrow X (main scanning) by the scanning lens  21 . At this time, the diameter of the laser beam  11  converged on the surface of the stimulable phosphor sheet  1  is double the beam diameter in the “high picture element density read-out”. 
     The parts of the stimulable phosphor sheet  1  exposed to the laser beam  11  emit stimulated emission  13   a  in proportion to the radiation energy stored thereon, and the stimulated emission  13   a  is guided to the photomultiplier  15   a.    
     The photomultiplier  15   a  has been applied with a voltage, which gives a sensitivity lower than that for “high picture element density”, by the electric voltage application means  39   a , detects the stimulated emission  13   a  at the sensitivity for “low picture element density”, and converts the stimulated emission  13   a  to an analog image signal QA. 
     The analog image signal QA output from the photomultiplier  15   a  is input into the logarithmic amplifier  16   a . The analog image signal QA is converted by the logarithmic amplifier  16   a , which has been set to have a frequency characteristic for “low picture element density”, to a logarithmic image signal QA′. The logarithmic image signal QA′ output from the logarithmic amplifier  16   a  is input into the adder  43   a.    
     The data D 1  for shading correction for “high picture element density” stored in the memory  41   a  is converted to an analog signal D 1 ′ by the D/A convertor  42   a  at a sampling rate governed by a clock for “low picture element density” which is half the reference clock in frequency. That is, a frequency divider  61  provided in the characteristic changing means  60  divides the frequency of the reference clock and generates a clock for “low picture element density” which is double the reference clock in cycle. The selector  62  has been switched to selectively input the clock for “low picture element density” output from the frequency divider  61  into the D/A convertor  42   a  and the A/D convertor  36   a . As a result, the analog signal D 1 ′ for “low picture element density” is half the analog signal D 1 ′ for “high picture element density” in the number of picture elements in the main scanning direction. 
     The analog signal D 1 ′ for shading correction is input into the adder  43   a  and added to the logarithmic image signal QA′ input from the logarithmic amplifier  16   a , whereby the logarithmic image signal QA′ is converted to a shading-corrected image signal Q 1 . The shading-corrected image signal Q 1  is input into the anti-aliasing filter  35   a.    
     The anti-aliasing filter  35   a  has been switched to the low-density filter by the characteristic changing means  60  and the image signal Q 1  input into the anti-aliasing filter  35   a  is properly removed with aliasing noise by the low-density filter. The image signal Q 1 ′ removed with aliasing noise is input into the A/D converter  36   a.    
     The A/D converter  36   a  converts the image signal Q 1 ′ to a digital image signal S 1  at a sampling rate governed by the clock for “low picture element density”, which is input into the A/D converter  36   a  at this time by the selector  62  of the characteristic changing means  60 . 
     As can be understood from the description above, in the radiation image read-out system of this embodiment, a low picture element density image signal whose picture element density is ½ of that of the high picture element density in both the main scanning direction and the sub-scanning direction and is ¼ of that of the high picture element density in total can be obtained without changing the main scanning speed of the laser beam. At the same time, since the characteristic changing means 60 changes, according to the parameters which determine the picture element density, the position of the scanning lens  21 , the control signal to the electric voltage application means  39   a , the sampling clock for the D/A convertor  42   a , the frequency characteristic of the logarithmic amplifier  16   a , and the anti-aliasing filter  35   a , a low picture element density image signal can be read out with the amount of energy applied to the stimulable phosphor sheet per one picture element, the sensitivity of the photomultiplier, the shading correction and suppression of aliasing noise set properly according to the read-out picture element density. 
     Though, in the embodiment described above, the system is set for the high picture element density reading in the initial state, the system may be arranged to be initially set for the standard picture element density reading and to be switched for the high picture element density reading and the low picture element density reading. 
     When the picture element density in the standard picture element density reading is 10 pix/mm and the main scanning speed (main scanning frequency) is 160 Hz, the sampling intervals in the main scanning direction, the pitches of picture elements in the sub-scanning direction, the sub-scanning speed and the cut-off frequency of the anti-aliasing filter may be, for instance, as follows. 
     sampling intervals in the main scanning direction: 100 μm 
     pitches of picture elements in the sub-scanning direction: 100 μm 
     sampling cycles in the main scanning direction: 1.0 μsec 
     sub-scanning speed: 16 mm/sec 
     cut-off frequency of the anti-aliasing filter: 500 kHz (Since being an analog filter, the cut-off frequency is preferably not higher than 500 kHz, e.g., 400 kHz) 
     In the case of high picture element density reading where the picture element density is 20 pix/mm, these factors are changed to as follows with the main scanning speed kept unchanged at 160 Hz. 
     sampling intervals in the main scanning direction: 50 μm 
     pitches of picture elements in the sub-scanning direction: 50 μm 
     sampling cycles in the main scanning direction: 0.5 μsec 
     sub-scanning speed: 8 mm/sec . 
     cut-off frequency of the anti-aliasing filter: 1000 kHz 
     In the case of low picture element density reading where the picture element density is 5 pix/mm, these factors are changed to as follows with the main scanning speed kept unchanged at 160 Hz. 
     sampling intervals in the main scanning direction: 200 μm 
     pitches of picture elements in the sub-scanning direction: 200 μm 
     sampling cycles in the main scanning direction: 2.0 μsec 
     sub-scanning speed: 32 mm/sec 
     cut-off frequency of the anti-aliasing filter: 250 kHz 
     Though, in the third embodiment, the read-out picture element density is set stepwise like “high picture element density” and “low picture element density”, the system may be arranged so that the read-out picture element density can be freely set by selecting the parameters (m, n) which determine the read-out picture element density.