Image reading apparatus

Abstract: A color reading apparatus is disclosed. In the apparatus, a reflection type one dimensional blazed diffraction grating for color separation is provided on an optical path between an imaging optical system for focusing an color image on a photosensor and the photosensor in which three line sensor are disposed on a same substrate. The pitch of the diffraction grating is varied in a direction of the pitch so as to control the reflective diffraction angle from reflection points resulting from different incident angles on the diffraction grating and to prevent the occurrence of a blur of ±1st-order diffracted lights in the pitch cross section on the diffraction grating.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a color image reading apparatus, and more particularly to a color image reading apparatus provided with color separation means composed of a one-dimensional reflective blazed diffraction grating, and light receiving means including three line sensors (photosensors) mounted on the same substrate, capable of preventing image blur on the photosensors resulting from aberration in the reflective diffraction angle caused by a difference in the incident light angle thereby enabling to read color image information of an original image with a high precision, and adapted for use in a color scanner, a color facsimile apparatus or the like. Related Background Art There have been proposed various apparatus for focusing the color image information of an original image onto line sensors (CCD) through an optical system and reading the color image information in digital manner, utilizing the output signals of said line sensors. As an example, Fig. 1 schematically illustrates the optical system of a conventional color image reading apparatus, wherein the light from a color image on an original image plane 11 is condensed by an imaging lens 19, then split into three colors, for example red (R), green (G) and blue (B) by a 3P prism 20, and guided to respective line sensors 21, 22, 23, and the color images focused on said line sensors are respectively read by scanning in the sub scanning direction. Fig. 2 also illustrates schematically the optical system of another conventional color image reading apparatus, wherein the light from a color image on the original image plane 11 is condensed by an imaging lens 29, and is split into three light beams corresponding to three colors, by beam splitters 30, 31 with dichroic transmission films, and said three color beams are respectively focused on three line sensors, formed on so-called monolithic 3-line sensor 32 and are respectively read by scanning in the sub scanning direction. Fig. 3 is a schematic perspective view of the monolithic 3-line sensor 32 shown in Fig. 2. Said sensor 32 is provided with three line sensors (CCD's) 25, 26, 27 in mutually parallel manner and with a certain mutual distance, on a same substrate, and said line sensors are respectively provided thereon with unrepresented color filters corresponding to the colors of the light beams. The distances S1, S2 of said line sensors 25, 26, 27 are generally in a range of 0.1 - 0.2 mm based on various conditions of preparation, and the pixel sizes W1, W2 of each single element is for example in a range of 7 x 7 to 10 x 10 µm. The color image reading apparatus shown in Fig. 1 is complex and expensive as it requires not only three independent line sensors but also a 3P prism which necessitates a high precision and is difficult to manufacture. Also the assembling and adjustment are cumbersome since the alignment between the light beams and the line sensors has to be made three times independently. Also in the color image reading apparatus shown in Fig. 2, the distance between the line sensors is represented by 2 2 t, for a plate thickness t of the beam splitters 30, 31. Thus, for a distance of 0.1 to 0.2 mm, which is preferred for the manufacture of line sensors, the plate thickness t for the beam splitters 30, 31 has to be in a range of 35 to 70 µm. It is in general very difficult to produce a beam splitter of a satisfactory optical flatness with such small thickness, and the color image focused on the line sensors becomes inevitably deteriorated when beam splitters of such small thickness are employed. On the other hand, in the monolithic 3-line sensor, the distances S1, S2 of the lines 25, 27 from the central line 26, as shown in Fig. 4, are generally selected as mutually equal and as an integral multiple of the pixel size W2 (cf. Fig. 3) in the sub scanning direction, for the following reason. In case of reading a color image with the above-mentioned monolithic 3-line sensor through an ordinary imaging optical system 45 as shown in Fig. 4, three line sensors 25, 26, 27 read three different positions 25', 26', 27' on the original image plane 11 at the same time. Stated differently, the three-color (R, G, B) signal components of a given position on the original image plane 11 cannot be read simultaneously but have to be obtained by time adjustments after signal reading with three line sensors. More specifically, three color signal components can be relatively easily obtained by selecting the distances S1, S2 of the three line sensors as an integral multiple of the pixel size W2, and for example delaying the G, R signals (corresponding to the lights of green and read colors) with respect to the B signal (corresponding to the light of blue color) by means of redundant line memories. For this purpose, the distances S1, S2 of the line sensors 25, 27 from the central line sensor 26 are selected as an integral multiple of the pixel size W2 in the sub scanning direction. However, in such color image reading apparatus, for providing the redundant line memories corresponding to the gaps in the three line sensors, there are required plural units of expensive line memories. Such configuration is extremely disadvantageous in cost, and also complicates the entire device. Also there is known, as disclosed in the Patent US-A-4,277,138 or in EP-A-383 307, a color image reading device employed a blazed diffraction grating as the color separation means, instead of dichroic mirrors. However, the disclosed configuration is only designed for the light coming from a point in the object, and does not consider so-called image angle characteristics arising from a fact that the object has a finite reading width in the main scanning direction. SUMMARY OF THE INVENTION The object of the present invention is, in reading a color image by color separation with a reflective one-dimensional blazed diffraction grating, to suitably modify the grating pitch of said diffraction grating on the grating plane in order to control the reflective diffraction angles from the reflection points of different incident angles on said diffraction grating and to effectively prevent image blur caused by ±1st order diffracted lights on the photosensors in the cross-sectional direction of the grating pitch, thereby providing a color image reading apparatus capable of digitally reading a color image with a high precision, by means of lights of three colors, for example R, G and B. The color image reading apparatus of the present invention is featured, in focusing a color image by an imaging optical system onto photosensor means including three line sensors on a same substrate and reading said color image with said photosensor means, by the presence, in the optical path between said imaging optical system and said photosensor means, of color separation means consisting of a reflective one-dimensional blazed diffraction grating for separating the incident light beam into three color light beams, and by a fact that the pitch of said blazed diffraction grating varies in continuous manner in the direction of said pitch. It is also featured by facts that said three line sensors are provided in mutually parallel manner, that said imaging optical system is composed of a telecentric system at the exit side, and that said color separation means color-separates the incident light beam in a direction perpendicular to the direction of array of pixels in said line sensors. It is furthermore featured by a fact that the pitch of said reflective one-dimensional blazed diffraction grating is continuously increased or decreased from a peripheral area to the other in the sub scanning cross section, and is continuously increased from the center of the optical axis of said imaging optical system to the peripheral part in the image angle thereof. It is furthermore featured by a fact that the grating thickness of said blazed diffraction grating increases in succession from the optical axis center of said imaging optical system to the peripheral part in the image angle thereof. Furthermore, the color image reading apparatus of the present invention is featured, in focusing a color image by an imaging optical system onto photosensor means including three line sensors on a same substrate and reading said color image with said photosensor means, by the presence, in the optical path between said imaging optical system and said photosensor means, of color separation means consisting of a reflective one-dimensional blazed diffraction grating for color-separating the incident light beam into three color light beams, and the presence, between said color image and said photosensor means, of light amount correction means for controlling the amount of transmitted light in such a manner that the numerical aperture in the sub scanning direction becomes smaller than that in the main scanning direction. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partial schematic view of the optical system of a conventional color image reading apparatus; Fig. 2 is a partial schematic view of the optical system of another conventional color image reading apparatus; Fig. 3 is a schematic view of a monolithic 3-line sensor; Fig. 4 is a partial schematic view of the optical system of a conventional color image reading apparatus; Figs. 5A and 5B are respectively a plan view and a lateral view of the optical system of a first embodiment of the color image reading apparatus of the present invention; Fig. 6 is a partial view of the optical path after the imaging optical system shown in Figs. 5A and 5B; Figs. 7A and 7B are partial magnified views of a reflective one-dimensional blazed diffraction grating shown in Fig. 6; Fig. 8 is a chart showing optical paths of the reflective diffracted lights of different orders of diffraction toward the 3-line sensor; Fig. 9 is a view showing the diffraction pattern of a +1st order light; Fig. 10 is a view showing the diffraction pattern of a 0th order light; Fig. 11 is a view showing the diffraction pattern of a -1st order light; Fig 12 is a partial schematic view of the optical system of a second embodiment of the color image reading apparatus of the present invention; Fig. 13 is a partial schematic view for comparison with the configuration of the present invention; Fig. 14 is a schematic view of the diffraction grating of the second embodiment of the present invention; Fig. 15 is a schematic view of photosensor means in the optical system of a third embodiment of the color image reading apparatus of the present invention; Fig. 16 is a schematic view of the diffraction grating of the third embodiment of the present invention; the image reading apparatus of the present invention is defined in claim 1. Fig. 17 is a perspective view of the diffraction grating in the optical system of a fourth embodiment of the color image reading apparatus of the present invention; Figs. 18A and 18B are respectively a plan view and a lateral view of the optical system in a fifth embodiment of the color image reading apparatus of the present invention; Fig. 19 is an elevation view of a diagram shown in Fig. 18; Fig. 20 is a view showing the diffraction pattern of a +1st order light in the fifth embodiment; Fig. 21 is a view showing the diffraction pattern of a 0th order light in the fifth embodiment; and Fig. 22 is a view showing the diffraction pattern of a -1st order light in the fifth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 5A and 5B are respectively a plan view along the main scanning cross section and a lateral view along the sub scanning cross section, perpendicular to said main scanning cross section, of a first embodiment of the color image reading apparatus of the present invention. Fig. 6 is a partial view showing the optical path after the imaging optical system shown in Figs. 5A and 5B, and Figs. 7A and 7B are partial magnified views of the reflective one-dimensional blazed diffraction grating shown in Fig. 6. In these drawings, an original image plane 1 bears a color image constituting an object. An imaging optical system 2 is constructed, in the present embodiment, as so-called exit telecentric system in which the principal ray at the exit side emerges parallel to the optical axis, and focuses a light beam, coming from said color image, onto photosensor means (monolithic 3-line sensor) through a reflective one-dimensional blazed diffraction grating to be explained later. Color separation means 3 is composed of said reflective one-dimensional blazed diffraction grating, of which grating pitch is continuously increased or decreased in the direction of said pitch (sub scanning direction). Said blazed diffraction grating 3 separates the incident light beam, by reflective diffraction, into the lights of predetermined colors, for example three elementrary colors of red (R), green (G) and blue (B), in a direction perpendicular to the direction of array of pixels of the line sensors. Photosensor means 4 is composed of so-called monolithic 3-line sensor, having three line sensors (CCD's) 8, 9, 10 in mutually parallel manner on a same substrate. Said line sensors 8, 9, 10 are respectively provided with color filters (not shown) corresponding to the incident color lights, and the distances of said line sensors are selected at different values corresponding to the directions of separated lights obtained from the color separation means 3. In the present embodiment, the color image on the original image plane 1 is line scanned in the sub scanning direction by scanning means composed for example of an unrepresented scanning mirror, and the light beam obtained from said color image is condensed by the imaging optical system 2, then separated by the one-dimensional blazed diffraction grating 3 into lights of three colors, which are respectively focused on the line sensors 8, 9, 10, and color image signals are obtained in digital manner by the photosensor means 4. Thus the image information on the original image plane 1 is read by mutual movement, in the sub scanning direction, between the original image plane 1 and the image reading device (imaging optical system 2, one-dimensional blazed diffraction grating 3 and photosensor means 4). On the photosensor means 4, the sensor arrays 8, 9, 10 extend in the main scanning direction, represented as X-direction in the drawing. Said photosensor means 4 is constructed as a monolithic 3-line sensor in which three one-dimensional sensor arrays are formed on a same substrate, with finite distances therebetween in a direction perpendicular to the direction of arrays. The one-dimensional blazed diffraction grating 3 is provided in the optical path between the imaging optical system 2 and said photosensor means 4, and serves to separate the light from the object into lights of plural colors and to guide thus separated lights to the respectively corresponding sensor arrays. The original image plane 1 is illuminated by an unrepresented light source. The one-dimensional blazed diffraction grating 3 separates the light from the objects in the sub scanning direction, which is perpendicular to the direction of arrays. As shown in Fig. 6, the blazed diffraction grating 3 of the present embodiment separates the reflection diffracted light into three directions, namely into a -1st order light 5, a 0th order light 6 and a +1st order light 7, and focuses said lights, as beams of converging spherical wave formed by the imaging optical system 2, respectively on the line sensors 8, 9, 10. In the following there will be explained the image blur in case a converging light beam enters the reflective one-dimentional diffraction grating. In the cross section along the direction of grating pitch (or in the sub scanning cross section shown in Figs 5B and 6), among the converging spherical wave, a light beam entering the diffraction grating 3 from the upper side of the drawing (hereinafter called "upper ray"), a light beam entering the diffraction grating 3 along the optical axis, and a light beam entering the diffraction grating 3 from the lower side of the drawing (hereinafter called "lower ray") have respectively different incident angles to said diffraction grating 3. The incident angles  1 ,  0 ,  2 respectively of the upper ray, axial ray and lower ray satisfy a relation  1 >  0 >  2 . The 1st order reflected diffraction angle ' is related with the incident angles  in the following manner: sin' - sin = ±λ/P wherein λ is wavelength; P is grating pitch; and sign + or - respectively stands for +1st order or -1st order diffraction. Consequently, the reflected diffraction angle ' can be determined by: ' = sin -1 (±λ/P + sin) Said reflected diffraction angle ' varies for different rays in the aforementioned sub scanning cross section. Consequently, when a one-dimensional blazed diffraction grating is positioned in the converting spherical wave formed by the imaging optical system, there will result, on the image plane (photosensor means), a blur corresponding to the aberration in the reflected diffraction angles ' of the diffracted lights of different orders. In the following the above-mentioned situation will be further explained by a numerical example, in which the incident angle  0 of the axial ray is 45°, and the numerical aperture NA of the incident light beam of the converging spherical wave to the one-dimensional blazed diffraction grating is defined by NA = sinβ. There are also assumed conditions of a converging angle β of the converging spherical wave of 5.5°, a grating pitch P of 160 µm, and an axial distance L 0 of 35.2 mm from the diffraction grating to the photosensor (three line sensors). Under these conditions and for the +1st order diffracted light of a wavelength λ +1 = 606 nm, the upper ray has an incident angle  1 = 50.5° and a reflected diffraction angle ' 1 , +1 = 50.8°; the lower ray has an incident angle  2 = 39.5° and a reflected diffraction angle ' 2,+1 = 39.8°; and the axial ray has an incident angle  0 = 45° and a reflected diffraction angle ' 0,+1 = 45.3°. Fig. 8 shows the optical paths of ±1st and 0th order reflected diffraction lights to the surface of the 3-line sensor 4. As will be apparent from Fig. 8, the upper and lower rays show a certain aberration in this cross section. The +1st order lights converse in front of the face of the 3-line sensor 4, thus generating a blur of about 75 µm on said photosensor. Similarly the -1st order diffracted lights with a wavelength λ -1 = 471 nm generate a blur. The upper and lower rays of the -1st order diffracted lights converge behind the face of the 3-line sensor 4, thus generating a blur of 58 µm on said sensor in geometrical optics. On the other hand, the 0th order diffracted lights do not generate such image blur, since the reflective one-dimensional blazed diffraction grating merely functions as a mirror surface. The reflective one-dimensional blazed diffraction grating employed in the above-mentioned numerical example has grating thicknesses d 1 = d 2 = d 3 = 749.5 nm, step widths W 1 = W 2 = W 3 = W 4 and a pitch P represented by: Also the efficiency peak wavelengths λ for the diffracted lights of different orders are determined, for the ±1st order lights, by: 2d·cos 0 ≃ (m ∓ 1/n)·λ ±1 wherein (n, m) = (4, 2) so that λ +1 = 606 nm and λ -1 = 471 nm, and for the 0th order light, by: 2d·cos 0 = m·λ 0 so that λ 0 = 530 nm. The above-mentioned amounts of blur are obtained by geometrical optics, but the point image intensity distributions based on simulations by stricter Kirchhoff diffraction are shown in Figs. 9 to 11. As will be shown in these drawings, the amounts of blur in both methods mutually coincide in relatively satisfactory manner. In general, for reading a color image information with a high precision in a color image reading apparatus, the above-mentioned blur cannot be tolerated as it deteriorates the resolving power in the sub scanning direction in the image reading. In the present invention, in order to resolve the above-mentioned drawback, the grating pitch of said one-dimensional blazed diffraction grating is continuously decreased from one peripheral portion to the other peripheral portion, i.e. from the upper end of said grating to the lower end in the drawing, thereby controlling the reflective diffraction angles at the different reflection points, in such a manner that all the light beams, subjected to reflectively diffracted on said blazed diffraction grating, are properly focused on the photosensors. In this manner there is effectively prevented the blur on the photosensor face, caused by the aberration in the reflected diffraction angles ', resulting from different incident angles  to the reflective one-dimensional blazed diffraction grating. More specifically, in the foregoing numerical example, in order to focus the +1st order light of the wavelength λ +1 of 606 nm, with the reflected diffraction angle ' 1,+1 = 50.8° for the upper ray, onto the 3-line sensor without image blur, said angle ' 1,+1 has to be 50.75°, so that the grating pitch P1 at this point has to be 198.5 µm. Also in order to focus the lower ray, with the reflected diffraction angle ' 2,+1 = 39.8°, onto the 3-line sensor without image blur, said angle ' 2,+1 has to be 39.86°, so that the grating pitch P2 at this point has to be 135.0 µm. For this reason, in the present first embodiment, the grating pitch is decreased continuously from the upper side to the lower in such a manner that, as shown in Fig. 7A, the grating pitch P1 at an uper position A is 198.5 µm and that P0 on the axial position B is 160 µm, and as shown in Fig. 7B, the grating pitch P2 at a lower position C is 135.0 µm. In this manner all the light beams coming from the color image and passing through the imaging optical system 2 are properly focused on the photosensor, without generating blur. In the present embodiment, as explained in the foregoing, in the color separation of a converging light beam by a reflective one-dimensional blazed diffraction grating, the grating pitch in the sub scanning cross section (in which the light beam is color separated) is selected larger in a portion (A) where the incoming beam has a larger incident angle and smaller in a portion (C) where the incoming beam has a smaller incident angle. Although the foregoing description has been limited to the +1st order light, but the above-explained variation in the grating pitch has a similar effect of the -1st order light with a wavelength λ -1 = 471.1 nm. In the following there will be explained the features of the reflective one-dimensional blazed diffraction grating of the present embodiment, particularly in comparison with a transmissive one-dimensional blazed diffraction grating. As discussed in Applied Optics, 17 , 15, p. 2273 - 2279 (Aug. 1, 1978), the transmissive diffraction grating separates the incident light beam principally into three directions by transmissive diffraction. For a blazed wavelength λ 0 , such transmissive diffraction grating requires a grating thickness d T = m·λ 0 (n λ0 - 1) , wherein n λ0 is the refractive index of the medium. Calculated for m = 2, λ 0 = 530 nm as in the foregoing embodiment and for a refractive index n λ0 = 1.5, the transmissive diffraction grating requires a thickness d T = 2120 nm. On the other hand, the reflective one-dimensional blazed diffraction grating of the present invention (with an incident angle of 45° for the axial ray) has a grating thickness d R = 749.5 nm as explained before. Consequently the transmissive grating requires a thickness d T about three times as large as the thickness d R of the reflective grating. Such large step imposes significant difficult in the preparation of the diffraction grating. Although this difficulty is somewhat alleviated by the user of a medium of a higher refractive index, the medium for the one-dimensional blazed diffraction grating generally has a refractive index of about 1.5, such as SiO 2 , in consideration of the working properties, cost and other factors. Also in consideration of the efficiency of space utilization in the device, the reflective diffraction grating is advantageous as it easily allows compactization of the entire device. Also in the present embodiment, the imaging optical system 2 is constructed as an exit telecentric system in the main scanning cross section, as shown in Fig. 5A, for the following reason. In case the incident angle of the ray to the diffraction grating varies according to the field angle in the main scanning cross section, the blazed wavelength λ 0 varies according to the following equation: m·λ 0 = di x [ 1 cos 0 ·cos'α + 1-tan 2  0 -(tanα'/cos 0 ) 2 {1/cos 2  0 +(tanα'/cos 0 ) 2 } 1/2 ] More specifically, if a non-telecentric imaging optical system and a reflective one-dimensional blazed diffraction grating are employed for an incident angle α' = 20° in the main scanning cross-section, the brazed wavelength λ 0 is shifted by about 30 nm. In order to avoid such shift, the imaging optical system in the present embodiment is made telecentric at the exit side, whereby the principal rays from the imaging optical system corresponding to different field angles always enter the one-dimensional blazed diffraction grating perpendicularly in the main scanning plane. Fig. 12 is a partial schematic view of the optical system of a second embodiment of the image reading apparatus of the present invention, wherein the reflective one-dimensional blazed diffraction grating 40, serving as the color separation means, is constructed in a cylindrical form, so that an ordinary (non-telecentric) imaging optical system is employed. Also in this embodiment, the pitch of said diffraction grating 40 is continuously varied in the direction of pitch, as in the first embodiment. Let us consider a ray entering the imaging optical system 2 with a field angle α and emerging therefrom with an angle α'. With a non-cylindrical reflective one-dimensional blazed diffraction grating as shown in Fig. 13, the optical path length of the ray which is reflectively diffracted by said grating to a line sensor 4 of the photosensor means is L 0 in the axial position, and L 1 in an off-axial position with an emerging angle α', wherein L 1 = L 0 /cosα'. (It is to be noted that the optical paths are illustrated in broken-lined developed state in Figs. 12 and 13, and that α ≅ α' in the ordinary optical system). Thus, the optical path length between the diffraction grating and the photosensor is not constant but variable depending on the energing angle α'. (i.e. α' = 0 for on-axis, α' ≠ 0 for off-axis) The incident angle  to the reflective one-dimensional blazed diffraction grating and the position (separation distance) Z on the photosensor in the sub scanning direction are correlated by the following equation: Z = L·tan{sin -1 (±λ/p + sin) - } sing + : +1st order, sign - : -1st order, L : L 0 or L 1 . As represented by the equation (4), the position Z is not constant but varies according to the incident angle α' on the photosensor. Therefore, on each of the photosensor arrays arranged in parallel manner, the light beam of a constant wavelength cannot be focused properly. In order to resolve the above-mentioned drawback, in the present embodiment, the substrate bearing the reflective one-dimensional blazed diffraction grating 40 is formed as a cylindrical shape with the center at the exit pupil of the imaging optical system, in consideration of the emerging angle α' in the main scanning plane (finite field angle), whereby the principal rays corresponding to different emerging field angles always perpendicularly enter the diffraction grating 40. In this manner there is effectively prevented the shift in blazed wavelength, dependent on the incident angle in said main scanning plane. On the other hand, the optical path length from said diffraction grating 40 to the 3-line sensor 4 is L 0 = g - R in the axial position (α' = 0) but L 1 = g/cosα' - R in an off-axis position (α' ≠ 0). Thus the optical path length is not constant, in a similar manner as indicated by the equation (4), so that the reflectively diffracted lights are not properly focused on the photosensors. In order to avoid this drawback, the pitch p of said diffraction grating 40 is continuously decreased from the upper side to the lower side in the sub scanning plane as in the first embodiment, and symmetrically increased from the axial position to the off-axis position as shown in Fig. 14 (P to P' in Fig. 14). In this manner the 1st order diffraction angle is varied to achieve proper focusing on parallel linear sensor arrays, from the axial position to the off-axis position. As explained in the foregoing, in the present embodiment, the reflective one-dimensional blazed diffraction grating 40 is cylindrically shaped, and the grating pitch is suitably modified as explained above, whereby the color separation can be properly executed and the color image information can be precisely read for each color, each though an ordinary optical system is employed as the imaging optical system. Fig. 15 is a schematic view of the photosensor means 50 in a third embodiment of the image reading apparatus of the present invention. In contrast to the foregoing second embodiment in which the diffraction grating 40 is cylindrically formed in the main scanning plane, the present embodiment employs a monolithic 3-line sensor 50 as light receiving means in which both line sensors 51, 53 sandwiching a center line 52 are positioned in non-parallel manner, as shown in Fig. 15. In this case the diffraction grating substrate remains flat, and the shift in the Z-direction, represented by the equation (4) is absorbed by the positional shifts of the pixels constituting the line sensor arrays. Also in this case, the grating pitch P of said diffraction grating 40 is continuously varied in the direction of pitch (Z-direction), as in the first embodiment. On the other hand, the shift of the blazed wavelength depending on the incident angle α', resulting from the flat structure of the diffraction grating, is resolved, as shown in Fig. 16, by continuously increasing the grating thickness in the main scanning sectional direction of the diffraction grating 60, with a same ratio for each step, from the axial position toward the off-axis position (from a center portion to a peripheral portion). In a specific numeral example, in use of the diffraction grating of the first embodiment, the blazed wavelength λ 0 is maintained at a constant value of 530 nm by employing a grating thickness d 1 = 797.6 nm for an incident angle α' = 20° in contrast to a grating thickness d 1 = 749.5 nm for an incident angle α' = 0. Fig. 17 is a schematic view of the reflective one-dimensional blazed diffraction grating 70 employed as the color separation means in a fourth embodiment of the image reading apparatus of the present invention. The reflective diffraction grating 70 of the present embodiment has a three-dimensional structure, in which the grating pitch P is gradually increased in the direction of the main scanning cross-section from the center of the optical axis to the outside (Pc to Pe), and is continuously decreased in the direction of the sub scanning cross-section from the upper side to the lower side in a similar manner as in the first embodiment. In this manner there are prevented the aberration in the diffracted focus position on the photosensor and the generation of blur of the ±1st-order diffracted lights in the direction of the sub scanning cross-section. Also the grating thickness is continuously increased from the axial position toward the off-axis position (from hc at center to he at peripheral part) as in the third embodiment to effectively prevent the shift of the blazed wavelength depending on the image angle α'. Thus a color image reading device without a telecentric imaging optical system can be realized by a reflective diffraction grating 70 having a three-dimensional structure formed on a flat substrate. In the foregoing embodiments the variations in the grating pitch and in the grating thickness are assumed to be continuous, but similar effects can be attained by making said variations for example in plural steps. As explained in the foregoing, the present invention can provide a color image reading apparatus capable of reading a color image with a high precision, with color separation means consisting of a reflective one-dimensional blazed diffraction grating and photosensor means consisting of a monolithic 3-line sensor, by suitably modifying the grating pitch of said diffraction grating, thereby effectively eliminating the blur of ±1st-order diffracted lights in the sub scanning direction, caused by the aberration in the reflective diffraction angle resulting from the difference in the incident angle, into said diffraction grating, of the converging spherical wave emerging from the imaging optical system. Also there is provided a color image reading apparatus capable of employing a non-telecentric ordinary imaging optical system, by the use of a reflective diffraction grating with a three-dimensional structure. In the foregoing embodiments the image blur on the photosensor, caused by the aberration in the reflective diffraction angle, resulting from the difference in the incident angle, is prevented by the variation in the pitch of the diffraction grating. In the following there will be explained embodiments capable of preventing said image blur without the change in the grating pitch. Figs. 18A and 18B are respectively a schematic plan view in the main scanning direction and a schematic lateral view in the sub scanning direction of the optical system of a 5th embodiment of the color image reading apparatus of the present invention, and Fig. 19 is an elevation view of a diaphragm SP shown in Figs. 18A and 18B. As the basic configuration of the apparatus is similar to that shown in Figs. 5A and 5B, the components equivalent in function to those in Figs. 5A are represented by same numerals and will not be explained further. The paths of the light beams after passing the imaging optical system are similar to those shown in Fig. 6. In these drawings, an original image plane 1 bears a color image. Light amount control means SP consists of a diaphragm and controls the amount of passing light in such a manner that the numerical aperture (N.A.) in the sub scanning direction, perpendicular to the main scanning direction, becomes smaller than the numerical aperture (N.A.) in said main scanning direction. As shown in Fig. 19, the diaphragm SP of the present embodiment is so constructed that the aperture diameter in the direction of array of pixels of the line sensors (main scanning direction) is larger than that in the sub scanning direction. An imaging optical system 2 is constructed, in the present embodiment, as so-called exit telecentric system, in which the principal rays emerge at the exit side parallel to the optical axis, and serves to focus the light beams from the color image onto photosensor means (monolithic 3-line sensor) through a reflective one-dimensional blazed diffraction grating to be explained later. Color separation means 3, consisting of a reflective one-dimensional blazed diffraction grating, separates the incident light beams into lights of predetermined colors, for example three elementary colors of red (R), green (G) and blue (B), by reflective diffraction in a direction perpendicular to the direction of array of pixels of the line sensors. The pitch of said diffraction grating is maintained constant in the direction of pitch. Photosensor means 4 consists of so-called monolithic 3-line sensor in which three line sensors (CCD's) 8, 9, 10 are formed on a same substrate in mutually parallel manner. The distances among said line sensors 8, 9, 10 are selected differently, corresponding to the directions of color separation of the color separation means 2. In the present embodiment, the color image on the original image plane 1 is line scanned by scanning means consisting for example of an unrepresented scanning mirror, and the passing light quantity of the light beam from said color image is controlled by the diaphragm SP, then converged by the imaging optical system 2, and separated by the one-dimensional blazed diffraction grating 3 into three color light beams, which are respectively focused on the line sensors 8, 9, 10, whereby the photosensor means 4 digitally reads the color images corresponding to said color lights. The reflective one-dimensional blazed diffraction grating 3, used for color separation in the present embodiment, separates, as shown in Fig. 6, the incident light beam into a -1st-order light 5, a 0th-order light 6 and a +1st-order light 7 in three directions, and focuses these lights of converging spherical wave respectively onto the photosensors 8, 9, 10. In general, for reading a color image information with a high precision by a color image reading apparatus, the image blur shown in Figs. 9 and 11 cannot be tolerated as it deteriorates the resolving power in the sub scanning direction. If said blur is in the order of 80 µm for the +1st-order light and 60 µm for the -1st-order light as explained before, such blur is totally unacceptable because the pixels constituting the line sensors are generally sized in the order of 10 x 10 µm. Also the amount of blur for the +1st-order light in the sub scanning direction can be represented as follows, based on geometrical optics which has been proven to provide the approximately correct amount of blur as explained before: X' = L 0 ±tanβ·tan 0 - 1 ·[tan[sin -1 {sin( 0 ±β) + λ+1 P } - ( 0 ±β)] sign + : upper side sign - : lower side and the amount of blur ΔX' +1 can be determined as the difference of the values of the equation (5) respectively corresponding to different signs. Also for the -1st-order light, there stands: X' = L 0 ±tanβ·tan 0 - 1 ·tan[sin -1 {sin 0 ±β) - λ-1 P } - ( 0 ±β)] and the amount of blur ΔX' -1 can be determined as the difference of the values of the equation (5') respectively corresponding to different signs. On the other hand, the numerical apertures in the main and sub scanning directions are respectively represented by sin γ, sinβ wherein γ and β are converting angles of the converging spherical wave respectively in the main and sub scanning directions. As will be apparent from the equations (5) and (5'), the amount of blur ΔX' can be suppressed by reducing the converging angle β in the sub scanning direction. The illumination intensity on the photosensor, corresponding to the required amount of light received thereby, is proportional to the square of the numerical aperture at the converging side. Consequently, a required amount of light may not be obtained if the converging angle β in the sub scanning direction is simply reduced in the designing of the color image reading apparatus. In the present embodiment, therefore, the required amount of light is obtained by selecting the numerical aperture in the main scanning direction larger than that in the sub scanning direction, and the high precise color image reading is enabled by selecting the converting angle β in such a manner that the amount of blurs for the ±1st-order lights on the photosensor remains in the tolerable range. More specifically, in the present embodiment, a diaphragm SP having predetermined shape is provided as the light amount correcting means in front of the imaging optical system 2, thereby controlling the amount of passing light in such a manner that the numerical aperture in the sub scanning direction becomes smaller than that in the main scanning direction, thus maintaining β at a low value. The diaphragm SP has a rectangular apertures, as indicated by a hatched area α (light intercepting portion) in Fig. 19, which is made longer in the main scanning direction, whereby the amount of passing light is made larger in the main scanning direction than in the sub scanning direction. By means of the diaphragm SP of the above-mentioned shape, the converging angle β in the sub scanning direction is reduced for example to 3.0°, while the converting angle γ in the main scanning direction is maintained at 5.5°. The point image intensity distribution in this state is shown in Figs. 20 to 22. As will be apparent from these charts, the amount of blur of the +1st-order light is about 20 µm at a level of 10 % of the peak intensity, and such blur is sufficiently tolerable in consideration of the signal blunting resulting in the scanning operation of the photosensor, due to a certain aperture in the sub scanning direction. On the other hand, in relation to the lower limit of the converging angle β in the sub scanning direction, there stands the following relation among the numerical aperture of the imaging optical system 2, the wavelength γ and the limit of diffraction:  A ≃ 1.64 x λ x 1/2NA wherein NA = sinβ, and  A is the diameter of diffraction spot. According to said equation (6), for example for a wavelength λ +1 of the +1st-order light of 606 nm and a diffraction spot diameter  A of ca. 10 µm (equal to the pixel size), the NA has to be 0.05 or larger. Consequently, the image blur resulting from the numerical aperture of the imaging optical system cannot be maintained within the tolerable level unless the converging angle β is maintained at about 2.8° or larger. In the present embodiment, the incident angle  0 of the axial ray is selected at 30°, since the image blur on the photosensor is also dependent on said incident angle as indicated by the equation (5). The above-mentioned figures are selected because perpendicular incident is is difficult to achieve due to mechanical interference etc. when a reflective one-dimensional blazed diffraction grating is employed as the color separation means. Also in the present embodiment, a slit of a predetermined shape is provided as the light amount correcting means at an arbitrary position between the color image (original image plane) and the photosensor means, so that the amount of passing light is controlled in such a manner that the numerical aperture in the sub scanning direction becomes smaller than that in the main scanning direction, whereby the blur of the ±1st-order lights on the photosensor in the direction perpendicular to the direction of pitch of the diffraction grating is maintained within the signal blunting in the photosensor, resulting from the finite aperture in the sub scanning direction. In this-manner effects similar to those in the foregoing embodiments can be attained by so forming the slit that the amount of passing light becomes larger in the main scanning direction than in the sub scanning direction. In the present invention, the above-mentioned slit may also be positioned in front of or inside the imaging optical system, or in the optical path between the reflective one-dimensional blazed diffraction grating, and the imaging optical system, or on the surface of said diffraction grating. Furthermore, the light amount correcting means may be composed of any optical member that can control the amount of passing light in such a manner that the numerical aperture becomes smaller in the sub scanning direction than in the main scanning direction. As explained in the foregoing, the present invention can provide a color image reading apparatus capable of reading a color image with a high precision, in which apparatus in reading a color image by a photosensor means consisting of a monolithic three-line sensor through a reflective type one-dimensional blazed diffraction grating as color separation means, since there is provided, between the color image and the photosensor means, light amount correcting means for limiting the amount of passing light in such a manner that the numerical aperture of the imaging optical system becomes smaller in the sub scanning direction than in the main scanning direction, thereby maintaining the image blur of the ±1st-order lights in the sub scanning direction, caused by the aberration in the reflective diffraction angle resulting from the difference in the incident angle, to said diffraction grating, of the converging spherical wave emerging from said imaging optical system, within the signal blunting in the photosensor means, resulting from the finite aperture in the sub scanning direction thereof, whereby the resolving power in the sub scanning direction is maintained at a high level.

Claims: An image reading apparatus comprising: a plural-line sensor (4) including plural one-dimensional sensor arrays (8, 9, 10) disposed on the same substrate; an optical imaging system (2) for focussing an image of an object on said plural-line sensor (4); and a reflective blazed diffraction grating (70) positioned between said optical imaging system (2) and said plural-line sensor (4) and adapted to separate light from said object into plural colors; characterized in that the grating pitch (P) of said reflective blazed diffraction grating (70) is continuously decreased in the subscanning direction from the one periphery side to the other periphery side. An image reading apparatus according to claim 1, characterized in that said pitch (P) of said reflective blazed diffraction grating (70) is varied in relation to the incident angle of the light beam entering said reflective blazed diffraction grating (70) in the direction of said pitch (P). An image reading apparatus according to claim 1, characterized in that said pitch (P) of said reflective blazed diffraction grating (70) is varied from an end thereof to the other in the direction of said pitch (P). An image reading apparatus according to claim 1, characterized in that said plural one-dimensional sensor arrays (8, 9, 10) are separated by finite distances in a direction perpendicular to the direction of said sensor arrays (8, 9, 10). An image reading apparatus according to claim 1, characterized in that said reflective blazed diffraction grating (70) is adapted to separate the light from said object into plural colors in a direction perpendicular to the direction of said sensor arrays (8, 9, 10). An image reading apparatus according to claim 1, characterized in that said object is scanned in a subscanning direction perpendicular to the direction of said sensor arrays (8, 9, 10). An image reading apparatus according to claim 1, characterized in that said reflective blazed diffraction grating (70) is three-dimensional. An image reading apparatus according to claim 1, characterized in that said optical imaging system (2) is an exit telecentric optical system in which the principal ray at the exit side emerges parallel to the optical axis. An image reading apparatus according to claim 1, characterized in that the thickness of said reflective blazed diffraction grating (70) is continuously increased from the center of the optical axis of said optical imaging system (2) towards the periphery thereof. An image reading apparatus according to claim 1, characterized in that the grating pitch of said reflective blazed diffraction grating (70) is gradually increased in the main scanning direction from the center of the optical axis of said optical imaging system (2) towards the outside.

Inventors: SETANI MICHITAKA, SETANI, MICHITAKA, Setani, Michitaka, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N1
IPC codes: H04N1