Image reader

0001The present invention relates to an apparatus for reading a color image by using a solid-state imaging device or the like, and more particularly to a color image reader which directs a light from an object to a sensor such as a solid-state imaging device through a focusing optical system and color decomposition means.

0002A prior art apparatus which line-scans an object such as a document sheet in a sub-scan direction and color-reads an image thereof by a solid-state imaging device (such as CCD) array is shown in Fig. 1. In Fig. 1, an information light on a portion of a document sheet 1 illuminated by a light from an illumination light source (not shown) is decomposed and separated to three colors by a three-pieces (3p) prism 20 through an imaging optical system 19, and they are focused to and read by three one-line CCD sensors 21, 22 and 23.

0003In the prior art apparatus, three independent sensors are required. Further, since the 3p prism 20 requires high precision to manufacture, a cost is high. In addition, adjustment of the focusing light beam and the sensors 21, 22 and 23 is required for each of the three sensors and the difficulty in the manufacture is high.

0004It has been proposed to arrange three one-line sensors in parallel on one substrate with a predetermined spacing therebetween so that they function as a monolithic 3-line sensor.

0005Such a 3-line sensor 24 is shown in Fig. 2. Spacings S₁ and S₂ between the three lines 25, 26 and 27 are, for example, 0.1 - 0.2 mm by various manufacturing conditions, and widths W₁ and W₂ of a single element 28 are, for example, 7 µm x 7 µm and 10 µm x 10 µm.

0006A known color image reader which uses such a monolithic 3-line sensor as the photo-sensing device is shown in Fig. 3. In Fig. 3, when information on a document sheet 1 is read by line-scan along a sub-scan direction, a light from the document sheet 1 is decomposed and separated to three light beams of three colors by color decomposing beam splitters 30 and 31 having dichroic selective transmission films applied thereto through a focusing optical system 29, and they are focused onto line-elements on the monolithic 3-line sensor 32.

0007As shown in Fig. 3, where a thickness of the beam splitters 30 and 31 is X, a line-to-line distance on the sensor 32 is 2√2X. Thus, if the line-to-line distance (2√2X) is 0.1 - 0.2 mm as described above, the thickness (X) is 35 - 70 µm. This thickness is not easy to manufacture when a required flatness of the plane is taken into consideration.

0008On the other hand, distances between the center line 26 of the monolithic 3-line sensor and two other lines 25 and 27 are usually equal and integer multiples of a pixel size in the sub-scan direction (W₂ in Fig. 2).

0009The reason is as follows. As seen from Fig. 4, when the image is read by the monolithic 3-line sensor by using only a conventional focusing optical system 45, three positions 25', 26' and 27' (Fig. 4) on the document sheet are simultaneously read by the three lines 25, 26 and 27. Since three color (R, G, B) signal components for one point are not simultaneously read, it is necessary to collate and compose them after reading.

0010Where the line-to-line distances S₁ and S₂ are integer multiples of the pixel size and a redundancy line memory is provided to delay the G and R signals, for example, relative to the B signal, the three-color composition is attained relatively easily. For this reason, the integer multiple distance is selected. However, since the redundant line memory is required for each of the line-to-line distance, a plurality of expensive line memories are required. This is disadvantageous from a cost standpoint and an inexpensive color reader cannot be provided.

0011A color image reader which uses a blazed diffraction grating as color decomposition means is disclosed in USP 4,277,138 (counterpart DE 2,645,075).

0012However, it pays no attention to the construction of a sensor which reads an image of an object.

0013Moreover, document EP-A-0 383 308 (post-published) discloses an image reading device comprising a plural-line sensor containing plural one-dimensional sensor arrays formed on a substrate, a telecentric imaging optical system for forming the image of an object on the sensor, and a blazed diffraction grating positioned in the optical path between the imaging optical system and the sensor and adapted to separate the light from the object into plural color components and to guide thus color-separated lights to respectively corresponding sensor arrays.

0014A further prior art arrangement is known from document EP-A-0 240 000, disclosing an apparatus for separating electromagnetic waves. A projected image is separated into its three color components with spacial and spectral precision by means of a trichromatic beam splitter consisting of dichroic beam splitter plates. The thus color-separated image is guided to a three linear array photosensor comprising a monolithic sensor having three parallel photodiodes spaced precisely to receive the color separated images of the beam splitter.

0015It is an object of the present invention to provide an inexpensive and high performance color image reader.

0016In order to achieve the above object, the image reader of the present invention comprises a multi-line sensor having a plurality of one-dimensional sensor arrays being line shaped in a first direction and substantially parallel to each other, arranged on one substrate with different spacings, said different spacings between the one-dimensional arrays extending in a second direction perpendicular to the first direction; a focusing optical system for focusing an object onto said sensor; and a blazed diffraction grating arranged in a light path between said focusing optical system and said sensor for color-decomposing the light from the object into a plurality of color component lights and directing the decomposed lights to corresponding sensor arrays, and being adapted for performing an image reading operation by scanning an original image in a main scanning direction corresponding to said first direction and in a sub-scanning direction corresponding to said second direction.

0017More specifically, in case of a 3-line sensor, the distances from the center sensor array to the two other sensor arrays correspond to separation distances from the center sensor array on the 3-line sensor which are primarily determined by a wavelength at a maximum diffraction efficiency of two ±1-order diffracted lights of the one-dimension blazed diffraction grating.

0018The one-dimension blazed diffraction grating may be of reflection type or transmission type.

0019In this manner, the restriction to keep the line-to-line distances of the sensor equal to each other and to an integer multiple of the pixel size is eliminated and an inexpensive and high performance line scanning digital image reader is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

0020<ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 shows a prior art apparatus,</li><li>Fig. 2 shows a construction of a monolithic 3-line sensor,</li><li>Fig. 3 shows another prior art apparatus,</li><li>Fig. 4 illustrates prior art 3-line reading,</li><li>Fig. 5A shows a sub-scan sectional view of a first embodiment of the present invention,</li><li>Fig. 5B shows a partial enlarged view of Fig. 5A,</li><li>Fig. 6 shows a spectrum distribution of spectroscopically separated components in the first embodiment,</li><li>Fig. 7 shows a sub-scan sectional view of a second embodiment, and</li><li>Fig. 8 shows a spectrum distribution of spectroscopically separated components in the second embodiment.</li></ul>

0021Fig. 5 shows a first embodiment of a color image reader of the present invention. Fig. 5 shows a sectional view along a sub-scan which is perpendicular to a main scan cross-section. In Fig. 5, image information of a document sheet 1 as an object is line-scanned in a sub-scan direction (vertical direction on the sheet of Fig. 5A) by a mirror (not shown) arranged between the document sheet 1 and a focusing optical system 2, and the image information light is directed to a 3-color decomposing reflection type one-dimension blazed diffraction grating 3 through the focusing optical system 2. The information light from one point (one line) on the document sheet 1 is separated into three-color (R, G, B) light beams 5, 6 and 7 in the so-called color reading, laterally in Fig. 5A by reflection diffraction, and they are focused onto sensor arrays or line sensors 8, 9 and 10 on a monolithic 3-line sensor 4. The document sheet 1 and the image reader (focusing optical system 2, diffraction grating 3 and sensor 4) are relatively moved in the sub-scan direction to sequentially read the image information of the document sheet 1.

0022The sensor arrays 8, 9 and 10 on the sensor 4 extend in the main scan direction normal to the sheet of Fig. 5A. The sensor 4 is a monolithic 3-line sensor having three one-dimension sensor arrays arranged on one substrate with the arrays spaced from each other in a direction (main scan direction) perpendicular to the array direction by a predetermined distance.

0023The one-dimension blazed diffraction grating 3 is arranged in a light path between the focusing optical system 2 and the sensor 4, closer to the sensor 4 than to an exit pupil of the focusing optical system 2 to color-decompose the light from the object and direct the color-decomposed lights to the corresponding sensor arrays.

0024The document sheet 1 is illuminated by an illuminating light source (not shown) and the image information is read by the image reader.

0025The three-color decomposing one-dimension blazed diffraction grating is shown in Applied Optics, Vol. 17, No. 15, pages 2273 - 2279 (August 1, 1978), and the shape thereof is shown in an enlarged view in Fig. 5B, which shows a sub-scan sectional view.

0026In the reflection type one-dimension blazed diffraction grating 3 shown in Fig. 5B, distances 16 and 15 (where the light is separated and focused) on the sensor plane 4 between the 0-order light 6 and the reflected, diffracted and separated ±1-order diffracted lights 7 and 5 are given by<maths id="math0001" num="(1)"><math display="block"><mrow><msup><mrow><mtext>Z = ℓ x tan{sin</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>1</mtext></mrow></msup><msub><mrow><mtext>(±λ/p + sin θ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><msub><mrow><mtext>) - θ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext>}</mtext></mrow></math><img file="EP0440169B1_D0001.tif" /></maths> where λ is a wavelength of the separated and focused information light, θ₀ is an incident angle to the blazed diffraction grating 3, p is a grating pitch and ℓ is a distance between the grating and the photosensing plane along an optical axis.

0027For example, where the reflection type blazed diffraction grating 3 is of stepped shape and a first step has a depth h₁ = 909 nm and a second step has a depth h₂ = 1818 nm, then a center wavelength of the 0-order light is λ₀ = 525 nm, the wavelength of the +1-order diffracted light is λ₊₁ = 592 nm, and the wavelength of the -1-order diffracted light is λ_-1 = 472 nm. They follow the following formula.<maths id="math0002" num="(2)"><math display="block"><mrow><msub><mrow><mtext>λ = 2h</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msub><mrow><mtext>·cos θ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><msub><mrow><mtext>/m = 2h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mtext> cos θ/2m</mtext></mrow></math><img file="EP0440169B1_D0002.tif" /></maths> where λ₊₁ is calculated by m = 3-1/3, λ_-1 is calculated by m = 3+1/3, and λ₀ is calculated by m = 3 (approximated values for λ±1).

0028Where the grating pitch of the diffraction grating is p = 130 µm, the grating - photosensing plane distance along the optical axis is ℓ = 45 mm and the incident angle is θ₀ = 30 degrees, then Z λ₊₁ = 0.171 mm and Z λ_-1 = -0.136 mm.

0029Namely, the line-to-line distances 16 and 15 between the sensor line 9 (G) and the two other lines of the monolithic 3-line sensor 4 are selected to 0.171 mm for the +1-order light (λ₊₁) (R) (that is, the distance 16 to the line 10), and 0.136 mm for the -1-order light (λ_-1) (B) (that is, the distance 15 to the line 8) so that the 3-line sensor 4 has asymmetric line-to-line distances. In this manner, an efficient color reader which eliminates the need for the interpolating redundant memory described above is provided.

0030In determining the center wavelengths of the respective order lights based on the relationship between the grating thicknesses h₁ and h₂ of the step and the center wavelengths of the respective order lights, m is set to 3 in the above example although any positive integer may be selected in principle. However, m is preferably no smaller than 3 because the shortage of the light intensity of the R and B components, in the present embodiment, likely occurs according to the 1-order diffraction efficiency. (Particularly when a lamp used for a black body radiation such as a halogen lamp is used as the illumination light source, the shortage of the light intensity of the B component likely occurs.)

0031As described above, in accordance with the present invention, the color image reader comprises a 3-line sensor having three one-dimension sensor arrays arranged on one substrate with each array being spaced by a predetermined distance along a direction perpendicular to the array direction, a focusing optical system for focusing an image of an object onto the sensor, and a one-dimension blazed diffraction grating arranged in a light path between the focusing optical system and the sensor, closer to the sensor than to an exit pupil of the focusing optical system for spectroscopically separating a light from the object by a diffraction effect to color-decompose it into three colors perpendicularly to the array and directing the color-decomposed lights to the corresponding sensor arrays. Distances from a center sensor array of the 3-line sensor to two other sensor lines are different from each other.

0032Further, the distances from the center sensor array to the two other sensor arrays correspond to separation distances from the center sensor array on the 3-line sensor which are primarily determined by a wavelength at a maximum diffraction efficiency of two ±1-order diffracted lights of the one-dimension blazed diffraction grating.

0033Fig. 6 shows a spectrum distribution of the spectroscopically separated components when the reflection type one-dimension blazed diffraction grating 3 of the present embodiment and a halogen lamp are used.

0034Fig. 7 shows a second embodiment of the color image reader of the present invention. It shows a sub-scan sectional view. In the first embodiment, the reflection type one-dimension blazed diffraction grating 3 is used to attain the three-color separation effect. In the second embodiment, a similar effect is attained by a transmission type one-dimension blazed diffraction grating 3'.

0035The like numerals to those shown in Figs. 5A, 5B designate the like function elements or diffracted lights. The sensor plane of the sensor 4 is parallel to the line scan direction (sub-scan direction).

0036As shown in Fig. 6, where a diffraction grating 23 is arranged perpendicularly to the optical axis of the optical system, a relationship between the center wavelengths of the respective order diffracted lights and the shapes of the stepped grating is given by<maths id="math0003" num="(3)"><math display="block"><mrow><msub><mrow><mtext>λ = (nλ - 1) h</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msub><mrow><mtext>/m = (nλ - 1)·h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mtext>/2m</mtext></mrow></math><img file="EP0440169B1_D0003.tif" /></maths> where the symbols are same as those used for the first embodiment, λ₀ is determined by m = 3, λ₊₁ is determined by m = 3 - 1/3 and λ_-1 is determined by m = 3 + 1/3, nλ is a refractive index of the grating 23, and the formula gives approximated values for λ±1.

0037Where the center wavelength of the 0-order diffracted light is to be same as that in the first embodiment, that is λ₀ = 525 nm, and n ≃ 1.5, then    h₁ = 3150 nm, h₂ = 6300 nm

0038From the formula (3), λ₀ = 525 nm, λ₊₁ = 591 nm and λ_-1 = 473 nm. Thus, the center wavelengths of the G, R and B diffracted lights are substantially the same as those in the first embodiment. For the transmission type diffraction grating 23, the distances 16 and 15 on the sensor plane 4 spaced by the grating-photosensor plane distance ℓ along the optical axis are given by<maths id="math0004" num="(4)"><math display="block"><mrow><msup><mrow><mtext>Z = ℓ x tan{sin</mtext></mrow><mrow><mtext>-1</mtext></mrow></msup><mtext>(±λ/p)}</mtext></mrow></math><img file="EP0440169B1_D0004.tif" /></maths> Like in the first embodiment, where the grating pitch is p = 0.18 mm and the grating-photosensor plan ℓ along the optical axis is ℓ = 45 mm, then Z λ₊₁ = 0.148 mm and Z λ_-1 = -0.118 mm.

0039Thus, the line-to-line distances 16 and 15 between the sensor line 9 (G) and the two other lines of the monolithic 2-line sensor 4 are selected to 0.148 mm for the +1-order light (λ₊₁) (R) (that is, the distance 16 to the line 10), and 0.118 mm for the -1-order light (λ_-1) (B) (that is, the distance 15 to the line 8) so that the 3-line sensor 4 has asymmetric line-to-line distances. In this manner, a similar effect to that of the first embodiment is attained.

0040In the transmission type one-dimension blazed diffraction grating 3', the grating thicknesses h₁ and h₂ are larger than those of the reflection type diffraction grating 3, accordingly, the easiness in manufacturing the grating is reduced but since the optical axis can be kept straight, the optical system is simplified.

0041In the first embodiment, the G component light having λ₀ = 525 nm is used for the 0-order diffracted light component 6. In general, the B component light which is likely short in terms of light intensity may be used as the 0-order diffracted light component to compensate for the shortage.

0042For example, in the reflection type one-dimension blazed diffraction grating 3, where the grating pitch P and the incident angle θ₀ are same as those in the first embodiment and the stepped grating having the grating thicknesses h₁ = 831 nm and h₂ = 1662 nm are used, the center wavelengths of the respective order diffracted lights are determined by the formula (2) as follows.    λ₀ = 480 nm, λ_-1 = 540 nm and λ₊₁ = 617 nm (where λ₀ is determined by m = 3, λ_-1 is determined by m = 3 - 1/3 and λ₊₁ is determined by 3 - 2/3).

0043A spectrum distribution of the spectroscopically separated components when a halogen lamp is used as the light source is shown in Fig. 8. As seen from Fig. 8, the B component light which is likely short composed to the spectrum distribution of Fig. 6 is compensated.

0044The line-to-line distances of the sensor are determined by the formula (1). The distance to the sensor line 9 (B) is 0.156 mm for the -1-order light (λ_-1) (G) (that is, the distance 15 to the line 8), and 0.178 mm for the +1-order light (λ₊₁) (R) (that is, the distance 16 to the line 10).

0045In the above embodiments, the three-line sensor having the -1-order light sensor line (B), the 0-order light sensor line (G) and the +1-order light sensor line (R), and the three-line sensor having the -1-order light sensor line (G), the 0-order light sensor line (B) and the +1-order light sensor line (R) are used, although other combinations may be used. In any case, the distances from the center sensor array of the three-line sensor to two other sensor arrays are different from each other.

0046As described above, in accordance with the color image reader of the present invention which reads the image information by the line scan, the monolithic 3-line sensor is arranged in parallel to the line scan direction (sub-scan sectional direction) and the color decomposing one-dimension blazed diffraction grating is mounted between the focusing optical system and the sensor plane to decompose the light into three color components. The light beams are separated and focused to the corresponding line sensors to color-read the image. The distances between the center line and the two other lines are asymmetric as determined by the grid configuration of the one-dimension blazed diffraction grating and the arrangement condition. As a result, high quality color reading is attained without the redundant line memory which is required in the conventional 3-line reading to interpolate the lines.

0047The present invention also suppresses, in principle, the color shift due to the mechanical vibration by the scan-reading which is likely to occur in the conventional 3-line reading.

0048An image reader comprises a monolithic 3-line sensor having three one-dimension sensor arrays such as solid-state imaging devices arranged on one substrate with each array being spaced by a predetermined distance along a direction perpendicular to the array direction, a focusing optical system for focusing an image of an object onto the sensor, and a one-dimension blazed diffraction grating arranged in a light path between the focusing optical system and the sensor, closer to the sensor than to an exit pupil of the focusing optical system for spectroscopically separating a light from one point on the object by a diffraction effect to color-decompose it into three colors perpendicularly to the array and directing the color-decomposed lights to the corresponding sensor arrays. Distances from a center sensor array of the 3-line sensor to two other sensor lines are different from each other.