Optical image reader

An optical image reader includes a transparent light guide plate having an object placement surface, an illumination light source for emitting light to an end face of the plate, a holographic diffraction grating element disposed at a surface of the plate opposite to the object placement surface via a low refraction index layer, a sensor unit disposed near an end face of the diffraction grating element via a light-condensing optical system, and a control circuit connected to the diffraction grating element and sensor unit. The control circuit performs controlling so that linear areas to which an electrical field is applied are successively selected in the extension direction of the object placement surface, successively receives electrical signals output in linear area units from the sensor unit, converts them into pieces of linear image information, and joins these pieces of linear image information to produce a two-dimensional image.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical image reader making use of a diffraction effect by a hologram.

2. Description of the Related Art

An optical image reader which is generally and conventionally known as a flat head type operates on the basic principle of linearly reading an object image with an optical system primarily comprising an illuminator, a mirror, a lens, and a charge coupled device (CCD) line sensor. The optical image reader forms a two-dimensional image by successively repeating imaging of linear images and joining these linear images. The imaging is carried out while mechanically driving and unidirectionally moving a carriage of the optical system. (For example, refer to Japanese Unexamined Patent Application Publication Nos. 9-116703 and 2000-115455.) In order to move the carriage, such an optical image reader requires, for example, a motor for converting electrical energy into mechanical energy, a rail for supporting the carriage, and a shaft for converting rotational energy generated by the motor into a driving force in a unidimensional direction.

However, in the above-described optical image reader, the structure for mechanically moving the carriage is complicated. In addition, it is very difficult to downsize the optical image reader to a portable size.

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the above-described drawbacks, and has as its object the provision of an optical image reader which can carry out two-dimensional imaging without using a driving mechanism and which can be reduced in size.

To this end, according to a first aspect of the present invention, there is provided an optical image reader comprising a transparent light guide plate having an object placement surface, an illumination light source for emitting light to an end face of the transparent light guide plate, a holographic diffraction grating element disposed at a surface of the transparent light guide plate opposite to the object placement surface via a transparent low refraction index layer having a refraction index that is lower than that of a material of the transparent light guide plate, a sensor unit disposed near an end face of the holographic diffraction grating element via a light-condensing optical system, and a control circuit connected to the holographic diffraction grating element and the sensor unit. The transparent light guide plate receives the light emitted from the illumination light source from the end face thereof, and causes the light that propagates through the transparent light guide plate and that is scattered by an object placed on the object placement surface to exit towards the low refraction index layer from the surface opposite to the object placement surface. The low refraction index layer totally reflects the light which propagates through the light guide plate, transmits a portion of the scattered light which has exited from the light guide plate, and causes the light to exit towards the holographic diffraction grating element. The holographic diffraction grating element has a diffraction efficiency changing layer whose diffraction efficiency changes by an applied electrical field strength, an electrical field being applied to selected linear areas of the diffraction efficiency changing layer and the scattered light that has exited from the low refraction index layer being diffracted at the selected linear areas to which the electrical field has been applied. The light-condensing optical system is disposed on an optical axis extending in a direction in which the scattered light is diffracted at the linear areas and focuses the diffracted light on the sensor unit in linear area units. The sensor unit converts the intensities of the focused light into electrical signals and outputs the electrical signals to the control circuit. The control circuit performs a controlling operation so that the linear areas to which the electrical field is applied are successively selected in the direction of extension of the object placement surface, successively receives the electrical signals output in linear area units from the sensor unit, converts the electrical signals into pieces of linear image information, and joins these pieces of linear image information in order to produce a two-dimensional image.

In the optical image reader of the first aspect, light (illumination light) emitted to the transparent light guide plate from the illumination light source propagates through the transparent light guide plate while being repeatedly totally reflected by the surfaces of the light guide plate. In addition, the low refraction index layer disposed between the surface of the transparent light guide plate opposing the object placement surface and the holographic diffraction grating element makes it possible to increase propagation by total reflection of the illumination light through the transparent light guide plate.

A portion of the illumination light propagating in the transparent light guide plate is scattered by striking a portion of an object placed on the object placement surface, and a portion of the scattered light passes through the low refraction index layer and impinges upon the holographic diffraction grating. In the holographic diffraction grating, the diffraction efficiency is increased by successively applying an electrical field to linear areas of the diffraction efficiency changing layer by the control circuit, the light which has exited from the low refraction index layer is diffracted by the linear areas to which the electrical field has been applied, and the diffracted light passes through areas where the electrical field has not been applied in order to be focused on the sensor unit by the light-condensing optical system in linear area units. The sensor unit converts the intensities of the focused light into electrical signals, and outputs the electrical signals to the control circuit. The control circuit can produce a two-dimensional image by successively receiving the electrical signals output from the sensor unit in linear area units, converting the electrical signals into pieces of linear image information, and combining these pieces of linear image information.

According to a second aspect of the present invention, there is provided an optical image reader comprising a first holographic diffraction grating element having an object placement surface and a diffraction efficiency changing layer whose diffraction efficiency changes by an applied electrical field strength, an electrical field being applied to selected linear areas of the diffraction efficiency changing layer; an illumination light source for emitting light to an end face of the first holographic diffraction grating element; a second holographic diffraction grating element disposed at a surface of the first holographic diffraction grating element opposite to the object placement surface via a transparent low refraction index layer having a refraction index that is lower than that of the opposite surface; a sensor unit disposed near an end face of the second holographic diffraction grating element via a light-condensing optical system; and a control circuit connected to the first and second holographic diffraction grating elements and the sensor unit. The first holographic diffraction grating element receives the light emitted from the illumination light source from the end face thereof, diffracts the light which propagates through the first holographic diffraction grating element towards the object placement surface by the linear areas to which the electrical field has been applied, and causes the diffracted light that is scattered by an object placed on the object placement surface to exit towards the low refraction index layer from the surface opposite to the object placement surface. The low refraction index layer totally reflects the light which propagates through the first holographic diffraction grating element, transmits a portion of the scattered light which has exited from the first holographic diffraction grating element, and causes the light to exit towards the second holographic diffraction grating element. The second holographic diffraction grating element has a diffraction efficiency changing layer whose diffraction efficiency changes by an applied electrical field strength, an electrical field being applied to selected linear areas of the diffraction efficiency changing layer and the scattered light that has exited from the low refraction index layer being diffracted at the selected linear areas to which the electrical field has been applied. The light-condensing optical system is disposed on an optical axis extending in a direction in which the scattered light is diffracted at the linear areas and focuses the diffracted light on the sensor unit in linear area units. The sensor unit converts the intensities of the focused light into electrical signals and outputs the electrical signals to the control circuit. The control circuit performs a controlling operation so that the linear areas to which the electrical field is applied of the first and second holographic diffraction grating elements are successively selected in the direction of extension of the object placement surface, successively receives the electrical signals output in linear area units from the sensor unit, converts the electrical signals into pieces of linear image information, and joins these pieces of linear image information in order to produce a two-dimensional image.

In the optical image reader of the second aspect, it is possible to concentrate more illumination light at object imaging areas with the first holographic diffraction grating element instead of the transparent light guide plate used in the first aspect, so that, for example, the S/N ratio of the image after imaging is increased and the power of the illumination light source is saved.

The optical image reader may be formed so that the diffraction efficiency changing layer of the (or each) holographic diffraction grating element is sandwiched between a pair of transparent substrates, transparent electrodes for applying the electrical field to the (or each) diffraction efficiency changing layer are disposed on the inner sides of the transparent substrates, and the (or each) diffraction efficiency changing layer has first areas having an isotropic refraction index and second areas having a refraction index that is different from that of the first areas by the application of the electrical field.

Since the (or each) holographic diffraction grating element comprises a diffraction efficiency changing layer comprising first areas having an isotropic refraction index and second areas having a refraction index that is different from that of the first areas by application of an electrical field, when a voltage is applied between selected transparent electrodes, the electrical field is applied to the linear areas between the transparent electrodes to which the voltage has been applied, thereby producing a difference between the refraction indices of the first and second areas disposed in the linear areas. Therefore, the scattered light that has exited from the low refraction index layer undergoes Bragg diffraction. Here, there is a close relationship between the degree of the diffraction efficiency and the extent of the difference between the refraction index of the first areas and that of the second areas. Only light having, for example, a wavelength and an angle of incidence satisfying particular conditions is diffracted instead of any light.

The optical image reader may be formed so that the low refraction index layer totally reflects the light having an angle of incidence that is greater than sin−1(nh/nl), and transmits the light having an angle of incidence that is equal to or less than sin−1(nh/nl) (where nhis the refraction index of the transparent light guide plate or the surface of the first holographic diffraction grating element adjacent to the low refraction index layer, and nlis the refraction index of the low refraction index layer).

The optical image reader may be formed so that the light to be diffracted by the linear areas to which the electrical field (or electrical fields) has (have) been applied satisfies the following Bragg condition:
2Λ·sin θi′=λ′
where Λ represents the refraction index distribution period of the (or each) diffraction efficiency changing layer, θi′ represents the angle between an equal refraction index surface in the (or each) diffraction efficiency changing layer and a light incidence path in the (or each) diffraction efficiency changing layer, and λ′ represents the effective wavelength of the light in the (or each) diffraction efficiency changing layer.

The optical image reader may be formed so that a transparent low refraction index layer is disposed at the object placement surface.

According to the present invention, it is possible to provide an optical image reader which can perform two-dimensional imaging without using a driving mechanism and which can be reduced in size.

This is due to the following reasons. Whereas, in the related flat head optical image reader, a reading location is scanned by moving the carriage, in the present invention, two-dimensional imaging can be achieved by successively scanning linear areas having high diffraction efficiency of the holographic diffraction grating. In addition, whereas, in the related reader, it is necessary to illuminate a limited area near a reading location with illumination light, in the present invention, only light having, for example, a wavelength and an angle of incidence satisfying particular conditions (more specifically, light which propagates from a particular direction and satisfies the Bragg diffraction condition) is diffracted, so that the location of illumination does not need to be necessarily limited.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Obviously, the present invention is not limited to the embodiments that are described below, and parts are drawn to different scales that allow the parts to be easily denoted.

FIG. 1is a schematic view of the structure of an optical image reader of a first embodiment of the present invention.FIG. 1illustrates typical paths of illumination light and an object in order to describe the function of the optical image reader of the first embodiment.

The optical image reader of the embodiment generally comprises a transparent light guide plate5having an object placement surface5a, an illumination light source4for emitting light (illumination light)10to an end face5cof the transparent light guide plate5, a holographic diffraction grating element100disposed at a side of the transparent light guide plate5opposing the object placement surface5athrough a transparent low refraction index layer6having a lower refraction index than the material of the transparent light guide plate, a sensor unit8disposed near an end face of the holographic diffraction grating element100through a light condensing optical system7, and a control circuit (not shown) connected to the holographic diffraction grating element100and the sensor unit8.

The illumination light source4is disposed beside the end face5cof the light guide plate5, and primarily comprises a light emitter and a light guide. The light emitter is, for example, a laser diode (LD), a light emitting diode (LED), or a cold cathode tube (CCFL). The light guide efficiently guides the light emitted from the light emitter to the transparent light guide plate5. The illumination light10emitted from the light emitter may be any one of a monochromatic light, white light, and light which is a synthesis of three primary colors (red, green, and blue). In the embodiment, monochromatic light is used as the illumination light10. When the illumination light10is monochromatic light, an image after imaging is monochromatic.

It is desirable that the light guide be formed of a material that is sufficiently transparent to visible light. Therefore, optical glass (synthetic quartz or alkali free glass) is used for the material of the light guide.

The material of the transparent light guide plate5(the transparent light guide plate material) is optical glass or plastic. It is sufficiently transparent to the visible range (380 nm to 780 nm) and is desirably transparent to a wavelength range of from the wavelength of ultraviolet light (330 nm or more) to the wavelength of near infrared light (1600 nm or less), and has a refraction index of from 1.4 to 1.9 in the visible range.

When using the optical image reader of the embodiment, an object14is disposed in close contact with the object placement surface5aof the transparent light guide plate5.

The low refraction index layer6is disposed at a surface (light-exiting surface)5bof the transparent light guide plate5opposing the object placement surface5a, has a refractive index that is lower than that of the transparent light guide plate material in the range of from the wavelength of visible light to the wavelength of near infrared light, and is transparent.

The low refraction index layer6is used to efficiently transmit the illumination light10in the transparent light guide plate5by total reflection and to restrict direct impingement upon the holographic diffraction grating element100of light other than a scattered component of the light from the object14placed on the object placement surface5a.

The low refraction index layer6may be formed by, for example, coating the light-exiting surface5bof the transparent light guide plate5with low refraction index resin that is transparent to light, such as fluorocarbon polymer, or forming an air gap between the holographic diffraction grating element100and the light-exiting surface5bof the transparent light guide plate5.

The low refraction index layer6totally reflects light having an angle of incidence that is greater than sin−1(nh/nl) with respect to a normal line direction of the layer, and transmits light having an angle of incidence that is equal to or less than sin−1(nh/nl) with respect to the normal line direction. (nhis the refraction index of the transparent light guide plate5, and nlis the refraction index of the low refraction index layer6.)

When light impinges upon a layer having a low refraction index from a layer having a high refraction index, and the incident angle θisatisfies the relationship of Expression (1) below, the light is totally reflected. Therefore, it is desirable that the light guide of the illumination light source4be such that light which satisfies the following Expression (1) exits therefrom.
θi>sin−1(nh/nl)  (1)
(In the expression, nhrepresents the refraction index of the high refraction index layer, and nlrepresents the refraction index of the low refraction index layer.)

By virtue of the above-described structure, when imaging of the object14is carried out, it is possible for the illumination light10to be emitted from the illumination light source4and to enter the transparent light guide plate5from the end face5c. The illumination light10that has entered the transparent light guide plate5propagates through the transparent light guide plate5, and portions of the illumination light10are scattered at locations of the object14that is disposed in close contact with the light guide plate5. Of the portions of the light scattered from the object14, light portions10bthat are incident upon the low refraction index layer6at an angle equal to or less than sin−1(nh/nl) pass the interface between the low refraction index layer6and the light guide plate5and reach the holographic diffraction grating element100, whereas light portions that are incident upon the low refraction index layer6at an angle that is greater than sin−1(nh/nl) are totally reflected at the interface between the low refraction index layer6and the light guide plate5.

In the holographic diffraction grating element100, a pair of transparent substrates33aand33bsandwich a medium layer30(diffraction efficiency changing layer) whose diffraction efficiency changes successively by the strength of an applied electrical field and which has a property which is the same as or equivalent to that of the transparent light guide plate5. The inner sides of the transparent substrates33aand33bare patterned with transparent electrodes34aand34b, so that an electrical field can be applied to predetermined linear areas of the diffraction efficiency changing layer30.

Of the transparent electrodes34aand34b, the transparent electrodes34bare disposed in the form of strips on the transparent substrate33b. The transparent electrode34amay be formed over the entire surface of the transparent substrate33a, or a plurality of the transparent electrodes34amay be disposed in the form of strips. When the transparent electrodes34aand the transparent electrodes34bare disposed in the form of strips, they are disposed so as to overlap each other as viewed from one side of the element100. The pitch of the electrodes34bis equal to a width corresponding to the resolving power.

As shown inFIG. 2, the diffraction efficiency changing layer30is formed of a hologram recording material and has a structure in which first areas31and second areas32are periodically repeated. Each first area31has an isotropic refraction index. Each second area32has an anisotropic refraction index (uniaxial or biaxial) which differs from that of each first area31due to an application of an electrical field.

In the embodiment, each first area31is formed of polymer resin, and each second area32is formed of liquid crystals. A high density of the polymer resin is disposed in each first area31indicated by slanted lines inFIG. 2, and a high density of the liquid crystal molecules is disposed in each second area32shown as an elliptically shaped area inFIG. 2. The liquid crystals used in the second areas32are nematic liquid crystals and have a positive or a negative dielectric anisotropy. When an electrical field is applied to the liquid crystals, the liquid crystals respond to the electrical field by the dielectric anisotropy that they have and change their orientation states.

The direction in which the orientation states of the liquid crystals change depends upon whether the dielectric anisotropy of the liquid crystals is positive or negative. When the dielectric anisotropy is positive, liquid crystal molecules that move parallel to the electrical field direction are used, whereas when the dielectric anisotropy is negative, liquid crystal molecules that move perpendicular to the electrical field direction are used. By such movements of the liquid crystal molecules, the refraction index of each second area32changes.

PDLC disclosed in PCT Japanese Translation Patent Publication No. 2000-515996 may be used as the liquid crystals of the second areas32.

The diffraction efficiency changing layer30is a volume hologram. Light satisfying the Bragg diffraction condition expressed by Expression (2) below is considerably diffracted (diffraction efficiency is high), and light that does not satisfy the Bragg diffraction condition is substantially not diffracted (diffraction efficiency is very low or zero).
2Λ·sin θi′=λ′  (2)
(In the expression, Λ represents the refraction index distribution period of the diffraction efficiency changing layer30, θi′ represents the angle between an equal refraction index surface in the diffraction efficiency changing layer and a light incidence path in the diffraction efficiency changing layer, and λ′ represents the effective wavelength of the light in the diffraction efficiency changing layer.) The equal refraction index surface is a surface in which equal refraction indices are continuously provided in a refraction index distribution of a cross section taken along line II—II ofFIG. 2when, for example, voltage is applied to a portion of the diffraction efficiency changing layer30shown inFIG. 2.

The diffraction efficiency changing layer30is formed so that the refraction index of the liquid crystals of the second areas32is substantially equal to the refraction index of the polymer resin of the neighboring first areas31when an electrical field is not applied under the Bragg condition with respect to the wavelength of the illumination light10. Therefore, when an electrical field is not applied to the liquid crystals, the light is scattered at the object14and passes through the diffraction efficiency changing layer30without being diffracted even if some of the light portions10breaching the diffraction efficiency changing layer30satisfy the Bragg condition. Since a light component that does not satisfy the Bragg condition is not diffracted, it only passes through the diffraction efficiency changing layer30regardless of the state of the liquid crystals.

When an electrical field is applied to the liquid crystals of the second areas32, the liquid crystal molecules move and change their orientations. This causes the refraction index of the liquid crystal areas that sense light under the Bragg condition to differ from the refraction index of the neighboring polymer resin (that is, the first areas31), so that the light is diffracted. If the refraction index of the second areas32changes so that the difference between the refraction index of the second areas32and that of the neighboring first areas31is small, the diffraction efficiency is small. In contrast, if the refraction index of the second areas32changes so that the difference between the refraction index of the second areas32and that of the neighboring first areas31is large, the diffraction efficiency is large.

Therefore, if an electrical field is applied to only any linear areas42of the diffraction efficiency changing layer30, only particular light satisfying the Bragg condition is diffracted at these areas. Such particular light corresponds to light scattered and exiting from locations of the object14corresponding to the particular linear areas42of the diffraction efficiency changing layer30to which an electrical field has been applied.

The light condensing optical system7is disposed on an optical axis extending in the diffraction direction of light11. The light11is obtained after the light portions10bhave passed through the low refraction index layer6, have reached the holographic diffraction grating element100, and have been diffracted at the linear areas42to which an electrical field has been applied. The light condensing optical system7is designed so that the light11, which is obtained by diffraction in linear area units, is focused on the sensor unit8. Accordingly, the light11is focused on the sensor unit8.

The sensor unit8is a sensor array in which light-receiving sections are disposed in a unidimensional direction, and uses a semiconductor line sensor unit, such as a CCD or a CMOS. The sensor unit8converts the intensities of the focused light into electrical signals, and outputs the electrical signals to the control circuit.

The control circuit carries out a controlling operation so that linear areas42to which an electrical field is applied are successively selected in the direction of extension of the object placement surface (that is, in a direction parallel to the object placement surface), successively receives electrical signals output in linear area units from the sensor unit8, converts them into pieces of linear image information, and successively joins these pieces of linear image information regarding portions of the object14(linear portions) above the linear areas42to which the electrical field has been applied in order to produce a two-dimensional image of the object14.

An electrical field is applied to linear areas42disposed at locations where the upper transparent electrode34aand the lower transparent electrodes34bcorresponding to the linear areas42overlap by applying a voltage to these transparent electrodes34aand34b.

Here, a method for applying an electrical field to the liquid crystals will be described.

FIG. 3Ashows an applied voltage waveform when a positive or a negative electrical field is applied for a predetermined sufficiently longer period of time than the response speed of the liquid crystal molecules.

FIG. 3Bis a schematic view showing changes in the diffraction efficiency when the electrical field having such a waveform is applied to the liquid crystals. Actually, a delay in the rising and falling in the waveform occurs by the response speed of the liquid crystals. Since the nematic liquid crystals respond to an effective value of the electrical field, a difference in change of the diffraction efficiency is not produced due to a difference in the polarity of the electrical field. However, this method has a definite disadvantage. That is, when ionic impurities which move by the electrical field are mixed among the liquid crystals, field reversing caused by the ionic impurities weakens the effective electrical field applied to the liquid crystals.

To overcome this problem, a driving method shown in FIG.4is provided.

FIG. 4Ashows an applied voltage waveform when the positive and negative polarities of the electrical field are reversed in the form of pulses for a sufficiently shorter period of time than the response speed of the liquid crystals. Since the nematic liquid crystals respond to an effective value of the electrical field, they do not follow polarity changes of the electrical field in a short time. This gives rise to changes in the diffraction efficiency of the holographic diffraction grating element100as shown inFIG. 4B. The feature of this method is that it is possible to reduce the effects of the field reversing caused by the ionic impurities described with reference toFIG. 3when the period of the polarity reversal of the electrical field is longer and their movement is sufficiently faster than the response speed of the liquid crystals.

According to the embodiment, since two-dimensional imaging can be carried out by successively scanning linear areas of the holographic diffraction grating having a high diffraction efficiency, a driving mechanism such as a carriage is not required. Therefore, it is possible to provide a downsized optical image reader. When 500 electrodes34bare disposed at a pitch of 50 μm, and when the response speed of the PDLC is 200 μs, the optical image reader of the embodiment can carry out imaging of an object at an imaging speed of 100 ms.

FIG. 5is a schematic view of the structure of an optical image reader of a second embodiment of the present invention.FIG. 5illustrates typical paths of illumination light and an object in order to describe the function of the optical image reader of the second embodiment.

The optical image reader of the embodiment comprises a holographic diffraction grating element200(which may also be called a first holographic diffraction grating element in the embodiment) instead of the transparent light guide plate5of the optical image reader of the first embodiment shown inFIG. 1.

In the structure of the first holographic diffraction grating element200, a diffraction efficiency changing layer60, which is similar to the diffraction efficiency changing layer30in the first embodiment, is sandwiched by a pair of transparent substrates63aand63b, and the inner sides of the transparent substrates63aand63bare patterned with transparent electrodes64aand64b, so that an electrical field can be applied to predetermined linear areas62of the diffraction efficiency changing layer60.

Of the transparent electrodes64aand64b, the transparent electrodes64bare disposed in the form of strips on the transparent substrate63b. The transparent electrode64amay be formed over the entire surface of the transparent substrate63a, or a plurality of the transparent electrodes64amay be disposed in the form of strips. When the transparent electrodes64aand the transparent electrodes64bare disposed in the form of strips, they are disposed so as to overlap each other as viewed from one side of the element200.

The diffraction efficiency changing layer60is formed of a hologram recording material and has a structure in which first areas and second areas similar to those in the first embodiment are periodically repeated. Each first area has an isotropic refraction index. Each second area has an anisotropic refraction index (uniaxial or biaxial). Even in the second embodiment, as in the first embodiment, each first area is formed of polymer resin, and each second area is formed of liquid crystals.

The top surface of the holographic diffraction grating element200(that is, the surface of the transparent substrate63a) corresponds to an object placement surface200a.

An illumination light source4is disposed beside the holographic diffraction grating element200so that illumination light10can be emitted to an end face200cof the element200.

Through a low refraction index layer6, a holographic diffraction grating element100(which may be called a second holographic diffraction grating element in the embodiment) is disposed at a surface200b(which is also a light-exiting surface as well as a surface of the transparent substrate63b) opposing the object placement surface200aof the holographic diffraction grating element200. A sensor unit8is disposed near an end face of the holographic diffraction grating element100through a light-condensing optical system7. A control circuit (not shown) is connected to the holographic diffraction grating elements100and200and to the sensor unit8.

The low refraction index layer6used in the embodiment is formed of a transparent material that has a lower refraction index than transparent substrates at the holographic diffraction grating element100.

Another low refraction index layer6is disposed at the object placement surface200aof the holographic diffraction grating element200. Disposing the low refraction index layers6at both the top and bottom surfaces (that is, the object placement surface and the light-exiting surface) of the holographic diffraction grating element200increases light propagation by total reflection in the holographic diffraction grating element200.

The control circuit used in the embodiment carries out a controlling operation so that linear areas42and linear areas62to which an electrical field is applied of the holographic diffraction grating elements100and200are successively selected in the direction of extension of the object placement surface200a, successively receives electrical signals output in linear area units from the sensor unit8, converts them into pieces of linear image information, and joins these pieces of linear image information in order to produce a two-dimensional image of an object14.

An electrical field is applied to linear areas62disposed at locations where the upper transparent electrode64aand the lower transparent electrodes64bcorresponding to the linear areas62overlap by applying a voltage to the transparent electrodes64aand64b.

An electrical field is applied to linear areas42disposed at locations where an upper transparent electrode34aand lower transparent electrodes34bcorresponding to the linear areas42overlap by applying a voltage to the transparent electrodes34aand34b.

By virtue of the above-described structure, when carrying out imaging the object14, it is possible for the illumination light10to be emitted from the illumination light source4and enter the diffraction efficiency changing layer60from the end face200c. The illumination light10that has entered the diffraction efficiency changing layer60propagates through areas62bto which an electrical field is not applied, and a component of the propagation light that satisfies the Bragg diffraction condition is diffracted in the direction of the object placement surface200a(a predetermined direction) at linear areas62to which an electrical field is successively applied and becomes diffracted light10a. The diffracted light10astrikes a portion of the object14disposed on the object placement surface through the low refraction index layer6, and is scattered there. Portions10bof the scattered light pass through the areas62bof the holographic diffraction grating element200to which the electrical field is not applied, exit from a light-exiting surface200btowards the low refraction index layer6disposed below the light-exiting surface200b(or adjacent the element100), pass through the low refraction index layer6, and impinge upon the holographic diffraction grating100. In the holographic diffraction grating100, the control circuit successively applies the electrical field to the linear areas42of the diffraction efficiency changing layer in order to increase the diffraction efficiency. Of the light portions10bthat have exited from the low refraction index layer6, any light portion10bthat satisfies the Bragg diffraction condition is successively diffracted at the associated linear area42to which the electrical field has been applied, and becomes diffracted light11. The diffracted light11is focused on the sensor unit8in linear area units by the light-condensing optical system7. The sensor unit8converts the intensities of the focused light into electrical signals, and outputs the electrical signals to the control circuit. The control circuit successively receives the electrical signals output in linear area units, converts them into pieces of linear image information, and successively joins these pieces of linear image information regarding portions (linear portions) of the object14above the linear areas42to which the electrical field has been applied in order to produce a two-dimensional image of the object14.

It is desirable that the location of each linear area42to which the electrical field is applied be controlled so that each linear area42is disposed at the center of the light that is diffracted by the linear areas62to which the electrical field is applied, that strikes a portion of the object14and is scattered by the object14, and that exits towards the element200. (The center of the light is where the intensity of the scattered light is greatest.)

As can be understood from the foregoing description, in the optical image reader of the second embodiment, a larger amount of illumination light can be concentrated at the imaging areas of the object14by using the holographic diffraction grating200instead of the transparent light guide plate5used in the first embodiment. As a result, it is possible to, for example, increase the S/N ratio of an image after imaging and to save power of the illumination light source.

Similarly to a related optical image reader, the optical image reader of the present invention is primarily used to read information on, for example, characters or figures, printed or hand-written on a sheet serving as an object, as two-dimensional image data. Since the optical image reader can read finger print information if a finger instead of a sheet is placed as an object on the object placement surface, it can function as a finger print sensor.

If the illumination light source is turned off and the holographic diffraction grating element is in a nondiffraction state, the transparency is maintained. Therefore, for example, if the optical image reader is disposed on a display surface of a portable information terminal of, for example, a cellular phone having a liquid crystal display element disposed at its display surface, a portable information terminal having an optical image reader can be provided.