JPH0453286B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention scans a sample to be observed two-dimensionally with a light beam focused in the form of a minute spot, and uses reflected light or transmitted light from the sample. The present invention relates to a scanning microscope imaging device that captures images.

(Prior Art) A scanning microscope device has been put into practical use that images a sample by two-dimensionally scanning the sample to be observed with a light beam focused into a minute spot. In this conventional microscope device, light from a laser light source is deflected at high speed in the main scanning direction by a first deflection mirror consisting of a galvanometer mirror, and is deflected at low speed in the sub-scanning direction by a second deflection mirror before being directed to the objective lens. It is incident. This scanning beam is focused into a minute spot by an objective lens and is incident on the sample. Therefore, the sample is two-dimensionally scanned by a light beam focused into a minute spot, and the reflected light from the sample is received by the objective lens, passes through the second and first deflection mirrors again, and then is converted into a photographic image. incident and converted into an electrical signal. Since this known scanning microscope device constitutes a confocal microscope device, it is possible to capture clear images with high resolution. Moreover, since it is configured to obtain optical information from the sample as an electrical signal, it is possible to electrically adjust the brightness and contrast of the image, and it has a wide range of uses when observing the sample on a monitor.

(Problems to be Solved by the Invention) The above-described scanning microscope device allows light from a sample to pass through a second deflection mirror that performs sub-scanning and a first deflection mirror that performs main scanning, and then enters the photoprinter. Therefore, a stationary spot of light is incident on the photographic image.

On the other hand, it is extremely difficult to deflect a light beam with high precision and high speed. In particular, when beam deflection is performed using a mechanical drive mechanism such as a galvano mirror,
It is impossible to perform high-speed deflection at television rates, while even low-speed deflection has problems such as uneven scanning speed and fluctuations in scanning length. When these uneven scanning speeds and fluctuations in scanning length occur, an inconvenience arises in that the image is distorted. In particular, the first deflection device that performs high-speed deflection in the main scanning direction tends to cause high-speed unevenness and fluctuations in the scanning beam.
Scanning speed unevenness and fluctuations caused by the deflection device directly appear as image distortion.

On the other hand, a method can also be considered in which a two-dimensional solid-state imaging device is used as the photoelectric conversion device and the reflected light from the sample is made incident on the two-dimensional solid-state imaging device without passing through the deflection device. However, it is difficult to obtain a two-dimensional solid-state imaging device with high resolution, and the resolution is insufficient for applications such as defect inspection equipment, for example. Moreover, when performing photoelectric conversion using a two-dimensional solid-state imaging device, stray light is likely to enter each light receiving element, resulting in a problem that the S/N ratio decreases.

Furthermore, in the field of microscope imaging devices, there is a strong demand for the ability to capture both reflected and transmitted images of a sample. However, in a scanning microscope imaging device, it is necessary to pass the reflected light and transmitted light from the sample through the deflection device again, and to make the light that has passed through the deflection device enter the photoelectric conversion device. When trying to capture an image, it is extremely difficult to design the structure of the optical system. For these reasons, in conventional microscope imaging devices, it is not possible to share a photoelectric conversion device and a deflection device in the reflection imaging mode and the transmission imaging mode, and as a result, optical systems must be provided separately for each mode. First, the device has the drawback of becoming larger and more expensive.

Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks, to be able to capture high-resolution images that are less susceptible to the influence of deflection devices, and to have no image distortion. An object of the present invention is to provide a scanning microscope imaging device that can share an optical converter and an optical conversion device.

(Means for Solving the Problems) A scanning microscope imaging device according to the present invention includes a light source that generates a light beam, a first deflection device that deflects the light beam generated from the light source at high speed in the main scanning direction, and a parallel It has first and second beam deflection mirror surfaces formed on the front and back sides of a flat plate, and the scanning beam deflected by the first deflection device is deflected by the first beam deflection mirror surface into a sub direction perpendicular to the main scanning direction. A second deflection device that deflects in the scanning direction, a condenser lens that two-dimensionally scans the sample by irradiating the scanning beam from the second deflection device onto the sample as a minute spot, and a plurality of light-receiving elements. A linear image sensor that is arranged one-dimensionally along the main scanning direction and that receives transmitted light from a sample and outputs a photoelectric output signal; and an optical system that forms an image as a minute spot on the linear image sensor through a beam deflection mirror surface, and the linear image sensor is configured to be scanned by the transmitted light from the sample.

Furthermore, the scanning microscope imaging device according to the present invention includes a light source that generates a light beam, and a first deflector that deflects the light beam generated from the light source at high speed in the main scanning direction.
and a first deflection device formed on the front and back sides of the parallel plane plate.
and a second beam deflection mirror surface;
a second deflection device that deflects the scanning beam deflected by the deflection device in a sub-scanning direction perpendicular to the main scanning direction using a first beam deflection mirror surface; A condenser lens that scans the sample two-dimensionally by irradiating it as a minute spot on the sample, and a plurality of light-receiving elements are arranged one-dimensionally along the main scanning direction and receive reflected light or transmitted light from the sample. a linear image sensor that outputs a photoelectric output signal using a linear image sensor; an optical path switching unit that switches an optical path between a transmitted image observation mode and a reflected image observation mode; a first optical system that forms an image as a minute spot on the linear image sensor through the beam deflection mirror surface of the sample and the optical path switching means; and a first beam deflector of the second deflection device that directs the reflected light from the sample a second optical system that forms an image as a minute spot on the linear image sensor via a mirror surface and the optical path switching means, and configured to scan the linear image sensor with transmitted light or reflected light from the sample. It is characterized by what it did.

(Operation) In the present invention, a light beam emitted from a light source is deflected in a main scanning direction and a sub-scanning direction perpendicular to the main scanning direction, and the deflected scanning beam is projected as a minute spot onto a sample through an objective lens. Therefore,
The sample is scanned two-dimensionally with a tiny spot-shaped light beam. The reflected light or transmitted light from the sample is received by the objective lens, and is imaged as a minute spot on the linear image sensor via the second deflection device. Since the light incident on the linear image sensor does not pass through the first deflection device that performs main scanning, this incident light is directed in the direction corresponding to the main scanning direction, that is, in the direction in which the many light receiving elements that make up the linear image sensor are arranged. Therefore, the linear image sensor is scanned one-dimensionally by the light focused into a minute spot from the sample. Therefore, the sample is scanned by a light beam in the form of a minute spot, and the light from the sample is received as light focused in the form of a minute spot and photoelectrically converted. The device can be configured to capture high-resolution images.

In addition, since the light from the sample passes only through the second deflection device and enters the linear image sensor, the effects of speed unevenness and fluctuations caused by the first deflection device are removed, resulting in clear images with little image distortion. can be imaged. That is, since the linear image sensor has a charge accumulation effect, the photoelectric output signal can be read out independently from beam scanning. As a result, even if there is unevenness in the scanning speed, the brightness changes only slightly, and even if fluctuations occur due to the deflection device, no image distortion occurs. In particular, the first deflection device deflects at high speed in the main scanning direction,
When the second deflection device performs two-dimensional scanning by performing low-speed deflection in the sub-scanning direction, the fluctuation of the first deflection device that performs high-speed scanning is reduced, so image distortion caused by the deflection device can be removed. can. Also, since the light from the sample does not pass through the first deflector, the first
The degree of freedom in selecting a deflection device is expanded, and a deflection device such as an acousto-optic device that performs high-speed beam deflection at television rate using an electric signal can be used. Therefore, if an acousto-optic element is used as the first deflection device, main scanning can be performed at television rate, and real-time imaging becomes possible.

Furthermore, in the present invention, an optical path switching device is provided to switch the optical path according to the imaging mode, and the optical path from the light source to the sample is shared between the reflection imaging mode and the transmission imaging mode, and the reflected light from the specimen and Since an optical system is provided that images a minute spot on the linear image sensor via a second deflection device that deflects transmitted light in the sub-scanning direction and an optical path switching device, it can be used in reflective imaging mode and transmission imaging mode. A light source, a deflection device, and a photoelectric conversion device can be used in common.

Furthermore, in the present invention, an optical path switching device is provided in the optical path from the light source to the sample, so that in the reflection imaging mode, the scanning beam is incident from the surface side of the sample, and in the transmission imaging mode, the scanning beam is incident from the surface side of the sample. Since the optical system from the sample to the linear image sensor is shared, the light source, deflection device, and photoelectric conversion device can be shared by using the optical path switching device.

(Example) FIG. 1 is a diagram showing the configuration of an example of a scanning microscope imaging device according to the present invention. A light beam emitted from a laser light source 1 is expanded by an expander 2, then reflected in the right angle direction by right angle prisms 3 and 4, and is incident on an acousto-optic element 5, which is a first deflection device. This acousto-optic element 5 vibrates a light beam at high speed, and the light beam vibrates at high speed to scan the sample surface at high speed in the X direction (main scanning direction) at a scanning frequency of ₁ . In this example,
Set scanning frequency ₁ to TV rate. The light beam deflected by the acousto-optic element 5 is focused by a condenser lens 6, passes through a relay lens 7, right angle prisms 8, 9 and 10, and enters a beam splitter 11. This beam splitter 11 has a half mirror surface 11a, and the incident light beam passes through this half mirror surface 11a and enters the imaging lens 12. This imaging lens 12 acts as a relay lens for the light beam directed toward the sample, and acts as an imaging lens for the light beam emitted from the sample. The light beam emitted from the imaging lens 12 enters a vibrating mirror 13, which is a second deflection device, and is deflected in the Y direction (sub-scanning direction) orthogonal to the X direction of the sample surface. This vibrating mirror 13 also functions as an optical path switching device, and its position is different when capturing a reflected image of the sample and when capturing a transmitted image.
It is configured to switch by 90°, and is located at position a when capturing a reflected image, and located at position b when capturing a transmitted light image. Further, this vibrating mirror 13 has a configuration in which both the front and back surfaces thereof form total reflection surfaces. First, a case will be described in which a reflected image of a sample is captured. The light beam reflected by the vibrating mirror 13 at position a is reflected in the right angle direction by the right angle prisms 14, 15, and 16, passes through the relay lens 17, is reflected again by the right angle prism 18, and is then directed to a minute spot by the objective lens 19. It is converged into a shape and incident on the sample 20. The light beam that is incident on the sample 20 and converged into a minute spot is deflected by the first deflection device and the second deflection device in the X and Y directions of the sample surface at predetermined intervals, so that the sample 20 has a minute spot shape. The spot-shaped light beam is scanned in the X direction (main scanning direction) and the Y direction (sub scanning direction) at a predetermined scanning frequency. The reflected light reflected by the sample 20 is condensed by the objective lens 19, and is again transmitted to the right angle prism 18, the relay lens 17, and the right angle prisms 16, 1.
5 and 14 and enters the vibrating mirror 13. Then, it is reflected in the right angle direction by this vibrating mirror 13, passes through the imaging lens 12, and passes through the beam splitter 11.
It is reflected in the right angle direction by the half mirror surface 11a of
The light passes through a right-angle prism 21 and enters an optical path switching device 22 . This optical path switching device 22 is configured to switch the optical path by rotating a right angle prism by an angle of 90° with respect to the optical axis. That is, when the reflective surface 22a of the right-angle prism is placed at a position making an angle of 45° with respect to the optical axis, and when a reflected image is to be captured, the reflective surface 22a is positioned at position a to capture a transmitted image. is placed at position b. This switching is performed by rotating the right-angle prism by an angle of 90° around the optical axis using a vibrating device (not shown).
This is done in conjunction with the switching operation. The light beam incident on the optical path switching device 22 is reflected in a perpendicular direction, then converged into a minute spot and incident on the linear image sensor 23. This linear image sensor 23 is arranged at the imaging position of the imaging lens 12, and each light receiving element is arranged in a direction corresponding to the X direction of the sample surface so that the reflected light from the sample 20 is received line by line in the main scanning direction. Sample 2
The structure is such that the reflected light from 0 is received by each element and photoelectrically converted, and the amount of charge generated in each light receiving element is read out line by line at a readout frequency _{of 2} . That is, since the reflected light from the sample 20 passes through the second deflection device and then enters the linear image sensor 23, the light incident on the linear image sensor becomes a beam that is one-dimensionally deflected in the main scanning direction. Therefore, the linear image sensor scans the sample one-dimensionally at a TV rate frequency using a beam focused into a minute spot from the sample. _Each light-receiving element of the linear image sensor accumulates a charge corresponding to the amount of reflected light from the sample every time one line is scanned, and in this example, the charge accumulated in each light-receiving element is Read sequentially. With this configuration, a confocal type imaging device is configured, and it is possible to capture a high-resolution image. Moreover, linear image sensor 2
Since the charges accumulated in each of the light receiving elements 3 are read out separately from the scanning frequency, image distortion does not occur even if uneven scanning speed or the like occurs due to the first deflection device.

Next, a case will be described in which a transmission image of a sample is captured. The configuration from the laser light source 1 to the vibrating mirror 13 is the same as in the case of capturing a reflected image, so a description thereof will be omitted. When capturing a transmission image, first,
The vibrating mirror 13 and the optical path switching device 22 are switched to the position b. The light beam that has passed through the imaging lens 12 and entered the vibrating mirror 13 is reflected in the right angle direction and enters the pentagonal prism 24, and is reflected by the two reflective surfaces 24a and 24b of this pentagonal prism 24 in the right angle direction. bent, right angle prism 2
5, relay lens 26, right angle prism 27 and 2
8, it is converged into a minute spot by the condenser lens 29, and enters the back surface of the sample 20, and the sample 2
0 in the X and Y directions. In this example, the pentagonal prism 24 is used to adjust the number of reflections so that images with opposite left and right directions are not formed in the case of reflection and the case of transmission. The light beam that has passed through the sample 20 is focused by the objective lens 19 and enters the vibrating mirror 13 through an optical path that captures a reflected light image. Since the vibrating mirror 13 has a total reflection surface formed on its back surface, the incident transmitted light is reflected in the right angle direction by the total reflection surface on the back side, passes through the imaging lens 30, and passes through the right angle prisms 31 and 32. The light then enters the optical path switching device 22. Optical path switching device 22
Since the reflecting surface 22a is at the position b, the transmitted light is reflected in the right angle direction and the linear image sensor 23
incident on . In this case, the configuration is such that the linear image sensor 23 is located at the imaging point of the imaging lens 30. Therefore, the linear image sensor 23 is one-dimensionally scanned by the transmitted beam from the sample 20.

FIG. 2 is a plan view showing the relationship between the beam spot imaged on the linear image sensor 23 and each light receiving element constituting the linear image sensor. The reflected light from the sample 20 is transmitted to the image sensor 23
In this example, the diameter of the projected beam spot 40 is determined by each element 23a.
The spot diameter is configured to be slightly larger than the light receiving surface of ~23n. Projected beam spot 4
0 is deflected at a TV rate in the X direction, which is the arrangement direction of the elements 23a to 23n, so the light from the sample 20 is one-dimensionally received by each element 23a to 23n and converted into a photoelectric output signal. With this configuration, there is always a one-to-one correspondence between the pixels of the sample 20 and each light-receiving element constituting the linear image sensor 23, so there is no unevenness in the scanning speed of the acousto-optic element 5 in the main scanning direction. Even if this occurs, the amount of light received by each element simply changes too much, and unlike conventional imaging devices, pixel distortion due to uneven scanning speed can be effectively prevented from occurring. Furthermore, if the configuration is such that the reflected light from the sample 20 is incident as a spot diameter larger than the light receiving surface of each element of the image sensor 23 as in this example, the image sensor 25
It is stable against external vibrations and when there is a positional error in the incident light relative to the object. In particular, when imaging with zoom, the spot diameter of the light beam is likely to fluctuate, so this is effective for imaging apparatuses equipped with a zoom imaging function.

FIG. 3 is a graph showing the relationship between the readout frequency of the linear image sensor 23 and the amount of charge accumulated in each element. Since the linear image sensor 23 has a charge storage ability, the amount of charge is generated according to the amount of received light until the amount of charge reaches saturation, and the amount of generated charge can be accumulated sequentially. Figure 3A
The configuration is such that when the readout frequency ₂ of the linear image sensor is equal to the scanning frequency ₁ in the main scanning direction of the light beam, that is, the amount of charge accumulated in the element is read out each time the sample is scanned once with the light beam. Figure B shows the amount of accumulated charge when ₂ = _1/2 , that is, when the sample is scanned twice with the light beam and then the amount of charge accumulated in the element is read out. , and C in the same figure shows the amount of accumulated charge when ₂ = ₁ /3, that is, when the sample is scanned three times with the light beam and then the amount of charge accumulated in the element is read out.

In the present invention, since a confocal type microscope device is configured, a high-resolution sample image can be taken. On the other hand, as described above, if the light from the sample scans the linear image sensor multiple times using the charge accumulation effect of the linear image sensor, and then the accumulated charges are read out, the S/ N
The ratio can be further improved. That is, since the laser light includes brightness fluctuations, when the linear image sensor is scanned once, the brightness fluctuations of the laser light appear as noise on the image signal. On the other hand, if scanning is performed multiple times, the brightness fluctuations of the laser light itself are averaged, and the S/N ratio of the image signal can be improved.

Next, resolution will be explained. FIG. 4A is a diagram schematically showing the scanning state on a sample by the conventional optical scanning microscope imaging device, and FIG. 4B is a diagram schematically showing the scanning state on the sample by the microscope imaging device according to the present invention. FIG. In conventional scanning microscope devices, when using a light source with low output, the scanning speed must be slowed down and the scanning line density must be set low, which eliminates the inconvenience of missing optical information between the scanning lines. was occurring. On the other hand, the scanning frequency ₁ in the main scanning direction of the light beam is set to the image sensor 2.
By setting the readout frequency to be approximately an integral multiple of the readout frequency ₂ of 3, a photoelectric output signal of approximately the same magnitude can be obtained even if the main scanning speed is increased and the scanning line density is increased. As a result, the scanning line density can be set equivalently high without deteriorating the S/N ratio of the photoelectric output signal or slowing down the scanning speed of the light beam, and the optical information of the sample can be reproduced more accurately.
In particular, when pattern defects of photomasks or reticle patterns are inspected using a conventional optical scanning microscope, minute defects often exist between scanning lines and defects are often overlooked. Being able to set the scanning line density to an equivalently high value is extremely effective for pattern defect inspection equipment.

Next, prevention of shading will be explained. Normally, when a deflected light beam is incident on an objective lens, it is subjected to a shading effect by the lens as shown in Figure 5, and the amount of transmitted light of the light beam incident on the periphery of the objective lens is reduced by that of the light beam incident on the center. It will be less than the amount of light. As a result,
This creates an inconvenience in that the central part of the image is reproduced brightly and the peripheral part becomes dark. In such cases, conventional imaging devices that generate photoelectric output signals using photomultiply
If an attempt is made to correct this by changing the scanning speed of the deflection means, distortion will occur in the image. Furthermore, even if electrical correction is attempted, it is extremely difficult to make an unambiguous correction because the scanning speed of the deflection means is uneven. In contrast, in the present invention, the image of the sample and each element constituting the linear image sensor are 1:1
Therefore, the scanning speed by the deflection means is changed so that the scanning speed becomes faster when the light beam enters the center of the objective lens.
If the scanning speed is set to be slow when entering the peripheral area, it is possible to correct the shading caused by the lens, and electrically, if the photoelectric output signal is amplified by taking into account the shading characteristics caused by the objective lens, it can be further improved. It can be corrected accurately.

The present invention is not limited to the embodiments described above, but can be modified and modified in many ways. For example, in the embodiment described above, an optical path switching device is placed in the optical path between the light source and the sample, and the scanning beams are incident on the front surface side of the sample and from the front side, respectively, and the optical system from the sample to the linear image sensor is shared. However, on the contrary, an optical path switching device is placed in the optical path from the sample to the linear image sensor, and the optical system from the light source to the sample is shared, and the reflected light and transmitted light from the sample are The light can also be made to enter the linear image sensor through another optical path.

Further, in the above-described embodiments, an example of use as a microscope imaging device has been described, but the present invention can also be applied to imaging devices other than a microscope, such as an imaging device that captures a life-sized image, an enlarged image, a reduced image, or the like.

Furthermore, in the above embodiment, the oscillating mirror that performs sub-scanning also functions as an optical path switching device, but it is also possible to use a separate optical path switching means to configure the oscillating mirror to perform only sub-scanning. It is. In this case, another optical path switching device can be placed after the vibrating mirror.

(Effects of the Invention) As explained above, according to the present invention, reflected light or transmitted light from a sample is not allowed to pass through the first deflection means, but is incident on the linear image sensor only through the second deflection means. Because of this configuration, the influence of scanning speed unevenness, scanning length fluctuation, etc. caused by the first deflecting means can be removed, and the occurrence of image distortion can be reduced. This is particularly effective when high-speed scanning is performed using the first deflection means, since uneven scanning speed is likely to occur. Furthermore, since the degree of freedom in selecting the first deflection means increases, it is possible to use a high-speed deflection device that does not have a mechanical drive mechanism, such as an acousto-optic device, and as a result, it is possible to perform high-speed scanning with high precision, and Since main scanning at TV rate is possible, real-time imaging is also possible.

Furthermore, in the present invention, since the optical path switching device is disposed in the optical path from the light source to the sample or in the optical path from the sample to the linear image sensor, the light source, polarization, and equipment and linear image sensors can be shared;
As a result, a reflection/transmission type confocal microscope imaging device having a simple and compact structure can be realized.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the configuration of an example of a microscope imaging device according to the present invention, Fig. 2 is a plan view showing the relationship between the beam spot projected on the linear image sensor and the element, and Figs. 3 A to C are linear diagrams. FIG. 4A is a graph showing the relationship between the readout frequency of an image sensor and the amount of accumulated charge. FIG. FIG. 5 is a diagram schematically showing the state of the scanning line on the sample of the microscope imaging device according to the present invention, and is a graph showing the shedding effect by the objective lens. 1... Laser beam, 2... Expander, 3,
4,. 8, 9, 10, 14, 15, 16, 18,
25, 27, 28, 31, 32... Right angle prism, 5... Acousto-optic element, 6... Condenser lens,
7, 17, 26... Relay lens, 11... Beam splitter, 12, 30... Imaging lens, 13
...Vibrating mirror, 19...Objective lens, 20...
Sample, 22... Optical path switching device, 23... Linear image sensor, 24... Pentaprism, 29
...condenser lens.

Claims (1)

[Claims] 1. A light source that generates a light beam, and a first light source that deflects the light beam generated from the light source in the main scanning direction at high speed.
and a first deflection device formed on the front and back sides of the parallel plane plate.
and a second beam deflection mirror surface;
a second deflection device that deflects the scanning beam deflected by the deflection device in a sub-scanning direction perpendicular to the main scanning direction using a first beam deflection mirror surface; A condenser lens that scans the sample two-dimensionally by irradiating it as a minute spot on the sample, and a plurality of light-receiving elements are arranged one-dimensionally along the main scanning direction to receive the transmitted light from the sample and output photoelectric output. A linear image sensor that outputs a signal, and an optical system that images transmitted light from the sample as a minute spot on the linear image sensor through a second beam deflection mirror surface of the second deflection device. . A scanning microscope imaging device, characterized in that the linear image sensor is configured to scan using transmitted light from the sample. 2. A light source that generates a light beam, and a first light source that deflects the light beam generated from this light source in the main scanning direction at high speed.
and a first deflection device formed on the front and back sides of the parallel plane plate.
and a second beam deflection mirror surface;
a second deflection device that deflects the scanning beam deflected by the deflection device in a sub-scanning direction perpendicular to the main scanning direction using a first beam deflection mirror surface; A condenser lens that scans the sample two-dimensionally by irradiating it as a minute spot on the sample, and a plurality of light-receiving elements are arranged one-dimensionally along the main scanning direction and receive reflected light or transmitted light from the sample. a linear image sensor that outputs a photoelectric output signal using a linear image sensor; an optical path switching unit that switches an optical path between a transmitted image observation mode and a reflected image observation mode; a first optical system that forms an image as a minute spot on the linear image sensor through the beam deflection mirror surface of the sample and the optical path switching means; and a first beam deflector of the second deflection device that directs the reflected light from the sample a second optical system that forms an image as a minute spot on the linear image sensor via a mirror surface and the optical path switching means, and configured to scan the linear image sensor with transmitted light or reflected light from the sample. A scanning microscope imaging device characterized by: 3. The second deflection device is capable of switching the optical path according to a transmitted image observation mode and a reflected image observation mode, and in the transmitted image observation mode, the scanning beam is incident on the first surface side of the sample and the reflected image is In observation mode, the scanning beam is directed to the second
The scanning microscope imaging device according to claim 2, characterized in that the imaging device is configured so that the light is incident on the surface side.