JP7820189B2 - Gas Sensor - Google Patents

Description

The present invention relates to a gas sensor.

NDIR (Non-Dispersive InfraRed) gas sensors are commonly used in high-precision gas concentration measurement devices due to their high reliability and long-term measurement reproducibility.

For example, Patent Document 1 discloses a gas sensor that uses one light source and two photodetectors.

Japanese Patent Application Laid-Open No. 2004-138499

Conventional gas sensors such as those disclosed in Patent Document 1 have difficulty measuring gas concentrations with high accuracy when there is absorption by other gases (hereinafter sometimes referred to as interfering gases) near the absorption wavelength of the gas being measured.

For example, if the gas being measured is methane, the absorption wavelength of methane is between 3.1 and 3.5 μm. Furthermore, the absorption wavelength of water vapor, which is present in considerable amounts in typical environments, is around 2.7 to 3.3 μm. This causes interference between the absorption wavelengths of the gas being measured (methane) and water vapor, making it difficult to measure methane gas concentrations with high accuracy.

The objective of the present invention is to provide a gas sensor that can measure gas concentrations with high accuracy even when interfering gases are present in addition to the gas being measured.

A gas sensor according to a first aspect of the present invention comprises:
a first light-emitting unit and a second light-emitting unit each outputting infrared light;
a first light receiving section and a second light receiving section each outputting an output signal in response to incident infrared light;
a first optical path region through which light emitted from the first light-emitting unit passes before being incident on the first light-receiving unit;
a second optical path region through which the light emitted from the second light-emitting unit passes before being incident on the second light-receiving unit,
the first optical path region and the second optical path region have at least a portion of a common region;
an optical path length of the first optical path region is longer than an optical path length of the second optical path region;
a rate of change in an output signal of the first light receiving unit with respect to a change in concentration of a predetermined amount of the target gas to be measured is greater than a rate of change in an output signal of the second light receiving unit with respect to a change in concentration of the predetermined amount of the target gas to be measured;
a rate of change in the output signal of the second light receiving unit with respect to a change in concentration of the predetermined amount of interference gas is greater than a rate of change in the output signal of the first light receiving unit with respect to a change in concentration of the predetermined amount of interference gas;
The peak sensitivity wavelength of the second light receiving portion overlaps with the absorption wavelength of water vapor.

The present invention provides a gas sensor that can measure gas concentrations with high accuracy even when an interfering gas is present in addition to the gas being measured.

FIG. 1 is a schematic diagram for explaining a gas sensor according to one embodiment of the present invention. FIG. 2 is a schematic diagram for explaining a gas sensor according to one embodiment of the present invention. FIG. 3 is a graph showing the correlation between the output signal level of the first light receiving element and the output signal level of the second light receiving element at various relative humidities when the ambient temperature is 60° C. and the methane gas concentration is 0 ppm. FIG. 4 is a graph showing output fluctuation values when humidity correction based on the signal from the second light receiving section is not applied. FIG. 5 is a graph showing output fluctuation values when humidity correction is applied using the signal from the second light receiving section.

The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention as claimed. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

<Gas sensor>
The gas sensor of this embodiment includes a first light-emitting element and a second light-emitting element, each of which outputs infrared light; a first light-receiving element and a second light-receiving element, each of which outputs an output signal corresponding to the incident infrared light; a first optical path region through which light emitted from the first light-emitting element passes before entering the first light-receiving element; and a second optical path region through which light emitted from the second light-emitting element passes before entering the second light-receiving element.

The optical path length of the first optical path region is longer than the optical path length of the second optical path region. Here, optical path length refers to the length of the path through which the density of light passing through the optical path region is highest (or the optically shortest distance from the light emitter to the light receiver).

The rate of change in the output signal of the first light receiving unit in response to a change in concentration of a predetermined amount of target gas is greater than the rate of change in the output signal of the second light receiving unit in response to a change in concentration of a predetermined amount of target gas. Furthermore, the rate of change in the output signal of the second light receiving unit in response to a change in concentration of a predetermined amount of interference gas is greater than the rate of change in the output signal of the first light receiving unit in response to a change in concentration of a predetermined amount of interference gas. Furthermore, the peak sensitivity wavelength of the second light receiving unit overlaps with the absorption wavelength of water vapor. Here, "the peak sensitivity wavelength of the second light receiving unit overlaps with the absorption wavelength of water vapor" means that the peak sensitivity wavelength of the second light receiving unit is included in at least a portion of the absorption wavelength of water vapor.

As will be described in more detail below, by incorporating the above configuration requirements, it is possible to optimize the detection performance (e.g., SNR: signal-to-noise ratio) of the interfering gas and the target gas to the desired performance, and by using the output signals from each light-receiving element, it is possible to reduce the influence of the interfering gas, determine the presence or absence of the target gas, and measure its concentration with high accuracy. Furthermore, the sensitivity peak wavelength of the second light-receiving element overlaps with the absorption wavelength of water vapor, thereby reducing the influence of water vapor, which is the most influential of the interfering gases, and enabling the presence or absence of the target gas to be determined and its concentration to be measured with high accuracy.

<Light receiving section>
In the gas sensor of this embodiment, the first light receiving unit and the second light receiving unit each output an output signal corresponding to the intensity of the incident infrared ray. When a gas that absorbs the infrared ray output from the light emitting unit is present in the first optical path region and the second optical path region, the infrared ray is attenuated and enters the first light receiving unit and the second light receiving unit, respectively. Examples of the light receiving unit include a pyroelectric sensor, a thermopile, a phototransistor, and a photodiode. From the viewpoint of achieving high responsiveness, the light receiving unit is preferably a quantum infrared sensor represented by a phototransistor or a photodiode, and in some cases a photodiode is more preferable.

In order to ensure that the magnitude relationship between the rate of change in the output signals of the first and second light receiving elements in response to changes in the concentrations of the target gas and interference gas satisfies the above-mentioned conditions, it may be preferable for the spectral sensitivity characteristics of the first and second light receiving elements to be different.

Here, spectral sensitivity characteristics refer to the sensitivity characteristics for each wavelength of incident light.

There are no particular limitations on the method for achieving the desired spectral sensitivity characteristics. For example, if the light-receiving section is a photodiode, methods include adjusting the material and composition ratio of the active layer to achieve the desired band gap, or providing an optical filter in the optical path region.

An optical filter can be used to ensure that the magnitude relationship between the rate of change in the output signals from the first and second light receiving units in response to changes in the concentrations of the target gas and interference gas satisfies the above-mentioned conditions. When an optical filter is used, the optical filter may be provided in either the first or second light path region, or optical filters with different wavelength transmission characteristics may be provided in each of the first and second light path regions.

In addition, as a method of improving long-term reliability while maintaining the accuracy of the gas sensor, it may be preferable to achieve low power consumption by differentiating the response speeds of the first and second light receiving elements. In particular, when the gas to be measured is a combustible gas such as methane gas, and the interference gas is water vapor, which is commonly present in the environment, changes in the concentration of methane gas or combustible gas, which is not commonly present in the environment, are sudden changes caused by leakage from storage locations, etc. In contrast, changes in the concentration of water vapor, which is commonly present in the environment, are very gradual. Therefore, by differentiating the response speeds of the first and second light receiving elements, low power consumption and sufficient accuracy can be ensured, and long-term reliability can be improved.

When the gas being measured is a combustible gas, such as methane, it exhibits absorption wavelengths for infrared rays with wavelengths of 3-4 μm and 7-8.5 μm, and particularly high absorption wavelengths for infrared rays around 3.3 μm. Water vapor, which is commonly present in the environment as an interference gas, exhibits absorption wavelengths for infrared rays with wavelengths of 2-3 μm and 5-8 μm, and high absorption wavelengths for infrared rays around 2.7-3.3 μm.

Therefore, the peak sensitivity wavelength of the second light receiving element should overlap with the absorption wavelength of water vapor, and is preferably 2 to 3 μm or 5 to 8 μm. Furthermore, to reduce the influence of flammable gases, the peak sensitivity wavelength of the second light receiving element should preferably be 2 to 3 μm or 5 to 7 μm, which does not overlap with the absorption wavelength of flammable gases. On the other hand, the peak sensitivity wavelength of the first light receiving element should preferably overlap with the absorption wavelength of the gas to be measured, and is preferably 3 to 4 μm or 7 to 8.5 μm.

Here, by making the sensitivity peak wavelength of the first light receiving element closer to 3.3 μm than the sensitivity peak wavelength of the second light receiving element, it is possible to make the rate of change of the output signal of the first light receiving element greater than the rate of change of the output signal of the second light receiving element in response to a given change in concentration of the gas to be measured.

Furthermore, by making the peak sensitivity wavelength of the second light receiving element closer to 2.7 μm than that of the first light receiving element, it is possible to make the rate of change of the output signal of the second light receiving element greater than the rate of change of the output signal of the first light receiving element in response to a change in the concentration of a predetermined amount of interference gas.

<Light-emitting part>
The first light-emitting unit and the second light-emitting unit in the gas sensor of this embodiment are not particularly limited as long as they are light sources that can output infrared light at the absorption wavelength of the measurement target gas. Examples include thermal light sources such as light bulbs and MEMS heaters, and quantum light sources such as laser light sources and LEDs. Infrared LEDs may be preferable from the perspective of response speed. Furthermore, if the measurement target gas or interference gas is a flammable gas, quantum light sources may be preferable because they generate less heat and are safer.

In order to ensure that the magnitude relationship between the rate of change in the output signals of the first and second light receiving units in response to changes in the concentrations of the target gas and interference gas satisfies the above-mentioned conditions, it may be preferable for the first and second light emitting units to have different emission spectra.

Here, emission spectrum refers to the light intensity distribution for each wavelength of emitted light.

In the case of infrared LEDs, adjusting the materials and composition ratio of the light-emitting layer can be preferable because it is possible to tailor the emission spectrum (central emission wavelength, wavelength band, etc.) to the desired value.

<Optical path area>
In the present invention, the optical path region refers to the space from the light-emitting unit to the light-receiving unit. That is, the first optical path region refers to the space through which light emitted from the first light-emitting unit passes before entering the first light-receiving unit. The second optical path region refers to the space through which light emitted from the second light-emitting unit passes before entering the second light-receiving unit. The first optical path region and the second optical path region may be collectively referred to as the "optical path region."

When the target gas and interference gas enter this optical path region, light is absorbed by the target gas and interference gas, changing the amount of light received by the light receiving unit, resulting in an output signal corresponding to the concentration of the target gas and interference gas. In Figure 1, the light beam L1 (the optical path, sometimes referred to as the "first optical path") emitted from the first light-emitting unit 101 is shown schematically by a dashed-dotted line. Also in Figure 1, the light beam L2 (the optical path, sometimes referred to as the "second optical path") emitted from the second light-emitting unit 201 is shown schematically by another dashed-dotted line.

Furthermore, as shown in Figure 2, the first optical path and the second optical path may intersect. From the perspective of space saving, it may be preferable for the first optical path and the second optical path to intersect.

Furthermore, crossing the first and second optical paths may be preferable because it reduces the impact on measurement results due to differences in the concentration distribution of the measurement target gas and interference gas in the optical path space compared to separating the first and second optical paths.

Typically, the optical path region is formed by arranging a highly reflective member (also called a mirror) so that light emitted from the light-emitting unit reaches the light-receiving unit. Alternatively, it may be formed by arranging the light-emitting surface of the light-emitting unit and the light-receiving surface of the light-receiving unit so that they face each other.

<Example of a specific configuration of the gas sensor according to this embodiment>
FIG. 1 is a schematic diagram of a gas sensor 1000 according to one embodiment of the present invention.

The gas sensor shown in Figure 1 comprises a first light-emitting unit 101 and a second light-emitting unit 201, each of which outputs infrared light; a first light-receiving unit 102 and a second light-receiving unit 202, each of which outputs an output signal corresponding to the incident infrared light; a first optical path region 10 through which light emitted from the first light-emitting unit 101 passes before entering the first light-receiving unit 102; and a second optical path region 20 through which light emitted from the second light-emitting unit 201 passes before entering the second light-receiving unit 202.

The first optical path region 10 and the second optical path region 20 have a common region 30 in which part of one region is spatially shared with part of the other region.

The first light receiving unit 102 and the second light receiving unit 202 are designed so that the rate of change in the output signal of the first light receiving unit in response to a change in concentration of a predetermined amount of target gas to be measured is greater than the rate of change in the output signal of the second light receiving unit in response to a change in concentration of a predetermined amount of target gas to be measured, and so that the rate of change in the output signal of the second light receiving unit in response to a change in concentration of a predetermined amount of interference gas is greater than the rate of change in the output signal of the first light receiving unit in response to a change in concentration of a predetermined amount of interference gas.

Furthermore, the optical path length of the first optical path region (the optical path length indicated by light ray L1 from the first light emitter 101 to the first light receiver 102) is longer than the optical path length of the second optical path region (the optical path length indicated by light ray L2 from the second light emitter 201 to the second light receiver 202).

The light-emitting drive unit 400 is provided to drive the first light-emitting unit 101 and the second light-emitting unit 201. The light-emitting drive unit 400 may supply constant current pulses or constant voltage pulses to the first light-emitting unit 101 and the second light-emitting unit 201.

The concentration calculation unit 500 may also calculate the concentration of the gas to be measured using the output signals from the first light receiving unit 102 and the second light receiving unit 202 and the output signal from the temperature sensor 300. From the perspective of highly accurate measurement, it may be preferable to perform temperature correction using the output from the temperature sensor.

The light-emitting driver 400 and the concentration calculator 500 may be synchronized to detect the output signals of the first light-receiving unit 102 and the second light-receiving unit 202. The frequency at which light is emitted from the first light-emitting unit 101 and the first light-receiving unit 102 detects a signal may be different from the frequency at which light is emitted from the second light-emitting unit 201 and the second light-receiving unit 202 detects a signal, thereby preventing simultaneous light emission and signal detection. Alternatively, the light-emitting units may emit light at the same frequency, but the timing at which signals are detected by the light-receiving units may be shifted in time. This crosstalk elimination can be performed efficiently by using quantum light-emitting and light-receiving units, which may be preferable. Furthermore, if the light-emitting unit is a thermal light source and the light-receiving unit is affected by thermal radiation from the light source, the effect of the thermal radiation received by the light-receiving unit at the time of signal detection can be individually detected. This allows the effect of this thermal radiation to be accurately detected and suppressed through signal processing.

Furthermore, the first optical filter 103 and the second optical filter 203 may be bandpass filters that transmit light in the absorption wavelength bands of the interference gas and the measurement target gas, respectively.

When space-saving and long lengths are desired for the first and second optical path regions, the configuration shown in Figure 2 may be used. Furthermore, as shown in Figure 2, providing a common area for the first and second optical path regions can be preferable in some cases, as it suppresses the effects of differences in the concentration distributions of the measurement target gas and interference gas in the first and second optical paths, making it possible to remove the effects of interference gas with high precision.

Furthermore, the light emission driver 400 and concentration calculator 500 may be provided within the optical path area. This allows for further miniaturization.

<Example>
As shown in FIG. 2, the gas sensor was fabricated by arranging the first light-emitting unit, the first light-receiving unit, the second light-emitting unit, the second light-receiving unit, and multiple mirrors for forming the first optical path region and the second region so that the light beam from the first light-emitting unit and the light beam from the second light-emitting unit were approximately perpendicular to each other.

<First Light Emitting Unit, Light Receiving Unit, and First Optical Path Region>
The first light-emitting section and the first light-receiving section are an LED and a photodiode with a chip size of 0.7 × 0.7 μm2, each consisting of a P layer/active layer/N layer stacked structure formed on a GaAs substrate. The active layer is made of AlInSb with a thickness of 1.34 μm, and the Al content is 8.9%.

The LEDs were divided into seven rows and the photodiodes into 73 rows using an etching process, and finally a metal wiring layer was applied to electrically connect them in series.

The light input and output ports were located on the side of the GaAs substrate where the laminated structure was not formed, and were sealed with resin using SMT (Surface-mount Technology). An optical filter with a central transmission wavelength of 3.3 μm was installed on top of the photodiode package, and a PCB (Printed Circuit Board) was used to connect the LED and photodiode to the respective circuits. To achieve a high SNR, the signal processing circuit was installed near the photodiode.

In addition, light from the first light-emitting unit (LED) is reflected multiple times by 17 concave mirrors whose reflective surfaces are made of thin aluminum films, before reaching the first light-receiving unit (photodiode). The optical path length is 1 m, and it is designed so that more than 15% of the light emitted from the light-emitting unit reaches the light-receiving unit.

By driving the LED intermittently at a current value of 100mA, a resolution of several ppm in terms of methane concentration was achieved.

<Second Light Emitting Unit, Light Receiving Unit, and Optical Path Area>
The second light-emitting section, the second light-receiving section, and the second optical path region were configured to differ from the above-described first light-emitting section, the first light-receiving section, and the first optical path region in the following points.
Active layer thickness: 1 μm
Al composition of active layer: 14%
Number of photodiode stages: 41 stages Number of LED stages: 6 stages Optical filter central wavelength: 2.7 μm
Concave mirror: 5 pieces Optical path length: 80mm
The LED was driven intermittently at a current value of 100 mA, and a resolution of approximately 500 ppm in terms of water vapor concentration was achieved.

<Correction to remove the effects of water vapor>
When the correlation function between the output S2 from the second light receiving unit and the output S1 from the first light receiving unit obtained above is denoted by f, the following equation (1) is obtained under the condition that the methane concentration is 0 ppm.

S1=f(S2) | _0ppm ... (1)

The output fluctuation rate S1 _adj , which corrects for the influence of water vapor and extracts only the signal change due to methane from the first light receiving part, can be calculated as shown in equation (2).

S1 _adj = (S1-f(S2) | _0ppm )/f(S2) | _0ppm ... (2)

Here, "| _{0 ppm} " in Equation (1) and Equation (2) indicates that the correlation function f is obtained under the condition that the measurement target gas (methane gas) is 0 ppm.

In equation (2), when the methane concentration is 0 ppm, the output regulation rate S1 _adj is 0 ppm even if the water vapor concentration is 0 ppm or higher. In other words, even if the water vapor concentration changes, if the methane concentration does not change, the output regulation rate S1 _adj does not change. In equation (2), the output regulation rate S1 _adj changes in accordance with the change in the methane concentration only when the methane concentration changes.

Figure 3 shows the measurement results used in this example to derive the correlation function f(S2) shown in equation (1) above. In an environment with a methane concentration of 0 ppm and a temperature of 60°C, the measurement results of the output S1 from the first light receiving unit and the output S2 from the second light receiving unit when the relative humidity was changed to 0%, 40%, 60%, and 80% were plotted to derive the correlation function f(S2). Furthermore, to derive the correlation function f(S2), the output S1 from the first light receiving unit and the output S2 from the second light receiving unit may be temperature-corrected using temperature information from a temperature sensor. This makes it possible to derive a correlation function f(S2) that also has a temperature correction function.

Figure 4 shows the output fluctuation rate "(S1 - S1(0))/S1(0)" from the first light receiving element in response to changes in methane concentration in environments where the water vapor is equivalent to a relative humidity of 40%, 60%, and 80% RH at a temperature of 60°C, without water vapor correction (using only the output S1 from the first light receiving element). The output fluctuation rate is based on the output signal "S1(0)" from the first light receiving element when methane and water vapor are at 0 ppm, and the amount of signal attenuation "S1 - S1(0)" is expressed as the output fluctuation rate "S1 - S1(0)/S1(0)." As is clear from Figure 4, when the relative humidity changes, the output signal differs even for the same methane concentration, making it impossible to accurately determine the methane concentration from the output signal.

On the other hand, FIG. 5 shows the output fluctuation rate S1 adj obtained by correcting the influence of water vapor from the function shown in the above equations (1) and (2) using the output S2 from the second light receiving unit in addition to the output _signal from the first light receiving unit, and extracting only the signal change due to methane.

As shown in Figure 5, the output signal fluctuation rate corrected for each methane concentration remains approximately consistent even when the relative humidity RH changes, and it can be seen that the methane concentration can be determined with high accuracy from the corrected output signal.

Here, the concentration of the measurement target gas can be calculated by applying a concentration calculation table or exponential function fitting to the output fluctuation rate S1 _adj . For example, if g is a function showing the correlation with the output fluctuation rate S1 _adj when the concentration C of the measurement target gas is changed under a constant water vapor concentration, the concentration C of the measurement target gas can be obtained from the following equation (3):

C=g(S1 _adj )... (3)

Here, it is preferable that equations (2) and (3) used to calculate the concentration of the gas to be measured are polynomials.

The present invention has been described above using embodiments, but the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be clear to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included within the technical scope of the present invention.

In particular, while the explanation of this embodiment has focused on eliminating the effects of humidity (water vapor), it is also possible to eliminate the effects of other accuracy error factors. As an example, it is possible to eliminate the effects of temperature characteristics (output changes due to temperature during measurement).

10 First optical path region 20 Second optical path region 30 Common region 101 First light emitting unit 102 First light receiving unit 201 Second light emitting unit 202 Second light receiving unit 103 First optical filter 203 Second optical filter 300 Temperature sensor 400 Light emission driving unit 500 Concentration calculation unit 1000 Gas sensor L1 Light ray from first light emitting unit L2 Light ray from second light emitting unit

Claims (17)

a first light-emitting unit and a second light-emitting unit each outputting infrared light;
a first light receiving section and a second light receiving section each outputting an output signal in response to incident infrared light;
a first optical path region through which light emitted from the first light-emitting unit passes before being reflected by a first mirror and incident on the first light-receiving unit;
a second optical path region through which the light emitted from the second light-emitting unit passes before being reflected by a second mirror different from the first mirror and being incident on the second light-receiving unit,
the first optical path region and the second optical path region have at least a portion of a common region;
an optical path length of the first optical path region is longer than an optical path length of the second optical path region;
a rate of change in an output signal of the first light receiving unit with respect to a change in concentration of a predetermined amount of the target gas to be measured is greater than a rate of change in an output signal of the second light receiving unit with respect to a change in concentration of the predetermined amount of the target gas to be measured;
a rate of change in the output signal of the second light receiving unit with respect to a change in concentration of the predetermined amount of interference gas is greater than a rate of change in the output signal of the first light receiving unit with respect to a change in concentration of the predetermined amount of interference gas;
A gas sensor in which the peak sensitivity wavelength of the second light receiving portion overlaps with the absorption wavelength of water vapor. The gas sensor according to claim 1, wherein the first light receiving element and the second light receiving element have different spectral sensitivity characteristics. The gas sensor according to claim 1 or 2, wherein the response speeds of the first light receiving element and the second light receiving element are different. The gas sensor according to any one of claims 1 to 3, wherein the first light receiving section and the second light receiving section are quantum infrared sensors. The gas sensor described in any one of claims 1 to 4, further comprising an optical filter in either the first optical path region or the second optical path region. The gas sensor described in any one of claims 1 to 4 further comprises optical filters having different wavelength transmission characteristics in each of the first optical path region and the second optical path region. The gas sensor described in any one of claims 1 to 6, wherein the first light-emitting portion and the second light-emitting portion have different emission spectra. The gas sensor described in any one of claims 1 to 7, wherein the first light-emitting element and the second light-emitting element are infrared LEDs. The gas sensor described in any one of claims 1 to 8, wherein the sensitivity peak wavelength of the second light receiving element overlaps with 2 to 3 μm or 5 to 8 μm. A gas sensor according to any one of claims 1 to 9, wherein the peak sensitivity wavelength of the first light receiving element is closer to 3.3 μm than the peak sensitivity wavelength of the second light receiving element. 10. The gas sensor according to claim 1, wherein the peak sensitivity wavelength of the second light receiving portion is closer to 2.7 [mu ] m than the peak sensitivity wavelength of the first light receiving portion. The gas sensor according to any one of claims 1 to 11, wherein an equation that eliminates the influence of the interfering gas is calculated from the output signal of the first light receiving element and the output signal of the second light receiving element, and the concentration of the target gas is calculated using the equation. The gas sensor described in claim 12, characterized in that the equation is a polynomial. 14. The gas sensor according to claim 1, wherein a first optical path, which is a light beam emitted from the first light-emitting part, and a second optical path, which is a light beam emitted from the second light-emitting part, intersect with each other. 15. The gas sensor of claim 14, wherein at least some of the light rays of the first optical path and at least some of the light rays of the second optical path intersect at a substantially perpendicular angle. The gas sensor according to claim 1 , wherein the first light-emitting portion and the second light-emitting portion are disposed at different positions. The gas sensor according to claim 1 , wherein the first light receiving portion and the second light receiving portion are disposed at different positions.