Abstract:
A measurement unit used in an analyzing apparatus for measuring concentrations of component gases in a sample gas comprises a light emitting unit configured to emit a measurement light to the sample gas, a light receiving unit configured to receive the measurement light on a light receiving plane, a purge air introducing unit configured to introduce a purge air into a vicinity of at least one of the light emitting unit and the light receiving unit, and a condensing lens arranged in an optical path of the measurement light from the light emitting unit to the light receiving unit, the condensing lens being configured to condense the measurement light within the light receiving plane of the light receiving unit, a propagation path of the measurement light being varied by a thermal lens effect caused by a temperature difference between the sample gas and the purge air.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a gas analyzing apparatus and a measurement unit. More specifically, the present invention relates to a gas analyzing apparatus that analyzes a concentration of a predetermined component in a sample gas using a light absorption technique, and a measurement unit used in the gas analyzing apparatus. 
       BACKGROUND 
       [0002]    A combustion exhaust gas, which is expelled from a boiler that combusts coal or heavy oil, includes gases such as NOx, SOx, CO2, CO, etc. Gas analyzing apparatuses for analyzing contents of these components in the gas have previously been developed. Various types of apparatuses have been developed, such as an open-path type apparatus and a probe-type apparatus. 
       SUMMARY 
       [0003]    One example of a cylindrical measurement unit used in the above-described probe-type gas analyzing apparatus is disclosed in Patent Citation 1. The measurement unit disclosed in Patent Citation 1 emits a measurement light from a light source that is arranged at one end side of a cylindrical casing so as to pass the measurement light through a sample gas that is introduced into the inner space of the casing. The measurement light is reflected by a reflecting mirror that is arranged at another end side of the casing and the reflected measurement light is received by a light receiving sensor. An amount of the measurement light absorbed by the sample gas is derived by subtracting one information of the measurement light from another. One information is the information that can be derived from the light receiving sensor. Another is the information of the measurement light at the time when the measurement light is emitted from the light source. Then, the concentration of the predetermined component in the sample gas can be derived based on the amount of the measurement light absorbed by the sample gas. 
         [0004]    Due to such a measurement principle, in order to perform accurate analyses in the gas analyzing apparatus using the above-described measurement light, it is important to receive the measurement light within the light receiving plane of the light receiving sensor. From such a point of view, it is considered that the positioning of the optical components, such as the light source, the reflecting mirror, the light receiving, etc., can be performed in the probe-type gas analyzing apparatuses easier than the open-path type gas analyzing apparatus. This is because the optical components of the probe-type gas analyzing apparatus are fixed in a single casing as described above. On the other hand, these optical components of the open-path type gas analyzing apparatus are arranged separately. In other words, it is considered to be easier to set the irradiation point of the measurement light within the light receiving plane of the light receiving sensor in the probe-type gas analyzing apparatus. 
         [0005]    Patent Citation 1: U.S. Pat. No. 6,809,825 
       DISCLOSURE OF INVENTION 
     Technical Problem 
       [0006]    However, even with the above-described probe-type gas analyzing apparatus, situations arise in which the irradiation point of the measurement light cannot be set within the light receiving plane of the light receiving sensor. 
         [0007]    In measurement units, in which the optical components such as the above-described sensor, the reflecting mirror, etc. are used, there is the case in which a cleaning air (so-called purge air) is introduced around the optical components in the casing. The cleaning air is introduced with a predetermined pressure in order to avoid the contamination of the optical components due to dust contained in the sample gas, etc. 
         [0008]    While the temperature of the sample gas expelled from the above-described boiler is very high, the temperature of the purge air is typically the same as the ambient temperature. When there is a temperature difference between the purge air and the sample gas, spatial distribution of temperature occurs inside the casing of the measurement unit, such as on the path of the measurement light. When such a spatial distribution of temperature occurs, the spatial refractive index is changed proportionally to the spatial distribution of temperature. Then, the measurement light propagating in the space might be refracted since the change of the refractive index causes the effect equivalent to transitional optical lenses (so-called thermal lens effect). 
         [0009]    As shown in  FIG. 9 , the measurement light Lb 2  should propagate on a straight path R 1 . However, Lb 2  might propagate on a refracted path, like the path R 3 , due to the temperature difference between the sample gas Sg and the purge air Pa. 
         [0010]      FIG. 9  is an image view showing that the measurement light is improperly received in the conventional measurement unit. If the measurement light Lb 2  is refracted in such a manner, the measurement light Lb 2  cannot be received within the light receiving plane of the light receiving sensor. Therefore, it is sometimes difficult to perform an accurate analysis. 
         [0011]    In addition, the state of the refraction of the measurement light Lb 2  changes with time. This is due to the thermal lens effect changing because of changes in the flows of the sample gas Sg and the purge air Pa. As a result, even if the measurement light Lb 2  is emitted onto the light receiving plane of the light receiving sensor  54 , the irradiation point Lbp 2  of the measurement light Lb 2  on the light receiving plane may fluctuate, as shown in  FIG. 10 . 
         [0012]      FIG. 10  is a view showing that the irradiation point Lbp 2  fluctuates on the light receiving plane in the conventional measurement unit. In  FIG. 10 , a locus line Tr 2  is a movement locus of the irradiation point Lbp 2 . In  FIG. 10 , since the locus line Tr 2  snakes, it is shown that the irradiation point Lbp 2  fluctuates as described above. When the position of the irradiation point Lbp 2  moves in the light receiving sensor, stable signals might not be derived from the light receiving sensor, even if the intensity of the measurement light Lb 2  is constant, because the light receiving sensitivity of the light receiving sensor may be dependent on the positions of the light receiving plane. 
         [0013]    The measurement light might also be refracted, due to the thermal lens effect, in the open-path gas analyzing apparatus when using the purge gas in the same manner as the probe-type gas analyzing apparatus. In such a configuration, the irradiation point of the measurement light sometimes cannot be set properly within the light receiving plane of the light receiving sensor. 
         [0014]    The present invention was conceived in light of the above-described problems and the object of the present invention is to provide the measurement unit and the gas analyzing apparatus that can analyze the sample gas more accurately than the conventional techniques. 
       Technical Solution 
       [0015]    A measurement unit, according to one aspect of the present invention, is the measurement unit that is used in an analyzing apparatus for measuring concentrations of component gases in a sample gas. The measurement unit comprises a light emitting unit, a light receiving unit, a purge air introducing unit, and a condensing lens. The light emitting unit is configured to emit a measurement light to the sample gas. The light receiving unit is configured to receive the measurement light on a light receiving plane. The purge air introducing unit is configured to introduce a purge air into a vicinity of at least one of the light emitting unit and the light receiving unit. The condensing lens is arranged in an optical path of the measurement light. The optical path of the measurement light extends from the light emitting unit to the light receiving unit. The condensing lens is configured to condense, within the light receiving plane of the light receiving unit, the measurement light. In this case, the propagation path of the measurement light is variable due to a thermal lens effect caused by a temperature difference between the sample gas and the purge air. 
         [0016]    The measurement light can be properly received within the light receiving plane of the light receiving unit, even if the path of the measurement light is refracted due to the thermal lens effect. In addition, the measurement light can be stably emitted to the predetermined position of the light receiving plane. Therefore, the information of the measurement light can be accurately derived with the light receiving unit, and accurate analysis of the predetermined component gas, in the sample gas, can be performed based on such information. 
         [0017]    The condensing lens may be arranged immediately in front of the light receiving unit. The measurement unit may further include an optical window arranged immediately in front of the condensing lens. The optical window is configured to protect at least the condensing lens. The purge air introducing unit may introduce the purge air immediately in front of the optical window. 
         [0018]    Optical components can be properly protected by introducing the purge air in the appropriate position. Even if the thermal lens effect occurs when the purge air is introduced, the measurement light can properly be received by the light receiving plane of the light receiving unit by utilizing the condensing lens. 
         [0019]    The measurement unit may further comprise a cylindrical probe tube having openings to introduce the sample gas into the probe tube. The purge air introducing unit may introduce the purge air inside the probe tube. The light emitting unit may emit the measurement light to the sample gas introduced inside the probe tube. 
         [0020]    By utilizing the condensing lens, the measurement light can properly be received within the light receiving plane of the light receiving unit. This is especially true for a probe-type measurement unit, in which bending of the measurement light due to the thermal lens effect easily occurs. 
         [0021]    The measurement unit may further comprise a reflection mirror arranged at one end portion of the probe tube. The light emitting unit may be arranged at another end portion of the probe tube. The light emitting unit is configured to emit the measurement light toward the reflection mirror. The light receiving unit may be arranged at the another end portion of the probe tube. The light receiving unit is configured to receive the measurement light reflected by the reflection mirror. 
         [0022]    The numerical aperture of the condensing lens may be greater than or equal to 0.08. 
         [0023]    With this configuration, when the light receiving plane of the light receiving unit is formed with a multi-layered semiconductor, the interference of the measurement light can be inhibited. The interference is caused by the multiple reflections of the measurement light that occur in the semiconductor layers. The intensity of the measurement light can be detected accurately with the light receiving unit. 
         [0024]    The light receiving unit may be tilted with respect to the condensing lens so that an angle between the light receiving plane and an image formation plane of the condensing lens is 10 degrees or more. 
         [0025]    With this configuration, when the light receiving plane of the light receiving unit is formed with a multi-layered semiconductor, the interference of the measurement light can be inhibited. The interference is caused by the multiple reflections of the measurement light that occur in the semiconductor layers. The intensity of the measurement light can be detected accurately with the light receiving unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is an external view of the measurement unit  1  according to the first embodiment. 
           [0027]      FIG. 2  is a cross-sectional view showing the inner structure of the measurement unit  1  according to the first embodiment. 
           [0028]      FIG. 3  is an image view showing that the measurement light Lb 1  refracted in the measurement unit  1  is guided within the light receiving plane of the light receiving unit  24  by the condensing lens  23 . 
           [0029]      FIG. 4  is a view showing that the movement of the irradiation point Lbp 1  on the light receiving plane is inhibited in the measurement unit  1 . 
           [0030]      FIG. 5  is a cross-sectional view showing the detailed structure of the light receiving unit  24 . 
           [0031]      FIG. 6  is a graph showing the stabilities of the electrical signals of the light receiving unit  24  derived at each of settings of the condensing lens  23  and the light receiving unit  24  in the measurement unit  1  according to the first embodiment. 
           [0032]      FIG. 7  is an enlarged view of a part of  FIG. 6 . 
           [0033]      FIG. 8  is a cross-sectional view showing the inner structure of the measurement unit  2  according to the second embodiment. 
           [0034]      FIG. 9  is an image view showing that the measurement light Lb 2  is not received properly in the conventional measurement unit. 
           [0035]      FIG. 10  is a view showing that the irradiation point Lbp 2  fluctuates on the light receiving plane in the conventional measurement unit. 
       
    
    
     DETAILED DESCRIPTION 
     Embodiment  
     First Embodiment 
       [0036]    A measurement unit  1  and a gas analyzing apparatus  100  using the measurement unit  1  will be explained below. The gas analyzing apparatus  100  is a so-called probe-type gas analyzing system and the measurement unit  1  is a so-called probe unit. First, the structure of the measurement unit  1  will be explained, referring to  FIG. 1  and  FIG. 2 .  FIG. 1  is an external view of the measurement unit  1  according to the first embodiment.  FIG. 2  is a cross-sectional view showing the inner structure of the measurement unit  1  according to the first embodiment.  FIG. 2  is a view that includes the A-A cross section of the measurement unit  1  shown in  FIG. 1 . As shown in  FIG. 1 , the measurement unit  1  includes a probe tube  11 , an optical unit  12 , and a flange  13 . 
         [0037]    The probe tube  11  is a cylindrical member in which introducing openings  111  are formed. The introducing openings  111  introduce a sample gas Sg inside the probe tube  11  by diffusion of the sample gas Sg. The probe tube  11  may be made of any metallic material appropriate for the environment where the measurement unit  1  is used. As shown in  FIG. 1 , the introducing openings  111  are formed as intermittent slits on the side plane of the probe tube  11 . As shown in  FIG. 2 , a reflection mirror  22  is arranged at one inner end side portion of the probe tube  11 . On the other hand, the other end side portion of the probe tube  11  is connected to the optical unit  12 . 
         [0038]    As shown in  FIG. 2 , the optical unit  12  is the optical apparatus that includes a light emitting unit  21 , a condensing lens  23 , a light receiving unit  24 , and an optical window  25 . The light emitting unit  21  is the light source apparatus that emits a measurement light Lb 1  to the inside of the probe tube  11 . The light emitting unit  21  is typically a light source apparatus that emits the light with a predetermined wavelength band, such as an infrared laser oscillating apparatus, an LED (Light Emitting Diode), or a deuterium lamp that emits an ultraviolet light. The light receiving unit  24  is the light receiving apparatus that receives the measurement light Lb 1  on the light receiving plane. The light receiving unit  24  is typically a photoelectric converting apparatus, such as a photodiode. The condensing lens  23  is the lens member that condenses the measurement light Lb 1  within the light receiving plane of the light receiving unit  24 . The condensing lens  23  is arranged immediately in front of the light receiving unit  24 . The light receiving unit  24  is electrically connected to a processing apparatus  15  and sends information (for example, an intensity) of the measurement light Lb 1  to the processing apparatus  30  as an electric signal. The optical window  25  is the planar member that is made of the material that transmits the measurement light Lb 1 . As shown in  FIG. 2 , the optical window  25  may be arranged at the point where the casing of the optical unit  12  and the probe tube  11  are connected. In other words, the optical window  25  may be disposed immediately in front of the light emitting unit  21  and the condensing lens  23 . The optical window  25  protects the light emitting unit  21 , and the condensing lens  23 . It should be noted that the above-described reflection mirror  22  is arranged inside the probe tube  11  in advance, so as to reflect the measurement light Lb 1 . Measurement light Lb 1  is emitted from the light emitting unit  21 , toward the light receiving unit  24 . 
         [0039]    The processing apparatus  30  controls the operations of the light emitting unit  21  and the light receiving unit  24  and calculates the concentration of the predetermined component in the probe tube  11  based on the signal received from the light receiving unit  24 . The processing apparatus  30  typically includes an information processing apparatus, such as a CPU (Central Processing Unit), etc., a storing apparatus, such as a memory, etc., an interface apparatus that receives the operations from a user, a displaying apparatus that displays results of the analysis, etc. The processing apparatus  30  performs the arithmetic processes based on the operations by the user and the program stored in the storing apparatus. 
         [0040]    As shown in  FIG. 2 , in the above-described probe tube  11 , a purge air introducing port  14  is arranged inside the probe tube  11 , the purge air introducing port  14  introduces a purge air Pa. The purge air introducing port  14  is arranged in the vicinity of the connection part at which the probe tube  11  and the optical unit  12  are connected, as shown in  FIG. 1  and  FIG. 2 . Arranged in such a manner, introducing the purge air Pa with the predetermined pressure from the purge air introducing port  14  prevents the sample gas Sg and dust inside the probe tube  11  from touching the optical window  25  of the optical unit  12 . Therefore, the contamination and corrosion of the optical window  25  can be inhibited. The flow paths of the purge air Pa are shown in thick black arrows in  FIG. 2 . In addition, the flow paths of the sample gas Sg is shown in white arrows in  FIG. 2 . It is preferable that the above-described purge air introducing port  14  is arranged so as to introduce the purge air immediately in front of the optical window  25 . The optical components such as the condensing lens  23 , etc. can be properly protected by introducing the purge air Pa at such an appropriate position. 
         [0041]    The probe tube  11  further includes a purge air introducing pipe  16  that introduces the purge air Pa in front of the reflection mirror  22  to protect the reflection mirror  22 . Such a structure avoids causing the sample gas Sg, and dust in the probe tube  11 , from coming into contact with the reflection mirror  22 . Therefore, the contamination and corrosion of the reflection mirror  22  can be inhibited. 
         [0042]    In addition, as shown in  FIG. 2 , holes  67  and  68  are formed at the both ends of the introducing openings  111 , and at the opposite side of the introducing openings  111  (at the side of the upper stream of the sample gas Sg) of the probe tube  11 . Flowing the sample gas Sg from these holes  67  and  68  can prevent the purge air Pa from flowing into the middle part of the probe tube  11 . The purge air Pa is expelled from the introducing openings  111  (SgPa) while mixing with the sample gas Sg. The introducing openings  111  are also used as an outlet for exhausting the purge air Pa. 
         [0043]    The flange  13  is the member that fixes the measurement unit  1  to a funnel  500  that expels the sample gas Sg or to a container that encapsulates the sample gas Sg (See  FIG. 2 ). The flange  13  is, for example, a disk-like member and arranged so as to be passed through by the probe tube  11  at the one end side (the side connected to the optical unit) of the probe tube  11 . The flange  13  is fixed to the funnel  500  with bolts, for example. 
         [0044]    Next, the optical path of the measurement light Lb 1  emitted from the light emitting unit  21  will be explained. The propagation path of the measurement light Lb 1  is shown in a chain line in  FIG. 2 . As shown in  FIG. 2 , the measurement light Lb 1  emitted from the light emitting unit  21  passes through the space inside the probe tube  11  and is reflected by the reflection mirror  22 . The probe tube  11  is filled with the sample gas Sg. The measurement light reflected by the reflection mirror  22  passes through the space inside the probe tube  11  and propagates toward the light receiving unit  24 . Thus, the measurement light Lb 1  reciprocates through the space inside the probe tube and is received by the light receiving unit  24 . 
         [0045]    Here, the measurement light Lb 1  reflected by the reflection mirror  22  might be refracted due to so-called thermal lens effect, and may propagate in a path different than straight from the reflection mirror  22  to the light receiving unit  24  in the probe tube  11 . In more detail, the sample gas Sg and the purge air Pa flow into the probe tube  11 . The spatial temperature gradient might be generated when the temperature difference between the sample gas Sg and the purge gas Pa exists. Thus, the change of the spatial refractive index might be generated in accordance with the spatial temperature gradient and therefore the measurement light Lb 1  might be refracted. 
         [0046]    Taking this point into consideration, the measurement unit  1  includes the condensing lens  23 . With the measurement unit  1  including the condensing lens  23 , the measurement light Lb 1  can be guided within the light receiving plane of the light receiving unit  24  by changing the propagation direction of the refracted measurement light Lb 1 , as shown in  FIG. 3 .  FIG. 3  is an image view showing that the measurement light Lb 1  refracted in the measurement unit  1  is guided to the light receiving plane of the light receiving unit  24  by the condensing lens  23 . Specifically, as shown in  FIG. 3 , the measurement light Lb 1 , which is refracted due to the thermal lens effect, enters the condensing lens  23  and propagates on a path R 2 . Then, the measurement light Lb 1  finally reaches within the light receiving plane of the light receiving unit  24 . Without the condensing lens  23 , the measurement light Lb 1 , which is refracted due to the thermal lens effect, propagates on a path R 3 . 
         [0047]    In addition, with the measurement unit  1 , the measurement light Lb 1  entering the condensing lens  23  is condensed to the predetermined condensing point, in accordance with the property of the condensing lens  23 . Therefore, unnecessary movement of the irradiation point Lbp 1  can be inhibited. The irradiation point Lbp 1  is a point where the measurement light Lb 1  intersects the light receiving plane of the light receiving unit  24 , as shown in  FIG. 4 .  FIG. 4  is a view showing that the movement of the irradiation point Lbp 1  on the light receiving plane is inhibited in the measurement unit  1 . In  FIG. 4 , the locus line Tr 1  shows the movement locus of the irradiation point Lbp 1 . Since the locus line Tr 1  does not snake in  FIG. 4 , it is shown that the movement of the irradiation point Lbp 1  is inhibited as described above. Thus, with the measurement unit  1  according to the present embodiment, the measurement light Lb 1  can be received within the predetermined area of the light receiving plane of the light receiving unit  24 . Therefore, a stable light receiving signal can be derived even with the light receiving unit  24  that has a positional dependence of the detection sensitivity. 
         [0048]    As described above, with the measurement unit  1 , the measurement light Lb 1  that has reciprocated inside the probe tube  11  can be properly received within the light receiving plane of the light receiving unit  24 . By receiving the measurement light Lb 1  within the light receiving plane of the light receiving unit  24 , an electrical signal that corresponds to the intensity of the measurement light Lb 1  can be derived. Therefore, the gas analyzing apparatus  100  comprising the measurement unit  1  can analyze the sample gas Sg accurately based on the electric signal that corresponds to the intensity of the measurement light Lb 1 . 
         [0049]    In probe-type gas analyzing apparatuses, the proportion of the purge air relative to the sample gas is greater than that in the open-path type apparatus. This is because the sample gas and the purge air are introduced into a limited space inside the probe tube. In other words, the refraction of the measurement light due to the thermal lens effect is greater in a probe-type gas analyzing apparatus than in an open-path type gas analyzing apparatus. Therefore, it is effective to apply the present embodiment to the above-described probe-type measurement unit  1  and the gas analyzing apparatus  100  using the probe-type measurement unit  1 . 
         [0050]    It is preferable that the numerical aperture NA (Numerical Aperture) of the lens used as the condensing lens  23  is 0.08 or more. It is preferable that the light receiving unit  24  is arranged such that the light receiving plane of the light receiving unit  24  is substantially perpendicular to the optical axis of the condensing lens  23 . The numerical aperture NA is the value expressed by the equation (1), where φ is the maximum angle of the light beam, which the condensing lens  23  condenses, relative to the optical axis of the condensing lens  23 , n is the refractive index of the medium between the condensing lens  23  and the light receiving unit  24 . 
         [0000]      NA=n sin φ  (1)
 
         [0051]    Namely, the numerical aperture NA is the value proportional to the condensing angle of the condensing lens  23 . 
         [0052]    In addition, the light receiving unit  24  is arranged while being tilted relative to the condensing lens so that a tilting angle ω is greater than or equal to 10 degrees, where the tilting angle ω is the angle of the light receiving plane of the light receiving unit  24  relative to the image forming plane of the condensing lens  23 . Thus, since the next multiple reflections can be inhibited without increasing the numerical aperture NA to an extremely large value, the space for setting the distance between the condensing lens  23  and the light receiving unit  24  can be increased. Moreover, it can prevent the incident light from reflecting, returning, and then becoming a noise in the signal. 
         [0053]    The reasons why it is preferable that the numerical aperture of the condensing lens  23  is greater than or equal to 0.08, and the tilting angle ω is greater than or equal to 10 degrees, will explained below. 
         [0054]    The light receiving plane of the above-described light receiving unit  24  has multiple layers of semiconductors as shown in  FIG. 5 .  FIG. 5  is a cross-sectional view showing the detailed structure of the light receiving unit  24 . Specifically, the light receiving unit  24  includes a package substrate  244 , an InP wafer layer  243  arranged on the principal surface of the package substrate  244 , an InGaAs absorbing layer  242  formed in the InP wafer layer  243 , and an AR (Anti Reflection) coating layer  241  formed on the surface of the InP wafer layer  243 . Gold plating is formed on the surface of the package substrate  244 . The plane where the AR coating layer  241  is formed is the light receiving plane of the light receiving unit  24 . The measurement light Lb 1  entering the light receiving surface of the light receiving unit  24  is absorbed by the InGaAs absorbing layer  242 . Then, the light receiving unit  24  generates an electric signal in accordance with the intensity of the light absorbed by the InGaAs absorbing layer  242 , and outputs this signal to the processing apparatus  30 . Several well-known techniques can be used as the technique with which the light receiving unit  24  performs the photoelectric conversion of the measurement light Lb 1 . 
         [0055]    In the conventional techniques, there has been the case in which an electric signal that corresponds to the intensity of the measurement light Lb 1  cannot be derived accurately. This is because the multiple reflection of the measurement light Lb 1  in the semiconductor layers shown in  FIG. 5  causes the interference of the measurement light Lb 1  (so-called etalon effect) when the measurement light Lb 1  is received by the light receiving unit  24 . In more detail, the measurement light Lb 1  entering the light receiving unit  24  propagates into the InP wafer layer  243 , while a part of the measurement light Lb 1  is reflected by the AR coating layer  241 . After a part of the measurement light Lb 1  is absorbed by the InGaAs absorbing layer  242  in the InP wafer layer  243 , the measurement light Lb 1  transmits through these layers and is reflected at the surface of the package substrate  244 . The measurement light Lb 1  reflected at the surface of the package substrate  244  transmits again through the InP wafer layer  243  and the InGaAs absorbing layer  242  and is then reflected again at the interface between the AR coating layer  241  and the InP wafer layer  243 . Thus, there has been the case in which, when the measurement light Lb 1  enters the light receiving plane of the light receiving unit  24  at the predetermined angle of incidence, the measurement light Lb 1  is repeatedly reflected in the semiconductor layers that form the light receiving unit  24 . Then the reflected measurement light and the incident measurement light interfere with each other. There has also been the case in which, when such interference occurs, even if the intensity of the measurement light Lb 1  is constant at the time when the measurement light Lb 1  enters the light receiving unit  24 , the magnitude of the electric signal derived from the light receiving unit  24  becomes unstable because the amount of the measurement light Lb 1  absorbed in the InGaAs absorbing layer  242  is unstable. 
         [0056]    Considering the above, it is preferable that the etalon effect is inhibited in the measurement unit  1 . In order to achieve this, it is preferable that the multiple reflections are inhibited by increasing the angle of incidence θ of the measurement light Lb 1  when the measurement light Lb 1  enters the light receiving plane of the light receiving unit  24 . Here, the angle of incidence θ can be large as the numerical aperture NA becomes large. In addition, the angle of incidence θ can also be adjusted by tilting the light receiving plane of the light receiving unit  24  with respect to the optical axis of the condensing lens  23 . The inventor considering this point has concluded, by performing the experiments that will be described later, that the numerical aperture NA of the condensing lens  23  is preferably greater than or equal to 0.08 and further the angle of incidence θ is preferably greater than or equal to 10 degrees. 
         [0057]    The results of the experiments derived by choosing various values of the numerical apertures NA of the condensing lens and the tilting angles ω in the measuring unit  1  will be presented below.  FIG. 6  is a graph showing the stabilities of the electrical signals of the light receiving unit  24 , derived at each of settings of the condensing lens  23  and the light receiving unit  24  in the measurement unit  1 , according to the first embodiment. The vertical axis of  FIG. 6  shows the difference value ΔE (a.u.) between the peak value and the bottom value of the electrical signals of the light receiving unit  24  measured at the corresponding numerical aperture NA and the corresponding angle of incidence θ(°). The horizontal axis of  FIG. 6  shows the angle of incidence θ (°). In  FIG. 6 , the chain line shows the difference value ΔE when the lens with NA value of 0.02 is used as the condensing lens  23  and the solid line shows the difference value ΔE when the lens with NA value of 0.08 is used as the condensing lens  23 . 
         [0058]    In addition, as shown in  FIG. 6  and  FIG. 7 , it has been found that the difference value ΔE can be converged to the value extremely close to 0 when the angle of incidence is greater than or equal to 10 degrees when the value of the numerical aperture is greater than or equal to 0.08.  FIG. 7  is an enlarged view of a part of  FIG. 6 . The vertical axis and the horizontal axis of  FIG. 7  show the same parameters as those in  FIG. 6 . In  FIG. 7 , the solid line shows the difference value ΔE when the lens with NA value of 0.08 is used as the condensing lens  23  and the chain double-dashed line shows the difference value ΔE when the lens with NA value of 0.14 is used as the condensing lens  23 . 
         [0059]    As shown in  FIG. 6  and  FIG. 7 , by setting the numerical aperture NA of the condensing lens  23  to be greater than or equal to 0.08, a more accurate electrical signal can be derived from the light receiving unit  24 . Additionally, by setting the angle of incidence θ to be greater than or equal to 10 degrees, a more accurate electrical signal can be derived. Therefore, in the gas analyzing apparatus  100  comprising the measurement unit  1  in which the condensing lens  23  and the light receiving unit  24  are set in such a manner, the analysis of the sample gas Sg can be performed more accurately based on the more accurate electrical signal. 
       Second Embodiment 
       [0060]    In the above-described first embodiment, the example in which the present invention is applied to the probe-type measurement unit has been shown. However, the present invention may be applied to an open-path measurement unit. The measurement unit  2  according to the second embodiment and the gas analyzing apparatus  200  using the measurement unit  2  will be explained below. The elements that are the same as those in the above-described first embodiment are assigned to the same numerals as those in the first embodiment, and the detailed explanations are omitted. 
         [0061]      FIG. 8  is a cross-sectional view showing the inner structure of the measurement unit  2  according to the second embodiment. As shown in  FIG. 8 , the measurement unit  2  includes an oscillator unit  32  and a detector unit  33  that are formed separately. The oscillator unit  32  is attached at one side plane of a funnel  500 . The sample gas Sg flows in the funnel  500 . The detector unit  33  is attached to a different side plane of the funnel  500 , so that the oscillator unit  32  and the detector unit  33  face each other. 
         [0062]    The oscillator unit  32  includes a light emitting unit  21 , an optical window  25 A, a purge air introducing port  14 A, and a flange  13 A. The optical window  25 A is arranged immediately in front of the light emitting unit  21  and the purge air introducing port  14 A introduces the purge air Pa into the space that is connected to the funnel  500  immediately in front of the optical window  25 A. The detector unit  33  includes a condensing lens  23 , a light receiving unit  24 , an optical window  25 B, a purge air introducing port  14 B, and a flange  13 B. The condensing lens  23  is arranged immediately in front of the light receiving unit  24 . The optical window  25 B is arranged immediately in front of the condensing lens  23 . The purge air introducing port  14 B introduces the purge air Pa into the space that is connected to the funnel  500  immediately in front of the optical window  25 B. 
         [0063]    The oscillator unit  32  and the detector unit  33  are attached to the funnel  500  via the flanges  13 A and  13 B, respectively, while their positions are adjusted in advance, so that the measurement light Lb 1  emitted by the light emitting unit  21  is emitted toward the light receiving unit  24 . 
         [0064]    With the above-described measurement unit  2 , like the first embodiment, the measurement light Lb 1  can be condensed properly within the light receiving plane of the light receiving unit  24  even if the measurement light Lb 1  is bent due to the thermal lens effect caused by the purge air Pa and the sample gas Sg. It is also preferable in the second embodiment that the numerical aperture NA is set to be greater than or equal to 0.08 and further the angle of incidence θ is set to be greater than or equal to 10 degrees. 
       INDUSTRIAL APPLICABILITY 
       [0065]    The measurement unit and the gas analyzing apparatus according to the present invention are useful for the measurement unit, the gas analyzing apparatus, etc. that can analyze the sample gas more accurately than the conventional ones. 
       EXPLANATION OF REFERENCE NUMERALS 
       [0066]      100 ,  200  gas analyzing apparatus 
         [0067]      1 ,  2  measurement unit 
         [0068]      11  probe tube 
         [0069]      12  optical unit 
         [0070]      13  flange 
         [0071]      14  purge air introducing port 
         [0072]      16  purge air introducing pipe 
         [0073]      21  light emitting unit 
         [0074]      22  reflection mirror 
         [0075]      23  condensing lens 
         [0076]      24  receiving unit 
         [0077]      30  processing apparatus 
         [0078]      241  AR coating layer 
         [0079]      242  InGaAs absorbing layer 
         [0080]      243  InP wafer layer 
         [0081]      244  package substrate 
         [0082]      32  oscillator unit 
         [0083]      33  detector unit 
         [0084]      54  light receiving sensor