JP2000019110A - Refractive index measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refractive index measuring apparatus which can be manufactured with a small number of components and at a low cost, and have high measurement accuracy. SOLUTION: In the refractive index measuring apparatus, a prism 2 where either or both of incident and emitting surfaces of a ray beam are made in a spherical shape is used. A sample 4 is put on this hemispherical prism 2 and a ray beam radiated from a light source 1 is made incident on the spherical part of the hemispherical prism 2. When the ray beam reflected on the plane part of the prism 2 is emitted from the opposite spherical part, the spherically shaped emitting surface acts as a lens and forms an image without using an additional optical component such as a lens to give a light-and-dark boundary image 3 shown as a boundary line at a critical refracting angle. The light-and- dark border is electrically detected by a CCD, a photodiode array, or the like, arranged at the position where the image is formed by the hemispherical prism 2 to determine the critical refracting angle, which can determine a refractive index n2 of the sample as a refractive index n1 of the prism 2 is known. In another way, the refractive index of the sample is determined by measuring the reflective index at a constant incident angle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring the optical refractive index of a substance which is used in measuring the concentration of a fluid such as a gas or a liquid, or which is measured as a physical property value of a solid.

[0002]

2. Description of the Related Art Heretofore, the refractive index has been measured by an apparatus mainly composed of a prism having a triangular or polygonal prism shape and an optical system using a lens attached thereto. The optical principle used by refractometers is
It is roughly divided into the following two. First, the object to be measured is attached to one surface of the prism, and the critical refraction angle of the interface formed by the prism and the object to be measured is changed by light beams of various angles incident from another surface of the prism or the object to be measured. The other is to utilize the fact that when light rays incident from another surface of the prism at a certain incident angle are reflected at the interface, the reflectivity changes according to the refractive index. .
In both cases, devices have been devised which have been subjected to various applications in order to improve accuracy and facilitate handling in realizing them.

[0003]

In both the case where the refractive index is measured by the critical refractive angle and the case where the refractive index is measured as a change in reflectance, the angle and positional accuracy of the prism and its peripheral optical components directly affect the measurement accuracy. However, it is very important to assemble a large number of parts with high precision, and it is expensive. In addition, a prism having a triangular prism or a polygonal prism structure is likely to generate distortion when manufactured by injection molding of plastic, and it is difficult to finish the prism with high precision.

[0004] Accordingly, an object of the present invention is to reduce the number of components, to reduce the cost of parts and the cost of assembling, and to configure the parts with a high measurement accuracy and a shape that can be processed well by injection molding. To provide a refractive index measuring device.

[0005]

According to the present invention, in order to attain the above object, according to the present invention, in a refractive index measuring device using a prism, a prism having both or one of a light incident surface and a light emitting surface having a spherical shape is used. A refractive index measuring device is provided. In a more specific preferred embodiment, the prism is a hemisphere cut by a plane passing through the center of a true sphere as shown in FIG. 1, and may be cut and polished from a sphere or injection-molded plastic. Often,
The shape other than the part where the light beam passes or reflects can be freely changed.

Further, as one mode of the present invention, there is provided a refractive index measuring device in which a CCD (Charge Coupled Device) or a photodiode array is arranged at an image forming position of the prism. Here, it is needless to say that the CCD or the photodiode can be replaced with another sensor capable of converting a light or darkness on a straight line or a plane at the image forming position of the prism into an electric signal.

On the other hand, as another aspect of the present invention, there is provided a refractive index measuring device, wherein at least one of a light emitting element and a light receiving element is arranged at an image forming position of the prism. The light emitting element referred to here may be an element that emits light at almost one point, such as a single LED (light emitting diode), or a light emitting element that can emit monochromatic light by attaching a filter to a light bulb.
In addition, a single light sensor such as a photodiode, a phototransistor, or a phototube is used as the light receiving element. In this embodiment, not only one set of light receiving and light emitting elements but also a plurality of elements can be provided.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION A refractive index measuring apparatus according to the present invention comprises:
Used as follows. That is, as shown in FIG.
The sample 4 is placed on the plane portion of the hemispherical prism 2 and the light source 1
The emitted light is incident on the spherical portion of the hemispherical prism 2. The light beam reflected by the plane portion exits from the opposite spherical portion. Preferably, a mask for covering the plane portion of the hemispherical prism 2 with a portion other than the portion for placing and dropping the sample 4 at the center portion, preferably a black color so as to form a clearer light-dark boundary image. The mask 5 is applied by an appropriate method such as printing or coating. As shown in FIG. 2, the light ray incident on the plane portion has different reflectivities depending on the incident angle α ₁ , the output angle α ₂ , the refractive index n _{1 of the} prism, and the refractive index n _{2 of the} sample. The reflectance of p-polarized light is γ _p and the reflectance of s-polarized light is γ
_Assuming that _s and the non-polarized light reflectance are γ, they are expressed by the following equations (1), (2) and (3), respectively.

Embedded image

Also, the refractive index n _{1 of the} prism and the sample
And n ₂ and the incident angle α ₁ and the outgoing angle α ₂ have the relationship of the following equation (4).

Embedded image

As the incident angle α ₁ is gradually increased from 0, the reflectance increases, and the angle α _c satisfying the following equation (5) is obtained.
Above, everything is reflected.

Embedded image This angle α _c is called a critical refraction angle. Since the light emitted from the hemispherical prism 2 has a spherical exit surface, an image is formed by this lens action without using another optical component such as a lens, and as shown in FIG. To create a light-dark boundary image 3 with. By observing this image and determining the position of the boundary, that is, the critical refractive angle α _c , the refractive index n _{1 of the} prism is known, and the refractive index n _{2 of the} sample is obtained.
Can be requested.

Among the embodiments of the present invention, the one in which a CCD or a photodiode array is arranged at the image forming position of a hemispherical prism electrically detects the light-dark boundary, and the CCD output drops sharply. The boundary is found at the starting position, and the refractive index of the sample is obtained. Further, among the embodiments of the present invention, the one in which at least one of the light emitting element and the light receiving element is arranged at the image forming position of the hemispherical prism has an incident angle α
The refractive index of the sample is obtained by measuring the reflectance according to the above equation (3) while keeping ₁ constant. Light rays other than a fixed angle of incidence are generated by the lens action due to the spherical entrance surface and exit surface. Prevention of entering the light-receiving element enables highly accurate measurement.

[0012]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
The present invention will be described in more detail. FIG. 3 shows an embodiment of a component arrangement in a case where the present invention is applied to a refractive index measuring device based on critical refraction angle measurement. In FIG. 3, reference numeral 1a denotes a light source, 2 denotes a hemispherical prism, and 6a denotes C
It is a CD. As the light source 1a, a monochromatic light source having a measurement wavelength to be used for measuring the refractive index is desirable, and a wide range of incident light including an incident angle which becomes a critical refractive angle at an interface with an object to be measured on a plane portion of the hemispherical prism 2 Must include angular rays. For this reason, it is desirable to configure the light source 1a to have a large light emitting surface or to increase the area by a scattering plate. The position of the light source 1a does not need to be specified as long as it can enter the plane portion of the hemispherical prism 2 at a wide range of incident angles. C
The CD 6a is arranged at a position where the emitted light near the center of the measurement range is vertically incident on the center of the light receiving surface and an image of a light-dark boundary generated before and after the critical refraction angle is formed. More specifically, the focal length of a spherical lens having the same radius as that of the prism 2 is obtained, and the focal point is located at a position apart from the center of the spherical surface of the hemispherical prism 2 by twice this distance. In the state where the sample is placed on the plane portion of the hemispherical prism 2, a bright / dark boundary image generated by the critical refraction angle is formed on the CCD surface without using other optical components. If the D position is converted and the boundary position is obtained by a CPU or the like, the critical refraction angle α _c can be easily obtained. Therefore, the unknown refractive index n ₂ of the sample can be obtained from the above equation (5). This embodiment can be applied to a relatively high-precision refractometer and a liquid / gas concentration meter.

FIG. 4 shows an embodiment of the arrangement of components when the present invention is applied to refractive index measurement by reflectance measurement. In FIG. 4, reference numeral 1b denotes a light source, 2 denotes a hemispherical prism, and 6b denotes a light receiving element. As the light source 1b, a monochromatic point light source is suitable, and specifically, L without a lens is used.
ED available. As the light receiving element 6b, a photodiode, a phototransistor, or the like can be used. Light source 1b
And the light receiving element 6b have the same angle of incidence and exit with respect to the plane of the hemispherical prism 2, and are separated from the center of the sphere by twice the focal length when the sphere of the hemispherical prism 2 is assumed to be a perfect sphere. It is located at two points on a plane passing through the center of the sphere and perpendicular to the plane (a plane including the vertical cross section of the hemispherical prism 2).
In this case, if the size of the light source 1b is sufficiently small, the light beam incident on the flat portion of the hemispherical prism 2 becomes a substantially parallel light beam due to the action of the spherical portion, and has a single incident angle. Therefore, as described above, the amount of light incident on the light receiving element 6b is determined by the refractive index of the solution dropped on the plane of the hemispherical prism 2 or the gas and the refractive index of the gas in contact with the plane of the hemispherical prism 2. Since the reflectance varies depending on the reflectance, if the reflectance is measured, α ₁ = α ₂ = constant. Therefore, the unknown refractive index n of the sample is calculated according to the equations (1) to (3).
You can ask for ₂ . Furthermore, if the relationship between the refractive index of a liquid or gas and the concentration of a substance contained therein is determined in advance,
The substance concentration can be obtained from the reflectance.

[0014]

As described above, according to the refractive index measuring device of the present invention, since the hemispherical prism itself also functions as a lens, other components are not required for image formation. Therefore, the number of parts can be reduced, the cost of parts and the cost of assembly can be reduced, and the measurement accuracy can be improved. Also,
The prism used in the present invention has a hemispherical structure, and is easy to manufacture and has a stable shape when cut from a true sphere or when injection molded.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing a basic concept of a refractive index measuring device of the present invention.

FIG. 2 is an explanatory diagram showing an incidence / emission angle at a boundary surface between a prism and a sample.

FIG. 3 is a schematic configuration diagram showing one embodiment of a component arrangement of an apparatus in which the present invention is applied to refractive index measurement by critical refraction angle measurement.

FIG. 4 is a schematic configuration diagram showing one embodiment of a component arrangement of an apparatus in which the present invention is applied to refractive index measurement by reflectance measurement.

[Explanation of symbols]

 1, 1a, 1b Light source 2 Hemispherical prism 3 Bright / dark boundary image 4 Sample 5 Mask 6a CCD 6b Light receiving element

Claims (3)

[Claims] 1. A refraction index measuring device using a prism, wherein a prism having a spherical shape on both or one of an input / output surface of a light beam is used. 2. The apparatus according to claim 1, wherein a CCD or a photodiode array is arranged at an image forming position of the prism. 3. The apparatus according to claim 1, wherein at least one of a light emitting element and a light receiving element is arranged at an image forming position of the prism.