JP2888766B2 - How to determine light / dark boundaries - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining a refractive index in a refractive index sensor.

[0002]

2. Description of the Related Art The basic structure of a refractive index sensor comprises the structure disclosed in International Application No. PCT / JP / 00470. The operation principle of this total reflection type refractometer is that the medium 1 having a different refractive index (refractive index n ₁ ) changes from the medium 2 having a different refractive index (refractive index n ₂ , n ₂ <
While light incident n ₁₎ to its boundary surface is refracted according to the laws of the so-called Snell, the principle that light incident at an angle greater than the critical angle determined by sinθ _c = n ₂ / n ₁ is completely reflected It is based on A simple and small sensor structure is obtained by using the incident light having a spread width. FIG. 5 shows an example of the structure of the sensor of the present invention. The sensor head
1 comprises an optical waveguide structure. This structure comprises an optical fiber 2 having a single or a plurality of incident angles at which light enters the waveguide layer.
Alternatively, a light incident surface 3 to which a light emitting element is connected, a detection surface 4 which totally reflects / transmits incident light having a spread angle from the optical fiber or the light emitting element, and forms a contact surface with the subject M, a detection surface 4 And outputs the reflected light from the
The light control means 7 is connected to the light detection means 6. As the light detection means 6,
A CCD sensor or the like is used.

FIG. 6 shows the light intensity ratio detected on the detection surface when light oil is measured by this sensor. This measurement light intensity ratio is obtained by dividing the measurement light amount by the blank light amount (light amount when the subject is air). The light intensity ratio, which shows a constant value at 100%, starts to undulate and starts decreasing near the center while increasing the amplitude centering on the 100% value toward the dark part (left side). Such a phenomenon that the amount of blank light increases more than the amount of measured light is always observed, and is caused by Fresnel diffraction.

[0006] As shown in FIG. 6, a change in a light-dark portion (bright-dark boundary) due to a change in the total reflection condition is not abrupt but a gentle one. Therefore, the light-dark boundary is a point at which the measured light amount and the blank light amount become equal first when the measured light amount is viewed from the dark part to the bright part (from left to right in FIG. 6) (the light intensity ratio becomes 100%). Point).

[0005]

FIG. 7 shows the result of simulating Fresnel diffraction in a half plane. The central axis is the light-dark boundary without diffraction. As shown in FIG. 7, the value at which the measured light amount and the blank light amount are initially equal has a deviation from the position at the light-dark boundary when there is no diffraction. For this reason, this shift causes an error when performing strict measurement.

[0006]

Means for Solving the Problems In view of the above problems, the present inventor has simulated Fresnel diffraction and made intensive studies, and as a result, has reached the following invention. That is, a waveguide layer having a waveguide structure is provided, and a light incident surface on which light is incident on the waveguide layer, a region that totally reflects incident light having a spread angle, and a region that transmits light, and the subject A detection surface that forms a contact surface; and a light exit surface that outputs reflected light from the detection surface and is connected to a light detection unit, and the total reflection light from the detection surface corresponding to the refractive index of the subject. In the refractive index sensor that detects the difference in the intensity of the emitted light due to the presence of the light, the value of the light intensity ratio represented by the measured light amount / blank light amount of the subject from the dark part to the bright part with respect to the position of the detection surface is first 100%
From the point x 'where the value of

## EQU1 ## (α ′; constant, λ; wavelength of light (mm), z ″; optical path length from the detection surface to the emission surface in the optical path above total reflection criticality (mm), θ ₀ : optical axis of incident light angle of incidence to the detection surface (degree), r _0; distance to the detection surface from the incident end of the optical axis of the incident light (mm), z _0; distance from the detection surface of the optical axis of the incident light to the emitting surface ( mm)) is a method for determining the boundary between light and dark, characterized by performing Δx correction, and the light intensity expressed as the measured light amount / blank light amount of the subject from the dark part to the bright part with respect to the position of the detection surface. From the point x 'where the value of the ratio first becomes 100%, toward the dark area,

## EQU2 ## (α ′; constant, λ; light wavelength (mm), z ′; light path from the detection surface to the light exit surface through a point x ′ at which the value of the light intensity ratio on the light exit surface becomes 100%) Optical path length (mm), θ ₀ : incident angle of the optical axis of the incident light on the detection surface (degre), r ₀ : distance (mm) from the incident end of the optical axis of the incident light to the detection surface, z ₀ A method for determining the boundary between light and dark, characterized by correcting the distance (mm) from the detection surface of the optical axis of the incident light to the emission surface.

[0007]

The light having a wavelength λ from a point light source as shown in FIG.
Assume a coordinate x ₀ on the (opening surface) and a coordinate x on the CCD (diffraction surface). At this time,

(Equation 3)

(Equation 4) Holds. The above equation takes into account the half-plane of the aperture and the reflectance of the subject. u (x) is the amplitude (mm) and λ is the light wavelength (m
m), r and z are optical path lengths (r is the optical path length from the incident surface to the detection surface
(mm), z is the optical path length from the detection surface to the emission surface (mm)), g (x ₀ )
The x ₀ = 0 to the opening function when there is a total reflection critical, r _p is the reflectivity of parallel deflection amount to the incident surface of the object, r _s is the reflectance of the vertical deflection amount to the incident surface of the object, x ₁ is the light-dark boundary position on the detection surface. r _p , r _s

(Equation 5) Ψ ₁ represents the angle of incidence (degree) of light on the detection surface, ψ ₂ represents the angle of refraction (degree), and ψ ₁ and ψ ₂ are Snell's law n
₁ sinψ ₁ = n ₂ sinψ ₂ (n ₂ ; refractive index of the subject) is satisfied. Therefore, r _p, r _s is expressed by a function of [psi _1, also x ₀ = r ₀ t
Since it can be approximated by an (θ ₀ -ψ ₁ ), it can be expressed as a function of x ₀ . α

(Equation 6) FIG. 2 is a graph obtained by solving equations (3), (4) and (5) and plotting the light intensity ratio and α. Α ′ in the present invention is a value at which the light intensity ratio initially becomes 100% when the light intensity ratio and α in the plot of FIG. 2 are plotted. Therefore, α 'is
If the specifications of the refractive index sensor (light wavelength λ, optical path length r ₀ , z ₀ ) and the target refractive index range are known, these values can be easily calculated in advance by simulation.

[0008] The position of the light-dark boundary is determined as follows. First, similarly to the conventional method, a point x '(a point at which the light intensity ratio becomes 100%) at which the measured light amount becomes first equal to the blank light amount is determined. Next, by solving the equation represented by equation (1), z ″,
Δx can be obtained. The optical path length z ″ is the optical path length from the detection surface to the emission surface in the optical path at x′−Δx, and θ
₀ is the angle of incidence of the incident light on the detection surface of the optical axis.

If the contribution of Δx to the optical path length z ″ can be neglected, the optical path length z ″ can be approximated by the optical path passing through x ′ and the optical path length z ′ from the detection surface to the emission surface, and the light intensity ratio becomes An optical path length z ′ from the detection surface to the emission surface in an optical path passing through a point x ′ that is 100%,
From the wavelengths of light λ and α ′, Δx is obtained using equation (2). Note that the contribution of Δx to the optical path length z largely depends on the shape of the refractive index sensor.

## EQU2 ## The thus determined Δx is corrected from x ′ toward the dark part. Hereinafter, an embodiment will be described.

[0009]

Example: λ = 850 nm, r ₀ = 16 mm, z ₀ = 16 mm, the incident angle θ ₀ = 75 ° of the optical axis of the incident light on the detection surface, n ₁ = 1.51, and the spread angle of the optical fiber as the light source is Design a refractive index sensor that is ± 4 °. The determination was made in consideration of the fact that the subject was light oil.
Light / dark boundary is -4 ° or + 4 ° of the divergence angle of the optical fiber
FIG. 3 shows a plot obtained by solving Equations (3), (4), and (5) at this time with respect to the position on the CCD. The central axis is the light-dark boundary position. FIG.
As described above, the value of the reading deviation from the light-dark boundary position at −4 ° is 0.067 mm, and the value of the reading deviation from the light-dark boundary position at + 4 ° is 0.049 mm, which is different.
Therefore, there is a deviation of 0.018 mm (a deviation of the order of 0.000n in terms of refractive index) at the light-dark boundary at both ends of the measurement range. If the average value of the reading deviation of 0.058 mm is used as a correction value for the light-dark boundary of the refractive index sensor, the obtained refractive index has an error on the order of 0.000n.

On the other hand, FIG. 4 shows a plot of α in equation (6). Thus, both at −4 ° and at + 4 °, they intersect approximately at α = 0.60 (= α ′). As described above, plotting with respect to α does not cause such a large shift as described above. Α 'of the refractive index sensor
The refractive index was designed to be 0.60, a refractive index sensor having the above-described design was manufactured, and the refractive index was measured. The light-dark boundary is determined at a point x 'at which the light intensity ratio becomes 100% as in the conventional method, and the value x' is determined by the optical path length z 'from the detection surface to the point on the emission surface, the wavelength of light.
From λ = 850 nm and α ′ = 0.60, it can be easily obtained by correcting the reading deviation value Δx obtained from the equation (2) toward the dark part. Further, z ″ and Δx can be obtained by solving the equation expressed by the equation (1), and can be more accurately obtained by correcting Δx toward the dark portion. In this case, the error in the refractive index of the subject is on the order of 0.0000n or less.

[0011]

The value Δx of the reading shift affects the precision of the refractive index of the subject at the fifth decimal place. Therefore,
The accuracy of the refractive index is improved from five decimal places to more by simple correction of the reading deviation as in the present invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a coordinate system used for calculations according to the present invention.

FIG. 2 is a diagram showing a plot of equation (2) with respect to α.

FIG. 3 shows a refractive index sensor having λ = 850 nm, r ₀ = 16 mm, z ₀ = 16 mm, θ ₀ = 75 °, and a divergence angle of an optical fiber of ± 4 °.
FIG. 9 is a diagram showing a light intensity ratio with respect to a position on a CCD when the total reflection criticality is -4 ° and + 4 ° of the spread angle of the optical fiber.

FIG. 4 shows a refractive index sensor having λ = 850 nm, r ₀ = 16 mm, z ₀ = 16 mm, θ ₀ = 75 °, and a divergence angle of an optical fiber of ± 4 °.
FIG. 6 is a diagram showing a light intensity ratio to α when the total reflection criticality is -4 ° and + 4 ° of the spread angle of the optical fiber.

FIG. 5 is a diagram showing an example of the structure of the sensor of the present invention.

FIG. 6 is a diagram showing a light intensity ratio detected on a detection surface when light oil is measured by this sensor.

FIG. 7 is a diagram showing a simulation result of Fresnel diffraction on a half plane.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sensor head 2 Optical fiber 3 Light incidence surface 4 Detection surface 5 Light emission surface 6 Light detection means 7 Operation control unit M Subject

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-8-68753 (JP, A) JP-A-48-102677 (JP, A) JP-A-58-27044 (JP, A) JP-A-7-78 10890 (JP, A) JP-A-6-29477 (JP, A) JP-A-6-18028 (JP, A) JP-A-2-118247 (JP, U) WO 94/24543 (WO, A1) ( 58) Fields surveyed (Int.Cl. ⁶ , DB name) G01N 21/41-21/45 JICST file (JOIS)

Claims (2)

(57) [Claims] A waveguide layer having a waveguide structure, a light incident surface on which light is incident on the waveguide layer, a region for totally reflecting and transmitting incident light having a spread angle, and a light receiving surface. A detection surface that constitutes a contact surface with the sample, and a light emission surface that outputs reflected light from the detection surface and is connected to the light detection unit; and a detection surface corresponding to the refractive index of the subject. In a refractive index sensor that detects the difference in the intensity of emitted light due to the presence of total reflected light, the value of the light intensity ratio expressed as the measured light amount / blank light amount of the subject from the dark part to the bright part with respect to the position of the detection surface From the point x ′ at which the first becomes 100%, toward the dark area, (α ′; constant, λ; wavelength of light (mm), z ″; optical path length from the detection surface to the exit surface in the optical path above total reflection critical (mm), θ ₀ ; to the detection surface of the optical axis of incident light angle of incidence (degree), r _0; than the distance from the detection surface of the optical axis of the incident light to the emitting surface (mm)); the distance to the detection surface from the incident end of the optical axis of the incident light (mm), z ₀ A method for determining the boundary between light and dark, characterized by performing Δx correction. 2. A waveguide structure comprising a waveguide layer having a waveguide structure, a light incident surface on which light is incident on the waveguide layer, a region for totally reflecting and transmitting incident light having a spread angle, and a light receiving surface. A detection surface that constitutes a contact surface with the sample, and a light emission surface that outputs reflected light from the detection surface and is connected to the light detection unit; and a detection surface corresponding to the refractive index of the subject. In a refractive index sensor that detects the difference in the intensity of emitted light due to the presence of total reflected light, the value of the light intensity ratio expressed as the measured light amount / blank light amount of the subject from the dark part to the bright part with respect to the position of the detection surface From the point x 'where the value of the first becomes 100%, toward the dark area, (α ′; constant, λ; wavelength of light (mm), z ′; optical path length from the detection surface to the emission surface in a light path passing through a point x ′ at which the value of the light intensity ratio on the emission surface becomes 100%. (mm), θ ₀ : the incident angle of the optical axis of the incident light on the detection surface (degree), r ₀ : the distance from the incident end of the optical axis of the incident light to the detection surface (mm), z ₀ : the incident light A method for determining a light-dark boundary, comprising correcting the distance (mm) from the detection surface of the optical axis to the emission surface.