JPS6112212B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a radiation thermometer that receives thermal radiation from an object and measures the temperature of the object based on the amount of radiation. FIG. 1 is a diagram showing an example of a conventional radiation thermometer installed in a continuous billet heating furnace used in steel works and the like. Continuous billet heating furnaces are widely used when continuously annealing steel billets, and in this equipment, the billet temperature is controlled from the viewpoint of quality control of the billets and optimization of the amount of heat supplied to the heating furnace. It is important to know the absolute value of . In FIG. 1, the radiation thermometer 1 is
A steel slab 3 existing in a heating furnace 2 is viewed vertically, and the temperature of the steel slab is measured by receiving radiation 4 from the surface of the steel slab. However, the reflectance of the surface of the steel slab 3 for infrared light in the vertical direction is usually about 0.2, which is not very small, and the temperature of the furnace wall 5 is usually about the same level or higher than the temperature of the steel slab 3, so the furnace wall The component of the radiation 6 reflected from the surface of the steel piece 3 and incident on the radiation thermometer 1 cannot be ignored compared to the radiation 4 from the surface of the steel piece. Since the radiation thermometer 1 calculates the temperature measurement by assuming that all the received radiant power is due to radiation from the surface of the steel slab 3, the component reflected from the surface of the steel slab 3 mentioned above causes an error in the temperature measurement value. give. In order to eliminate this drawback, this invention sets the angle at which the radiation thermometer looks into the steel piece to be a specific angle determined by the optical properties of the steel piece, and also provides a polarizing device in the light receiving part of the radiation thermometer. , Next, we will explain its principle. FIG. 2 is a diagram showing the relationship of light reflection on the surface of an object. Natural light such as thermal radiation from an object can be represented by a combination of two polarization components that are perpendicular to the direction of travel and orthogonal to each other. When considering the reflection of light, consider the plane containing the incident light 7 and the perpendicular line 9 of the object surface 8, that is, the plane of incidence, and the component in the plane perpendicular to this plane of incidence, and calculate the reflectance Rp for the polarized light component 10 in the plane of incidence. and the polarization component 11 in the plane perpendicular to the plane of incidence.
It is a well-known fact among optical engineers that the reflectance Rs for R is expressed as a function of the incident angle 12θ ₀ as follows.

[Table] However, rp = (η _1p - η _0p ) / (η _1p + η _0p ) r _s = (η _0s - η _1s ) / (η _0s + η _1s ) η _0p = 1/cosθ ₀ η _1p = (n −ik)/cosθ ₁ η _0s = cosθ ₀ η _1s = (n−ik) cosθ ₁ cosθ ₁ = ((α ² + β ² ) ^1/2 + α/2) ^1/2 −i((α ² + β ² ) ^1/2 - α / 2) ^1/2 α = 1 + (sin θ ₀ / n ² + k ² ) ² (k ² - n ² ) β = -2nk (sin θ ₀ / n ² + k ² ) ^-2 Note that the above formula n and k are the real part and imaginary part of the complex refractive index of the object, respectively, and i is the imaginary unit. When no light passes through an object, that is, when the incident light is divided into only the reflected component and the absorbed component, the emissivity and absorption rate of thermal radiation are equal due to thermal equilibrium, so the incident The emissivity εp of the component polarized in the plane and the emissivity ε _s of the component polarized in the plane perpendicular to the plane of incidence have the following relationship with Rp and R _s .

[Table] Figure 3 is a diagram showing an example of the dependence of reflectance R and emissivity ε on the angle of incidence θ _0. In the case of n=3 and k=0.1, Rp13, R _s 14, εp15, ε _s
The four values of No. 16 were obtained using the above equations (1) to (4). θ ₀ giving the minimum value of Rp13
The value θ _B is called the Brewster angle in the case of reflection from the surface of a dielectric material such as glass, where k=0, and is given by θ _B = arctan n (5). Even in the case of k=0, if n>1, there exists an angle θ _B that minimizes Rp between θ°<θ ₀ <90°. On the other hand, εp is Rp and the (3)
Since the relationship is as shown in the formula, Rp becomes maximum for the angle θ _B where Rp becomes minimum. Note that this angle θ _B is a value uniquely determined by the complex refractive index n−ik of the reflecting object. FIG. 4 is a diagram showing an embodiment of the invention. The radiation thermometer 17 looks at the steel slab 3 whose temperature is to be measured at an angle 18 of θ _B , and the light receiving window of the radiation thermometer 17 has a perpendicular line 19 between the optical path that the radiation thermometer 17 looks through and the surface of the steel slab. A polarizer 20 is provided that has a function of blocking components polarized in a direction perpendicular to the plane determined by . Here, θ _B is the angle of incidence that minimizes Rp, as mentioned earlier, and when looking at the surface of an object with this angle, the radiation thermometer 17
The reflectance Rp of the surface of the steel piece 3 for light polarized in the plane formed by the expected optical path and the perpendicular line 19 of the steel piece, that is, the plane of incidence, is small, and the surface of the steel piece 3 for light polarized within that plane. emissivity εp increases. On the other hand, the component polarized in the direction perpendicular to this plane of incidence is blocked by the polarizer 20,
It does not reach the detection section 21 within the radiation thermometer. Therefore, compared to the radiation thermometer of the conventional method shown in FIG. The ratio to component 22 is larger in the case shown in FIG. 4, and errors in temperature measurement values due to reflected light from the surface of the steel piece 3 can be reduced. FIG. 5 is a diagram for explaining a configuration example of the polarizing device 20 shown in FIG. 4. The input light 23 is reflected by a reflection plate 24 made of a dielectric material whose internal attenuation rate, that is, the imaginary part of the refractive index, is sufficiently small for the wavelength of the target light, and the reflected light 25 is detected by a radiation thermometer. The light enters the section 21. Here, input light 23
The angle of incidence 25θ _i on the reflecting plate 24 has the following relationship with the refractive index n _r of the reflecting plate 24: θ _i = arctan n _r (6), and the optical path of the input light 23 and the optical path of the reflected light 25 The plane determined by , that is, the plane of incidence on the reflection plate 24 is as described in the explanation of FIG.
The position and orientation of the reflecting plate 24 and the detection unit 21 are set so as to be perpendicular to the plane formed by the optical path of the radiation thermometer and the perpendicular line to the steel piece, that is, the plane of incidence on the steel piece. As is clear from what was stated above,
The input light 23 can be expressed as the sum of a polarized component in the plane of incidence with respect to the reflector 24 and a polarized component in a direction perpendicular to that component.
is set to the value given by equation (6), that is, the Brewster angle, so the reflected light 25 does not include a component polarized within the plane of incidence with respect to the reflector 24. Furthermore, since the plane of incidence on the steel piece and the plane of incidence on the reflection plate 24 are orthogonal, the polarization device shown in FIG. It has the function of blocking the By the way, since the reflection plate 24 is made of a material with a small attenuation rate for the target light, the transmitted light 27 from the back surface of the reflection plate also enters the detection unit 21. By providing a light shielding plate 28 with a low reflectance to such an extent that the fluctuation of the surface reflected light is sufficiently smaller than the fluctuation of the reflected light 25, the amount of transmitted light 27 from the back surface of the reflection plate is kept constant. value and does not affect the temperature readings of the radiation thermometer. In addition, the input light 23
The component 29 that passes through the reflection plate 24 is not reflected by the light shielding plate 28, so it does not affect others. This polarizing device has a similar function to a polarizing plate, but unlike infrared polarizing plates, which are expensive, it can be realized at a low cost by using a relatively inexpensive crystal plate such as CaF ₂ as a reflector. have In addition, although the case of a radiation thermometer used in a steel billet heating furnace has been described above, the present invention is not limited to this. It can be widely used when there is a need to reduce errors in the temperature measurement values of the meter. As described above, in the radiation thermometer according to the present invention, the angle at which the radiation thermometer is viewed from the object to be measured is set to a specific angle determined by the optical constant of the surface of the object to be measured, and the light receiving part of the radiation thermometer is exposed to polarized light. By providing the device, it is possible to reduce errors in temperature measurements due to background light reflected from the object surface.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an example of a conventional radiation thermometer installed in a continuous billet heating furnace, Figure 2 is a diagram showing the relationship of light reflection on the surface of an object, and Figure 3 is a diagram showing the reflectance R. , a diagram showing an example of the dependence of emissivity ε on the incident angle θ ₀ , FIG. 4 is a diagram showing an embodiment of the present invention, and FIG. 5 is a diagram for explaining an example of the configuration of a polarizing device. is a radiation thermometer, 3 is a steel piece, 4 is radiation from the surface of the steel piece, 6 is radiation from the furnace wall, 10 is a polarized light component in the plane of incidence, 11 is a polarized light component in a plane perpendicular to the plane of incidence, 20 2 is a polarizing device, 24 is a reflecting plate, and 26 is a light shielding plate. In the drawings, the same or corresponding parts are designated by the same reference numerals.

Claims (1)

[Claims] 1. In a radiation thermometer that receives thermal radiation from an object to be measured and measures the temperature of the object based on the amount of the received radiation, the radiation thermometer includes the optical path through which the radiation thermometer looks into the object to be measured and the perpendicular to the surface of the object to be measured. Position the radiation thermometer relative to the object to be measured so that the radiation thermometer looks into the object at an angle that minimizes the reflectance of light on the surface of the object to be measured with respect to components polarized in a plane, and A dielectric reflector plate is provided in the light receiving part with a reflective surface at an angle that minimizes the reflectance of light for components polarized in a direction perpendicular to the plane determined by the perpendicular line between the optical path and the surface of the object to be measured. A radiation thermometer characterized by measuring the temperature of an object to be measured using reflected light from a reflector.