RU2733391C1 - Method of measuring refractive indices of optical materials in solid state or in form of melt - Google Patents

Abstract

FIELD: measurement.

SUBSTANCE: invention relates to a method of measuring refractive indices of optical materials in a solid state or in form of a melt. Method is characterized by that a sample of the analysed material is heated to a temperature which enables to detect its heat radiation; determining angular diagrams of intensity of thermal radiation emanating from the surface element of the sample of the analysed material for two mutually perpendicular polarisations; determining the maximum value of the difference in thermal radiation intensity diagrams, depending on the refractive index of the sample of the analysed material, which is achieved in the range from the Brewster angle to the angle of total internal reflection; based on the value of the maximum difference in the heat radiation intensity diagrams, the refractive index of the sample of the analysed material is calculated; wherein the probing radiation used is the intrinsic thermal radiation of the sample of the analysed material.

EFFECT: technical result is wider range of methods of measuring optical properties of refractive index of optical material in the form of a melt at high temperatures, determining the refraction index of the analysed material in a wide spectral range without using external sources of probing radiation, simplifying the measurement circuit.

3 cl, 11 dwg

Description

TECHNICAL FIELD OF THE INVENTION

[001] The invention relates to the field of measuring the optical characteristics of semitransparent condensed media, namely to methods for measuring the refractive indices of optical materials in the solid state or in the form of a melt at high temperatures sufficient to observe their own thermal radiation.

LEVEL OF TECHNOLOGY

[002] The refractive index (hereinafter - RI) of a substance is an important optical characteristic associated with such physical properties as composition and crystal structure. Information on the RI of a material is necessary for scientific and practical applications in optical studies of objects consisting of these materials. In particular, in order to improve the control of the crystal growth process, it is important to have information about the internal structure of the crystal growth zone, the position of the crystallization front and the processes occurring near the crystallization front. For this, it is necessary to know the value of the PP of the melt.

[003] At present, the RI measurement is carried out by methods of refraction, interferometry, finding the angle of total internal reflection or Brewster for transparent substances in liquid and solid state at normal temperature.

[004] However, high-temperature melts of optical materials have a number of features that limit the application of these known methods to determine the RI of high-temperature melts of optical materials.

[005] Optical crystal melts are characterized by:

- high absorption coefficient (10 - 100 cm ^-1 and higher), which does not allow the use of methods with the transmission of probe radiation through the sample;

- chemical aggressiveness of the substance at high temperatures, therefore, the exclusion of the use of the container due to possible chemical contamination of the test substance and the associated problems with the placement of the sample;

- Intense intrinsic thermal radiation, which makes it difficult to use the probe radiation due to the presence of a high background;

- the need for powerful uniform heating of the substance for its melting, which complicates the measuring complex.

[006] At present, experimental values of the RI of optical crystal melts have been obtained only for sapphire melt. This is due to the difficulties in carrying out measurements at temperatures above 2000 K.

[007] In the international application WO2006 / 115042 (publication date 04/18/2005) discloses a method for determining the RI by the refraction method, characterized in that the measurement of RI of a sample is determined by the measured angle of refraction when the probe radiation passes through a prism containing a chamber with a measured sample placed inside prism shape.

[008] The disadvantage of this method for measuring the RI of a melt is that it is not designed to measure RI at a high absorption coefficient of the substance, and also the inability to use the container at a high temperature.

[009] In patent EP1066495 (published 02/23/1998), the refractive index of a gas filling a specially shaped chamber is determined by the method of multi-pass interferometry. In this case, in one arm of the interferometer there is a substance with a known RI, and in the other - with a measured substance. The interference of the beams allows the RI to be determined with high accuracy.

[010] The disadvantage of this method is that it is not suitable for measuring the RI of substances with a high absorption coefficient and requires the use of a container for the substance.

[011] In the patent KR101784336 (publication date 08.16.2016) a method of 3-D tomography is disclosed, which consists in the fact that to determine the RI distribution in the sample, the front of the probing radiation beam is first formed, then the radiation beam passes the sample, and the two-dimensional pattern of the intensity distribution the transmitted light is recorded and compared with the incident radiation. As a result of mathematical processing, the PP value is calculated at different points of the sample.

[012] The disadvantage of this method lies in the fact that for the calculation is used the probe radiation passed through the sample under study, i.e. the method is not intended for measuring the RI of substances with high absorption.

[013] Patent JP2003194710 (published on 12/21/2001) discloses a method for measuring RI by the Brewster angle, characterized in that to measure RI of a material with scattering losses, it is proposed to measure RI by the minimum value of the reflection intensity of polarized probe radiation when reflected from the surface of the material under study at Brewster's angle.

[014] The disadvantage of this method is the need to use an external source of probing radiation, which limits the set of data obtained in accordance with the wavelength range of the probing radiation source.

[015] Patent RU2353919 C1 (publication date 11.10.11.2007) describes an installation for high-temperature measurements of the PP of a substance by the known method of ellipsometry [Azzam R., Bashara N., "Ellipsometry and polarized light", Ed. "WORLD", M., 1981, p. 316].

[016] The operation of the installation consists in the fact that a sample is heated in a special chamber, the probe radiation is reflected from its surface, and the RI of the substance is calculated from the amplitude and phase of the polarization components of the reflected radiation.

[017] The disadvantage of measuring the RI using this installation is that it is not intended for the study of a substance in the form of a melt due to the contact of the test sample with the heater. In the case of high temperature melts, this will lead to chemical contamination of the test material. In addition, another disadvantage of the setup is the need to use external sources of probing radiation, which limits the range of data obtained.

[018] The closest in terms of the set of essential features and the achieved technical result to the claimed invention is the method for determining the PP disclosed in the article [J Am Ceram SOC, 74 [4] 881-83 (1991) Refractive Index of liquid Aluminum Oxide at 0.6328 pm Shankar Krishnan, J, K. Richard Weber, * Robert A, Schiffman, and Paul C. Nordine]. In the experiment described in the article, a drop of sapphire melted by CO ₂ laser radiation floated in a stream of oxygen or argon. The refractive index was determined by the ellipsometry method from the parameters of the reflected probe laser radiation at a wavelength of 0.6328 μm. As a result of the described experiment, the PP of sapphire melt n = 1.744 ± 0.016 was obtained at a temperature T = 2327 - 2600 K.

[019] The disadvantages of the method described in this article are a complex method of creating a sample for measurements in the form of a melt droplet floating in a gas flow, uneven heating of the droplet and the fundamental need to use external probing radiation, which leads to a complication of the measuring setup and limits the set of data obtained in communication with a limited set of suitable probing radiation sources.

SUMMARY OF THE INVENTION

[020] The technical problem to be solved by the claimed invention is to create a method for measuring the refractive index of optical materials in a solid state or in the form of a melt at high temperatures, in the wavelength range of their relative transparency at high temperatures, sufficient to observe their own thermal radiation.

[021] The technical result achieved by the implementation of the claimed invention consists in expanding the arsenal of methods for measuring the refractive indices of an optical material in a solid state or in the form of a melt at high temperatures according to the angular distributions of the intensity of the intrinsic thermal radiation of the melt from this material in two mutually perpendicular polarizations, which provides the ability to determine the RI of the investigated material in a wide spectral range without the use of external sources of probing radiation, and, in addition, allows measurements of thermal radiation spectra without using a container or gas flow to hold the material under investigation, thereby simplifying the measuring circuit.

[022] The claimed technical result is achieved due to the fact that the method for measuring the refractive indices of optical materials in a solid state or in the form of a melt is characterized by the fact that a sample of the material under study is heated to a temperature that allows recording its thermal radiation, angular diagrams of the intensity of thermal radiation are determined, The maximum value of the difference of these diagrams, depending on the refractive index of the material under study, which is achieved in the range from the Brewster angle to the angle of total internal reflection, is determined from the element of the surface of the sample of the material under study, and the value of the maximum value of the difference of these diagrams is calculated the refractive index of the material under study, while the intrinsic thermal radiation of the material under study is used as the probe radiation during measurements.

[023] In addition, in a particular case of implementation of the invention, measurements are carried out in the wavelength region of the relative transparency of the sample of the material under study.

[024] In addition, in the particular case of the invention, the angular diagrams of the radiation intensity of the test material are determined by measuring the fluxes of thermal radiation in the plane of the magnified optical image of the sample of the test material obtained in the paraxial beams of the sample's own thermal radiation.

INFORMATION CONFIRMING THE IMPLEMENTATION OF THE INVENTION

[025] The implementation of the claimed invention is confirmed by graphic materials on which:

Fig. 1 is a schematic diagram for implementing the proposed method;

Fig. 2 is an image of the growth region of the Er ³⁺ : YAG crystal fiber. The photograph shows a dashed horizontal line along which the thermal radiation spectra were measured at several points over a length of 0.715 mm. The line indicates the current position of the point R of registration of thermal radiation spectra. The size of the point designation R determined the size of the area on the melt surface (d = 37 μm), the radiation of which was recorded. The minimum division of the microscope scale is 50 mm., Β is the angle between the tangent to the generatrix of the surface of the melt drop and the Z axis. Β = 15 ° for the points of recording the spectra in the experiment;

Fig. 3 shows graphs of the thermal radiation spectra for two polarizations, measured at the point R, marked with a circle on the dashed line in Fig. 2. Jz - the axis of the polarizer is oriented along the cylindrical axis of the sample Z, Jx - the axis of the polarizer is directed horizontally, perpendicular to the axis Z. The spectral range for which the refractive index was determined is shown on the graph by the shaded area;

Fig. 4 shows the plane of incidence (orange) and the plane drawn through the points of the spectra measurement (the circle is indicated by the dotted line). Figure 4 denotes: r is the radius of the melt droplet in the plane of measurement of the spectra, R is the position of the measurement point of one of the spectra, γ is the angle of incidence of radiation from the side of the melt in the plane of incidence, ψ is the angle of refraction in air between the outgoing beam and the normal to the surface N;

Fig. 5 is a diagram of the path of rays in the melt in projection onto the XY plane, the x-coordinate of the point R along the X-axis;

6 - calculated functions of the spectral density of the intensity of thermal radiation for two polarizations Jx, Jz and their difference Jx-Jz along the x coordinate for the case β = 15 °, presented to demonstrate the measurement method;

Fig. 7 shows the profile of the spectral density of the intensity of thermal radiation in the range of 550-620 nm for two polarizations along the X coordinate. The experimental points at x = 0.27 mm are marked with circles, the intensity values of which were obtained from the spectra shown in Fig. 3;

Fig. 8 - calculated dependence of the relative value of the maximum of the function max (Q (x, n) on the refractive index of a dense medium;

Fig. 9 - experimental and calculated values of Q (x, n) for n = 1.84.

10 shows the plane of incidence (orange) and the plane perpendicular to Z passing through the scanning points of the radiation (light). In this plane, the angle ϕ defines the coordinates of the scanning point R ₁ . In the plane of incidence, the path of the ray in the melt at the angle of total internal reflection is indicated. The tangent to the surface of the melt, drawn in the XZ plane, makes an angle β with the Z axis.

Fig. 11 shows the position of the plane of incidence for the point R ₂ in the case of ϕ≈π / 6. The arrows show the direction of the electric field vector Ep, parallel to the plane of incidence, and Es - perpendicular to this plane.

CARRYING OUT THE INVENTION

[026] To implement the proposed method, an experimental scheme was used, consisting of an installation for growing monocrystalline fiber (MCF) by the LHPG (Laser Heating Pedestal Method) method (not shown in the drawings) and a measuring stand for recording the thermal radiation spectra of the Er ³⁺ : YAG sample during the growth of a crystalline fiber, a schematic diagram of which is shown in Fig. 1. A detailed description of the installation for growing a single crystal fiber by the LHPG method is contained in the article [G.А. Bufetova, V.V. Kashin, D.A. Nikolaev, S. Ya. Rusanov, V.F. Seregin, V.B. Tsvetkov, I.A. Shcherbakov, A.A. Yakovlev, Quantum Electronics, 2006, 36: 7, 616-619]. In this installation, the radiation of a CO ₂ laser (not shown in the drawings) with the help of a special optical system forms an axially symmetric heating region 10 at the surface of a sample of the material under study, consisting of a workpiece 5, a melt region 11 and a growing single-crystal fiber 6 (Fig. 1) ... As a result of heating the workpiece 5, a melt region 11 is formed.

[027] Figure 2 shows an enlarged view of a sample consisting of a preform 5, a melt region 11, and a growing monocrystalline fiber 6. The thermal radiation spectra of the melt 11 were sequentially recorded at a set of points along the dashed line shown in Figure 2.

[028] Thermal radiation from the region of the melt 11 was recorded using a measuring experimental stand (figure 1) in the plane of the optical image of the test sample.

[029] The scheme of the measuring experimental stand (figure 1) includes a spectrum analyzer 1, for example, Ocean Optics USB4000-UV-VIS-ES (OSA), OSA fiber input 2, which was an optical multimode fiber, a sample of the material under study, consisting of preforms 5, melt regions 11 and growing monocrystalline fiber 6 located along the vertical Z axis, spherical mirror 7, screen 8 and polarizer 9. The diameter of growing monocrystalline fiber 6 is 620 μm, preform 5 of the material under study is about 1200 μm, melt region 11 has a length approximately 1300 μm along the vertical Z-axis and was in the form of a drop (figure 2).

[030] The emission spectra of the heated region of the melt 11 of the Er ³⁺ : YAG crystal were recorded in the image plane of the region of the melt 11 on the screen 8 using an optical spectrum analyzer 1.

[031] To obtain an image of the region of the melt 11, a spherical mirror 7 with aluminum sputtering (F = 100 mm) was used, which displays the region of the melt 11 in the plane of the screen 8 with a linear magnification x2.7. The image of the melt region 11 was formed by paraxial beams of thermal radiation 3 incident on the mirror 7 with a deviation from the normal of no more than 3 °, beams of thermal radiation 3-4 were in the horizontal XY plane.

[032] In the plane of the image of the sample, consisting of preform 5, melt region 11 and growing monocrystalline fiber 6, end face 12 of optical fiber 2, which served as OSA input (spectral resolution of the device is not worse than 2 nm). Fiber input OSA 2 had the ability to shift in the plane of the screen 8 to record the thermal radiation spectra of the melt 11 in a sequence of points along the selected direction in the image of the melt 11. Additionally, a polarizer 9 was placed in front of the screen 8, the axis of which was oriented vertically (along the Z axis) or horizontally ( along the X axis). The core diameter (100 μm) of the multimode optical fiber 2 determined the size of the area (Fig. 2) on the surface of the melt 11 (d = 37 μm), the radiation of which was recorded. Accordingly, the spatial resolution of the thermal radiation spectra of melt 11 is 37 μm.

[033] During the experiment, using a spectrum analyzer 1, the emission spectra were recorded at different points on the surface of the melt region 11. This radiation was polarized in two mutually perpendicular directions: with the electric field vector oriented along and perpendicular to the Z axis. Jx - the polarizer 9 axis is directed horizontally, Jz - the axis of the polarizer 9 is directed vertically. The corresponding component was isolated using a polarizer 9 located in front of the input end 12 of the OSA 2 fiber input.

в различных точках изображения расплава 11 вдоль его диаметра для горизонтальной и вертикальной поляризаций. Величина

пропорциональна средней интенсивности излучения в спектральном диапазоне измерений (область интегрирования 550-620 нм, см.фиг.3).[034] The position of the area of the surface of the melt 11, the spectrum of which was recorded, for brevity is designated as point R (see figure 2). The angular aperture of the detected radiation from the melt 11 (0.011 rad) was significantly smaller than the angular aperture of the OSA 2 fiber entrance (0.22 rad). The depth of field of the optical radiation registration system made it possible to simultaneously obtain a clear image of both the central and edge zones of the investigated melt 11. In the experiments, the spectra of thermal radiation emanating from the surface element of the melt droplet 11 were measured at points located along the horizontal line (see Fig. 2, dotted line). The spectral sensitivity of the measuring path (OSA) 2 was calibrated against an absolute black body standard (metrological incandescent lamp TRU 1100-2350). Therefore, the spectra were actually used to determine the spectral density of the radiation intensity

at various points in the image of the melt 11 along its diameter for horizontal and vertical polarizations. The quantity

is proportional to the average radiation intensity in the spectral range of measurements (integration area 550-620 nm, see figure 3).

Where λ1 = 550 nm λ2 = 620 nm

[035] At the edges of the image, the step of registration of radiation over the melt 11 was 8 μm, in the center - 35 μm, the diameter of the melt in the plane of registration of radiation was 715 μm. For all points R along the line of radiation registration, the angle of inclination of the tangent to the surface with respect to the Z axis was β = 15 °.

для излучения, вышедшего из расплава 11 в воздух, имеет ярко выраженный максимум в интервале углов падения со стороны расплава 11 от угла Брюстера до угла полного внутреннего отражения (фиг.6). Величина этого максимума, как показывают простые расчеты по формулам Френеля, почти линейно зависит от ПП. Это дает возможность определить ПП расплава 11 путем измерения угловой зависимости интенсивности прошедшего света для двух поляризаций. Для получения угловой зависимости двух разных поляризаций можно измерить диаграмму направленности интенсивности излучения от одной и той же излучающей площадки в области расплава 11, смещая приемник по окружности вокруг нее. Но в случае исследования капли расплава 11 это трудно реализуемо. В то же время осевая симметрия и однородность расплава 11 в капле создает возможность получения аналогичной информации путем анализа излучения в точках, расположенных на линии перпендикулярной оси области расплава 11 (см. схему, представленную на фиг.5), при условии, что излучение распространяется в одном направлении (направлении наблюдения). Интервал от угла полного внутреннего отражения до угла Брюстера в плотной среде определяет рабочий интервал измерения спектров теплового излучения в зависимости от угла при определении ПП расплава 11 заявляемым способом (фиг.6).[036] From the calculation it follows that the difference in the intensity of the two polarizations

for radiation released from the melt 11 into the air, has a pronounced maximum in the range of angles of incidence from the side of the melt 11 from the Brewster angle to the angle of total internal reflection (Fig. 6). The magnitude of this maximum, as shown by simple calculations using the Fresnel formulas, almost linearly depends on the RI. This makes it possible to determine the RI of the melt 11 by measuring the angular dependence of the intensity of the transmitted light for two polarizations. To obtain the angular dependence of two different polarizations, it is possible to measure the directional diagram of the radiation intensity from the same emitting area in the region of the melt 11, displacing the receiver in a circle around it. But in the case of studying the melt droplet 11, this is difficult to implement. At the same time, the axial symmetry and homogeneity of the melt 11 in the drop makes it possible to obtain similar information by analyzing the radiation at points located on the line perpendicular to the axis of the region of the melt 11 (see the diagram shown in Fig. 5), provided that the radiation propagates in one direction (direction of observation). The interval from the angle of total internal reflection to the Brewster angle in a dense medium determines the working interval of measuring the spectra of thermal radiation depending on the angle when determining the PP of the melt 11 by the claimed method (Fig. 6).

[037] Measurement of the spectra of thermal radiation depending on the angle of incidence γ was carried out by moving the observation point R along a line perpendicular to the Z axis (figure 1 and 5). The position of the point R uniquely determines the angle of incidence of γ radiation on the interface between the melt 11 and air (figure 4). The movement of the observation point R corresponds to the movement of the end face 12 of the fiber inlet OSA 2 in the image of the melt drop 11 in the plane of the image 8 (Fig. 1).

[038] As a result, a significant simplification of the method implementation scheme is achieved due to some complication of the calculations, since the position of the plane of incidence in this case constantly changes relative to the XYZ reference system when the position of the point R changes (see Figs. 10 and 11).

и

вдоль линии сканирования, измеренные по изображению расплава 11. Для удобства данные для

и

на фиг.7 нормированы на 2·max(

). Рабочий диапазон 550-620 нм для определения n расплава 11 был выбран вне полос поглощения иона Er³⁺ и для расплава 11 и для кристалла Er³⁺:YAG. Форма графиков на фиг.7 имеет характерный вид для угловой зависимости пропускания поляризованного излучения в соответствии с формулами Френеля. Для разных поляризаций в средней части изображения расплава 11 (фиг.7) интенсивности

и

близки по величине, а на краях они заметно отличаются. Отличие по амплитуде видно на фиг.3, где приведены спектры для двух поляризаций, измеренные в точке x=0.27 mm.[039] FIG. 7 shows the profiles of the spectral density of the radiation intensity for two components of polarization

and

along the scanning line, measured from the melt image 11. For convenience, the data for

and

in Fig. 7 are normalized to 2 max (

). The working range of 550-620 nm for determining n of melt 11 was chosen outside the absorption bands of the Er ³⁺ ion for both melt 11 and the Er ³⁺ : YAG crystal. The shape of the graphs in Fig. 7 has a characteristic form for the angular dependence of the transmission of polarized radiation in accordance with the Fresnel formulas. For different polarizations in the middle part of the image of the melt 11 (Fig. 7), the intensities

and

are close in size, but at the edges they differ markedly. The difference in amplitude is seen in Fig. 3, which shows the spectra for two polarizations measured at the point x = 0.27 mm.

от показателя преломления расплава 11 позволяет по относительной величине наблюдаемого максимума этой функции определить величину показателя преломления n. [040] Calculations show (Fig. 8) that the dependence of the difference

from the refractive index of the melt 11 allows the relative value of the observed maximum of this function to determine the value of the refractive index n .

[041] The spectra of thermal radiation emanating from the surface element of the melt droplet 11 were recorded under the same conditions, so the intensity values of all measured spectra were obtained in the same relative units. This makes it possible to estimate the ratio of the absolute values of the intensity of the measured spectra and compare them with similar calculated results.

,

и их разности

-

была определена безразмерная функция Q(x,n), как функция показателя преломления n и координаты x[042] Based on the calculation of the values

,

and their differences

-

the dimensionless function Q (x, n) was defined as a function of the refractive index n and coordinates x

для каждого данного n связан с тем, что локальный максимум

при угле Брюстера регистрируется в измерениях наиболее точно (кривая

на фиг.7).[043] Further, the value was found n , at which the best agreement between the calculated and experimental values for max (Q (x, n)) was achieved (Fig. 9). Selection of the maximum intensity value as a normalizing value

for each given n is connected with the fact that the local maximum

at the Brewster angle is recorded in measurements most accurately (the curve

in Fig. 7).

[044] The calculated Q (x, n) curve (Fig. 9) was constructed from experimental data using the least squares method. The interval of PP values, taking into account the measurement accuracy, is indicated by two strokes in the calculated graph max (Q (x, n)) in Fig. 8. The temperature of melt 11 in the region of registration of thermal radiation spectra T = 2400 ± 30 K was determined from the slope of the spectra in the Wien coordinates.

[045] The resulting PP value is n = 1.84 ± 0.01. Variations in the angle of inclination β within ± 2 degrees and variations in the absorption coefficient α = 25-35 cm ^-1 with this method of determining n do not affect the result.

.[046] The values of the spectral density of the radiation intensity for two polarizations, measured in the experiment, are shown in Fig. 7. The difference between the right and left values of the functions Jx and Jz is due to the fact that in a real experiment the center of the heating region of the melt 11, which has a ring shape, can be slightly displaced relative to the axis of the growing single-crystal fiber 6. As a result, the heating of the melt droplet 11 and the intensity of thermal radiation on one side of the melt region 11 turned out to be slightly higher than on the other side (the temperature determined from the slope of the spectra in the Wien coordinates differed by 30K). When determining the function max (Q (x, n)), this difference does not play a significant role due to the fact that each wing of the intensity graphs Jx and Jz (Fig. 7), when processing experimental results, is normalized to its maximum value

...

[047] The accuracy of determining the RI by this method depends primarily on the stationarity of the shape and size of the melt droplet 11 during the growth of the crystal fiber.

[048] Thus, the claimed method measured the RI of the Er: YAG crystal melt at a temperature of about 2400 ° K, which turned out to be equal to 1.84 ± 0.01.

[049] Confirmation of the implementation of the proposed method is given for the melt of the test material, but for an optically isotropic material in a solid state at a high temperature, the proposed method can also be implemented, under conditions that do not change the essence of the proposed method, disclosed in the independent claim, namely - provided that the equipment is sufficiently sensitive for recording the thermal radiation of a heated sample made of the material under study and the optical quality of its surface, i.e. the surface is not rough and the scattering losses when passing through the interface between the media are negligible compared to the intensity of the transmitted light, which is described by the Fresnel formulas.

Claims (8)

1. A method for measuring the refractive indices of optical materials in the solid state or in the form of a melt, characterized in that a sample of the test material is heated to a temperature that allows recording its thermal radiation; determine the angular diagrams of the intensity of thermal radiation emanating from the surface element of the sample of the material under study for two mutually perpendicular polarizations; determine the maximum value of the difference between the diagrams of the intensity of thermal radiation, depending on the refractive index of the sample of the investigated material, which is achieved in the range from the Brewster angle to the angle of total internal reflection; the value of the maximum value of the difference in the intensity diagrams of thermal radiation calculate the refractive index of the sample of the material under study; in this case, the intrinsic thermal radiation of the sample of the investigated material is used as the probe radiation in the measurements. 2. The method according to claim 1, characterized in that the measurements are carried out in the wavelength region of the relative transparency of the sample of the material under study. 3. The method according to claim 1, characterized in that the angular radiation intensity diagrams of the sample of the test material are determined by measuring the fluxes of thermal radiation in the plane of the magnified optical image of the sample of the test material obtained in the paraxial beams of the sample's own thermal radiation.

Images