JP2011220903A - Refractive-index measurement method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve problems of a conventional refractive-index measurement method including a destructive inspection, usage of toxic materials and no availability other than on a flat surface, while a glass member and a plastic member require an accurately measured refractive-index distribution for their quality stability.SOLUTION: According to an interference signal obtained by a light sensor, the respective distances are measured between a standard reflective surface without an object to be inspected, a front face of the object to be inspected, a rear face of the object to be inspected, and at least one of the standard reflective surface after transmitting the object to be inspected and the standard reflective surface reflected with parallel light converged before the object to be inspected. A refractive-index of the object to be inspected is calculated based on the measured distances.

Description

  The present invention relates to a refractive index measuring method and a refractive index measuring apparatus for measuring the shape, thickness, refractive index and refractive index distribution of a glass member or a plastic member.

  Glass members and plastic members are used very widely such as front plates used for display panels and the like, molded lenses used for optical pickups, digital copiers, digital cameras, and the like. Further, the accuracy required for glass members and plastic members used for such applications is very high.

Glass members and plastic members are processed and molded by a thermal process, but the internal refractive index distribution may be uneven depending on the molding conditions.
In particular, molded lenses are superior in terms of manufacturability of aspherical lenses compared to glass-polished lenses and are low in cost. On the other hand, depending on the molding conditions, the instability of the refractive index distribution within the lens is likely to occur. It has special properties. The non-uniformity of the refractive index inside the lens has a great influence on the optical characteristics of the lens and may cause a deterioration in imaging performance. For this reason, in order to stabilize the quality of a glass member or a plastic member, it is necessary to measure the refractive index distribution with high accuracy.

As a conventional technique for measuring a refractive index, there is a method of measuring and obtaining a declination angle by a minimum declination method or the like, and it is known that this method can be measured with high accuracy.
In addition, as a method of measuring the refractive index of a specimen such as diamond or ore, immerse the specimen in a solution with a known refractive index, observe the Becke line appearing at the boundary line, and remove the solution until the Becke line disappears. A method is known in which the refractive index of the test object is indirectly measured from the refractive index of the solution when the line disappears after replacement.

  On the other hand, a refractive index measurement method using an interferometer is also known. As a method for measuring the refractive index using an interferometer, there is a Mach-Zehnder interferometer (see Patent Document 1) and a Michelson interferometer (see Patent Document 2).

FIG. 8 is a diagram showing a refractive index measuring apparatus using a conventional Mach-Zehnder interferometer.
As shown in FIG. 8, the light emitted from the laser light source 51 is converted into parallel light by the condenser lens 52 and then branched into reference light 54 and inspection light 55 by the half mirror 53. After the inspection light 55 is reflected by the mirror 56, the inspection light 55 is transmitted through the immersion bath 58 filled with a matching liquid having a refractive index substantially equal to that of the test object 57 and the test object 57 immersed therein, and the test object 55 An optical path difference corresponding to the refractive index distribution of 57 is generated, and after reaching the half mirror 59, enters the image sensor 61 through the imaging lens 60.

  On the other hand, the reference light 54 passes through the compensation plate 62, is reflected by the mirror 63, reaches the half mirror 59, and then enters the image sensor 61 through the imaging lens 60. The compensation plate 62 is for making the optical path length from the inspection light 55 to the image sensor 61 almost equal to the optical path length from the reference light 54 to the image sensor 61.

  In the image sensor 61, the inspection light 55 and the reference light 54 are combined, and an interference fringe 64 is generated according to the optical path difference. In this interference fringe observation, the optical thickness of the test object 57 in that portion can be calculated from the number of the fringes at the location where the interference fringes 64 are generated, and the difference in the refractive index of the matching liquid is calculated therefrom. Thus, the refractive index of the test object 57 can be obtained.

In addition, in a method using an interferometer that uses the matching liquid, there is a method that can measure a three-dimensional distribution by introducing a bathtub or a phase shift method that increases the temperature uniformity of the matching liquid.
FIG. 9 is a diagram showing a refractive index measuring apparatus using a conventional Michelson interferometer.

  Using a low-coherent light source 70, the light is condensed by a condensing lens 71 and irradiated to the boundary portion between the transparent plate-like test object 73 and the air, and the reference light mirror 74 and the front and rear surfaces of the test object 73 are irradiated. The intensity of the interference light is observed with the photodetector 72.

  The test object 73 or the condensing lens 71 and the reference light mirror 74 are moved so that the intensity of this interference becomes maximum, and the test object 73 or the light collection at the position of the maximum interference light intensity on the front and rear surfaces. The moving distance of the lens 71 and the reference light mirror 74 is obtained. Thus, the refractive index is obtained by simultaneously measuring the refractive index and the thickness.

JP 2001-21448 A JP-A-11-344313

However, in the conventional method of measuring the deflection angle, it is necessary to process the test object into a predetermined shape, and there is a problem that the optical element to be measured must be destroyed and polished.
Further, the method of immersing the test object 57 as shown in FIG. 8 in a matching liquid having a refractive index substantially equal to the test object can be measured nondestructively, but the non-uniformity of the refractive index of the matching liquid causes a large error. Therefore, there is a problem that the control method is difficult. Furthermore, the test object 57 is narrowed to the region where the refractive index of the matching liquid exists, and the matching liquid itself is often harmful to the environment and the human body, and there is a problem that the convenience of measurement is impaired.

  Further, in the Michelson interferometer as shown in FIG. 9, measurement is possible when the test object 73 is a flat surface, but measurement is difficult when the surface of the test object 73 is a curved surface. Moreover, it is necessary to move the test object 73 or the condensing lens 71 and the reference light mirror 74 at two locations, and the influence of the error in the amount of both movements occurs.

  The present invention solves the above-described conventional problems, and an object thereof is to provide a refractive index measuring method and a refractive index measuring apparatus capable of easily measuring a refractive index distribution of a test object in a non-destructive manner. .

  In order to solve the above-described problems, the refractive index measurement method of the present invention includes a light splitting step of splitting light from a light source into inspection light and reference light, and condensing the inspection light on a test object and a reference reflecting surface. An inspection light condensing step, a condensing position switching step of switching the condensing position of the inspection light to the front and back surfaces of the test object and the surface of the standard reflecting surface, and the reference light is reflected by the reference reflecting surface A reference light reflecting step, and an interference signal acquiring step of obtaining an interference signal by causing interference between the inspection light reflected from the test object and the standard reflecting surface and the reference light reflected from the reference reflecting surface And the reference reflection surface in a state where the test object does not exist, the surface of the test object, the back surface of the test object, and the reference reflection surface in a state where the test object exists, A distance measuring device that measures the optical distance of each of the optical distances based on the interference signal When, and having a refractive index calculation step of calculating a refractive index of the test object based on the optical distance measured by said distance measuring step.

  Further, the refractive index measuring apparatus of the present invention condenses the light source, light splitting means for splitting the light from the light source into inspection light and reference light, and the inspection light on the test object and the reference reflecting surface. A condensing optical system; condensing optical system driving means for moving the condensing optical system along the optical axis of the inspection light; and an object for moving the test object in a direction different from the optical axis of the inspection light. Specimen driving means, a reference reflecting surface that reflects the reference light, and interference caused by interference between the inspection light reflected from the test object and the standard reflecting surface and the reference light reflected from the reference reflecting surface A light receiving element for obtaining a signal, and based on an interference signal obtained by the light receiving element, a reference reflection surface in a state where the test object does not exist, a surface of the test object, and a back surface of the test object. Measure each optical distance and calculate the refractive index of the specimen based on these optical distances A calculation unit that, characterized by comprising a.

  By using the present invention, it is possible to easily measure the refractive index distribution of the test object in a non-destructive manner. Further, it is not necessary to process the test object into a predetermined shape, and since no matching liquid or the like is used, there is no harm to the environment and the human body.

The figure which shows the conceptual structure of the refractive index measuring apparatus which concerns on embodiment of this invention. It is a figure which shows the measurement state by the refractive index measuring method using the same refractive index measuring apparatus, (a) A figure in case a condensing position is a reference | standard reflective surface in the state in which a test object does not exist, (b) Collection The figure in the case where the light position is the reference reflecting surface after passing through the test object, (c) The figure in the case where the light collection position is the front surface of the test object, and (d) The light collection position is the test object. The figure in the case of the back side of the figure, (e) The figure in the case where the condensing position is in front of the test object The measurement state in the case where the test object is off-axis in the same refractive index measurement method is shown, (a) a diagram in which the inspection light is incident on the slope on the test object, (b) the slope on the test object Figure of reflection from In the same refractive index measurement method, the inspection light is transmitted through a test object that is off-axis and reflected by the reference reflecting surface, (a) the incident light is incident on the slope on the test object, (b) Illustration of reflection from the slope on the specimen In the same refractive index measurement method, it is a conceptual diagram of wavefront aberration in off-axis measurement, (a) a state in which the test object is shifted only in the Y direction from the optical axis, (b) in the state of FIG. (C) Diagram of interference fringes of reflected light in the state of FIG. 5 (a), (d) Peak position deviation and interference due to aberration in the state of FIG. 5 (a) Diagram showing the concept of signal rounding and rounding First flowchart of refractive index measurement in the same refractive index measurement method Second flowchart of refractive index measurement in the same refractive index measurement method The figure which shows the structure of the refractive index measuring apparatus using the conventional Mach-Zehnder type interferometer The figure which shows the structure of the refractive index measuring apparatus using the conventional Michelson type interferometer

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, FIG. 1 is a diagram illustrating a conceptual configuration of a refractive index measuring apparatus according to an embodiment of the present invention, and FIGS. 2A to 2E are diagrams illustrating a refractive index measurement state in the same embodiment. .

  As shown in FIG. 1, a refractive index measuring apparatus according to an embodiment of the present invention includes a light source 1 and a beam splitter (light splitting means) 2 that splits light from the light source 1 into inspection light 18 and reference light 19. And a condensing lens (condensing optical system) 3 that condenses the inspection light 18 and irradiates the test object 4 and the reference reflecting surface 6, and the condensing lens 3 in the optical axis direction (exit of the inspection light 18). And a test object drive for moving the test object (test lens) 4 in a direction perpendicular to the optical axis of the condensing lens 3 that is a condensing optical system. An optical sensor (light reception) that obtains an interference signal between the mechanism 8, the flat reference reflection surface 12 that reflects the reference light 19, the inspection light 18 reflected by the standard reflection surface 6, and the reference light 19 reflected by the reference reflection surface 12. Element) 17 and the camera 14 that obtains the interference image, and the interference signal obtained by the optical sensor 17. There are and a like controller 20 also has a function as a calculation unit for calculating the refractive index of the object 4. The condensing optical system driving mechanism 7 moves the condensing position of the condensing lens 3 to the front and back surfaces of the test object 4 and the surface of the reference reflecting surface 6.

  The control device 20 controls the light source 1, the condensing optical system drive mechanism 7, the test object drive mechanism 8, the camera 14, the optical sensor 17, and the like, and the reference reflection surface in a state where the test object 4 does not exist. 6, the front surface of the test object 4, the back surface of the test object 4, the reference reflection surface after passing through the test object 4, and the reference reflection surface that collects light before the test object 4 and reflects it with parallel light Are also provided with a function as a calculation means for calculating an accurate optical distance by removing an error effect due to aberration.

  Here, the test object 4 is a lens or the like made of a glass member or a plastic member (however, it is not limited to a lens, and any member having a property of transmitting and reflecting is to be measured as the test object 4). In the optical path in which the inspection light 18 is irradiated toward the reference reflecting surface 6 by the condenser lens 3 while being held by the holder 21. 1 is a holder reference surface for detecting the position of the holder 21 provided on the holder 21 that holds the condenser lens 3 as a condenser optical system, and 13 is reflected by the beam splitter 15. The imaging lens 18 forms an image on the camera 14 with the inspection light 18 and the reference light 19 transmitted through the beam splitter 15. Reference numeral 15 is the reference light reflected by the inspection light 18 transmitted through the beam splitter 2 and the beam splitter 2. A beam splitter that reflects the light 19, and 16 is a condenser lens that condenses the inspection light 18 and the reference light 19 reflected by the beam splitter 15 on the optical sensor 17.

  Further, in this embodiment, the ND filter 10 and the space between the beam splitter 2 that divides the light from the light source 1 into the inspection light 18 and the reference light 19 and the reference reflection plane 12 that reflects the reference light 19. A filter 11 is provided. Further, a spatial filter 9 is disposed between the beam splitter 2 and the condenser lens 3. Further, the condensing optical system driving mechanism 7 moves and adjusts the condensing lens 3 so that the condensing lens 3 collects light on the reference reflecting surface 6 and the like at a point.

  In the above configuration, as shown in FIG. 1, the light beam emitted from the light source 1 passes through the beam splitter 15 and is split into the inspection light 18 and the reference light 19 by the beam splitter 2 (light splitting step).

  The inspection light 18 passes through the spatial filter 9 and is then condensed by the condenser lens 3 (inspection light condensing step). The condensing point is adjusted by moving the condensing lens 3 by the condensing optical system driving mechanism 7 (condensing position switching step).

  FIG. 1 shows a state in which the inspection light 18 is condensed on the surface of the reference reflecting surface 6 by the condenser lens 3. In this state, the inspection light 18 that has passed through the condenser lens 3 passes through the test object 4 and the holder reference surface 5 and reaches the surface of the reference reflection surface 6. In this case, since the adjustment is made so that the light is condensed on the point by the reference reflecting surface 6 by the condensing optical system driving mechanism 7, the inspection light 18 reflected by the reference reflecting surface 6 again becomes the holder reference surface 5 The sample 4 passes through the specimen 4 and is returned as parallel light by the condenser lens 3. However, an optical aberration is generated by the configuration of the optical system after the condenser lens 3, and the parallel light is substantially parallel light with a wavefront slightly deviated.

  On the other hand, the reference light 19 split by the beam splitter 2 passes through the ND filter 10, passes through the spatial filter 11, reaches the reference reflecting surface 12 as parallel light, and is reflected by the reference reflecting surface 12 ( Reference light reflection step). The reference light 19 reflected by the reference reflecting surface 12 is combined with the inspection light 18 reflected by the beam splitter 2.

  Here, the inspection light 18 reflected by the beam splitter 2 and the reference light 19 transmitted through the beam splitter 2 are imaged on the camera 14 by the imaging lens 13. Since the inspection light 18 and the reference light 19 cause interference, the interference image can be observed by the camera 14.

  Further, the inspection light 18 transmitted through the beam splitter 2 and the reference light 19 reflected by the beam splitter 2 are both reflected by the beam splitter 15 and condensed on the optical sensor 17 by the condensing lens 16, and the optical sensor. 17 can obtain an interference signal (interference signal acquisition step). The reflection optical distance from the object reflected from the interference signal can be measured (distance measurement step).

  The light source 1 may be a white light source, a monochromatic laser such as helium neon, or a multi-wavelength light source such as an optical comb. However, in the case where the light source 1 is a white light source or a monochromatic laser, the reference reflection plane 12 and the reflection position of the target are made to coincide with each other according to the coherence and coherence optical distance. It is necessary to provide a moving mechanism. Therefore, here, a stable light source having a multi-wavelength such as an optical comb light source (a light source that generates light having a frequency continuously distributed at regular intervals like a comb) is desirable.

  The beam splitters 2 and 15 may be flat plate splitters or cubes. The spatial filters 9 and 11 are non-transparent concentric or slit-shaped filters that transmit only necessary light. By providing a rotation mechanism or a movement mechanism to the mechanism that supports the spatial filters 9 and 11 so as to be rotatable or movable, it is possible to transmit only necessary light and to perform more precise filtering. Similar effects can be obtained by using a variable filter such as a liquid crystal instead of using the spatial filters 9 and 11.

  It is desirable that the condenser lens 3 has a corrected aberration such as an objective lens and is close to zero or has a known aberration amount even if it is not zero. Further, when the reflected light cannot be sufficiently obtained due to the reflection angle of the test object 4, it is possible to use a diffractive optical system using a diffraction grating or the like as the condensing optical system instead of the lens system.

  The condensing optical system driving mechanism 7 and the test object driving mechanism 8 need only have submicron order accuracy, but the accuracy of straightness, pitching, yawing, and rolling affects the measurement accuracy. It is desirable that the accuracy is 1 μm or 1 minute or less.

  The reference reflecting surface 6 may be in the form of a mirror having a reflectance close to 100% with respect to the wavelength of the light source 1, but when the reflectance is close to the reflectance of the test object 4, the amount of reflected light is constant, and the test object Depending on 4, the signal processing error may be reduced. Further, it is desirable that the reference reflecting surface 6 has a surface accuracy of 1 / 10λ or less or a known aberration amount.

  Since the reflectivity of the test object 4 is generally smaller than the reflectivity of the reference reflecting surface 12, the ND filter 10 maximizes interference fringes and interference signals by matching the amount of light reflected from the test object 4 side. Is. Although this ND filter 10 can be measured even if it is not present, it is desirable to be present because it is strong against disturbance by maximizing the interference signal, and the ND filter 10 is more variable than the fixed concentration. Things are desirable. The transmission aberration of the ND filter 10 is preferably 1 / 10λ or less or a known aberration amount. Since the reference reflecting surface 12 is a reference surface (flat surface), it is desirable that the surface accuracy is 1 / 20λ or less or the amount of aberration is known.

  The imaging lens 13 is used for imaging light on the camera 14, and it is desirable that the aberration is small. However, even if it is not a camera lens, a single lens such as a doublet can be used. The camera 14 is an area sensor camera such as a CCD or CMOS, and preferably has a resolution of VGA (640 × 480 pixels) or more.

  Since the condensing lens 16 is for condensing light on the optical sensor 17 at one point, it is desirable that the aberration is as small as possible, but it is possible to use a single lens such as a doublet as well as an objective lens. . The optical sensor 17 is a single-point light quantity detector such as a photodiode, and preferably has a light quantity resolution of 10 BIT or more and a fast time response.

Next, the refractive index measurement method according to the embodiment of the present invention will be described with reference to FIG.
Here, a measurement method in the case where the center of the test object 4 is on the optical axis of the inspection light 18 will be described first.
2A to 2E respectively show states in which the condensing lens 3 is moved by the condensing optical system driving mechanism 7 and the condensing position of the inspection light 18 is changed. Here, FIG. 2A shows a case where the condensing position of the condensing lens 3 is the reference reflecting surface 6 when the test object 4 is not present, and FIG. FIG. 2 (c) shows a case where the reference reflecting surface 6 after transmission of the test object 4 is shown, and FIG. 2 (c) shows a case where the condensing position is the front surface of the test object 4 (surface on the condensing lens 3 side). FIG. 2D shows a case where the condensing position is the back surface of the test object 4 (the surface opposite to the condensing lens 3), and FIG. 2E shows the condensing position of the test object 4. This shows the case before this.

  As described above, the optical distance at this time is calculated by the control device 20 as an accurate distance from which the influence of the error due to the aberration is removed, and the optical distances are defined as La, Lb, Lc, Ld, and Le. (In other words, La is the optical distance when the collection position is the reference reflection surface 6 in the state where the test object 4 does not exist, and Lb is the reference reflection surface 6 after the transmission of the test object 4 at the collection position. In some cases, the optical distance Lc is the optical distance when the condensing position is the front surface of the test object 4, Ld is the optical distance when the condensing position is the back surface of the test object 4, and Le is the collection distance. This is the optical distance when the light position is in front of the test object 4). However, this optical distance is not an actual distance but an optical distance, and is affected by a refractive index of a lens or the like in the optical path.

Here, the lens thickness T of the test object 4 can be obtained by the following (formula 1).
T = (La−Lb) − (Ld−Lc) (Formula 1)
On the other hand, the refractive index N can be expressed by the following (formula 2) or the following (formula 3), and by calculating this, the lens thickness T and the refractive index N can be obtained.

N = (Lc−La) / T (Expression 2)
N = (Lc−La) / T + 1 (Expression 3)
However, the refractive index obtained here is a group refractive index, and when it is desired to obtain the phase refractive index, it can be obtained by conversion using the definition formula of dispersion of the glass material (lens material).

  As described above, the lens thickness T and the refractive index N can be obtained from the optical distances La to Ld. In addition to this, the number of data can be increased by measuring the optical distance Le in the state of FIG. By performing the measurement of FIG. 2E, higher accuracy can be achieved.

Here, the state shown in FIG. 2 (e) is different in the reflection state, but the measurement optical distance is measured at the same position as in FIG. 2 (b). ) Is also possible.
T = (La−Le) − (Ld−Lc) (Formula 4)
Thus, by obtaining the refractive index N (refractive index calculating step), it is possible to measure the same value not only in (Expression 1) but also in (Expression 4), thereby increasing the number of detected data points. From this result, the measurement accuracy can be further improved by performing an averaging process or an offset calculation process using two values.

  3A and 3B, the thickness and refractive index of the test object 4 are calculated when the center of the test object 4 is not on the optical axis of the test light 18 but at a position off the axis. The case is illustrated. In a state where the center of the test object 4 is off the optical axis of the inspection light 18, as shown in FIG. 3A, the inspection light 18 is incident on the slope portion of the test object 4, and therefore FIG. ), The reflection angle at the test object 4 is inclined. Therefore, the aperture is restricted by the condenser lens 3, and reflected light can be obtained only from the periphery of the thick line shown in FIGS. 3 (a) and 3 (b).

  As shown in FIGS. 4A and 4B, the same applies to the case where the center of the test object 4 is off the optical axis of the test light 18. In this case, the reference after passing through the test object 4 is the same. Although the reflection position on the reflecting surface 6 is detected, the incident optical axis is tilted by the refraction action of the test object 4, and the same phenomenon as shown in FIGS. 3 (a) and 3 (b) occurs. In this case, not only light vignetting but also wavefront aberration is generated, and it is necessary to remove the wavefront aberration.

  Therefore, in this case, it is necessary to first remove the influence of wavefront aberration due to the refraction angle. Here, since the shape of the test object 4 is known from the optical distance La to the optical distance Ld, it is possible to correct by using this measured value directly. Specifically, a phase refractive index is first assumed from a theoretical value, and a refraction angle is calculated based on the theoretical value. From the distance through which the light beam is refracted and transmitted obliquely, the actual specimen 14 is measured. Convert to thickness and optical distance. In this case, the influence of the error due to the assumption of the phase refractive index naturally occurs. However, if the difference between the theoretical value and the actual measurement value is small, the influence of the wavefront aberration can be eliminated even if the theoretical value is used as it is. If the difference between the theoretical value and the actual measurement value is large and it is necessary to determine the actual measurement value strictly, the error correction of the phase refractive index is performed again from the method obtained from the two calculation formulas using the optical distances La to Le. Then, it can be obtained again by regression calculation for calculating the group refractive index.

The details of the wavefront aberration correction method will be described with reference to FIGS.
FIG. 5A shows a case where the test object 4 is shifted only in the Y direction from the optical axis. The wavefront aberration in this case is shown in FIG. Here, the horizontal axis indicates the position of the incident light beam in the Y direction and the X direction, and the vertical axis indicates the amount of aberration. As can be seen from the left side of FIG. 5 (b), not only does the vignetting occur in the Y direction due to the tilt and the light does not return, but particularly the aberration occurs at the end of the incident light. Yes. As can be seen from the diagram on the right side of FIG. 5B, no tilt occurs in the X direction, but the influence of the spherical aberration of the specimen 4 appears.

  The interference fringes of these reflected lights are as shown in FIG. Since the interference signal is obtained as a whole set of light beams, if such an aberration occurs, the interference signal peak position shift a and the interference signal peak round b as shown in FIG. Will affect the detection accuracy.

  Therefore, as shown in FIG. 1, by arranging the spatial filters 9 and 11 to transmit only light having no aberration, a clear interference signal can be obtained, and high detection accuracy can be realized. In addition, what kind of spatial filter is used includes a method for obtaining a filtering shape from an interference image obtained from simulation and a method for obtaining a filtering shape by directly detecting an interference image with the camera 14.

The method for obtaining the thickness and refractive index of each position on the test object 4 has been described above. FIG. 6 shows a specific flowchart of refractive index measurement.
First, as shown to Fig.2 (a), the position of the reference | standard reflective surface 6 is measured in the state in which the test object 4 does not exist (step S1). Next, as shown in FIG. 2B, the test object 4 is moved to the initial measurement position and placed in the optical path (step S2). Next, as shown in FIG. 2C, the surface of the test object 4 is measured (step S3), and then as shown in FIG. 2D, the back surface of the test object 4 is measured ( Step S4).

  Next, the holder reference surface 5 attached to the holder 21 that holds the test object 4 is measured (step S5). By measuring the holder reference surface 5, even when there is an error in the straightness of the test object driving mechanism 8 of the test object 4, it is possible to measure at each measurement position.

  Next, a state (see FIG. 2B) that is transmitted through the test object 4 and reflected from the reference reflection surface 6 is measured (step S6), and is transmitted through the test object 4 from the reference reflection surface 6. The state of reflection (see FIG. 2 (e)) is measured (step S7). Here, in each of the measurement steps S6 and S7, the aberration is removed by using the above-described aberration removal method.

  Next, the measurement position is moved (step S8), and in step S9, it is determined whether all the measurement positions have been measured. In this case, when the measurement position still remains (No in step S9), the process returns to step S3, and the measurement at the new measurement position is sequentially performed. When measurement of all measurement positions is completed (Yes in step S9), the reference reflecting surface 6 is measured again without the test object 4 as shown in FIG. 6A (step S10). Although this step is not necessary, it is possible to confirm whether or not a change in position over time has occurred by performing this step, and if necessary, correction can be made from the first and last results. .

  The refractive index is calculated using each measurement optical distance measured in steps S3 to S7 at each measurement position and the measurement optical distance of the reference reflecting surface measured in steps S1 and S10 (step S11). Thus, the lens thickness and refractive index corresponding to the test object 4 at each measurement position are calculated. Thus, by calculating the refractive index at each measurement position, the refractive index distribution of the DUT 4 can be obtained.

FIG. 7 shows a flowchart of a refractive index measurement method according to another embodiment of the present invention.
First, as shown to Fig.2 (a), the position of the reference | standard reflective surface 6 is measured in the state in which the test object 4 does not exist (step S21). Next, as shown in FIG. 2B, the test object 4 is moved to the initial measurement position and placed in the optical path (step S22). Next, as shown in FIG. 2C, the surface of the test object 4 is measured (step S23), then the measurement position is moved (step S24), and all the measurement positions are measured. It is determined whether or not (step S25).

  If there is still a measurement position, the process returns to step S23, and measurement at the new measurement position is sequentially performed. When all the measurement positions have been measured, the reference reflection surface 6 is again measured in a state where the test object 4 is not present as shown in FIG. 2A (step S26). By adding this step, it is possible to check whether the specimen 4 has changed over time and, if necessary, correct it from the first and last results to improve accuracy. Become.

Hereinafter, the same series of flows as described above is performed by changing the measurement method.
About the measurement of the back surface of the test object 4, the measurement within a measurement range is performed by step S27 to step S31. Regarding the measurement of the holder reference surface 5 attached to the holder 21 that holds the test object 4, the measurement within the measurement range is performed from step S32 to step S36. Moreover, the measurement in the state (refer FIG.2 (b)) reflected on the reference | standard reflective surface 6 after permeation | transmission of the to-be-tested object 4 performs a measurement within a measurement range by step S37 to step S41. In the measurement of the state of being transmitted through the test object 4 and reflected from the reference reflecting surface 6 (see FIG. 2E), the measurement within the measurement range is performed from step S42 to step S46.

  The results of the measurement optical distances in steps S23, S28, S33, S38, and S43 at the measurement positions described above and the measurement optical distances of the reference reflecting surface 6 in steps S21, S26, S31, S36, S41, and S46 are used. Then, the refractive index is calculated (step S27).

  Thus, the lens thickness and refractive index corresponding to the test object 4 at each measurement position are calculated. Thus, by calculating the refractive index at each measurement position, the refractive index distribution of the test object 4 can be obtained. That is, the absolute distance of the positions of a plurality of positions including the position of the surface of the test object 4 is obtained from the interference, and the refractive index distribution of the test object 4 is non-destructive by considering the optical path change and wavefront aberration due to the refraction. It becomes possible to measure easily. In addition, as in the present embodiment, the test object drive mechanism 8 that moves the test object 4 in a direction orthogonal to the optical axis of the condensing lens 3 that is a condensing optical system is provided. Even if 4 is not planar, the refractive index of the test object 4 can be obtained satisfactorily.

  Note that measurement is possible in any of the measurement flows shown in FIG. 6 and the measurement flow shown in FIG. However, the refractive index measurement method shown in FIG. 6 with respect to FIG. 7 has an advantage that errors due to the test object driving mechanism 8 at each measurement position can be sequentially removed, but the moving stroke of the condensing optical system driving mechanism 7 is long. There is a drawback that it takes a long time to measure. Therefore, when a change with time is expected to be larger than a mechanical error, it is desirable to perform refractive index measurement based on the measurement flow shown in FIG. Conversely, when a mechanical error is expected to be larger than a change with time, it is desirable to perform refractive index measurement based on the measurement flow shown in FIG.

  According to the present invention, it is possible to easily measure the refractive index distribution of a test object in a non-destructive manner. As a result, for example, it can be used for a refractive index distribution measuring device of a molded lens used for a front plate used for a glass or plastic display panel, an optical pickup, a digital copying machine, a digital camera, or the like. is there.

DESCRIPTION OF SYMBOLS 1 Light source 2 Beam splitter 3 Condensing lens 4 Test object 5 Holder reference surface 6 Reference reflective surface 7 Condensing optical system drive mechanism 8 Test object drive mechanism 9, 11 Spatial filter 10 ND filter 12 Reference reflective surface 13 Imaging lens DESCRIPTION OF SYMBOLS 14 Camera 15 Beam splitter 16 Condensing lens 17 Optical sensor 18 Inspection light 19 Reference light 20 Control apparatus 21 Holder

Claims (6)

A light splitting step for splitting light from the light source into inspection light and reference light;
An inspection light condensing step for condensing the inspection light on the test object and the reference reflecting surface;
A condensing position switching step of switching the condensing position of the inspection light to the front and back surfaces of the test object and the surface of the reference reflecting surface;
A reference light reflecting step of reflecting the reference light by a reference reflecting surface;
An interference signal obtaining step of obtaining an interference signal by causing the inspection light reflected from the test object and the standard reflection surface to interfere with the reference light reflected from the reference reflection surface;
Each of the reference reflection surface in a state where the test object does not exist, the surface of the test object, the back surface of the test object, and the reference reflection surface in a state where the test object exists. A distance measuring step for measuring the optical distance of the optical signal based on the interference signal;
A refractive index calculating step of calculating a refractive index of the test object based on the optical distance measured in the distance measuring step. In the distance measurement step, the optical distance to the reference reflection surface when the light is collected before the test object and becomes parallel light on the reference reflection surface is measured, and the optical distance is added to refract the test object. The refractive index measurement method according to claim 1, wherein the refractive index is calculated. The condensing position switching step moves a condensing optical system that condenses the inspection light onto the front and back surfaces of the test object and the surface of the reference reflecting surface along the optical axis of the inspection light. The method for measuring a refractive index according to claim 1, wherein the method is performed. In the condensing position switching step, in addition to moving the condensing optical system along the optical axis of the inspection light, the inspection object is moved in a direction different from the optical axis of the inspection light and collected. 4. The refractive index measurement method according to claim 3, wherein the light position is switched. 5. The refractive index according to claim 4, wherein, in the refractive index calculation step, the refractive index is calculated using the distance calculated again after removing the error effect due to the aberration from the distance obtained in the distance measurement step. Measuring method. A light source;
A light splitting means for splitting light from the light source into inspection light and reference light;
A condensing optical system for condensing the inspection light on the test object and the reference reflecting surface;
Condensing optical system driving means for moving the condensing optical system along the optical axis of the inspection light;
A test object driving means for moving the test object in a direction different from the optical axis of the inspection light;
A reference reflecting surface for reflecting the reference light;
A light receiving element for obtaining an interference signal in which the inspection light reflected from the test object and the standard reflection surface interferes with the reference light reflected from the reference reflection surface;
Based on the interference signal obtained by the light receiving element, the respective optical distances of the reference reflecting surface, the surface of the test object, and the back surface of the test object in a state where the test object does not exist are measured. A refractive index measuring apparatus comprising: a calculating unit that calculates a refractive index of the test object based on these optical distances.