Refractive index measurement of spectacle lenses

Abstract: Apparatus for measuring the index of refraction of glass such as spectacle lenses without measuring the surface geometry of the glass. In particular, an embodiment of the present invention includes: (a) a source of a substantially spatially coherent beam of radiation; (b) a beamsplitter to provide a sample beam and a reference beam in response to the beam; (c) a translatable retroreflector which reflects the reference beam; (d) a holder to hold the material in the path of the sample beam, the holder including a retroreflector which reflects the sample beam back through the material and a caliper apparatus to measure the thickness of the material where the sample beam passes through; (e) a detector which detects the reflected reference beam and the reflected sample beam to produce a detector output signal; and (f) an analyzer which determines a position of the translatable retroreflector in response to the detector output signal and which determines the index of refraction of the material in response to the position of the translatable reflector means and the thickness of the material.

Description: Technical Field of the Invention The present invention pertains to method and apparatus for measuring a refractive index of spectacle lenses. Background of the Invention There are various methods in the prior art which are used to measure the index of refraction of glass. One method entails utilizing an Abbe refractometer to measure the critical angle of total reflection. In accordance with this method, the index of refraction is derived from the measured value of the critical angle. This method is illustrated in a book entitled "Optics" by Eugene Hecht and Alfred Zajac, published by Addison-Wesley Publishing Company, Inc. February, 1979 (Copyright 1974), pp. 81-84. Another method utilized to measure the index of refraction of glass entails measuring the reflectivity of glass, which reflectivity depends on the index of refraction. The relation between index of refraction and reflectivity is described, for example, by the Fresnel formula shown on p. 75 of the above -identified reference. Yet another method utilized to measure the index of refraction of glass entails measuring the deflection angle of a light beam passing through a sample block of glass. This method is illustrated on pp. 62-63 of the above identified reference. All of the above methods suffer from drawbacks. For example, all of the above methods require the surface geometry of the glass whose index of refraction is to be measured to be known. For example, the radii of curvature of the surfaces must be measured to an accuracy of about 0.1% to obtain a measurement of index of refraction of comparable accuracy. In addition, the first two methods are measurements of reflection. This is a drawback when measuring the index of refraction of spectacles since spectacles are often antireflection coated. In EP-0 277 496-A2 a Laserinterferometer-Refractometer for measuring the refractive index of gases is known. The devices comprises a two beam interferometer with a fixed reflector in the reference beam path and a chamber which can be evacuated and which is passed by the measuring beam twice by reflecting the measuring beam back after the first passage of through the chamber. For determining the refractive index of the gas the length of the chamber can be varied. This device in unsuited for measuring the refractive index of glasses. In light of the above, there is a need for a method and apparatus for measuring the index of refraction of glass such as the glass of spectacle lenses without measuring the surface geometry of the glass. This object is solved by apparatus according to the features of claims 1 and 12 and by methods having the features of claims 11 and 16. Embodiments of the present invention advantageously satisfy the above-identified need in the art and provide a method and apparatus for measuring the index of refraction of glass such as the glass of spectacle lenses without measuring the surface geometry of the glass. A first embodiment of the invention refers to an apparatus for measuring the index of refraction of a sample material which comprises: a source of a substantially spatially coherent beam of radiation, a beamsplitter for providing a sample beam and a reference beam; reflecting means for reflecting the sample beam back through the sample material a translatable reflector, a detector, disposed to detect the reflected reference beam and the reflected sample beam for producing a detector output signal in response thereto, and an analysis means in response to the detector output signal, for determining the index of refraction of the sample material, characterised in, that the translatable reflector reflects the reference beam, a sample thickness measuring means determines the thickness of the sample material where the sample beam passes through, the analysis means, in response to the detector output signal, determines a position of the translatable reflector and, in response to the position of the translatable reflector and the thickness of the sample material, determines the index of refraction of the sample material. In a specific instance of the first embodiment of the present invention, the source is a light emitting diode whose output is collimated and focused through a pinhole; the means for providing a sample and a reference beam is a beamsplitter; the translatable reflecting means is a retroreflector mounted on a stepper motor; the holder for the material is a caliper which holds the material at a predetermined distance from the beamsplitter and a retrorefelctor; and the detector means comprises a detector whose output is bandpass filtered, root means square filtered, and applied to a trigger. A second embodiment of the invention refers to an apparatus for measuring the index of refraction of a sample material which comprises: a source of a substantially specially coherence beam of radiation, beamsplitter means for providing a sample beam and a first reference beam and a second reference beam, a reflecting means for reflecting the sample beam back through the sample material, a translatable reflector, means for reflecting the second reference beam, wherein the means for reflecting the second reference beam are in a position which is fixed with respect to the reflecting means for reflecting the sample beam, a detector, disposed to detect the reflected reference beams and the reflected sample beam, for producing a detector output signal in response thereto, and an analysis means in response to the detector output signal, for determining the index of refraction of the sample material, characterised in, that the translatable reflector is disposed to reflect the first reference beam, and that the analysis means, in response to the detector output signal, determines a first and a second position of the translatable reflector and, in response to the first and a second positions, determines the index of refraction of the sample material. In a specific instance of the second embodiment of the present invention, the source is a light emitting diode whose output is collimated and focused through a pinhole; the means for providing a sample and a first and second reference beam is a first and a second beamsplitter; the translatable reflecting means is a retroreflector mounted on a stepper motor; the holder for the material is a caliper which holds the material and a first and a second retroreflector, and the detector means comprises a detector whose output is bandpass filtered, root means square filtered, and applied to a trigger. A complete understanding of the present invention may be gained by considering the following detailed description together with the accompanying drawings, in which: Fig. 1 shows, in pictorial form, a preferred embodiment of the present invention which is apparatus for measuring the index of refraction of glass such as the glass of spectacle lenses without measuring the curvature of the surface of the glass; Fig.2 shows, in graphical form, a signal produced by a detector in accordance with the present invention; FIG. 3 shows, in pictorial form, an alternate embodiment of the present invention; and FIG. 4, shows, in graphical form, a signal produced by a detector in accordance with the alternative embodiment of the present invention. Corresponding elements in each of the drawings have the same reference numbers. Detailed Description FIG. 1 shows, in pictorial form, embodiment 200 of the present invention which is apparatus for measuring the index of refraction of glass such as the glass of spectacle lens 8 without measuring the curvature of the surface of the glass. In the embodiment of the present invention shown in FIG. 1, a beam of light 205 output by light source 1 is collimated as beam 210 by collimator lens 2. In accordance with the present invention, output 205 from light source 1 has a short coherence length, which coherence length is preferably on the order of a few microns. A suitable light source may be, for example, a light emitting diode ("LED"). Collimated output 210 from lens 2 is focused by lens 21 having focal length f as beam 220 onto small pinhole 22 having a diameter substantially given by the following formula: d = 1.22λf/a where λ is the wavelength of the output from LED 1 and a is the radius of collimated beam 210 formed by lens 2. The result is that beam 230 output from pinhole 22 is spatially coherent. In other embodiments, beam 230 may be provided as the output from a superluminescent light source such as a superluminescent diode. Although a superluminescent diode is a diffraction limited source, it requires more critical alignment than does the extended LED source arrangement shown in FIG. 1. Beam 230 is again collimated, this time by collimator lens 23, into beam 240. Beam 240 impinges upon beamsplitter 3 to form sample beam 4 and reference beam 5. As shown in FIG. 1, reference beam 5 is directed to impinge upon reference mirror 6 which is mounted on movable stage 7. Reference mirror 6 can be, for example, a retroreflector such as a retroreflecting prism. As further shown in FIG. 1, sample beam 4 is directed to impinge upon spectacle lens 8 at its vertex point. Sample beam 4 passes through spectacle lens 8 and impinges upon retroreflecting prism 9 which is mounted on a movable arm of caliper 27. Retroreflected sample beam 4 and retroreflected reference beam 5 are superimposed in detector path 91 and the combined signal is detected by light detector 92. In accordance with the present invention, reference mirror 6 is moved back and forth by movable stage 7 at a constant linear speed v. In accordance with the present invention, as soon as the optical length of reference beam 5 is equal to the optical length L of sample beam 4, the signal at detector 92 is modulated with a frequency f which is given by the following: f = 2v/λ where λ is the wavelength of source 1. FIG. 2 shows, in graphical form, detector signal 300 produced by detector 92. In FIG. 2, vertical axis 400 represents the amplitude of detector signal 300 and horizontal axis 410 represents the displacement of reference mirror 7. As shown in FIG. 2, the signal length is about equal to the coherence length of light source 1 and, in an embodiment where light source 1 is an LED, the signal length is on the order of microns. The optical length L of sample beam 4 is given by: L = 2 (d 1 + nd + d 2 ) where d 1 is the distance from the upper surface of spectacle lens 8 to beamsplitter 3, n is the index of refraction of the glass material of spectacle 8, d is the thickness of spectacle lens 8, and d2 is the distance from the lower surface of spectacle lens 8 to retroreflector 9. In accordance with the present invention, n, the index of refraction of spectacle lens 8, is determined by solving eqn. 3 for n; this is done in accordance with the embodiment as described below. Part of the setup steps in carrying out the inventive method entails measuring thickness d of spectacle lens 8. Caliper 27 having pins 10 and 11 is utilized for this purpose. As shown in FIG. 1, retroreflector 9 is fixed to movable pin 11 of caliper 27 and pin 10 of caliper 27 is fixed to interferometer platform 101. The use of a fixed distance between pin 11 and retroreflector 9 and a fixed distance between pin 10 and interferometer platform 101 provides d 1 and d 2 of eqn. 3 as known, predetermined values. Then, in accordance with the present invention, the thickness d of spectacle lens 8 is measured by displacement sensor 12. Displacement sensor 12 measures the displacement of pin 11 of caliper 27 which holds retroreflector 9. Electrical readout signal 310 from displacement sensor 12 is transmitted to control unit 13 which outputs signal 320 to drive stepper motor 7. In accordance with the present invention, signal 300 from detector 92 is bandpass filtered by bandpass filter 14. Output signal 330 from bandpass filter 14 is an oscillating signal pulse with a frequency given by eqn. (2) and a pulse length which corresponds to the coherence length of light source 1. Output 330 from bandpass filter 14 is further filtered by root mean square filter 15 to obtain the envelope of the signal pulse produced by detector 92, i.e., signal 340. Next, output signal 340 from root mean filter 15 is applied as input to trigger unit 16, for example, a Schmitt trigger, to derive timing signal 350 from signal pulse 340. Timing pulse 350 is applied as input to control unit 13. In accordance with the present invention, in response to timing signal 350 from trigger 16, control unit 13 stores the position of stepper motor 7 at the moment of trigger pulse 350. The position of stepper motor 7 corresponds to the reference arm length which is equal to the optical path length of sample beam 4. This optical length is equal to L of eqn. 2 above. At this point, control unit 13 has the values needed to solve eqn. 3 for the index of refraction n: (a) d 1 and d 2 (as discussed above, d 1 and d 2 are obtained as predetermined values); (b) d (as discussed above, d is obtained from signal 310 output by displacement sensor 12); and (c) L (as discussed above, L is obtained from the position of stepper motor 7). Control unit 13 solves eqn. 3 for n, the index of refraction of the material of spectacle lens 8, and displays the value, for example, on display 17. Many methods and apparatus should be well known to those of ordinary skill in the art for providing control unit 13. For example, control unit 13 may be fabricated utilizing readily available microprocessor apparatus. FIG. 3 shows, in pictorial form, alternate embodiment 500 of the present invention which is apparatus for measuring the index of refraction of glass such as the glass of spectacle lens 8 without measuring the curvature of the surface of the glass. The following will concentrate on the matters which are different from embodiment 200 shown in FIG. 1 and described above. As shown in FIG. 3, beam 240 is a spatially coherent, collimated beam which is divided by beamsplitter 3 into sample beam 4 and reference beam 5. As further shown in FIG. 3, reference beam 5 is further split by beamsplitter 24 into a first reference beam 25 and a second reference beam 26. First reference beam 25 is reflected by reference retroreflector 6 which is mounted on stepper motor driven, movable stage 7. Second reference beam 26 is reflected by retroreflector 127 which is mounted on platform 28. As shown in FIG. 3, retroreflector 9 and caliper pin 11 are also mounted on platform 28. Backreflected, second reference beam 26 is reflected by beamsplitter 24 and beamsplitter 3 and, finally, is superposed with backreflected, first reference beam 25 and backreflected, sample beam 4 in detector beam path 91. The inventive measurement process takes place as follows. Spectacle lens 8 is brought between caliper pins 10 and 11 so that they are in close contact with spectacle lens 8. The geometrical path between beamsplitter 3, beamsplitter 24 and retroreflector 127 is chosen to be the same as the geometrical path between beamsplitter 3 and retroreflector 9. As described above, retroreflector 6 is moved back and forth by movable stage 7 at a constant liner speed v and detector 92 produces detector signal 600 shown in FIG. 4. In FIG. 4, vertical axis 610 represents the amplitude of detector signal 600 and horizontal axis 620 represents the displacement of reference mirror 7. The position of signal peak 1001 in FIG. 4 marks the position of retroreflector 127 relative to a zero point of caliper 27, i.e., the zero point of caliper 27 corresponds to a point where caliper pins 10 and 11 in FIG. 3 are in contact (no spectacle 8). As a result, the position of peak 1001 in FIG. 4 measures the thickness d of spectacle lens 8. The position of signal peak 2001 in FIG. 4 depends on the optical thickness nd of spectacle lens 8. Peaks 1001 and 2001 are determined by control 13 from signal 350 output from trigger 16 as described above in connection with FIG. 1. Then, control 13 uses peaks 1001 and 2001 to obtain the position of retroreflector 6 and, thereby, d and nd. Using these two numbers, d and nd, control 13 solves for n and displays the result on display 17. Alternative embodiment 500 shown in FIG. 3 is advantageous in that there is no need for displacement sensor 12 shown in FIG. 1 and there is no need to know d 1 (the distance from the upper surface of spectacle lens 8 to beamsplitter 3) or d 2 (the distance from the lower surface of spectacle lens 8 to retroreflector 9). In alternative embodiment 500 of FIG. 3, signal 300 from detector 92 is bandpass filtered by bandpass filter 14. Output signal 330 from bandpass filter 14 is two oscillating signal pulses, each with a frequency given by eqn. (2) and a pulse length which corresponds to the coherence length of light source 1. Output 330 from bandpass filter 14 is further filtered by root mean square filter 15 to obtain the envelope of the signal pulse produced by detector 92, i.e., signal 340. Next, output signal 340 from root mean filter 15 is applied as input to trigger unit 16, for example, a Schmitt trigger, to derive timing signal 350 from signal pulse 340. Timing pulse 350 is applied as input to control unit 13. In accordance with the present invention, in response to timing signal 350 from trigger 16, control unit 13 stores the position of stepper motor 7 at the moment of each of the pulses in timing pulse 350. As set forth above, the position of stepper motor 7 corresponding to peak 1001 produces d and the position of stepper motor 7 corresponding to peak 2001 produces nd.

Claims: Apparatus (200) for measuring the index of refraction of a sample material (8) which comprises: a source (1, 2, 21, 22) of a substantially spatially coherent beam of radiation (240), a beamsplitter (3) for providing a sample beam (4) and a reference beam (5); reflecting means (9) for reflecting the sample beam (4) back through the sample material (8) a translatable reflector (6), a detector (92), disposed to detect the reflected reference beam (5) and the reflected sample beam (4) for producing a detector output signal (300) in response thereto, and an analysis means (14, 15, 16, 13), in response to the detector output signal (300), for determining the index of refraction of the sample material (8), characterised in, that the translatable reflector (6) reflects the reference beam (5), a sample thickness measuring means (10, 11, 12) determines the thickness of the sample material (8) where the sample beam (4) passes through, the analysis means (14, 15, 16, 13), in response to the detector output signal (300), determines a position of the translatable reflector (6) and, in response to the position of the translatable reflector (6) and the thickness of the sample material (8), determines the index of refraction of the sample material (8). Apparatus of claim 1, wherein the source (1, 2, 21, 22) has a coherence length in the order of a few microns. Apparatus of claim 2, wherein the source (1) comprises light emitting diode. Apparatus of claim 2, wherein the source (1,2,21,22) comprises a superluminescent diode. Apparatus of claim 3 or 4, wherein the source further comprises means (2) for collimating the output of the light emitting diode (1) and means (21) for focusing the collimated output through a pinhole (22). Apparatus of one of the claims 1 to 5, wherein the translatable reflector (6) is a retroreflector (6) mounted on a stepper motor (7). Apparatus of one of the claims 1 to 6, characterised in that for holding the sample material (8) a caliper (27) is provided which holds a first surface of the sample material (8) at a predetermined distance (d1) from the beamsplitter (3) and s second surface of the sample material (8) at a predetermined distance (d2) from the reflecting means (9) reflecting the sample beam (4). Apparatus of claim 7, wherein the reflecting means (9) reflecting the sample beam (4) is a retroreflector and wherein the sample thickness measuring device (10, 11, 12) comprises means (12) for determining displacements of the caliper (27). Apparatus of one of the claims 1 to 8, characterised in that the output of the detector (92) is applied to a bandpass filter (14), a root means square filter (15) and applied to a trigger (16). Apparatus of claim 6, wherein the retroreflector (6) is moved at a substantially constant speed. Method for measuring the index of refraction of a sample material (8) comprising the steps of: forming a substantially spatially coherent beam of radiation (240), splitting the beam (240) into a sample beam (4) and a reference beam (5), holding the sample material (8) in the path of the sample beam (4) and reflecting the sample beam (4) back through the sample material (8), reflecting the reference beam (5), detecting the reflected reference beam (5) and the reflected sample beam (4) and producing a detector output signal (300), characterised by reflecting the reference beam (5) from a translatable reflector (6); determining the thickness of the material (8) where the sample beam (4) passes through, determining a position of the translatable reflector (6) in response to the detector output signal (300), and determining the index of refraction of the sample material (8) in response to the position of the translatable reflector (6) and the thickness of the sample material (8). Apparatus (500) for measuring the index of refraction of a sample material (8) which comprises: a source (1,2,21,22) of a substantially spatially coherence beam of radiation (240), beamsplitter means (3, 24) for providing a sample beam (4) and a first reference beam (25) and a second reference beam (26), a reflecting means (9) for reflecting the sample beam (4) back through the sample material (8), a translatable reflector (6), means (127) for reflecting the second reference beam (26), wherein the means (127) for reflecting the second reference beam (26) are in a position which is fixed with respect to the position of the reflecting means (9) for reflecting the sample beam (4), a detector (92), disposed to detect the reflected reference beams (25, 26) and the reflected sample beam (4), for producing a detector output signal (300) in response thereto, and an analysis means (14, 15, 16, 13), in response to the detector output signal (300), for determining the index of refraction of the sample material (8), characterised in, that the translatable reflector (6) is disposed to reflect the first reference beam (25), and that the analysis means (14, 15, 16, 13), in response to the detector output signal (300), determines a first and a second position of the translatable reflector (6) and, in response to the first and a second positions, determines the index of refraction of the sample material (8). Apparatus of claim 12, wherein the source (1, 2, 21, 22) has a coherence length in the order of a few microns. Apparatus of claim 13, wherein the source (1, 2, 21, 22) comprises a superluminescent diode. Apparatus of one of the claims 12 to 14, wherein the retroreflector (6) is moved at a substantially constant speed. Method for measuring the index of refraction of a sample material (8) comprising the steps of forming a substantially spatially coherent beam of radiation (240), splitting the beam (240) into a sample beam (4) and a first reference beam (25) and a second reference beam (26), holding the sample material (8) in the path of the sample beam (4) and reflecting the sample beam (4) back through the sample material (8), reflecting the first and the second reference beam (25, 26), wherein the position at which the second reference beam (26) is reflected is fixed with respect to the position at which the sample beam (4) is reflected, detecting the first and second reflected reference beams (25, 26) and the reflected sample beam (4) and producing a detector output signal (300), characterised by reflecting the first reference beam (25) from a translatable reflector (6); determining the thickness of the material (8) were the sample beam (4) passes through, determining a first and a second position of the translatable reflector (6) in response to the detector output signal (300), and determining the index of refraction of the sample material (8) in response to the first and second positions of the translatable reflector (6).

Inventors: CAMPBELL CHARLES E, HELLMUTH THOMAS DR, CAMPBELL, CHARLES E., HELLMUTH, THOMAS, DR.
Applicants: ZEISS CARL, ZEISS STIFTUNG, CARL ZEISS, CARL-ZEISS-STIFTUNG TRADING AS CARL ZEISS
Application year: 1995
Main classification: G01M11
IPC codes: G01B9, G01M11