Method and apparatus for determining the presence of an electrical charge on the surface of a lens mold

1. A method for determining the presence of an electrical charge on a surface of an ophthalmic lens mold ( 1 , 10 ), the method comprising the steps of

exposing a sensor crystal ( 22 ) of a sensor element ( 2 ) to a surface of an ophthalmic lens mold ( 1 ) on which an electrical charge generating an electrical field may be present,

converting a light beam ( 31 ) emitted from a laser light source ( 3 ) into a linearly polarized measurement light beam ( 34 ) by passing the light beam ( 31 ) through the sensor element ( 2 ),

processing the linearly polarized measurement light beam ( 34 ) exiting the sensor element ( 2 ) in a measurement unit ( 4 ),

wherein the step of converting the light beam ( 31 ) into a linearly polarized measurement light beam ( 34 ) by passing the light beam ( 31 ) through the sensor element ( 2 ) comprises:

converting the light beam ( 31 ) emitted from the laser light source ( 3 ) into a linearly polarized light beam ( 32 ) having a first polarization direction by passing the light beam ( 31 ) through a linear polarizer ( 21 ),

converting the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) into an elliptically polarized light beam ( 33 ) by passing the linearly polarized light beam ( 32 ) through the sensor crystal ( 22 ), the sensor crystal being selected from a non-centrosymmetrical crystallographic point group 4 3 m,

converting the elliptically polarized light beam ( 33 ) exiting the sensor crystal ( 22 ) into the linearly polarized measurement light beam ( 34 ) by passing the elliptically polarized light beam ( 33 ) through a quarter-wave polarizer ( 23 ), wherein the linearly polarized measurement light beam ( 34 ) has a second polarization direction forming a polarization angle (φ) with the first polarization direction of the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) and entering the sensor crystal ( 22 ), the polarization angle (φ) of the linearly polarized measurement light beam ( 34 ) being representative of the presence of the electrical field generated by the electrical charge on the surface of the lens mold ( 1 ) which the sensor crystal ( 22 ) is exposed to, and

wherein the step of processing the linearly polarized measurement light beam ( 34 ) in the measurement unit ( 4 ) comprises:

rotating the second polarization direction of the linearly polarized measurement light beam ( 34 ) by passing the polarized measurement light beam through a Faraday rotator ( 41 , 42 ) thereby forming a rotated linearly polarized measurement light beam ( 35 , 36 ),

converting the rotated linearly polarized measurement light beam ( 35 , 36 ) into an analyzer light beam ( 37 ) by passing the rotated linearly polarized measurement light beam ( 35 , 36 ) through an analyzer ( 43 ),

converting the analyzer light beam ( 37 ) into an electrical detection signal ( 441 ) with the aid of a photodetector ( 44 ) and evaluating the electrical detection signal ( 441 ) generated by the photodetector ( 44 ) with the aid of an evaluator ( 45 ) thereby determining the presence of the electrical charge on the surface of the ophthalmic lens mold ( 1 ).

2. The method according to claim 1 , wherein the propagation direction of the light beam ( 31 ) passing through the sensor element ( 2 ) is perpendicular to the surface of the ophthalmic lens mold ( 1 ) the sensor crystal ( 22 ) is exposed to.

3. The method according to claim 1 , wherein the sensor crystal ( 22 ) is selected from the non-centrosymmetrical crystallographic point group 4 3 m and has crystallographic faces along the crystallographic planes [1 1 0]×[110]×[001], wherein the electrical field ( 11 ) is parallel to the direction of the crystallographic plane [110], and wherein the propagation direction of the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) and entering the sensor crystal ( 22 ) has a propagation direction parallel to the crystallographic plane [1 1 0].

4. The method according to claim 2 , wherein the sensor crystal ( 22 ) is selected from the non-centrosymmetrical crystallographic point group 4 3 m and has crystallographic faces along the crystallographic planes [1 1 0]×[110]×[001], wherein the electrical field ( 11 ) is parallel to the direction of the crystallographic plane [110], and wherein the propagation direction of the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) and entering the sensor crystal ( 22 ) has a propagation direction parallel to the crystallographic plane [1 1 0].

5. The method according to claim 1 , wherein the analyzer ( 43 ) is a linear polarizer allowing light having a third polarization direction perpendicular to the first polarization direction of the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) and entering the sensor crystal ( 22 ) to pass through the analyzer ( 43 ).

6. The method according to claim 2 , wherein the analyzer ( 43 ) is a linear polarizer allowing light having a third polarization direction perpendicular to the first polarization direction of the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) and entering the sensor crystal ( 22 ) to pass through the analyzer ( 43 ).

7. The method according to claim 1 , wherein evaluating the electrical detection signal ( 441 ) generated by the photodetector ( 44 ) is performed in the evaluator ( 45 ) using a phase-locked loop, the evaluator ( 45 ) controlling a direct current driving the Faraday rotator ( 41 ) to rotate the second polarization direction of the linearly polarized measurement light beam ( 34 ) by an angle (α) determined by the electrical detection signal ( 441 ) generated by the photodetector ( 44 ) and evaluated in the evaluator ( 45 ).

8. The method according to claim 1 , wherein the second polarization direction of the linearly polarized measurement light beam ( 34 ) is rotated by passing the linearly polarized measurement light beam ( 34 ) through a first Faraday rotator ( 41 ) thereby forming a non-modulated rotated linearly polarized measurement light beam ( 35 ), and by thereafter passing the non-modulated rotated linearly polarized measurement light beam ( 35 ) through a second Faraday rotator ( 42 ) thereby forming a modulated rotated linearly polarized measurement light beam ( 36 ), with the first Faraday rotator ( 41 ) being driven by a direct current supplied by a direct current driver ( 411 ) controlled by the evaluator ( 45 ), and with the second Faraday rotator ( 42 ) being driven by an alternating current supplied by an alternating current driver ( 421 ) connected to an oscillator ( 47 ).

9. The method according to claim 7 , wherein the modulated rotated linearly polarized measurement light beam ( 36 ) is converted into the electrical detection signal ( 441 ), and wherein the direct current supplied by the direct current driver ( 411 ) controlled by the evaluator ( 45 ) and driving the first Faraday rotator ( 41 ) is varied by the phase-locked loop of the evaluator ( 45 ) until the modulated electrical detection signal ( 441 ) is symmetrical around a minimum value.

10. The method according to claim 8 , wherein the modulated rotated linearly polarized measurement light beam ( 36 ) is converted into the electrical detection signal ( 441 ), and wherein the direct current supplied by the direct current driver ( 411 ) controlled by the evaluator ( 45 ) and driving the first Faraday rotator ( 41 ) is varied by the phase-locked loop of the evaluator ( 45 ) until the modulated electrical detection signal ( 441 ) is symmetrical around a minimum value.

11. The method according to claim 1 , wherein the sensor crystal ( 22 ) is a single crystal selected from the group consisting of Bi 4 Ge 3 O 12 , Bi 4 Si 3 O 12 and GaAs.

12. The method according to claim 1 , wherein the photodetector ( 44 ) is a PIN-diode.

13. The method according to claim 2 , wherein the photodetector ( 44 ) is a PIN-diode.

14. The method according to claim 1 , wherein the laser light source ( 3 ) is a helium-neon laser or a laser diode.

15. The method according to claim 1 , wherein the surface of the lens mold ( 1 ) is a lens forming surface.

16. The method according to claim 1 , wherein the distance between the surface of the lens mold ( 1 ) and the sensor crystal ( 22 ) is in the range of 0.5 mm to 20 mm.

17. The method according to claim 2 , wherein the distance between the surface of the lens mold ( 1 ) and the sensor crystal ( 22 ) is in the range of 0.5 mm to 20 mm.

18. An apparatus for determining the presence of an electrical charge on a surface of an ophthalmic lens mold ( 1 ) comprising:

a laser light source ( 3 ) for emitting a light beam ( 31 ) defining a beam path,

a sensor element ( 2 ) comprising a sensor crystal ( 22 ) arranged in the beam path of the light beam ( 31 ), the sensor crystal ( 22 ) being exposable to a surface of an ophthalmic lens mold on which an electrical charge generating an electrical field ( 11 ) may be present, the sensor element ( 2 ) being configured for converting the light beam ( 31 ) emitted by the laser light source ( 3 ) into a linearly polarized measurement light beam ( 34 ) by having the light beam ( 31 ) emitted by the laser light source ( 3 ) pass through the sensor element ( 2 ),

a measurement unit ( 4 ) for processing the linearly polarized measurement beam ( 34 ) exiting the sensor element ( 2 ),

wherein the sensor element ( 2 ) comprises

a linear polarizer ( 21 ) having a first polarization direction and being arranged in the beam path of the light beam ( 31 ), the linear polarizer ( 21 ) being configured for converting the light beam ( 31 ) emitted from the laser light source ( 3 ) into a linearly polarized light beam ( 32 ) having a first polarization direction,

the sensor crystal ( 22 ) which is selected from the non-centrosymmetrical crystallographic point group 4 3m, the sensor crystal ( 22 ) being arranged in the beam path of the linearly polarized light beam ( 32 ) for converting the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) into an elliptically polarized light beam ( 33 ), and

a quarter-wave polarizer ( 23 ) being arranged in the beam path of the elliptically polarized light beam ( 33 ) exiting the sensor crystal ( 22 ), the quarter-wave polarizer ( 23 ) being configured for converting the elliptically polarized light beam ( 33 ) into the linearly polarized measurement light beam ( 34 ) having a second polarization direction forming a polarization angle (φ) with the first polarization direction of the linearly polarized light beam ( 32 ) exiting the linear polarizer ( 21 ) and entering the sensor crystal ( 22 ), the polarization angle (φ) of the linearly polarized measurement light beam ( 34 ) being representative of the presence of the electrical field generated by the electrical charge on the surface of the lens mold ( 1 ) which the sensor crystal ( 22 ) is exposable to,

and wherein the measurement unit ( 4 ) comprises

a Faraday rotator ( 41 , 42 ) arranged in the beam path of the linearly polarized measurement light beam ( 34 ) and configured for rotating the second polarization direction of the linearly polarized measurement light beam ( 34 ) to form a rotated linearly polarized measurement light beam ( 35 , 36 )

an analyzer ( 43 ) being arranged in the beam path of the rotated linearly polarized measurement light beam ( 35 ), the analyzer being configured for converting the rotated linearly polarized measurement light beam ( 35 , 36 ) into an analyzer light beam ( 37 ),

a photodetector ( 44 ) for converting the analyzer light beam ( 37 ) into an electrical detection signal ( 440 , 441 ), and

an evaluator ( 45 ) for evaluating the electrical detection signal ( 440 , 441 ) generated by the photodetector ( 44 ) to determine the presence of the electrical charge on the surface of the ophthalmic lens mold ( 1 ).

19. The apparatus according to claim 18 , wherein the sensor crystal ( 22 ) is selected from the non-centrosymmetrical crystallographic point group 4 3m and has crystallographic faces along the crystallographic planes [1 1 0]×[110]×[001].

20. The apparatus according to claim 18 , wherein the measurement unit ( 4 ) comprises

a first Faraday rotator ( 41 ) arranged in the beam path of the polarized measurement light beam ( 34 ) and connected to a direct current driver ( 411 ) controlled by the evaluator ( 45 ), for rotating the polarization direction of the linearly polarized measurement light beam ( 34 ) to form a non-modulated rotated polarized measurement light beam ( 35 ), and

a second Faraday rotator ( 42 ) arranged in the beam path of the non-modulated rotated linearly polarized measurement light beam ( 35 ) and connected to an alternating current driver ( 421 ) connected to an oscillator ( 47 ) to form a modulated rotated linearly polarized measurement light beam ( 36 ),

and wherein the analyzer ( 43 ) of the measurement unit ( 4 ) is a linear polarizer having a polarization direction perpendicular to the polarization direction of the linear polarizer ( 21 ) of the sensor element ( 2 ).