Instantaneous polarization measurement system and method
View Patent ↗The invention provides a highly sensitive measurement of retardance and slow axis orientation, accurately and instantaneously, across a full two-dimensional image. There are no moving parts and there need not be any electro-optic tuning as part of the measurement. It is ideally adapted to real-time imaging and is well-suited to use with biological and medical samples, including visualizing structures in oocytes. The invention splits a light beam into several beams, which are analyzed using elliptical polarizers and the resultant intensity is measured. It can be constructed using a single pixilated detector, or several detectors, to achieve high spatial resolution when this is desired.
1. A system for measuring retardance of a sample, comprising
a sample region for receiving the sample;
a source of substantially circularly polarized illumination light;
illumination optics for directing the illumination light toward the sample region;
analysis optics for receiving incident light from the sample region;
a plurality of photodetector regions;
beamsplitting optics for dividing the incident light into a plurality of sub-beams and for directing each sub-beam to a respective one of the plural photodetector regions;
a plurality of elliptical polarizers disposed in the sub-beams for transmitting incident light whose polarization state lies within a distance ε of a chosen pole on a Poincare sphere; and
a processor for determining retardance from intensity signals generated at the photodetector regions onto which the sub-beams are directed.
2. The apparatus of claim 1 , wherein the sample retardance is 50 nm or less.
3. The apparatus of claim 1 , wherein the sample retardance is 10 nm or less.
4. The apparatus of claim 1 , wherein ε is 35 degrees or less.
5. The apparatus of claim 1 , wherein ε is 20 degrees or less.
6. The apparatus of claim 1 , wherein the beamsplitting optics comprise a beamsplitter configured to operate by partial reflection at an interface for dividing the incident light into the sub-beams.
7. The apparatus of claim 6 , wherein the beamsplitter is substantially polarization neutral.
8. The apparatus of claim 6 , wherein the beamsplitter is a polka-dot type.
9. The apparatus of claim 1 , further comprising an optical retarder disposed adjacent an entrance face of the beamsplitting optics for transforming the polarization state of light passing therethrough.
10. The apparatus of claim 1 , wherein the beamsplitting optics comprises a plurality of prism facets which divide the incident light into the sub-beams according to the area of each facet.
11. The apparatus of claim 10 , wherein the beamsplitting optics comprises a single prism with multiple facets.
12. The apparatus of claim 10 , wherein the beamsplitting optics comprises an assembly of a plurality of prisms.
13. The apparatus of claim 10 , wherein the elliptical polarizers are located between the sample region and the beamsplitting optics.
14. The apparatus of claim 10 , wherein the beamsplitting optics are located between the sample chamber and the elliptical polarizers.
15. The apparatus of claim 1 , wherein at least one of the plural elliptical polarizers comprises a linear polarizer and at least one optical retarder.
16. The apparatus of claim 15 , wherein the optical retarder is an electrically variable retarder.
17. The apparatus of claim 16 , wherein the electrically variable retarder is a liquid crystal cell.
18. The apparatus of claim 1 , wherein at least one of the plural elliptical polarizers comprises a fixed linear polarizer and at least two retarder elements.
19. The apparatus of claim 18 , wherein at least one of the retarder elements is electrically variable.
20. The apparatus of claim 18 , wherein at least two of the retarder elements are electrically variable.
21. The apparatus of claim 1 , wherein the plural detector regions comprise a plurality of detectors.
22. The apparatus of claim 1 , wherein at least two of the plural detector regions comprise different regions on a single pixilated detector.
23. The apparatus of claim 1 , wherein the illumination light source is a pulsed lamp.
24. The apparatus of claim 23 , wherein the illumination light source is a flashlamp.
25. The apparatus of claim 1 , wherein the illumination light source is operable to emit monochromatic light.
26. The apparatus of claim 25 , wherein the illumination light source comprises a broadband light source and a filter.
27. A system for real-time imaging of retardance of a sample, comprising
a sample region for receiving the sample;
a source of substantially circularly polarized illumination light;
illumination optics for directing the illumination light toward the sample region;
analysis optics for receiving incident light from the sample region;
a plurality of photodetector regions;
beamsplitting optics for dividing the incident light into a plurality of sub-beams and for directing each sub-beam to a respective one of the plural photodetector regions;
a plurality of elliptical polarizers located in the sub-beams for transmitting incident light whose polarization state lies within a distance ε of a chosen pole on a Poincare sphere; and
a processor for calculating retardance from intensity signals generated at the photodetector regions onto which the sub-beams are directed;
wherein the sample is one of a biological cell, a tissue sample, and an oocyte.
28. The apparatus of claim 27 , wherein the sample is an oocyte.
29. The apparatus of claim 27 , wherein the beamsplitting optics comprise a beamsplitter configured to operate by partial reflection at an interface to divide the incident light into the sub-beams.
30. The apparatus of claim 29 , further comprising a waveplate located between the sample region and the beamsplitting optics.
31. The apparatus of 27 , wherein the beamsplitting optics comprise a plurality of prism facets which divide the incident light into the sub-beams according to the area of each facet.
32. The apparatus of claim 31 , wherein the plural elliptical polarizers are located between the sample region and the beamsplitting optics.
33. The apparatus of claim 31 , wherein the plural prism facets comprise a single prism with multiple facets.
34. The apparatus of claim 31 , wherein the plural prism facets comprise an assembly of a multiplicity of prisms.
35. The apparatus of claim 27 , further comprising a display unit for providing an image of the sample retardance.
36. The apparatus of claim 35 , wherein the display comprises a head-up display.
37. The apparatus of claim 35 , wherein the sample is viewable with a microscope and wherein the image of sample retardance provided by the display comprises an image viewed from within the eyepiece of the microscope.
38. A method for imaging retardance of a sample in real-time, comprising the steps of:
illuminating the sample with light that is substantially circularly polarized;
receiving light that has interacted with the sample;
dividing the received light into N sub-beams, where N≧2;
disposing elliptical polarizers in the N sub-beams, corresponding to states within a distance ε of a pole on a Poincare sphere;
analyzing a polarization state of each of the N sub-beams with the elliptical polarizers;
forming an image of the sample with each sub-beam;
measuring intensity at a plurality of points in the image at each of the N sub-beams; and
calculating the sample retardance based on the N image intensity measurements.
39. The method of claim 38 , further comprising the step of calculating a principal slow axis of the sample at a plurality of points.
40. The method of claim 38 , further comprising the step of taking a background measurement with no sample present.
41. The method of claim 40 , further comprising the step of storing background data derived from the background measurement.
42. The method of claim 41 , further comprising the step of correcting the calculation of retardance using the stored background data.
43. The method of claim 38 , wherein N is 5.
44. The method of claim 38 , wherein N is 4.
45. The method of claim 44 , wherein one of the elliptical polarizers transmits received light that is substantially circular in polarization state.
46. The method of claim 44 , wherein none of the elliptical polarizers transmit received light that is substantially circular in polarization state.
47. The method of claim 38 , wherein N is 3.
48. The method of claim 38 , wherein N is 2.
49. The method of claim 38 , wherein at least one of the elliptical polarizers is electrically variable.
50. The method of claim 38 , further comprising the step of taking calibration images to compensate for variations between optical responses of the N sub-beams.
51. The method of claim 50 , further comprising the step of correcting the image intensity measurements using the calibration images.
52. The method of claim 50 , wherein one of the polarization of the illumination light and the polarization state of at least one of the polarizers is altered between the calibration measurement and the sample measurement.