PolarCam and Advanced Applications Workshop Series 2013
Outline Polarimetry Background Stokes vector Types of Polarimeters Micro-polarizer Camera Data Processing Application Examples Passive Illumination Active Illumination Advanced Interferometric Applications 2
Polarimetry Background Stokes Vector -George Gabriel Stokes in 1852 Total Intensity Horizontal /Vertical ± 45 RHC/LHC The Stokes vector can fully describe any polarization state, both partial and purely polarized 3
Conventions The Stokes vector is defined relative to the following six flux measurements P performed with ideal polarizers in front of a radiometer (Shurcliff, 1962) : P H horizontal linear polarizer (0 ) P V vertical linear polarizer (90 ) P 45 45 linear polarizer P 135 135 linear polarizer P CR right circular polarizer P CL left circular polarizer 4
Mueller Matrix Output System Xfer Input 5
Types of Imaging Polarimeters Division of Time (rotating elements) Can get all parameters Takes time - Not for dynamic scenes Division of Amplitude (multiple cameras) Bulky, expensive, registration, calibration Division of Aperture (one sensor multi images) Registration issues, distortion, loss of resolution Division of Focal Plane (DoFP) Resolution loss, xtalk J. S. Tyo, D. L. Goldstein, D. B. Chenault, and J. A. Shaw, "Review of passive imaging polarimetry for remote sensing applications," Appl. Opt. 45, 5453-5469 (2006) 6
Micro-Polarizer (DoFP) Background 1. Chun, C. S. L., Fleming, D. L., Torok, E., J., Polarization sensitive, thermal imaging, SPIE Vol. 2234, 275-286 (1994). Proposed idea 2. Nordin, G. P., Meier, J. T., Deguzman, P. C., and Jones, M. W., Micropolarizer array for infrared imaging polarimetry, J. Opt. Soc. Am A, 16(5), (1999). Built in IR 3. Millerd, J. E., Brock, N. J., Hayes, J. B., North-Morris, M., Novak, M., and, Wyant, J. C., Pixelated phase-mask dynamic interferometer, Proc. SPIE Vol. 5531, 304-315 (2004). Built in visible.
Wiregrid Polarizer Very thin substrates ~1mm High Extinction: > 5000:1 Wide acceptance angle (0 to 50 degrees) Wide chromatic range (UV to IR) http://www.moxtek.com/optics-products.html
PolarCam Micro-Polarizer Array unit cell (super pixel) α=45 polarizer array matched to detector pixels α=0 α=135 α=90 Works with broadband source (multi-λ, or white light) Pixelated array provides: S 0, S 1, S 2 Linear Stokes Parameters US Patent 7,230,719 9
Patterned Wiregrid Polarizer Spectral Range 300nm 5µm Extinction >50:1 Transmission ~ 80% 7.4, 9.0 micron pixels
PolarCam Interline Transfer CCD 12 bit 1, 2, and 4 million pixels C-mount CameraLink, GigE Interface ~50 mm 100 s of cameras built into a variety of systems installed all over the world.
Overlap of intensity and polarization signal Power Spectrum Intensity data (s0) Pol. data (s1, s2) Spatial freq 0 fmax/2 Data overlap causes false signal Ideal applications: low freq intensity modulation (e.g. transparent or uniform objects) 12
Data Processing Processing Algorithms B A C D φ1 B A B φ 1 φ 2 B A C D φ2 C B D A C B φ 4 φ 5 Tan φ = A-C D-B Super pixel Data Array = 1/4 Tan φ = ΣA-ΣC ΣD-ΣB Convolution Data Array = (n-2) x (m-2) Sophisticated processing kernels can be used to separate intensity variation from polarization* * Charles F. LaCasse,* Russell A. Chipman, and J. Scott Tyo, Band limited data reconstruction in modulated polarimeters, OPTICS EXPRESS 149761 Vol. 19, No. 16, August 2011 4DTechnology Corporation 13
Angle of linear polarization Polarization Enhanced Image AOLP = ½ arctan (S2/S1) Ip = s0 +/- C*DoLP 14
Applications 15
Passive imaging Natural Scenes typically don t generate much circular pol. (i.e. S3 = 0) Polarization image can provide Target Detection Contrast Enhancement Haze Removal Glare Reduction Most Useful parameters S1, S2, Degree of linear polarization Polarization enhanced image (S0 +/- C*DoLP) 16
Passive Imaging (IR) J. S. Tyo, et. al., Appl. Opt. 45, 5453-5469 (2006) 17
Passive Illumination - visible Parsed Images DoLP Image
Passive Imaging Deglare Reference Camera Enhanced with DoLP Courtesy of Emergent Views, 2011 19
Passive Imaging Dehaze (40 miles over SF Bay) Reference Camera Enhanced with DoLP Courtesy of Emergent Views, 2011 20
Passive and Active Illumination Viktor Gruev,* Rob Perkins, and Timothy York, CCD polarization imaging sensor with aluminum nanowire optical filters, OPTICS EXPRESS, Vol. 18, No. 1830 August 2010 / 21
Active Illumination If illumination is controlled: Assume all light is polarized (random = 0) Use Mueller matrix calculation to solve for change, e.g. retardation, birefringence, pol. angle. Error sources External: o Ambient light, multi-path Internal: o Bandwidth of source, Polarization of source o Detector (linearity, gain, offset, noise, blooming, smear) o Mask (uniformity, contrast) Requires Calibration 22
Active Illumination Optic Birefringence Measurement Polarization state generator
Stress-Induced Birefringence 25mm dia. Window 0 70 nm birefringence mounting stress point Intensity image Birefringence Intensity Edge effects
Stress Birefringence Independent of orientation Optic with radial birefringence (spinning) Intensity image Birefringence 25
Other Measurements Biological samples (living Cells) 26
Other Measurements Biological Samples (human eye) 27
Conclusion PolarCam micropolarizer camera Enables whole-field, Dynamic polarimetry Wide variety of wavelengths and sensor formats Passive illumination: Target discrimination, Image enhancement Active illumination: Control illumination appropriate for measurement Model optical system under test, solve for desired parameter Linear Stokes adequate for many cases e.g. Birefringence, optical thickness, and surface shape Applications from glass fabrication to Biological imaging 28
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