{ "id": "ep7f0z6a0y", "slug": "circularly-polarized-light-detection", "title": "Circularly polarized light detection", "updated": "2026-09-30", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.astronomy", "label": "Astronomy", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.astronomy" }, { "id": "physical.astronomy.cosmos", "label": "Cosmology and observation", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.astronomy.cosmos" }, { "id": "physical.astronomy.cosmos.cosmos.techniques", "label": "Observational techniques: astrometry, photometry, spectroscopy", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.astronomy.cosmos.cosmos.techniques" } ], "geo": [ { "id": "geo.nongeo.t1800.physical.astronomy", "label": "Non-geographic · 1800 to 1945: Astronomy", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.astronomy", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1800", "label": "Non-geographic · 1800 to 1945", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800" }, { "id": "geo.nongeo.t1800.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical" }, { "id": "geo.nongeo.t1800.physical.astronomy", "label": "Astronomy", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.astronomy" } ] }, { "id": "geo.nongeo.t1946.physical.astronomy.cosmos", "label": "Non-geographic · 1946 to 2000: Cosmology and observation", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.astronomy.cosmos", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical" }, { "id": "geo.nongeo.t1946.physical.astronomy", "label": "Astronomy", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.astronomy" }, { "id": "geo.nongeo.t1946.physical.astronomy.cosmos", "label": "Cosmology and observation", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.astronomy.cosmos" } ] } ], "excerpt": "Circularly polarized light detection, or circular polarimetry, measures the circular component of polarization in light from stars and planets, probing magnetic fields and cloud scattering since 1970.", "snippet": "Circularly polarized light detection, or circular polarimetry, measures the circular component of polarization in light from stars and planets, probing magnetic fields and cloud scattering since 1970.", "node": "physical.astronomy.cosmos.cosmos.techniques", "markdown": "# Circularly polarized light detection\n\nCircularly polarized light detection (circular polarimetry) measures the circular component of polarization in light from stars, planets, and atmospheric phenomena, reporting the normalized Stokes parameter \\( P_{\\mathrm{C}} = V/I \\), where \\( V \\) is the circularly polarized intensity and \\( I \\) the total intensity.[1](https://ar5iv.labs.arxiv.org/html/1908.10431) In astronomy it probes magnetic fields through thermal emission in magnetized plasma[2](https://adsabs.harvard.edu/pdf/1970ApJ...162..169K); in planetary science it characterizes cloud scattering[3](https://www.aanda.org/articles/aa/full_html/2026/09/aa60315-26/aa60315-26.html); and in astrobiology it is pursued as a marker of homochiral biomolecules.[4](https://www.aanda.org/articles/aa/full_html/2021/07/aa40845-21/aa40845-21.html) A milestone was the 1970 report of the first known significant circular polarization in visible light from a star, apart from the polarization of narrow Zeeman-split lines in magnetic Ap stars and in sunspots, and the first such observation on a white dwarf, the white dwarf Grw+70°8247.[5](https://articles.adsabs.harvard.edu/pdf/1970ApJ...161L..77K) • [13](https://adsabs.harvard.edu/pdf/1970ApJ...161L..77K)\n\n| Key fact | Value |\n|---|---|\n| Measured quantity | Normalized Stokes parameter \\( P_{\\mathrm{C}} = V/I \\); linear polarization is \\( P_{\\mathrm{L}} = \\sqrt{q^{2}+u^{2}} \\)[1](https://ar5iv.labs.arxiv.org/html/1908.10431) |\n| Typical signal strength | Circular polarization is typically three orders of magnitude smaller than linear polarization[3](https://www.aanda.org/articles/aa/full_html/2026/09/aa60315-26/aa60315-26.html) |\n| Best broadband sensitivity | Of order 1 ppm (0.0001%) with POLISH2 on bright stars[6](https://iopscience.iop.org/article/10.3847/1538-4365/aca407) |\n| Conditions for ppm accuracy | Bright stars on telescopes with aperture ≥ 3 m[1](https://ar5iv.labs.arxiv.org/html/1908.10431) |\n| Founding observation | Circularly polarized light from the white dwarf Grw+70°8247, ApJ 161:L77–L79, August 1970[5](https://articles.adsabs.harvard.edu/pdf/1970ApJ...161L..77K) |\n| Calibration standard | Grw+70°8247, with \\( P_{\\mathrm{C}} \\approx -4.0\\% \\) in the B band[1](https://ar5iv.labs.arxiv.org/html/1908.10431) |\n| Exoplanet reflected light | Modeled intrinsic \\( P_{\\mathrm{C}} \\) at most \\( 3 \\times 10^{-4} \\); unresolved systems contribute at most \\( 10^{-8} \\)[3](https://www.aanda.org/articles/aa/full_html/2026/09/aa60315-26/aa60315-26.html) |\n\n## How it works\n\nThe polarization state of light is described by the Stokes vector \\( \\mathbf{S} = (I, Q, U, V)^{\\mathrm{T}} \\), containing the total intensity \\( I \\), the two linearly polarized intensities \\( Q \\) and \\( U \\), and the circularly polarized intensity \\( V \\).[3](https://www.aanda.org/articles/aa/full_html/2026/09/aa60315-26/aa60315-26.html) A circular polarimeter reports the fractional quantity \\( P_{\\mathrm{C}} = v = V/I \\), while linear polarization is \\( P_{\\mathrm{L}} = \\sqrt{q^{2}+u^{2}} \\) with position angle \\( \\theta = 0.5 \\arctan(u/q) \\).[1](https://ar5iv.labs.arxiv.org/html/1908.10431)\n\nCircular signals are far weaker than linear ones, typically by three orders of magnitude, which makes detection in unresolved observations difficult because of stellar contamination.[3](https://www.aanda.org/articles/aa/full_html/2026/09/aa60315-26/aa60315-26.html) The physical basis for magnetic-field work was set out in James C. Kemp's 1970 paper *Circular Polarization of Thermal Radiation in a Magnetic Field*, which showed that a gray-body radiating system in a magnetic field \\( B \\) emits with fractional circular polarization, to first order in \\( B \\), \\( q(\\omega) \\approx -(eB/m)/\\omega \\)[2](https://adsabs.harvard.edu/pdf/1970ApJ...162..169K), where \\( \\omega \\) is the optical frequency, predicting about \\( 10^{-4} \\) at \\( 10^{5} \\) G at visible wavelengths.[2](https://adsabs.harvard.edu/pdf/1970ApJ...162..169K)\n\nA practical advantage of the circular channel is that \\( V \\) is rotationally invariant, so no rotational zero-point calibration is needed; only modulation efficiency and sign must be calibrated.[6](https://iopscience.iop.org/article/10.3847/1538-4365/aca407)\n\n## How it is done\n\n**Modulation.** The instrument converts polarization into intensity modulations that a detector can record. In POLISH2, two photoelastic modulators (PEMs) at 40 and 50 kHz encode Stokes \\( Q \\) primarily at \\( 2 \\times 50 = 100 \\) kHz, Stokes \\( U \\) at \\( 50 \\pm 40 = 10 \\) and 90 kHz, and Stokes \\( V \\) at 50 kHz (odd harmonics of the 50 kHz PEM), with \\( I \\) from the time-averaged intensity.[6](https://iopscience.iop.org/article/10.3847/1538-4365/aca407) In spectropolarimeters such as the HARPS circular polarimeter, a quarter-wave plate rotated in 90° steps (from 45°) yields measurements of \\( I \\pm V \\) (and \\( I \\mp V \\)).[7](https://ar5iv.labs.arxiv.org/html/1010.0397)\n\n**Analysis and demodulation.** A Wollaston prism splits the beam onto two detectors; in POLISH2 this gives 70%–75% instrumental throughput, avoiding the 50% loss of a linear polarizer and enabling ppm-level sensitivity on bright targets.[6](https://iopscience.iop.org/article/10.3847/1538-4365/aca407) Spectropolarimetric data are demodulated with the \"double ratio\" method, and null spectra are computed to flag potential false signals.[7](https://ar5iv.labs.arxiv.org/html/1010.0397)\n\n**Calibration.** With a quarter-wave plate, circular polarization can be measured by taking two images at 45° and 135° positions and computing \\( P_{\\mathrm{C}} = v \\).[1](https://ar5iv.labs.arxiv.org/html/1908.10431) In the lab, right- and left-circular polarizers inserted in a beam from a lamp with a linear polarizer and quarter-wave Fresnel rhomb inject nearly 100% circularly polarized light; POLISH2 measures a modulation efficiency of 93% and correct recovery of the sign change between the two polarizers.[6](https://iopscience.iop.org/article/10.3847/1538-4365/aca407)\n\n## Origin\n\nThe theoretical foundation is James C. Kemp's 1970 [Astrophysical Journal](https://www.edgepedia.org/astrophysical-journal) paper, which proposed the circular polarization of thermal radiation in a magnetic field as a means of detecting magnetic fields in condensed stars such as white dwarfs.[2](https://adsabs.harvard.edu/pdf/1970ApJ...162..169K) In June 1970, Kemp's polarimetric observations at Pine Mountain Observatory twice detected a strong circular polarization signal in Grw+70°8247 with a 24-inch telescope, and the result was confirmed the same night with a 36-inch telescope at Kitt Peak.[8](https://arxiv.org/pdf/2008.01802) The report appeared in The Astrophysical Journal, volume 161, pages L77–L79.[5](https://articles.adsabs.harvard.edu/pdf/1970ApJ...161L..77K) Earlier work the method built on was a portable photoelectric filter polarimeter using a rapidly switched Pockels cell quarter-wave plate, a [Wollaston](https://www.edgepedia.org/wollaston) prism, and tunable interference filters with 30 Å bandpass to isolate spectral line wings, used in searches for weak fields in DA white dwarfs.[8](https://arxiv.org/pdf/2008.01802)\n\n## Variants\n\nBroadband photoelectric polarimeters dominate precision work. PlanetPol, using photoelastic modulators and avalanche photodiodes, achieves photon-noise-limited sensitivity of at least 1 in \\( 10^{6} \\) in fractional polarization and about 1% absolute accuracy on polarized standards.[9](https://iopscience.iop.org/article/10.1086/507955) HIPPI, a successor to PlanetPol, uses a [ferroelectric liquid crystal](https://www.edgepedia.org/ferroelectric-liquid-crystal) modulator at 500 Hz with PMT detectors, optimized for 400–700 nm on the 3.9 m Anglo-Australian Telescope, with accuracy of 1.5–10 ppm depending on brightness.[1](https://ar5iv.labs.arxiv.org/html/1908.10431) HIPPI-2 reaches better than 3.5 ppm in SDSS g′ on the 3.9 m AAT.[10](https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/hippi2-a-versatile-highprecision-polarimeter/758F4BFD38209FAE755B2A19A9141DE6)\n\nPOLISH2 measures \\( q \\), \\( u \\), and \\( v \\) simultaneously in UBV passbands; its accuracy is quoted as of order 1 ppm (0.0001%) in its own instrument paper.[6](https://iopscience.iop.org/article/10.3847/1538-4365/aca407) Higher-resolution optical and infrared spectropolarimeters such as ESPaDOnS and SPIRou on the CFHT provide Stokes V data for [stellar magnetic field](https://www.edgepedia.org/stellar-magnetic-field) studies.[11](https://arxiv.org/pdf/2504.00179)\n\n## Applications\n\n**Magnetic fields.** The founding application remains the detection of white-dwarf magnetism: fields large enough to generate an easily detected circular polarization signal of \\( V/I \\approx 0.5\\% \\) are present in perhaps 5% of white dwarfs.[8](https://arxiv.org/pdf/2008.01802) Grw+70°8247, with \\( P_{\\mathrm{C}} \\approx -4.0\\% \\) in the B band, serves as a standard for calibrating the sign of circular polarization.[1](https://ar5iv.labs.arxiv.org/html/1908.10431) In cooler stars, Zeeman-sensitive Stokes V [spectropolarimetry](https://www.edgepedia.org/spectropolarimetry) with instruments such as ESPaDOnS and SPIRou is used for stellar magnetic field studies.[11](https://arxiv.org/pdf/2504.00179)\n\n**Planetary atmospheres.** High-precision polarimetry was motivated by the goal of detecting the polarization signature of unresolved extrasolar planets; nearby stars within 32 pc show polarizations of only a few \\( \\times 10^{-6} \\).[9](https://iopscience.iop.org/article/10.1086/507955) [Monte Carlo](https://www.edgepedia.org/monte-carlo) radiative transfer simulations of 20 cloud condensates show that the intrinsic degree of circular polarization of starlight reflected by giant exoplanets is at most \\( 3 \\times 10^{-4} \\), and no modeled cloud composition produced \\( P_{\\mathrm{C}} \\) above \\( 6 \\times 10^{-4} \\) even for one hemisphere.[3](https://www.aanda.org/articles/aa/full_html/2026/09/aa60315-26/aa60315-26.html)\n\n**Biosignatures.** Airborne spectropolarimetry has been used to detect photosynthetic life on Earth. FlyPol, an adaptation of the TreePol instrument, measures \\( V/I \\) as a function of wavelength over 400–900 nm at sensitivity below \\( 10^{-4} \\) and accuracy below \\( 10^{-3} \\).[4](https://www.aanda.org/articles/aa/full_html/2021/07/aa40845-21/aa40845-21.html)\n\n## Limitations and alternatives\n\n**Cross-talk.** The main systematic is the transformation of high linear polarization into circular polarization in telescope or polarimeter optics, which must be carefully investigated and taken into account.[1](https://ar5iv.labs.arxiv.org/html/1908.10431) Superachromatic half-wave plates with retardance of \\( 0.50 \\pm 0.01 \\) waves cause wavelength-dependent linear-to-circular crosstalk with efficiency between \\( \\pm 12.5\\% \\), producing spurious circular polarization up to \\( \\pm 0.1\\% \\) on a star with about 1% linear polarization; after correction, the residual on-sky spurious circular polarization has a standard deviation of 0.02%.[6](https://iopscience.iop.org/article/10.3847/1538-4365/aca407)\n\n**Telescope polarization.** The telescope itself adds polarization: the [William Herschel Telescope](https://www.edgepedia.org/william-herschel-telescope)'s on-axis polarization is typically \\( \\sim 1.5 \\times 10^{-5} \\), measured with an accuracy of a few parts in \\( 10^{7} \\),[9](https://iopscience.iop.org/article/10.1086/507955) and the AAT's is \\( 48 \\pm 5 \\times 10^{-6} \\) in the g′ band.[12](http://academic.oup.com/mnras/article/449/3/3064/2893054)\n\n**Sensitivity ceiling and comparison with alternatives.** Accuracy at the \\( 10^{-5} \\) to \\( 10^{-6} \\) level is only possible for bright stars on telescopes with aperture ≥ 3 m.[1](https://ar5iv.labs.arxiv.org/html/1908.10431) Linear polarimetry enjoys signals roughly a thousand times larger, which is why instruments such as HIPPI and PlanetPol were built primarily as linear polarimeters; circular work trades signal strength for rotational invariance and direct magnetic-field sensitivity.\n\n## References\n\n1. [Optical polarimetry: Methods, Instruments and Calibration Techniques (Berdyugin, Piirola & Poutanen, 2019)](https://ar5iv.labs.arxiv.org/html/1908.10431)\n2. [Circular Polarization of Thermal Radiation in a Magnetic Field (Kemp, ApJ 162:169, 1970)](https://adsabs.harvard.edu/pdf/1970ApJ...162..169K)\n3. [Circular polarization as a probe of cloud properties and asymmetries in giant exoplanet atmospheres](https://www.aanda.org/articles/aa/full_html/2026/09/aa60315-26/aa60315-26.html)\n4. [Biosignatures of the Earth - I. Airborne spectropolarimetric detection of photosynthetic life](https://www.aanda.org/articles/aa/full_html/2021/07/aa40845-21/aa40845-21.html)\n5. [Circularly Polarized Light from a White Dwarf (Kemp, Swedlund, Landstreet & Angel, ApJ 161:L77, 1970)](https://articles.adsabs.harvard.edu/pdf/1970ApJ...161L..77K)\n6. [A Decade of Linear and Circular Polarimetry with the POLISH2 Polarimeter](https://iopscience.iop.org/article/10.3847/1538-4365/aca407)\n7. [The HARPS polarimeter](https://ar5iv.labs.arxiv.org/html/1010.0397)\n8. [Historical review of white-dwarf magnetic-field discovery (Landstreet, 2020)](https://arxiv.org/pdf/2008.01802)\n9. [PlanetPol: A Very High Sensitivity Polarimeter](https://iopscience.iop.org/article/10.1086/507955)\n10. [HIPPI-2: A versatile high-precision polarimeter](https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/hippi2-a-versatile-highprecision-polarimeter/758F4BFD38209FAE755B2A19A9141DE6)\n11. [Spectropolarimetric analysis (arXiv 2504.00179)](https://arxiv.org/pdf/2504.00179)\n12. [HIPPI: a high-sensitivity polarimeter using a ferro-electric liquid crystal modulator](http://academic.oup.com/mnras/article/449/3/3064/2893054)\n13. [adsabs.harvard.edu](https://adsabs.harvard.edu/pdf/1970ApJ...161L..77K)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Cosmology and observation › Observational techniques: astrometry, photometry, spectroscopy*\n\n*Initially written Sep 29, 2026 · Reviewed: Sep 30, 2026 · Edited: Sep 30, 2026 · Last review: Sep 30, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/circularly-polarized-light-detection", "markdown_url": "https://www.edgepedia.org/circularly-polarized-light-detection.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Circularly polarized light detection\", Edgepedia (EdgeChat), https://www.edgepedia.org/circularly-polarized-light-detection. Edgepedia Community License 1.0.", "credit_md": "\"[Circularly polarized light detection](https://www.edgepedia.org/circularly-polarized-light-detection)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/circularly-polarized-light-detection](https://www.edgepedia.org/circularly-polarized-light-detection). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Circularly polarized light detection\", Edgepedia (EdgeChat), https://www.edgepedia.org/circularly-polarized-light-detection. Edgepedia Community License 1.0.", "speakable": "Circularly polarized light detection, or circular polarimetry, measures the circular component of polarization in light from stars and planets, probing magnetic fields and cloud scattering since 1970." }