REVIEW 3 major objections 5 minor 40 references
Modeling of optical scattering from topographic surface measurements of high-quality mirrors
T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read From measured surface maps of KAGRA's core mirrors, the paper derives a scattering profile that puts the arm's scattered-light noise below the design-phase estimate.
desk verdict Useful engineering estimate of KAGRA mirror scattering, but the headline profile is an extrapolated worst-case envelope, not a validated prediction for coated mirrors. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The engine is the Rayleigh-Rice relation, Eq. (2), which links the two-sided 2D power spectral density $S_2(f_x,f_y)$ of surface height errors to the scattering probability density per unit solid angle. With normal incidence, small scattering angles, and $P_s\simeq P_i$, it reduces to $dP/d\Omega_s \simeq 16\pi^2 S_2(f)/\lambda^4$, where $f\simeq\theta_s/\lambda$. To turn measured maps into a model, the paper circularly averages the raw 2D PSD, fits an ABC/K-correlation form $S_2(f)=A'/[1+(Bf)^2]^{(C+1)/2}$, and converts the fitted PSD into the angular profile analytically. An independent branch compresses the 2D PSD to a one-sided 1D PSD through the Abel transform, fits the corresponding spectral model, and cross-checks the 2D result. Together these pieces carry the argument from raw interferometric maps to Eq. (17).
What would settle it
Measure the actual scattering distribution of one KAGRA core mirror with a goniometer over $\theta_s$ from roughly $4\times10^{-5}$ to $10^{-3}$ rad; if the measured profile falls off more slowly than $\theta_s^{-3.2}$ or lies above Eq. (17) beyond the fitted uncertainties, the central claim fails. A second, less direct check is measuring the surface PSD up to $\sim100\ \mathrm{mm}^{-1}$ with a microscope; if the power-law slope changes beyond the measured $\sim1\ \mathrm{mm}^{-1}$ band, the extrapolation underlying the full-angle profile is not supported.
Extended reading notes
Core claim
The paper's central result is Eq. (17): after cropping, detrending, windowing, and circularly averaging the two-sided 2D power spectral density (PSD) of the four KAGRA core mirror maps, a worst-case ABC/K-correlation fit gives $dP/d\Omega_s \simeq 2.5\times 10^{2}\,[1+(\theta_s/(7.1\times10^{-5}\,\mathrm{rad}))^2]^{-1.6}\ \mathrm{sr}^{-1}$. An equivalent inverse-power-law limit, Eq. (18), is $dP/d\Omega_s \simeq 1.3\times10^{-11}/\theta_s^{3.2}\ \mathrm{sr}^{-1}$. These formulas describe an isotropic scattering envelope that bounds the actual mirror behavior for ray-tracing studies. Compared with the design-phase profile $2\times10^{-6}/\theta_s^2$, the new profile scatters less light at $\theta_s > 48\,\mu\mathrm{rad}$, and also less than the design curve between 37 and 48 $\mu$rad, so the scattered-light noise in the KAGRA arm is expected to be smaller than previously estimated. The consistency between the independent 1D-PSD Abel-transform fit and the 2D-PSD fit supports treating the roughness as circularly symmetric.
Load-bearing premise
The result stands on the assumption that the Rayleigh-Rice formula, developed for bare surfaces, still describes scattering from the multilayer-coated KAGRA mirrors; the paper itself flags that this is not obvious.
Editorial extensions
If this is right
- KAGRA's scattered-light noise is lower than the design-phase estimate, so the existing baffle system needs no immediate change.
- Eq. (17) can be fed directly into ray-tracing simulations to set upper limits on stray-light contamination inside the arm cavity.
- The same preprocessing-and-fit flow can be applied to any smooth mirror with a topographic height map, including optics for future gravitational-wave detectors.
- The power-law form of Eq. (18) provides a simple conservative envelope for worst-case noise studies.
Reading between the lines
- If the Rayleigh-Rice relation extends to multilayer-coated mirrors, the same flow would give comparable scattering envelopes for other interferometric detectors from their metrology data, enabling direct cross-detector stray-light comparisons.
- The unresolved high-spatial-frequency behavior is the most promising place to test the model; a microscopic measurement of the coated surface roughness would sharpen or refute the large-angle tail of the profile.
- A consistency check the paper does not perform is integrating Eq. (17) over the full sphere and comparing the total scattered fraction with the independently measured 50-100 ppm optical loss of the KAGRA core mirrors.
- The circular-symmetry assumption could be tested by examining the azimuthal dependence of the raw 2D PSD; coating-induced anisotropy would break the azimuthal invariance of the scattering profile.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper revisits the traditional Rayleigh-Rice method for converting a measured 2D topographic height map of a mirror surface into an angular scattering profile. It provides self-contained derivations of the Abel transform between 2D and 1D power spectral densities and of the ABC (K-correlation) spectral model, then describes a practical data-processing pipeline (crop, detrend, window, FFT, circular mean, curve fit). As a case study, the authors apply the pipeline to measured surface maps of the four KAGRA core mirrors and obtain a parametric scattering profile, Eqs. (17) and (18), which they compare to the previously assumed 2×10^-6/θ_s^2 model. They conclude that the updated profile predicts less scattered-light noise at the angles relevant to KAGRA's arm cavities, so no immediate changes to the baffle design are needed.
Significance. If the derived profile is valid, it provides the first topography-based scattering model for the KAGRA core mirrors and a simple parametric input for ray-tracing simulations of stray light. The paper also collects and derives standard formulas (Abel transforms, ABC model conversion) in a clear way that may be useful to practitioners. A strength is that the data-processing flow is explicitly described, and the cross-check between the 1D and 2D fits supports the assumed circular symmetry of the surface roughness. However, the central result rests on two explicitly acknowledged assumptions—the applicability of the Rayleigh-Rice single-surface relation to multilayer-coated mirrors and the extrapolation of the PSD fit beyond the measured spatial-frequency band—and on an internal inconsistency in the normalization of the quantity called 'scattering PDF'.
major comments (3)
- [Section 4.2] Section 4.2 concedes that 'Due to the multilayer, it is not obvious whether the actual scattering angular profile would follow the relation in Eq. (2)' and cites Ref. [40] without applying it. Since Eq. (17) is obtained by inserting the top-surface PSD into Eq. (2), the central claim is an unvalidated single-surface surrogate for the actual coated-mirror scattering. The authors should either implement the multilayer scattering calculation of Ref. [40] to quantify the deviation, or explicitly state in the abstract and conclusion that the profile is conditional on the single-surface approximation; as written, the phrase 'first model-based estimation' overstates the evidence.
- [Section 4.2 / Eq. (17)] The measured PSD data in Figs. 4 and 5 cover spatial frequencies up to only ~1 mm^-1 (θ_s ≲ 1 mrad), while the text notes that full arm-cavity coverage requires angles up to ~0.1 rad (~100 mm^-1). The profile Eq. (17) and the noise comparison in Section 4.1 therefore depend on extrapolating the fitted ABC/power-law model by more than two orders of magnitude in frequency. The paper should provide a sensitivity analysis or a conservative upper bound for the extrapolated region (e.g., the Lambertian diffusive-scattering floor mentioned in Section 4.2) and discuss how the conclusions in Section 4.1 would change if the high-frequency PSD deviates from the fitted form.
- [Eq. (1) vs. Eq. (17)] Eq. (1) defines dP/dΩ_s as a probability density with ∫ dP/dΩ_s dΩ_s = 1, but the fitted profile Eq. (17) integrates to roughly 7×10^-6 over the sphere (using the small-angle approximation appropriate for the KAGRA geometry), not unity. The quantity in Eq. (17) is therefore a differential scattering coefficient normalized to incident power, not a normalized PDF. The label 'scattering PDF' in Fig. 4 and the text is misleading, and any downstream ray-tracing use that assumes unit normalization will be incorrect. The authors should either renormalize the profile or explicitly present it as (1/P_i)dP_s/dΩ_s, state that its integral equals the scattered-power fraction represented by the fitted angular range, and provide guidance on how it should be used in ray-tracing simulations.
minor comments (5)
- [Abstract / Introduction] The Introduction contains a typo: 'ray-tracging' should be 'ray-tracing'.
- [Section 3.2] The sentence 'the read data are the measured 2D maps of the KAGRA mirrors' is awkward; consider rephrasing to 'the input data are the measured 2D maps'.
- [Fig. 4] The right vertical axis label 'Scattering PDF dP/dΩ_s (1/sr)' is inconsistent with the normalization issue raised in the major comments; consider using a neutral term such as 'differential scattering coefficient'.
- [Eq. (17)] The fitted parameters A', B, C are quoted with uncertainties in Section 3.2, but Eq. (17) gives only central values; propagation of the parameter uncertainties into the profile would be helpful for users.
- [References] Reference [10] is an internal LIGO document that is not publicly accessible; consider citing a published equivalent or indicating how interested readers can obtain it.
Circularity Check
No significant circularity: Eq. (17) is an openly labeled fit to the measured PSD, converted via the external Stover Rayleigh-Rice relation; the multilayer-coating caveat is an acknowledged validity risk, not a circular step.
full rationale
The claimed derivation chain is linear and transparent: the four KAGRA mirror height maps are measured externally (Ref. [8]); the 2D PSD is computed by FFT with windowing and circular mean (Eqs. 5, 15, 16); an ABC model is freely curve-fitted to the worst-case S⋄2 values (Eq. 13, parameters A' = 2.049, B = 15.00, C = 2.198); and the profile is obtained through the Rayleigh-Rice relation quoted from Stover's external textbook, Eq. (2), reduced to Eq. (3), dP/dΩs ≃ 16π2 S2(f)/λ4 with f = θs/λ. Eq. (17) is indeed algebraically identical to the fitted S2 rescaled by 16π2/λ4, but the paper is explicit about this: Section 3.2 says 'fit parameters' are obtained and then 'the scattering PDF estimate can be obtained through Eq. (3)', and Section 4.1 calls Eqs. (17)-(18) a 'model-based estimation', not an independently verified prediction; no goniometric or parameter-free validation is claimed. The load-bearing premises — that the single-surface Rayleigh-Rice relation holds for multilayer-coated mirrors, and that the PSD fit can be extrapolated beyond the measured ≤1 mm−1 band up to ~100 mm−1 — are validity assumptions, explicitly conceded in Section 4.2 ('it is not obvious whether the actual scattering angular profile would follow the relation in Eq. (2)'). These are correctness risks, not circularity, because the target profile is never used as an input to the fit. The 1D-vs-2D cross-check is used only to support the circular-symmetry assumption, which is its legitimate scope. Self-citations ([12], [22], and the multilayer study [40], which has Akutsu as co-author) are present but not load-bearing: [40] is cited precisely because it is not applied. Two non-circular defects are flagged for completeness: the normalization inconsistency between Eq. (1) (PDF normalized to unity over the sphere) and Eq. (17) (which integrates to roughly 7×10−6, so it is actually a per-incident-power differential scattering coefficient), and the absence of any external benchmark, which the paper acknowledges. The score of 2 reflects only the presence of minor, non-load-bearing self-citations in a derivation that is otherwise self-contained against external data and external theory.
Assumptions & free parameters
free parameters (4)
- A' (2D ABC amplitude) =
2.049 ± 0.141 nm^2 mm^2
- B (ABC corner spatial frequency inverse) =
15.00 ± 1.22 mm
- C (ABC spectral index) =
2.198 ± 0.132
- 1D fit parameters A, B, C =
A=0.479±0.031 nm^2 mm, B=14.04±0.69 mm, C=2.234±0.043
assumptions (5)
- domain assumption The Rayleigh-Rice relation, Eq. (2), from Ref. [13] describes scattering from the measured topographic height maps of KAGRA core mirrors.
- domain assumption The mirror surface height errors are isotropic, so the two-sided 2D PSD is circularly symmetric.
- domain assumption Scattered power is concentrated near the specular direction with normal incidence, so Eq. (2) reduces to Eq. (3) with Q≈1 and cos factors≈1.
- domain assumption The measured PSD band (≈0.03 to 1 mm^-1) is representative of the full spatial frequency range that contributes to scattering angles of interest, and the ABC model extrapolates outside this band.
- standard math Standard Fourier analysis identities, Abel transform pairs, beta and gamma function relations, and DLMF hypergeometric identities used in Section 2 are correct.
Cite this review
Pith. "Pith review of Modeling of optical scattering from topographic surface measurements of high-quality mirrors." pith.science (2026). https://pith.science/paper/3SDHFADT
@misc{pith2026241115733,
author = {Pith},
title = {Pith review of: Modeling of optical scattering from topographic surface measurements of high-quality mirrors},
year = {2026},
howpublished = {\url{https://pith.science/paper/3SDHFADT}},
note = {Machine review of arXiv:2411.15733}
}
read the original abstract
In this paper, we revisit computational methods to obtain an angular profile of optical scattering from a smooth surface, given a two-dimensional map of topographic height errors of the surface. Quick derivations of some traditional equations and relevant references are organized to shorten the search time. A practical data-processing flow of the methods is discussed. As a case study of this flow, the core mirrors of the KAGRA interferometer are examined, and we obtain a representative scattering profile that is easily applicable to ray-tracing simulations.
Figures
Figures from the paper (2 more)
Reference graph
Works this paper leans on
-
[40]
S. Zeidler, T. Akutsu, Y. Torii,et al., “Calculation method for light scattering caused by multilayer coated mirrors in gravitational wave detectors,” Opt. Express25, 4741 (2017)
work page 2017
-
[1]
Advanced LIGO,
J. Aasi, B. P. Abbott, R. Abbott,et al., “Advanced LIGO,” Class. Quantum Gravity32, 074001 (2015)
2015
-
[2]
Advanced Virgo: a second-generation interferometric gravitational wave detector,
F. Acernese, M. Agathos, K. Agatsuma,et al., “Advanced Virgo: a second-generation interferometric gravitational wave detector,” Class. Quantum Gravity32, 024001 (2014)
work page 2014
-
[3]
Overview of KAGRA: Detector design and construction history,
T. Akutsu, M. Ando, K. Arai,et al., “Overview of KAGRA: Detector design and construction history,” Prog. Theor. Exp. Phys.2021, 05A101 (2020)
2020
-
[4]
GW170817: ObservationofGravitationalWavesfromaBinaryNeutron Star Inspiral,
B.P.Abbott,R.Abbott,T.D.Abbott, et al.,“GW170817: ObservationofGravitationalWavesfromaBinaryNeutron Star Inspiral,” Phys. Rev. Lett.119, 161101 (2017)
work page 2017
-
[5]
A. G. Abac, R. Abbott, I. Abouelfettouh,et al., “Observation of Gravitational Waves from the Coalescence of a 2.5–4.5𝑀⊙ Compact Object and a Neutron Star,” The Astrophys. J. Lett.970, L34 (2024)
work page 2024
-
[6]
Modeling and reduction of high frequency scatter noise at LIGO Livingston,
S. Soni, J. Glanzer, A. Effler,et al., “Modeling and reduction of high frequency scatter noise at LIGO Livingston,” Class. Quantum Gravity41, 135015 (2024)
work page 2024
-
[7]
A. Longo, S. Bianchi, G. Valdes,et al., “Scattered light monitoring system at the Virgo interferometer: performance improvement and automation based on O3 data,” Class. Quantum Gravity41, 015004 (2024)
work page 2024
Show all 40 references
-
[8]
Characterization of core optics in gravitational-wave detectors: Case study of KAGRA sapphire mirrors,
E. Hirose, G. Billingsley, L. Zhang,et al., “Characterization of core optics in gravitational-wave detectors: Case study of KAGRA sapphire mirrors,” Phys. Rev. Appl.14, 014021 (2020)
2020
-
[9]
Impact of upconverted scattered light on advanced interferometric gravitational wave detectors,
D. J. Ottaway, P. Fritschel, and S. J. Waldman, “Impact of upconverted scattered light on advanced interferometric gravitational wave detectors,” Opt. Express20, 8329 (2012)
2012
-
[10]
Noise due to light scattering in interferometric gravitational wave detectors. I: Handbook of Formulae, and their Derivations,
E. E. Flanagan and K. S. Thorne, “Noise due to light scattering in interferometric gravitational wave detectors. I: Handbook of Formulae, and their Derivations,” (2011). Internal document; Semi-final Draft, provided to LCGT on September 24, 2011
2011
-
[11]
Displacement noise from back scattering and specular reflection of input optics in advanced gravitational wave detectors,
B. Canuel, E. Genin, G. Vajente, and J. Marque, “Displacement noise from back scattering and specular reflection of input optics in advanced gravitational wave detectors,” Opt. Express21, 10546 (2013)
2013
-
[12]
Vacuum and cryogenic compatible black surface for large optical baffles in advanced gravitational-wave telescopes,
T. Akutsu, Y. Saito, Y. Sakakibara,et al., “Vacuum and cryogenic compatible black surface for large optical baffles in advanced gravitational-wave telescopes,” Opt. Mater. Express6, 1613 (2016)
2016
-
[13]
J. C. Stover,Optical Scattering: Measurement and Analysis (SPIE Press, 1995), 2nd ed
1995
-
[14]
Multiple factors contribute to this difficulty, including the limited angular resolution of an optical goniometer, where a photodetector is illuminated by specularly reflected light from a surface under test. Due to the finite light-sensitive area of the photodetector, the mea...
-
[15]
The prediction of BRDFs from surface profile measurements,
E. L. Church, P. Z. Takacs, and T. A. Leonard, “The prediction of BRDFs from surface profile measurements,” Proc. SPIE 1165, 136 (1989)
1989
-
[16]
The two-sided 1D PSD𝑆△ 1(𝑓𝑥)≡ ∫∞ −∞𝑆2(𝑓𝑥, 𝑓𝑦)𝑑𝑓𝑦 for 𝑓𝑥∈(−∞ ,∞) satisfies𝑆△ 1(−𝑓𝑥) =𝑆△ 1(𝑓𝑥) due to∫∞ −∞𝑆2(−𝑓𝑥, 𝑓𝑦)𝑑𝑓𝑦 = ∫∞ −∞𝑆2(−𝑓𝑥,−𝑓′𝑦)𝑑𝑓 ′𝑦 for the conversion𝑓′𝑦 =−𝑓𝑦, and the fact𝑆2(−𝑓𝑥,−𝑓𝑦) = 𝑆2(𝑓𝑥, 𝑓𝑦)
-
[17]
In general, the 2D PSD𝑆2 of a map𝑧(𝑥,𝑦) is obtained by applying the Fourier transform to the autocorrelation of𝑧(𝑥,𝑦). For isotropic maps, by definition, the autocorrelation depends solely on the distance between any two points, given by𝛿=((𝑥1−𝑥2) 2+( 𝑦1−𝑦2) 2) 1/2, and not on...
-
[18]
R. N. Bracewell,Fourier Analysis and Imaging (Springer, 2003), 3rd ed
2003
-
[19]
R. N. Bracewell,The Fourier Transform and Its Applications (McGraw-Hill, 2000), 3rd ed
2000
-
[20]
Fractal surface finish,
E. L. Church, “Fractal surface finish,” Appl. Opt.27, 1518 (1988)
1988
-
[21]
NIST Digital Library of Mathematical Functions,
“NIST Digital Library of Mathematical Functions,”https://dlmf.nist.gov/, Release 1.2.1 of 2024-06-15. F. W. J. Olver, A. B. Olde Daalhuis, D. W. Lozier, B. I. Schneider, R. F. Boisvert, C. W. Clark, B. R. Miller, B. V. Saunders, H. S. Cohl, and M. A. McClain, eds
2024
-
[22]
1D PSD of mirror maps,
H. Yamamoto, “1D PSD of mirror maps,” LIGO-T1100353-v1 (2011)
2011
-
[23]
Effects of Scattering on X-Ray Imaging,
R. J. Noll, “Effects of Scattering on X-Ray Imaging,” Proc. SPIE0184, 203 (1979)
1979
-
[24]
Effects of Scattering on X-Ray Imaging,
R. J. Noll, “Effects of Scattering on X-Ray Imaging,” Opt. Eng.19, 192249 (1980)
1980
-
[25]
The role of spatial bandwidth limits in the measurement and interpretation of second-order statistical properties,
E. L. Church, “The role of spatial bandwidth limits in the measurement and interpretation of second-order statistical properties,” inProceedings of the 26th Conference on the Design of Experiments in Army Research Development and Testing,(U. S. Army Research Office, 1980), p. 387
1980
-
[26]
Mirror surface autocovariance functions and their associated visible scattering,
R. J. Noll and P. Glenn, “Mirror surface autocovariance functions and their associated visible scattering,” Appl. Opt. 21, 1824 (1982)
1982
-
[27]
Spectral analysis of the finish of polished optical surfaces,
E. L. Church and H. C. Berry, “Spectral analysis of the finish of polished optical surfaces,” Wear83, 189 (1982)
1982
-
[28]
Statistical and Signal Processing Concepts in Surface Metrology,
E. L. Church and P. Z. Takacs, “Statistical and Signal Processing Concepts in Surface Metrology,” Proc. SPIE0645, 107 (1986)
1986
-
[29]
See, for example,https://www.mathworks.com/help/stats/pearson-distribution.html
-
[30]
Note that Eq.(4.23) in [13] corresponds to𝑆△ 1(𝑓𝑥) here, so the factor of 2 differs
-
[31]
prysm: A python optics module,
B. Dube, “prysm: A python optics module,” J. Open Source Softw.4, 1352 (2019)
2019
-
[32]
POPPY: Physical Optics Propagation in PYthon,
M. Perrin, J. Long, E. Douglas,et al., “POPPY: Physical Optics Propagation in PYthon,” Astrophysics Source Code Library, record ascl:1602.018 (2016)
2016
-
[33]
Simulating point spread functions for the James Webb Space Telescope with WebbPSF,
M. D. Perrin, R. Soummer, E. M. Elliott,et al., “Simulating point spread functions for the James Webb Space Telescope with WebbPSF,” inSpace Telescopes and Instrumentation 2012: Optical, Infrared, and Millimeter Wave, vol. 8442 ofSociety of Photo-Optical Instrumentation Engine...
2012
-
[34]
Zernike polynomials and atmospheric turbulence,
R. J. Noll, “Zernike polynomials and atmospheric turbulence,” J. Opt. Soc. Am.66, 207 (1976)
1976
-
[35]
scikit-image: image processing in Python,
S. van der Walt, J. L. Schönberger, J. Nunez-Iglesias,et al., “scikit-image: image processing in Python,” PeerJ2, e453 (2014)
2014
-
[36]
They are acronyms forinput test mass x or y or end test mass x or y
-
[37]
Interferometer techniques for gravitational-wave detection,
C. Bond, D. Brown, A. Freise, and K. A. Strain, “Interferometer techniques for gravitational-wave detection,” Living Rev. Relativ.19, 3 (2017)
2017
-
[38]
Compilation of Metrology Data for the LIGO Large Optics,
R. Weiss, “Compilation of Metrology Data for the LIGO Large Optics,” LIGO-T980065 (1998)
1998
-
[39]
Development of advanced photon calibrator for Kamioka gravitational wave detector (KAGRA),
Y. Inoue, B. H. Hsieh, K. H. Chen,et al., “Development of advanced photon calibrator for Kamioka gravitational wave detector (KAGRA),” Rev. Sci. Instruments94, 074502 (2023)
2023
Reviewed August 12, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.