Roughness / Scattering
Real interfaces are never perfectly smooth, and the residual roughness scatters a little light out of the specular beam. Roughness / Scattering estimates that loss as the Total Integrated Scatter (TIS) and shows you the specular R and T that remain after it. You give each interface an RMS roughness σ; the tool treats the interfaces as uncorrelated, combines them into an effective roughness σ_eff (where σ_eff² is the sum of the individual σ²), and computes the scatter fraction at every wavelength.
The analysis runs for the surface mode set in the Design Editor. Front uses the front-stack interfaces, back uses the back-stack interfaces, and total sums the roughness across both stacks.
Settings
Section titled “Settings”λ range / step: the wavelength grid, in nanometres.
AOI: angle of incidence.
T / R: which curves to draw, with avg, s and p inside each. Each one is drawn twice, ideal and specular. Every polarization is computed together, so switching one on adds a pair rather than replacing the averaged one.
ppm / frac: the units for the TIS axis: parts per million or fraction.
Roughness
Section titled “Roughness”The Roughness strip below the plot is where the σ values are set. Choose Uniform σ to apply the same roughness to every interface, or Per-interface to set each one individually. In uniform mode a single σ field drives every interface; in per-interface mode each interface is listed, named by the two media it separates, with its own σ.
Reset clears all roughness back to zero.
How to read it
Section titled “How to read it”The chart overlays two things on different axes. The left axis, in percent, shows the specular R and T after scatter loss as solid lines, with the ideal (zero-roughness) R and T drawn faintly dotted behind them so you can see how much the roughness costs. The right axis shows TIS(λ) on its own ppm or fraction scale.
Scatter loss rises sharply toward short wavelengths (TIS scales as 1/λ²), so a roughness that is harmless in the infrared can be serious in the ultraviolet. The Results strip reports the effective roughness σ_eff and the number of interfaces contributing to it, and opens the spectrum wavelength by wavelength.
References
Section titled “References”- H. A. Macleod, Thin-Film Optical Filters, 5th ed., §16 (Eq. 16.30).
- H. E. Bennett & J. O. Porteus, J. Opt. Soc. Am. 51, 123 (1961).