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

λ range / step — the wavelength grid, in nanometres.

AOI / pol — angle of incidence and polarization (s, p, or averaged).

ppm / frac — the units for the TIS axis: parts per million or fraction.

Roughness model — choose Uniform σ to apply the same roughness to every interface, or Per-interface to set each one individually in the table. In uniform mode a single σ field and slider drive 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

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 toolbar reports the effective roughness σ_eff and the number of interfaces contributing to it, and the sidebar summarizes TIS at the band edges.

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).