Swanepoel Envelope Analyzer

Extract refractive index n(λ), extinction coefficient k(λ), and physical thickness d from a coated-film transmission spectrum plus a bare-substrate reference, including a closed-form low-index (n < s) branch derived in-house for porous-silica LRI coatings.

Drop in two transmission spectra (bare substrate + coated film) and the tool extracts the Cauchy dispersion of both the film and the substrate, the physical thickness d, and the extinction coefficient — using the Swanepoel envelope method. Handles the standard high-index regime (n > s) plus the low-index (LRI) regime where the textbook Swanepoel formula breaks down. The scientific kernel is a TypeScript port of a Python reference; cross-language parity is enforced on every CI run.