The inversion algorithm works by first estimating angle-dependent P-wave reflectivities for the input-partial stacks. For high quality rings, reflectivities larger than 99.999% (R = 1-10 ppm) are indispensable. The resulting formulae for the reflection coefficients and reflectivities became known as the "Fresnel equations". Reflectivities greater than 99.5% are routinely achieved in dielectric mirrors. In this way, reflectivities of up to ~50% can be attained.