public static RgbSpectrum FresnelApproxEta(RgbSpectrum i)
{
RgbSpectrum intensity = new RgbSpectrum(i);
intensity.Clip(0f, 0.999f);
return (RgbSpectrum.UnitSpectrum() + intensity.Sqrt()) / (RgbSpectrum.UnitSpectrum() - intensity.Sqrt());
}