Layer stacks respond to the following arguments:
(Reflectance at 540 nm): this function returns the reflectance of the layer stack at 540 nm (normal incidence of light)
(Transmittance at 540 nm): this function returns the transmittance of the layer stack at 540 nm (normal incidence of light)
(Absorption at 540 nm): this function returns the absorption ( = 1-R-T) of the layer stack at 540 nm (normal incidence of light)