A dichroic coating is a stack of metal-oxide layers evaporated on a fraction of a wavelength at a time, chosen to reflect one band of light and pass the rest — which is exactly what a mirrored lens is. The lens is also the ideal shape for it. A goggle lens is toric: it wraps around the face in plan and curves again in section, so its surface normal sweeps a wide arc in two directions at once. Because a film's hue depends on the angle light crosses it at, that lays the whole sequence out across one object — gold where you look straight in, running through green to violet at the rim where the path through the stack is longest. A flat coated sheet gives you one colour and a promise.