Patent ID: 8570647

Claim:
An optical element, comprising: a substrate made of sapphire, the substrate comprising a first surface and a second surface opposite to the first surface; and a film covered on the first surface and configured for increasing transmissivity of ultraviolet lights; the film stacked by a first layer and a second layer in an order facing away from the first surface; the first layer is one high refractive index layer, the second layer is one low refractive index layer, a material of the high refractive index layer being hafnium oxide (HfO 2 ) having a refractive index from about 2.0 to about 2.15, and a material of the low refractive index layer being magnesium fluoride (MgF 2 ) having a refractive index of about 1.38, a thickness of the first layer is about 70±8 nm, and a thickness of the second layer is about 50±8 nm.