The surface of the early telescope lens was not coated with anti-reflection coating, and the white glass surface would lose about 4% of the energy for each reflection. Taking the Forester 10X50 Porro prism as an example, there are exactly 10 air contact surfaces inside the lens barrel on each side. Coating consumes 40% of the energy. Zeiss company proposed the concept of anti-reflection coating in the 1930s, coating a layer of magnesium fluoride (mgf2) on the surface of the lens, the thickness is a quarter of 550nm, so that the middle band of visible light can be completely transmitted, visible light in the edge band cannot be completely transmitted and there is residual reflection. The reflection in the blue band is relatively strong, so the coating we see shows blue reflection, commonly known as blue film. The average reflectance of the blue film is about 1.5%. This coating is the first generation of anti-reflection coating, because there is only one layer of coating, also called single-layer coating. Still taking the Forester 10X50 as an example, if all the glass-air contact surfaces are coated with blue film, the energy loss of the entire telescope due to the reflection of the contact surface will be about 15%, which is much less than that without coating.