BRIGHTENING LENSES WITH ANTI-REFLECTION COATINGS.
In the considerations regarding the brightness of the lenses, it is assumed, that the illumination of the image depends only on the area of the entrance pupil and the focal length. However, not all of the light falling on the lens reaches the plate or film, because it is partially reflected on the lens surfaces. Part of the light is also absorbed in the mass of glass, but the losses are small, because optical glass is generally very transparent. Significant loss of lens brightness, on the other hand, occurs due to reflection. 4% to 7% of the light energy is reflected from each surface of the glass adjacent to the air. A lens with two surfaces bordering the air loses 10% its brightness, on four surfaces - approx. 17%, at six - approx. 25%, and eight is fine. 35% or more.
So-called bright lenses (with a large relative aperture) are characterized by a very complicated structure. They consist of more lenses, therefore, there are sometimes even greater losses of light energy from reflection and a reduction in the actual brightness of the lenses. In addition, the light reflected from the surface of the glass inside the lens is reflected back towards the object, it is partially reflected on the encountered surfaces of the lenses and, returning to the image, reduces the contrast of the photo.
These harmful reflections on glass surfaces are combated by coating them with an anti-reflective layer.
If the lens surface is coated very thinly (of thickness 1/4 light wavelength) and an even layer of a durable substance with a refractive index intermediate between that of air and glass, then, due to the phenomenon of light interference in thin layers, the reflected light will be extinguished, powering the luminous flux reaching the image. Such coatings, called brightening or anti-glare, not only increase the actual brightness of the lens, but they also improve image clarity. The coatings are applied by two methods: physical and chemical. The physical method consists in sputtering thin layers by evaporation of fluorides onto the glass surface. Physical coatings, used by some companies, until recently, they were relatively resistant to abrasion and therefore great care had to be taken when cleaning lenses brightened in this way. They are now permanent.
The chemical method of brightening consists in the production of anti-reflective layers by etching the glass in acids or by forming films from solutions on rotating lenses. Then a layer of silicon dioxide with a refractive index lower than the refractive index of glass is formed on the glass. The chemical method of applying two layers gives good results: one of titanium dioxide, the other from silicon dioxide.
Of course, complete removal of glare is possible only for light of one wavelength. Usually, glare is removed for yellow or green light. A mixture of red lights, blue and purple, that are not completely removed, gives a characteristic purple color in the light.
Single chemical coatings reduce the loss of light to 2%, two-layer physical and chemical coatings up to 1%. Because chemical coatings are much more resistant to abrasion, therefore, they are usually applied to the outer surfaces of the lenses.
Chemical coatings are also, and even more resistant to abrasion than the surface of lenses equipped with them. They can be carefully cleaned with a soft brush or a completely clean one without fear, cloth that has been washed many times.