The surfaces of optical lenses are covered in thin films called coatings, which play an important role in enhancing the performance of lenses.
Ordinary glass lenses transmit most of the light that hits them, but even so, about 4% of this light is lost to surface reflection. Since lenses have front and rear surfaces, this means that the overall loss of light from passing through one lens element is 8%. Most camera lenses are made up of five to 10 elements, and so in the end, the total amount of light getting through the lens is reduced by about 50%. Lens coatings were developed to prevent surface reflection and boost light transmission. Coating lenses enables more light to pass through them.
Surface reflection reduces the amount of light transmitted through a lens, but this is not the only adverse effect. Reflection within the lens also causes such problems as image duplication, and the transmission of non-image light to the image: phenomena known as ghosts and flares, respectively. Ghosts are created when light reflected from the rear surface of a lens is reflected once again from the front surface, resulting in a faint second image slightly displaced from the primary image. Flares appear when light from the back of the lens barrel is reflected from the lens surface onto the image. Ghosts and flares caused by surface reflection reduce the quality of the image produced.
Surface reflection can be reduced by applying coatings to the lens surface. You might think that coating the lens surface would block light, but in fact it increases light transmission. This is because light is reflected first by the coating surface, and then by the lens surface itself. The light reflected by the coating surface and that reflected by the lens surface have a phase difference of twice the coating thickness.
If the thickness of the coating is one quarter of the wavelength of the light to be suppressed, light of that wavelength reflected by the coating surface and light reflected by the lens surface will cancel each other out. This reduces the overall amount of light reflected. In short, coatings make use of light wave interference phenomena to eliminate reflections.
Magnesium fluoride (MgF2) or silicon monoxide (SiO) are used as coating materials, with very thin coatings being applied evenly over the surface through such techniques as vacuum deposition or plasma sputtering. However, light is made up of many different wavelengths, and one coating cannot possibly cut out all reflected light. To cut down reflections of light of various wavelengths requires many layers of coatings. Such multilayer coatings are applied to high-end lenses. The technology for applying coatings of over 10 layers has been developed, and Canon's high-end lenses featuring such coatings provide light transmission of 99.9% over a range that extends from ultraviolet to near-infrared light.
Lens coatings are used not only to boost light transmission, but also to filter light. Lenses coated to reflect ultraviolet light are commonly used in eyeglasses and sunglasses. It is also possible to create coatings that allow light of only a specific wavelength to pass through, and reflect all other wavelengths. In video cameras, light is first split into RGB elements (red, green and blue) before being converted into electrical signals to form an image. This splitting of light is accomplished by lens coatings that permit only light of the required red, green and blue wavelengths through.
The latest technologies are also being used in lens coating.
SWC (Subwavelength Structure Coating), developed by Canon, is a new type of technology that uses aluminum oxide (Al2O3) as the structural material of the coating in order to align countless wedge-shaped nanostructures only 220 nm high, which is smaller than the wavelength of visible light, on a lens surface. This nano-scale coating provides a smooth transition between the refractive indexes of glass and air, successfully eliminating the boundary between substantially different refractive indexes. Reflected light can be limited to around 0.05%.
Furthermore, it has displayed excellent reflection-prevention properties not seen in conventional coating even for light with a particularly large angle of incidence. Currently, SWC is being used in a broad range of lenses, not only wide angle lenses, which have a large curvature factor, but also large-diameter super telephoto lenses, greatly reducing the occurrence of flare and ghosting caused by reflected light near the peripheral area, which had been difficult in the past.