Anti-reflective coating

Antireflective coatings ( short AR coating ) are used to suppress the reflection from optical surfaces of lenses, lenses, prisms or plates and increase the transmission. With lenses and eyepieces with such a coating is referred here by a payment arrangement in glasses, windows, or picture tubes by a anti-reflective coating. As the inventor of optical compensation shall be the Ukrainian physicist Alexander Smakula.

Basics

The reduction in reflectance of the coated surface is achieved by a destructive interference of the reflected rays.

For the simplest case of a single, homogeneous coating layer, we consider a beam of a specific wavelength that is incident perpendicular (in the picture for clarity obliquely drawn ). He is partially reflected at the surface of the coating layer (r1 ), and partly it passes through the layer and then at the next interface partially reflected (r2). Thus, the two partial beams r1 and r2 are completely interfere destructively, their amplitudes have the same size ( amplitude condition ) and of phase ( phase difference) to each other be ( phase condition ).

From the Fresnel formulas shows that the refractive index of the coating layer

Must be so that the amplitudes of R1 and R2 are the same. Here, the refractive index of the material (usually glass) and a refractive index of the medium in front of the surface (usually air). It is here neglected, that the beam is r2 on the surface of the coating layer again reflected; it is taken exactly infinitely often reflected back and forth.

Because the reflection takes place at both point A and B in a phase change of place, actually a sign change of the amplitude, which does not affect the interference. For the necessary phase difference of needs so the optical length of the beam in the coating layer

Amount. If you use the thinnest possible layer ( ) concerns with the optimum layer thickness:

If the beam is incident is not perpendicular but at an angle to the surface, the optical path length changes in the coating layer at the same thickness according to the Snell's law and the outside through the outlet laterally offset, so that there is a higher optimal layer thickness and for given d, a reduction of the appropriate wavelength. In the case of destructive interference and is (see )

Different wavelengths are increasingly reflected or not but interfere completely destructively. That is the reason why the (weak) reflection of hardened surfaces is colored and the color angle-dependent.

Example of a single compensation

As an example we consider the normal incidence of yellow- green light ( 550 nm wavelength) on soda lime glass (). The external medium is air ( ≈ 1.0). Without compensation of reflectance is according to the Fresnel equations

According to the formula in the previous section would be the ideal value for the AR layer; however, no durable material available. Used is often magnesium fluoride ( MgF2 ). The λ/4-Schicht for the yellow- green light which is about 100 nm thick and results in a normal incidence reflectance ( derivation see pike or Pedrotti ) of

Multi-coated optics

Next, and over a wider wavelength range and angle the reflection can be reduced by using multiple layers with different refractive indices. For the optimum film thickness for a given choice of materials, there is no simple formula. These parameters are therefore determined with the aid of simulation programs.

Production

The preparation of anti-reflection coatings is carried out by coating methods of the thin film technology. The most commonly used include physical vapor deposition, such as thermal evaporation and sputter deposition. The choice of coating method is mainly depending on the desired coating material, for example, there are materials which are not suitable for the thermal evaporation.

The anti-reflective layers have high requirements on the uniformity of the layer thickness. For unevenly tempered glasses show gradients or even Newtonian rings and are therefore unsuitable for many applications. Hardened surfaces are also susceptible to contamination (fingerprints, traces of cleaning agent ), as they also represent a thin layer, thereby affecting the optical properties.

Applications

Lenses and lenses

Lenses with ten or more lenses, such as zoom lenses, would be without antireflection coatings practically not usable, because going through the reflections per lens about 8 % of the incident intensity is lost. But above all, can emerge along with the useful light from the lens and disturbing light spots ( reflections ) cause the picture or decrease as a diffuse veil the contrast of the image light after double reflection. At lens surfaces, there are pairs of surfaces, where this can occur, i.e., the effect increases with the square of surface number.

The reduction of the reflectance of the individual surfaces by a factor of reducing the intensity of the reflections by a factor, since the light is reflected twice more. The positive effect of the compensation that is used will also square.

In good camera lenses all air-glass surfaces are multicoated. In photographic lenses coated lenses are used since the 1930s, the multi-layer coating but has prevailed until the 1970s in high-quality lenses and today, except for very simple cameras and lenses standard.

Some optical materials for the infrared spectral region, such as thermal imaging cameras have a high refractive index, such as single crystal germanium, or zinc selenide, and therefore have high reflectivities on uncoated.

With lenses for photography, the type of remuneration in modern MC- layers has only a very small influence on the color reproduction, because the proportion of the more reflected light to the total energy of the radiation passing through is very low, and because grants, within a lens to combined so that the total reflection depends on all surfaces, only little on the wavelength. However, lenses, because of their specific total transmission quite "warmer " or " colder" draw something, but this is only in the Diafotografie of importance in practice usually. By changing the film thickness is affected by the dependence of the wavelength, wherein the color appearance of the lens front lenses formed.

For eyeglasses colored reflections are particularly undesirable. Therefore, there used broadband effective multi-coated optics. Particularly important are anti-glare glasses for driving in the dark.

High-performance optics

The materials used for coating generally have higher absorption than the materials of the optical components. The destruction threshold of an anti -reflective coating by thermal stress is, therefore, less than that of an uncoated interface in the rule. Is carried out at the beam Hochleistungslichtleitkabeln therefore before entering or after exiting the fiber first into a block of glass with the same refractive index, until he has a larger diameter. There, an interface may be, which can also be anti -reflection coated.

The damage threshold of coated and uncoated laser optics is the case of continuous radiation (CW ) with a maximum power density (eg watts per cm2 ) and pulsed radiation with a maximum energy density indicated (eg joules per cm2).

Continuously varying refractive index

A smooth transition of the refractive index reduces the reflectance without strong wavelength and angular dependence. However, a refractive index for the transition to n = 1 it is necessary close to 1. A team of researchers at Rensselaer Polytechnic Institute has developed a coating of silicon rods ( cf. Black silicon), which has a refractive index of only 1.05. The antireflection nanostructures on the surface is also called moth-eye effect.