Nov 17 2007
Introduction
Relationship between Refractive Index and Anti-Reflective Coatings
Wave Mechanics and Anti-Reflective Coatings
Difference Between Single and Multiple Coatings for Anti-Reflective Coatings
Introduction
Anti-reflective coatings significantly reduce the loss of light. Anti-reflective coatings also reduce the amount of glare from lights. Anti-reflective coatings are made possible by using techniques such as changes in phases and the dependence of reflectivity on refraction index. Coatings found on microscopes and camera lenses for example are similar to anti-reflective coatings.
Anti-reflective coatings are made from multiple layers of metal oxides coated to the surfaces at the front and back of the lens.
Relationship between Refractive Index and Anti-Reflective Coatings
When there is a variation in the refractive index as a wave travels from one material to another, then reflection will happen. It is important to consider the refractive index of glass and the refractive index of air when manufacturing anti-reflective coatings. Hence not just any coating materials can be used as anti-reflective coatings.
If the refractive index of the anti-reflective coating is the same as glass, which is about 1.6, then the color ‘red’ will not be reflected and there will not be any anti-reflective effect. As a result, the behavior of the lens will be similar to as if it has been uncoated. On the other hand, if the refractive index of the anti-reflective coating is the same as air, which is 1.0, then the color ‘blue’ will not be reflected, and the lens will appear as it is uncoated.
The refractive index of an anti-reflective coating will therefore required to be in the middle between the refractive index of air and glass. This refractive index value will also make the blue and red reflection equal and can cancel one another.
The refractive index of a perfect single anti-reflective coating would be around 1.3.
Wave Mechanics and Anti-Reflective Coatings
The theory behind anti-reflection coatings is that the generation of a double interface by means of a thin film that can produce two reflected waves. If these waves are considered to be out of phase, then these waves can partially or totally cancel each other.
If the coating has a thickness of a quarter wavelength and the coating has a refractive index less that the glass that it is coating then the two reflections is out of phase by 180 degrees. Because of this, anti-reflective coatings are sometimes known as ‘quarter wave’ coatings.
Difference Between Single and Multiple Coatings for Anti-Reflective Coatings
The elimination of reflection at one wavelength can be made possible by a single layer coating with optimum index. Multi-layer coatings are significantly more efficient and the reduction of losses over the visible spectrum can be achieved.
A single coating must have a refractive index in the middle between the refractive index of glass and air. For multi-layer coating to produce similar effect, the material is required to have an even progression of refractive indices. For instance, if 5 layers of an anti-reflective coating is required, then 5 different materials with progressive refractive indices are required.
Single layer coating can handle the majority of the reflections, while multi-layer coating is a refinement to improve its effectiveness. The difference between single and multi-layer coatings becomes significant when there are a large number of reflective surfaces in the lens.
Source: AZoOptics