Reflective Coatings

Utilizing the phenomena of constructive and destructive interference, engineers may create a multitude of thin-film coatings with different reflective properties. A camera lens, for example, should reflect as little light as possible so that the maximum amount of light passes through the lens to the film. Because the lens (glass) has a higher index of refraction than the surrounding air, a thin film coating must be used to reduce the reflection. Observe the difference between a coated and uncoated lens:


Ideally, one could produce a surface with any desired reflectance. Using a material with index of refraction n = 1.2247, one could completely cut out the reflection from the above lens. Due to the limited availability and cost of materials, however, an exact match to the desired index of refraction is not always possible; magnesium fluoride has an index of refraciton n = 1.35 and can reduce the reflection to less than 1 %.

Set index of refraction:    for middle layer

Set film width:    Width of middle layer


For applications that require mirrors with very high reflectance (such as a laser mirror), several layers of coating may be used. Often, many layers of alternating indices of refraction may be used to increase the reflectance to more than 98%. In the following example, the mirror is made of alternating layers of zinc sulfide (n=2.3) and magnesium fluoride (n=1.35) film (For an excellent discussion of these and other coating methods see Fowles 100). Add layers and observe how the reflectance changes.



References:

Fowles, Grant R. Introduction to Modern Optics. 2nd ed. New York: Dover Publications, 1975.