Reflective Coatings

Thin dielectric films can be used to greatly reduce unwanted reflections at a surface.  A camera lens, for example, should reflect as little light as possible so that the maximum amount of light passes through. The lens (glass) has a higher index of refraction than the surrounding air, and reflects approximately 8 % of the light intensity.  A quarter wavelength thick  MgFl film reduces this reflection. Observe the difference between a coated and uncoated glass:


 

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 refraction n = 1.35 and can reduce the intensity reflection to less than 1 %.

Note: The table shows the electric field.  The intensity of the wave is n*E*E.

Set index of refraction:    of the antireflection layer

Set film width:    of the antireflection layer


References:

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