Anti-reflective Lens Coatings - Pros and Cons

Anti-Reflective Coating: Are you missing out on a clearer picture?

Anti-reflective coatings reduce unwanted reflections from surfaces and are commonly used on spectacle, photographic and optical instrument lenses. Reflections are a problem for spectacle wearers. Power rings, which are particularly noticeable in high minus lenses, are produced by multiple reflections at the lens surfaces. If these internal and external reflections are reduced, then the light transmission is increased, therefore increasing clarity. The decreased reflection also reduces glare and 'ghost' images which are observed with night driving or working in front of the computer monitor, dramatically reducing eye fatigue. Cosmetically speaking, an antireflective coating allows a spectacle wearer's eye and face to be seen more clearly by the observer.

In the Beginning

The basic principle underlying an anti-reflective coat is called 'destructive interference'. Anti-reflective coatings are possible because light travels as a wave and we can put to the use the destructive interference technique. If the coating is a quarter wavelength thick, then the reflections from the front and the back of the coating layer will be out of phase with each other, therefore will cancel each other out and there will be no visible reflection.

For an anti-reflective coating to work, the refractive index of the coating material needs to be equal to the square root of the refractive lens material. Anti-reflective coatings are particularly suited to high index lenses.

Cons

One of the major complaints by spectacle wearers concerning anti-reflective coatings is that they are difficult to keep clean. However, they are unaware that without the coating, the unwanted reflection effectively masks any dirt, smears or smudges on the lens.

This is also the case for scratches which become more noticeable on an anti-reflective coated lens. The scratching is of course, made worse by the frequent cleaning, therefore it is important to not use a dry tissue or your shirt to clean the lenses, particularly if they are dusty.

Pros

Anti-reflective coatings today for the most part include an additional hydrophobic coating that repels water and grease. This will prevent water marking and fogging and by reducing static build-up makes them easier to clean.

Benefits of the 'new' Anti-reflective coatings include:

Easy to clean
Scratch resistant
Durability
Heat resistance
Visual Clarity
Cosmesis (lenses look invisible - like the TV presenters)
Improved light transmission aiding night driving