How Do AR (Anti-reflective) Coatings Work?
The Problem: No Time for Reflection
As light passes through a lens from air, it experiences a change in index of refraction. When that occurs, some of the incident light is transmitted through the lens medium and refracted while some of the light is reflected. This reflected light is perceived by others as glare and represents a loss of light transmitted through to the eye.
As the refracted light continues through the lens material and reaches the back surface of the lens, there is another index change (lens to air) and again refraction and reflection occur. Reflected light here can bounce off the internal surfaces of the lens and be seen by the wearer as glare, blurred or ghost images. Others may see internal reflections as multiple rings inside the lens (most prevalent in high minus powers). Blurred or ghost images can become intensified at night around bright lights common in dusk or night time driving conditions, and can significantly impair vision. Also, this backside reflection represents further loss of light transmitted through to the eye.
Light coming from the back of the wearer to the back surface of a lens will also undergo a certain amount of reflection. Light here can be reflected directly back to the eye. The results can be a distraction to the wearer or can, in certain conditions, impair vision. For example, bright sun light hitting the back surface of a sun lens that is not AR coated, depending on the angle, can either be reflected directly back into the eye or can "fill" the lens with reflected light. Either case can result in significant vision impairment.
AR coating can minimize front and back lens surface reflections, significantly reducing or eliminating the problems discussed above, reducing eye strain, while allowing more light to reach the eye, improving contrast and clarity.
The Solution: Fight Reflections with Reflections
AR coatings reduce lens surface reflections by actually generating reflections of its own. The index of refraction of the AR layer is in between that of the lens medium and that of air. Light incident upon an AR coated lens experiences reflection at both the AR layer and the surface of the lens. However, the thickness of the AR layer is such that the light waves reflected from the AR surface are 180° out of phase with light waves reflected from the surface of the lens. Consequently, the reflected light waves undergo destructive interference and effectively cancel each other. The Law of Conservation of Energy states that energy can neither be created nor destroyed. So, what happens to the energy from the cancelled light waves? It is transferred through the lens medium to the patient's eyes improving contrast and clarity!