Purpose of Optical Coatings:
When light passes from one medium to another, such as from air to glass, some of the light is transmitted and some is reflected. The amount that is reflected is determined by the difference in the refractive index between the materials and by the angle of incidence of the incoming light.
For a light beam in air striking uncoated glass at normal incidence, the strength of the reflection is typically just a few percent of incident optical power. For example, in the case of the common optical glass BK7, which has a refractive index of 1.52, the reflection is 4.25% at each air-glass boundary.
Although this may not seem like much, for optical designs using more than a few components, losses in transmitted light can rapidly multiply. For BK7, half the signal is lost passing through 8 elements (16 surfaces) For germanium, a high-index substrate, more than half the signal is lost passing through a single element. The lost signal is not absorbed, but reflected, so light can appear at unexpected locations in the system. In imaging equipment, these reflections can cause a significant loss in image contrast as ghost images, since the reflections may get superimposed on the primary image.
How do optical coatings work?
The optical properties of a coating are governed by optical interference between the reflections from the upper and lower surfaces of the film. With a stack of thin films, reflections from each of the layers need to be considered to understand the coating’s optical characteristics.
A simple anti-reflection coating can be made by choosing a film of a certain thickness and refractive index so that the reflections from its upper and lower surfaces are out of phase and will interfere destructively. In reality, this coating is often made from a single film with an optical thickness equal to a quarter of the incident light's wavelength. These “quarter-wave” films are a common building block in many coating designs.
Optical coatings generally fall into three main spectral regions:
- Ultraviolet (UV)
- Visible and near infrared (VIS-NIR)
- Infrared (IR)