Single Anti Reflection Coating

The simplest interference AR coating consists of a single quarter-wave layer of transparent material whose refractive index is the square root of the substrate's refractive index; this, theoretically, gives zero reflectance at the center wavelength and decreased reflectance for wavelengths in a broad band around the center.

The most common type of optical glass is crown glass, which has an index of refraction of about 1.52. An optimum single layer coating would have to be made of a material with an index equal to about 1.23. Unfortunately, there is no material with such an index that has good physical properties for an optical coating. The closest 'good' materials available are magnesium fluoride, MgF2 (with an index of 1.38), and fluoropolymers (which can have indices as low as 1.30, but are more difficult to apply). MgF2, on a crown glass surface, and bare glass give reflectances of about 1% and 4%, respectively. MgF2 coatings perform much better on higher-index glasses, especially those with index of refraction close to 1.9. MgF2 coatings are commonly used because they are cheap, and when they are designed for a wavelength in the middle of the visible band they give reasonably good anti-reflection over the entire band.
 
Substrate Substrate
fused silica, BK7, Broad float and so on
Surface Figure
<λ/10 @ 632.8nm
Surface Quality
40-20
Chamfer
0.25mm* 45deg
Coating Material
Electron beam deposited dielectric multi-layers
Incident
0deg, 45deg
Clear Aperture
> Central 90% of diameter
 
Wavelength (nm)
Resldual Reflectance
Wavelength (nm)
Resldual Reflectance
266
>99.5%
355
>99.5%
650
>99.7%
1064
>99.7%
400-700
>99%
420-680
>99%
650-1050
>99%
1050-1620
>99%