Transmission & Anti-Reflection Coatings

All windows and viewports suffer Fresnel reflectivity losses
 

    Reflectivity  r = [(n-1)/(n+1)]²   
                                                                                         

^{(where n is the refractive index)}

which can be on the order of 6% per face for high refractive index windows such as Sapphire. The reflection is different for p and s polarised waves, and is a function of angle.

This problem can be a severe limitation in many applications requiring optical fidelity, viewing of low luminosity against a high ambient, power transmission etc.

<- However the problem can be mitigated using anti-reflection coatings. In the case of Sapphire, due to a fortuitous relationship between the refractive indices, a considerable improvement can be obtained using a simple single QWOT (Quarter Wave Optical Thickness) of MgF₂ as shown below.

<- For more conventional viewport materials such as Kodial and fused quartz this simple approach would provide less obvious, but still useful benefit.

More complex multi-layer dielectric coatings enable a wide variety of tailored transmission / reflection characteristics giving for example virtually zero reflection at a single design wavelength (‘V’ coating) or at two wavelengths (‘W’ coating), or over a wide band BBAR or a neutral density attenuation, and many others.

<- A 4 layer Broad Band AR Coating – optimised typically but not exclusively for the visible range and giving the best general performance, with a reflected luminosity of only 0.13% and transmitted luminosity of 99.86%

<- For applications requiring the best transmission at a single wavelength, such as high power laser applications, TSL can provide a ‘V’ Coating having just about zero reflection at a design wavelength 550, 1064 nm etc. for example

TSL can also provide more exotic coated viewports such as MgF2, CaF2, ZnSe, with coatings to provide optimum transmission in the Infrared or UV end of the spectrum