The reflection of an uncoated surface decreases the transmission of an optic. So windows need an anti-reflecting coating on both sides.
Optics with a filter coating on the front surface need an AR on the rear surface as well which is often forgotten in customer’s requests.
The example, right, shows the reflection of an uncoated surface depending on the refractive index of the substrate.
The values are calculated from the Fresnel equations with an angle of incidence AOI = 0° and against air, so the reflection becomes:
The values for the given materials refer to a wavelength of 1064 nm.
The impact of polarization changes with the angle of incidence. The diagram shows the reflection for different polarizations depending on the AOI for Fused Silica against air at 1064 nm.
Please notice that working with p-polarized light and the Brewster angle (arctan(n), for FS AOI = 55.4°) the reflection falls down to zero and no antireflective coating will be necessary. This effect is used for TFP rear surfaces.