If very low reflection is needed, but at only one specific
wavelength, v-coating, a two-layer anti-reflective coating, is
often the best solution. By using two layers with contrasting
indices of refraction, it is possible to reduce the reflection at a
specific wavelength to near zero. A drawback of this technique
is that it actually increases reflection at wavelengths other than
that for which the coating is optimized (evident on figure 2). If
the actual goal is to minimize reflections at multiple
wavelengths, v-coating will not produce the desired result.