  The nature of atomic vapors, their natural alignment with interatomic
transitions, and their ease of use make them highly suited for spectrally
narrow-banded optical filters. Atomic filters come in two flavors: a filter
based on the absorption of light by the Doppler broadened atomic vapor, i.e., a
notch filter, and a bandpass filter based on the transmission of resonant light
caused by the Faraday effect. The notch filter uses the absorption of resonant
photons to filter out a small spectral band around the atomic transition. The
off-resonant part of the spectrum is fully transmitted. Atomic vapors based on
the Faraday effect allow for suppression of the detuned spectral fraction.
Transmission of light originates from the magnetically induced rotation of
linear polarized light close to an atomic resonance. This filter constellation
allows selective acceptance of specific light frequencies. In this manuscript,
we discuss these two types of filters and elucidate the specialties of atomic
line filters. We also present a practical guide on building such filter setups
from scratch and discuss an approach to achieve an almost perfect atomic
spectrum backed by theoretical calculations.
