= Optical oscillator strength
{title2=$f_j$}
An <optical oscillator strength> is a dimensionless weight assigned to an optical resonance. In an independent-resonance <Lorentz oscillator model> using a reference <electron mass> $m_e$ and a specified participating <number density> $n$,
$$
\epsilon_r(\omega)=1+\frac{ne^2}{\epsilon_0m_e}
\sum_j\frac{f_j}{\omega_j^2-\omega^2-i\gamma_j\omega}.
$$
Its normalization must be stated: assigning one unit of total strength per participating <Electron> means $\sum_jf_j=1$. A one-resonance approximation with $f_1=1$ represents the whole chosen electron population, rather than asserting that the first allowed <interband transition> carries all optical strength. The smallest <band gap> therefore need not equal the fitted resonance energy.
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