A hyperfine filter for the 3776 Å thallium resonance line

A magnetic hyperfine filter for the resonance line ²P_{$\text{1}\text{2}$} → ²S_{$\text{1}\text{2}$} of thallium is described. This line is a hyperfine triplet and this filter transmits only the 0→1 component connecting the lower sublevel of the ²P_{$\text{1}\text{2}$} ground state. This filter may thus be used for obtaining a population inversion in the ground state of Tl.