In the past several years important developments have been made in the construction and theory of birefringent filters. Tunable filters that operate from 4500-7000 Å with full widths at half maximum of .05 Å and maximum transmissions of almost 20 percent have been constructed. Inexpensive large aperture (≥15 cm) achromatic waveplates have been fabricated that allow full use of the resolution-solid angle products 102 that of Fabry-Perots and 104 that of grating systems have been constructed.
David L. KingSusan P. Worswick