Large rear-screen TV projection systems are typically enclosed in a large box. To reduce the volume of the box two mirror folds have been used. A new wide angle lens covering a 70-degree field has been developed which further reduces the volume, yet only requires one mirror. The design evolved from the earlier design types, having a relatively weak first group, a strong positive second group, and a negative field flattener. It is dependent upon the use of aspherical surfaces, having no analogous all-spherical type.
Ellis Betensky, Ellis Betensky,
"A New Wide-Angle TV Projection Lens Having Aspherical Surfaces", Proc. SPIE 0554, 1985 International Lens Design Conference, (14 February 1986); doi: 10.1117/12.949266; https://doi.org/10.1117/12.949266