A radial ERS
fixture has the lamp enter the reflector
housing along a radius of the ellipse as opposed to an axial
fixture where the lamp enters the reflector on axis of the ellipse. The radial fixture
was a long time standard, though the reflector is less efficient than an axial reflector. The most popular radial ERS was probably the Altman Cat#360-6x9
, pictured below.
Radial ERS fixtures are also occasionally called "top loaders" due to the positioning of the lamp cap
The reason that the lamp had to enter the reflector off axis is because incandescent
lamps of the period had a "burn position" specified by the manufacturer, due to the way the large and heavy filament
was supported. Lamps used in ERSs were "BU30," meaning "Base
Up, +/- 30° ". Thus when the fixture was hanging above and pointing down toward the stage
, the lamp ends up more or less vertical. Until the advent of T/H lamps and fixtures to take advantage of their properties (one of which was the ability to burn in
any position), all ERS fixtures were radial until about 1970. With the axial Altman 360Q having been introduced in 1974, the 360 line
was discontinued around 1991.