GE Lamps Brochure page 97

Biplane or Monoplane?
The filament format of a lamp will have an effect on the beam performance of a luminaire.
In Fresnel optics a biplane filament will, due to its smaller area, produce a narrower spot of slightly
increased peak intensity, compared to an equivalent monoplane filament. However, in intermediate and
flood positions better light collection is obtained from a monoplane source, as the additional light
collected by the rear mirror is largely obscured with a biplane source. A wider angle beam for a given
intensity is thus provided by a monoplane filament.
Ellipsoidal optics are designed around a specific filament area. Larger areas will allow some of the light
to fall outside the gate and be lost. A smaller filament area will concentrate the light on the centre of
the gate producing a hot spot. The choice between a monoplane and a smaller equivalent biplane is,
therefore, dependent on luminaire design and customer preference.
GE pursue a policy of allowing the customers to make this choice by offering both monoplane and
biplane versions of relevant lamp types.
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