||Following Thorn's pioneering role to introduce the first 70W high pressure sodium lamp to European markets in 1976, the company added a 50W lamp to the range in 1982, somewhat later than its competitors.
Thorn produced the vast majority of its 50W and 70W SON lamps in a pearl frosted ellipsoidal bulbs, whose photometric characteristics made them suitable for replacing low wattage mercury fluorescent lamps in similar luminaires. A market soon emerged for low wattage HPS lamps in clear envelopes to achieve better optical control for floodlighting and streetlighting applications, and at first this was catered for by offering clear ellipsoidal lamps. However the large diameter bulb limits the distance between the arc tube and the luminaire's reflector, which limits the optical efficiency of the system. This drawback was overcome following the introduction of tubular clear 70W lamps around 1980. Surprisingly, this simple adaptation was not extended to the 50W rating until 1987, and this lamp is representative of the first design.
With the earlier pearl lamps the exact position of the arc tube within the diffuse bulb was not critical, but for the new floodlighting applications, improved alignment of the arc tube in the small external reflector was desired. This was achieved in the first lamps with Thorn's rather complex 'Butterfly clip' top support. Expansion of the arc tube is accommodated within a ceramic bushing, which is fixed on the axis of the outer bulb by a 4-limb metal strap. Although technically excellent and showing outstanding vibration and impact resistance, this construction was rather expensive, and was simplified around 1990.