||For many years after the introduction of the enhanced MBF mercury lamp having a fluorescent outer bulb, a small market still remained for the original style which was produced without the colour-correcting fluorescent coating. At the time of the introduction of the MBF lamp this was understandable because the phosphor-coated types were significantly more expensive than their uncoated predecessors. However, this particular lamp was made in 1978, and for many years leading up to this both lamps had been offered at the same price.
The principal reason for the continued use of MB lamps was that they worked better in the optical systems of many lanterns. The higher wattage types were made in clear tubular bulbs and the arc tube could be precisely located at the focal point of the reflector. Lower wattage types were offered in Pearl bulbs only, because the glare from such a small and intense arc tube was unacceptably high. Despite the diffuse glass, the central 'hot spot' of light emission was still quite small. With the fluorescent coated lamps, the entire bulb surface was the point of light emission and the large source area meant that the light was not so easy to control in optical systems. Light from a lantern would be thrown out at a much wider angle and although a greater area was illuminated, the intensity at any given point was considerably lower. Because this was unacceptable in many cases, the uncoated MB lamps continued to be offered.
While Mazda and Philips offered Pearl MB bulbs in the ellipsoidal bulb shape, GEC continued to make them to the original design in round bulbs. This ensured that the light centre length was maintained at the correct position.