3.4.1 Lighting

Luminescent lighting, commonly called fluorescent lighting, has been in regular use for decades since it was first introduced in the 1930s. Its dominance in the illumination of offices, department stores and public spaces arises from the fact that it has a much higher efficiency than incandescent lighting. For instance, a 60 W incandescent light tube generates only 15 lm W ^?1 whilst a 40 W luminescent light bulb yields 80 lm W ^?1.14

Luminescent light is generated using ultraviolet radiation from a low-pressure mercury discharge, the mercury vapour being present in the noble gas, which fills the tube under low pressure. The emitted radiation consists of 85% at 254 nm and 12% at 185 nm, the remainder being mainly in the visible region. The lamp phosphors, which coat the inside of the tube, convert the 254 nm and 185 nm light into visible light (Figure 3.6). The light emitted must be white light in order to match the ambient natural lighting from the sun. Since the sun is a black body radiator, a terminology has been developed for the lighting...