Sodium vapour lamps: Efficient specialists
Sodium vapour lamps are another member of the discharge lamp family and are distinguished by their very high luminous efficacy. They operate very efficiently and are used predominantly for outdoor lighting; types with improved colour rendering properties are increasingly installed in production facilities and sales premises.
The gas discharge in a sodium vapour lamp produces visible, yellow light. Being virtually monochromatic, it permits very limited colour vision but relatively good contrast vision. Sodium vapour lamps are available as high-pressure and low-pressure models. They differ in the design of their tube.
High-pressure sodium vapour lamps: marked improvement in colour rendering
High-pressure sodium vapour lamps score points with a very high luminous efficacy rating of 130 lm/W and a long life of around 16,000 hours. They consume only half as much energy as high-pressure mercury vapour lamps. Another major point in their favour is their particularly warm-white light with no UV content. One long-standing disadvantage used to be their poor colour rendering (Ra ≤ 39), which is why they were formerly used only for outdoor lighting.
For a number of years, however, types with a substantially improved colour rendering index of Ra 80 to 85 have been available. These are also suitable for indoor use, e.g. in production facilities or as shop lighting. The better colour rendering is achieved at the expense of luminous efficacy, although this is still high at 50 lm/W. High-pressure sodium vapour lamps require appropriate ballasts and igniters.
Low-pressure sodium vapour lamps: maximum luminous efficacy
Low-pressure sodium vapour lamps have the highest luminous efficacy of all artificial light sources: up to 178 lm/W. On the down side, their colour rendering properties are very poor. Colour vision is not possible.
Because their monochromatic yellow light is good at penetrating fog and mist, these lamps are used for port, tunnel and security lighting.
Spent discharge lamps must not be disposed of as household waste. They require professional disposal.
