Low-pressure discharge lamps

Fluorescent lamps are low-pressure mercury discharge lamps. They have been used for many decades due to their high luminous efficacy and service life and cover a high percentage of our light requirement (ca. 70%) in existing installations. Regarding newer-generation tri-phosphor fluorescent lamps, the drop in luminous flux is about 8% after 10,000 operating hours, and about 12% after 20,000 operating hours when using an electronic control gear unit ECG (see chapter “Discharge lamp luminaire operation”). Since the late 1990s, more efficient tri-phosphor fluorescent lamps with a diameter of 16 mm (T5 lamps) have also been used besides main-series 18 W, 36 W and 58 W fluorescent lamps with a diameter of 26 mm (T8 lamps) in many applications. Their advantages are higher luminous efficacy, a smaller drop in luminous flux during the operating period, their smaller diameter and ideal luminous flux at ambient temperatures between 35°C and 38°C as they usually occur in indoor luminaires. These lamps can only be operated with ECG.

Fluorescent substances convert the UV radiation generated through low-pressure mercury vapour discharge into visible light. These substances consist of “rare-earth” group elements.

The fluorescent lamps’ luminous flux is highly dependent on the ambient temperature (see figure). The maximum luminous flux of T5 lamps is achieved between 34°C to 38°C, meaning the temperature range inside luminaires. Due to international standards, however, the (lower) lamp luminous flux for 25°C is specified. Therefore, light output ratios turn out higher. Examples for differences in luminous flux are shown in table  for T5 lamps with light colour 840.

Compact fluorescent lamps are smaller-construction tri-phosphor fluorescent lamps. Lamps in lower wattage ranges from 5 W to e.g. 23 W with integrated electronic control gear unit and base E14 or compact fluorescent lamps with plug-in bases require a separate magnetic or electronic control gear unit which is integrated in the luminaire.

Instead of tubular fluorescent lamps, more powerful compact fluorescent lamps, e.g. 18 W, 24 W, 36 W, 40 W and 55 W, are operated in compact round, square or rectangular luminaires which provide additional opportunities in terms of the lighting installation’s architectural design due to their design.

Low-pressure sodium lamps lamps create a monochromatic yellow light colour at very high luminous efficacies (up to about 180 lm/W) at which colours are virtually no longer perceived. Therefore, it is not permitted to use them for indoor lighting according to EN 12464-1 [48]. They were used e.g. for illumination of locks, harbour installations, object protection and places where colour recognition is not important.

Induction lamps are low-pressure mercury vapour lamps, similar to fluorescent lamps but without electrodes. The UV radiation generated through the discharge is also converted into visible radiation by fluorescent substances. Gas discharge is triggered by coupled-in high-frequency electric or magnetic fields. Due to high frequencies, special protective measures are required regarding electromagnetic compatibility (EMC) which are defined in international standards.