Fluorescent lamps are low pressure gas discharge lamps in which the invisible UV radiation generated by the discharge is converted into visible radiation (i.e. light) with the aid of phosphors. (Principle of fluorescence)
 
There are tubular, ring-shaped and U-shaped fluorescent lamps and also compact fluorescent lamps. The tube diameter is often expressed in eighths of an inch (e.g. T5 = 5/8'' = 16 mm). In the lamp catalogues, diameters are also given in millimetres (e.g. 16 mm for T5 lamps). Most lamps are internationally standardized.
 
Like all discharge lamps, fluorescent lamps cannot be operated directly on mains voltage because of their negative internal resistance characteristics. Suitable control gear between the mains supply and the lamp, limits and control the lamp current and ensures reliable starting under specific conditions. Fluorescent lamps have various operating modes, depending on the way in which the electrodes are brought up to the temperature:

• Current-controlled preheating in switch-start mode, preferred in countries with a high mains voltage (200 V or more). Used in most electronic control gear (ECG).
• Voltage-controlled preheating with additional transformer for "rapid start" mode.
• No preheating (cold start, used for example with slimline lamps). This type of starting reduces the lamp life more than any other type and is therefore not recommended for systems with frequent on/off switching.
• Electronic control gear (ECG) converts the mains frequency into a high frequency supply in the region of 35 to 50 kHz. As a result, the 100 Hz flicker that may appear as a stroboscopic effect, for example with rotating machines, is much less noticeable or virtually eliminated.

Another advantage of ECG operation is the additional energy savings of around 25% for the same luminous flux, comprising:

• 10% high luminous efficacy from the fluorescent lamp operating at a high frequency
• Much lower losses in the ECG (less than half) compared with conventional control gear (CCG)

Dimming

Dimmable ECGs make use of the property of a choke whereby its inductance increases as the frequency increases. As the operating frequency is made to increase, the choke connected in series with the lamp supplies a progressively lower current. The frequency is controlled via a 1-10 V or a DALI interface. The dimmable ECGs must also ensure that in the dimmed state the cathodes are constantly heated so that they continue to emit electrons even when the lamp current is at a low level.

Lamp life and resistance to switching transients

If fluorescent lamps are operated with CCGs and conventional starters their life is shortened considerably as the frequency of on/off switching increases. The same phenomenon can be observed with cold-start ECGs, which have the ability of starting fluorescent lamps instantly. However, the rapid transition from glow discharge temperature to emission temperature seriously damages the cathodes, so frequent switching cycles reduce the life of fluorescent lamps. Warm start control gear behave in a completely different way. In this case, the cathodes are heated before ignition, which virtually eliminates switching damage to the cathodes. The associated delay in ignition, of around a second is not a serious inconvenience.

Thermal behaviour

The physical properties of fluorescent lamps depend on their ambient temperature. This in turn depends on the characteristic temperature sensitivity of the mercury vapour pressure in the lamp. At low temperatures it is too low so there are too few atoms that can be excited. At excessively high temperatures the high vapour pressure results increasingly in self-absorption of the generated UV radiation. The lamps achieve their maximum luminous output at an ambient temperature of approx. 25°C. In the case of T5 lamps with a tube diameter of 16mm (FH, FQ) the rated luminous flux is specified at 25°C, as with other fluorescent lamps but their maximum luminous flux is achieved at an ambient temperature of 33°C to 37°C. In other words, one advantage of T5 lamps is the higher light output from ‘hot’ compact luminaires.