How does neon light work

_How does a fluorescent lamp work_ in detail

3 back

 

In these explanations we deal with discharge lamps, in particular with the two fluorescent lamps:
- The Fluorescent tube (FL tube) with low voltage
- The Neon tube with high voltage

 

The principle of the discharge lamp

As in the case of an incandescent lamp, the electric current is conducted through a metal wire, in the case of a discharge lamp this is done by a gas or metal vapor, which must be conductive. In their normal state, gases and vapors are non-conductive, but by nature there are always a small number of charged particles (free electrons) in the gas. Under the influence of the electrical voltage, these drifting electrons start moving rapidly. The gas or vapor still contains large amounts of bound electrons in the flask, but these are not readily available. The attraction of the positive atomic nucleus binds the negative electrodes in the atom.

 

The ionization

When the bond between atomic nucleus and electron is broken, electrons are released and function as conductors of electrical current. This happens through collision with the rapidly moving free electrons. The electrons released in this way also get high speed and collide with other atoms so that the process spreads quickly (electron avalanche). An atomic nucleus that has been deprived of an electron is called an ion, the separation itself is called ionization.
The number of free electrons gradually increases and with it the conductivity of the gas. On the other hand, under the influence of the greater conductivity of the gas, the current strength increases even more. This interaction causes the ionization to continue at an accelerated pace. Without an upstream current stabilizer or current limiter (transformer, ballast), the ionization would continue indefinitely; the consequence of this would be an avalanche of electrons and the rapid destruction of the lamp.

 

The light generation

However, the splitting into atomic nucleus and electron (ionization) does not yet generate light. The collision between a free electron and an atom at rest is not always so violent that an electron is torn loose again. However, without ionization occurring, electrons can be thrown out of their normal orbit around the atomic nucleus.
Every collision is paired with a transfer of energy. The ionizing collision gives the electron energy to free itself from the atomic system. If an electron is not released, but pushed into an outer orbit further away from the nucleus, it is said that the atom has reached a higher energy level. But the electron immediately falls back into its original path and releases the energy absorbed during the collision, in the form of radiation energy.
If this radiation energy falls into the visible area, we perceive it as light. The result of this second type of collision is called the excitation of the gas.

 

The light color

One can think of an atom as a solar system in miniature, with the nucleus representing the sun and the electrodes representing the planets. The planets are forced by the gravitational pull of the core to move in certain orbits around the core. You cannot stand between these lanes. Depending on the energy gained during a collision, the electron is pushed in orbits that follow one another in steps. Now the size of the rebound and the original path determine the wavelength of the radiation, or in other words, the color of the light. The two most important phenomena in a discharge lamp are:

1. The conduction of gas or vapor by ionization;

2. The transition of the electrons bound to the atomic nucleus into orbits further out,

The kinetic energy of the fine electrodes is finally converted into radiation energy, which is called the excitation of the gas. The "size" of the radiation energy determines the wavelength of the radiation or the color of the light. The "size" of the radiation energy generally depends on the particular noble gas mixture.

Neon (gas) itself only produces a bright orange-red light; By filling the tubes with other noble gases as an alternative, additional color tones can be created: Argon has a bluish glow, krypton purple, and helium and xenon give off a white light. For financial reasons, however, neon and, above all, argon are still used almost exclusively as gases today, with all other color variations being achieved using additional fluorescent materials (fluorescent powder) or colored glass tubes.

 

The construction of a discharge lamp

A discharge lamp consists of a hermetically sealed glass tube, at the ends of which power supply lines, the electrodes, are melted down. A gas suitable for the discharge (neon, argon, helium, etc.) or a small amount of metal (mercury, sodium) is placed in the tube or lamp bulb, which evaporates at the beginning of the discharge. If it is a lamp for direct current, the negative electrode is called the cathode and the positive electrode is called the anode. In AC lamps, the electrodes are designed in such a way that they serve alternately as cathode and anode.
The dimensions of the lamp are now based on the distance between the electrodes, on the amount of gas or metal and their pressure, and on the electrical voltage on the electrodes.

 

The stabilization of the discharge (use of ballasts and transformers)

It has already been mentioned that the ionization would continue indefinitely and the current intensity would become excessively large if the state desired for constant light excitation was not stabilized. For this purpose, the electrical current must be limited and a fluorescent lamp must always be equipped with a device to limit the current intensity (ballast, transformer).
According to Ohm's law, the strength of a current flowing through a conductor can only increase if the voltage increases at the same time. A conductor that behaves according to Ohm's law has a positive resistance characteristic. It can now be seen that a discharge lamp has a negative characteristic, i.e. that the voltage decreases with increasing current intensity. To stabilize the current, it is therefore necessary to connect the lamp in series with a suitable resistor so that the negative characteristic is converted into a positive one.
An ohmic resistor could be used for this. However, such a resistor takes up so much energy that the advantage of the high luminous efficacy of the lamps could be largely negated. That is why an inductive resistor, e.g. a choke coil, is used for this purpose. The energy losses are then very low.

 

The fluorescence

A phosphor has the property of converting radiation of a certain wavelength into radiation of greater wavelength (never the other way around); one could therefore call it a wavelength transformer. The discharge of some lamps filled with mercury vapors delivers not only visible but also invisible ultraviolet radiation, which, however, is absorbed by the glass of the lamp bulb, i.e. does not escape to the outside. This invisible radiation can now be used for lighting by converting it into visible radiation through a fluorescent layer (fluorescent powder).
For this purpose, the phosphor is applied to the inside of the lamp bulb. The type of phosphor determines the color of the radiation converted into light. With the use of suitable phosphors, light of any color composition can be achieved.

 

The fluorescent tube (fluorescent lamp for low voltage)

For electrical, light and technical reasons, the standard lengths of the most commonly used FL tubes are 60, 100, 120 and 150 cm. These lamps are produced in a wide variety of colors, but mainly in daylight, white and warm white. By far the most common and most economical fluorescent lamp for low voltage is the 40 W lamp. About 60% of all fluorescent lamps manufactured in the world are of this type. This lamp has an internationally standardized length of 120 cm with a tube diameter of 38 mm.
The fluorescent powder is evenly distributed over the inner wall of the FL tube. This gives the lamp the appearance of a frosted glass tube. The luminance of the FL tube is relatively low and is even below the glare threshold. The luminous efficacy of the fluorescent lamp for low voltage gradually decreases with the service life. Characteristic is the decrease in light during the first 100 burning hours of around 10%. With this in mind, the nominal luminous flux of the tube is understood to mean the luminous flux after 100 burning hours. In general, the luminous flux of these lamps is guaranteed at 75% of the nominal luminous flux after 7500 burning hours.
The service life of the FL tube can be viewed as the period of time during which the lamp still has a satisfactory light output. With prolonged use, the lamp becomes, so to speak, too expensive to consume in relation to the amount of light it still emits. Then it is more advantageous to replace the old lamp with a new one.

 

"Neon tube" (fluorescent lamp for high voltage)

Over the years the term "neon tube" has become the collective name for the fluorescent lamps to be connected to the high voltage, regardless of whether the tubes are actually filled with neon or another gas.
The light color of the neon tube depends mainly on the type of gas or metal vapor used and on the color of the glass; in the case of fluorescent high-voltage tubes, it also depends on the type of powder in the tube. Since the light output of the high-voltage tubes is quite high, they are not only used as illuminated advertising, but also to illuminate various rooms. Especially when the correct illumination requires a deformation of the tube.
Neon tubes are connected in series to a high voltage transformer. They can also be connected piece by piece to a transformer that is calculated for the ignition voltage of a tube length.
A distinction is made between neon tubes, among other things, between those without fluorescent powder (non-fluorescent) and with fluorescent powder (fluorescent tubes). The light output and also the number of possible color variations are greater with the fluorescent tubes than with the usual high-voltage tubes. Furthermore, the luminous efficacy of both types of neon tubes depends on the current strength and the tube diameter. In general, doubling the current intensity means doubling the luminous flux for the same tube diameter. If the electrical current remains the same, increasing the tube diameter means reducing the luminous flux, because the arc voltage is then lower and with it the power consumed in watts.

 

Advantages and disadvantages of comparing fluorescent tubes and neon tubes

High-voltage tubes have the advantage that they have a long service life (approx. 20,000 hours depending on the color); they ignite directly and without a starter. Their disadvantages are that a high-voltage system is required. The installation costs are considerably higher compared to the FL tube, but the operating costs are significantly lower than with the low-voltage fluorescent lamps.