fluorescent lamps (original) (raw)

Definition: lamps which emit fluorescent light, usually generated by irradiation of a phosphor with light from an electric gas discharge

Alternative terms: fluorescent lights, fluorescent bulbs

Category: article belongs to category non-laser light sources non-laser light sources

Related: gas discharge lampsmercury vapor lampsfluorescencephosphorscolor rendering indexlight-emitting diodesincandescent lampshalogen lamps

Page views in 12 months: 623

DOI: 10.61835/qg3 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

📦 For purchasing fluorescent lamps, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What is a Fluorescent Lamp?

Fluorescent lamps are devices used for illumination purposes which emit fluorescent light. That fluorescence occurs in some phosphor (fluorescent material), which is usually excited by ultraviolet light from an electric discharge in some gas, usually in mercury vapor. Such lamps usually emit white light.

Mercury Discharge Lamps

The most common fluorescent lamps contain mercury vapor (mixed with argon or xenon) inside a glass tube (fluorescent tube) with a length between 20 cm and 2.5 m and a diameter between 1 cm and a few centimeters. As in other mercury vapor lamps, an electric discharge excites the mercury atoms via inelastic scattering of electrons: Electrons are accelerated by the electric field in the tube, and during a collision of an electron with a mercury atom, part of the kinetic energy of an electron is converted into excitation energy of the mercury atom. Thereafter, the mercury atom can emit light, mostly in the ultraviolet spectral region at wavelengths of 254 nm and 185 nm. This part of the process is called cathodoluminescence (→ luminescence).

Subsequently, a fluorescent coating (called the phosphor) on the inner surface of the tube absorbs the ultraviolet light and converts it into fluorescent light, mostly in the visible spectral region. (Any remaining ultraviolet light is normally absorbed in the glass tube.) The phosphor contains several active (light-emitting) substances, which are mixed in such a way that the overall emission spectrum results in the perception of white light.

Color Tones and Color Rendering

The emission spectrum of any fluorescent lamp, as recorded with an optical spectrum analyzer (e.g. a spectrograph), can easily be distinguished from that of an incandescent lamp: It is much more structured, with a high power spectral density in some spectral regions and much lower values at other wavelengths. Nevertheless, the visual impression for the human eye can be similar to daylight. This is essentially because the eye has only three different kinds of photoreceptors for distinguishing colors; only the relative excitation levels for these kinds of photoreceptors matter for the color impression. In addition, the human visual sense quickly adapts its color calibration according to the spectrum of the ambient light; for that reason, even strong changes in the spectrum of daylight, e.g. between noon and late afternoon, are hardly perceived. Only when the change in color tone is rapid, it is easily noticeable.

By varying the composition of the phosphor, different color tones can be obtained. A “warm” (soft) color tone is often preferred for residential lighting; it mimics the warmer color tone of sunlight in the evening, which can contribute to a relaxing atmosphere. On the other hand, light with a “cold” color tone (with a higher color temperature) is more suitable for, e.g., offices; it better matches sunlight at noon and supports concentrated work.

Even when the color tone is optimized for the application, the non-uniform optical spectrum of a fluorescent lamp may somewhat modify the color appearance of objects which it illuminates. This is essentially because the optical properties of an object are probed primarily in the wavelength regions of strong emission, whereas the properties in some regions with weak emission do not contribute substantially to the overall visual impression. Such effects can be quantified with a color rendering index (CRI). Some fluorescent lamps have a low color rendering index and are therefore not suitable when subtle color differences can matter, e.g. in a photo studio. However, this depends strongly on the composition of the phosphor; many modern lamps using a triphosphor mixture containing europium and terbium can achieve a very high color rendering index. Various marketing terms such as “natural full spectrum”, “high definition” and “daylight” are used for lamps with a high color rendering index.

Electrical Aspects

The electric discharge occurs between two tungsten filaments. In most fluorescent lamps, these are coated with various oxides favoring thermionic emission. In the start-up phase, the electrodes are heated with a stronger than normal current, and the discharge is started with a high-voltage spike. Once the gas is ionized, its electrical conductivity becomes substantial, and the voltage required for sustaining the discharge drops to well below 200 V. The electric current needs to be stabilized with some electric ballast, as a higher current in the gas discharge would reduce the resistance, allowing the current to rise further.

We first describe a simple kind of circuit, which was very dominant for many years, while more recently more sophisticated electronic solutions are common. Such older fluorescent lamps usually use an inductor coil as the ballast, which is possible for operation with the usual AC current. The inductor also serves to provide the high-voltage spike for start-up. During that phase, the gas discharge is electrically bypassed, causing a relatively large current to flow through the electrodes and the inductor. After a few seconds, the electric bypass is removed, and the sudden drop in current causes the inductor to produce the voltage spike which starts the discharge. In the steady state, the current is much lower than in the start-up phase, so that the heating of the electrodes is weak. The start-up process is controlled by a small starter (automatic starting switch), containing a small glow tube and a bimetallic switch. See Figure 1 for a typical electrical circuit.

circuit for operating a fluorescent lamp

Figure 1: Layout of a typical circuit for operating fluorescent lamps.

Instead of a simple inductor, a more sophisticated electronic circuit is used in many modern fluorescent lamps as a ballast and for start-up. This can have several advantages:

On the other hand, some people are concerned about generated electrosmog (see below).

Dimming (reducing the brightness) is possible only with special electronic devices. Electronic dimmers made for incandescent lamps are not suitable for dimming fluorescent lamps.

There are so-called cold cathode fluorescent lamps (CCFL), where thermionic emission of the cathode is not used: the electrodes have no oxide coating, and the operation temperature is below the thermionic emission temperature, even though it may not be low. One accepts a higher voltage drop at the cathode, which somewhat reduces the power efficiency. Advantages of this concept are that the lifetime can be higher, and that instant on-off switching is possible.

Compact Fluorescent Lamps

compact fluorescent lamp

Figure 2: Compact fluorescent lamp with E27 socket as used in central Europe.

Conventional straight or curved fluorescence tubes are often too large to replace incandescent lamps. Therefore, compact fluorescent lamps (fluorescent light bulbs) were developed which have a size comparable to an incandescent lamp, and can be used in the same type of light fixture. That compact form was made possible by developing relatively small tubes (sometimes in a wound form) and compact electronic ballasts, which also enhance the power efficiency and reduce flicker. There are also non-integrated compact lamps, used with a separate electronic ballast, so that the lamp can be exchanged after its lifespan, and the long-life electronic ballast can be reused.

Meanwhile, compact fluorescent lamps have largely been replaced by lamps based on light-emitting diodes (LEDs), because these are still far more compact, even more efficient, and avoid the mercury toxicity problem.

Lifetime

The lifetime of a fluorescent lamp can easily exceed 10 000 hours when it is operated during long time intervals. Frequent switching on and off can substantially reduce the device lifetime, unless an optimized electronic ballast is used, which makes such lamps much less vulnerable to switching.

Temperature Influence

Fluorescent lamps can be operated in a wide range of temperatures. However, their light output can be significantly diminished in cold environments. For such use, one may apply a housing with reduced thermal dissipation, which at least for longer operating times increases the lamp temperature.

Energy Efficiency and Environmental Aspects

Although part of the energy of the ultraviolet light from the mercury emission is lost in the phosphor, fluorescence lamps are still several times more energy-efficient than incandescent lamps. Instead of quoting the energy conversion efficiency, it is more sensible to consider the luminous efficacy, which takes into account the wavelength-dependent sensitivity of the eye. Luminous efficacies of fluorescent lamps can reach the order of 100 lumens per watt. (When a high color rendering index is needed, somewhat lower efficacies may have to be accepted.) This can be compared with roughly 10–20 lm/W for incandescent bulbs (where higher values apply to higher-power devices) and somewhat over 20 lm/W for halogen lamps, while LED-based lamps can reach values far over 100 lm/W. The use of a fluorescent lamp instead of an incandescent bulb can thus easily reduce the electric power consumption for a given luminosity by 80% compared with an incandescent lamp, although an LED lamp can be still somewhat better. The savings in energy costs can be a multiple of the increased price of the lamp, and the reduced energy consumption is associated with reduced pollution and reduced emission of greenhouse gases (carbon dioxide) from power plants.

On the other hand, the energy consumption for producing a fluorescent lamp is several times higher than that for an incandescent lamp. However, that amount of energy is still much smaller than that used during operation, and also the device lifetime (see above) is much longer. Therefore, the additional amount of energy for production is compensated within a small fraction of the lifetime, and does not constitute a valid argument against using fluorescent lamps.

The higher energy efficiency of fluorescent lamps is also associated with a correspondingly lower emission of heat. While heat emission may be welcome in heated buildings, that contribution to heating is relatively inefficient due to the energy losses in typical power plants (or compared with heat generation in a heat pump). In air-conditioned buildings, heating by lamps is undesirable, as it can significantly increase the electricity consumption of the cooling devices, so that the energy savings by using efficient fluorescent lamps are larger than the direct savings in the lamp. Note that the heating by lamps can also have unwanted local side effects, such as creating the risk of fire in wooden buildings.

After their lifespan, fluorescent tubes should be recycled, mainly because they contain some amount of the poisonous mercury: a few milligrams for compact fluorescent lamps, and somewhat more for long tubes. (For comparison, clinical thermometers used to contain roughly 1 g of mercury, and many other electrical devices, which can partly still be sold, contain similar amounts.) In the case of compact fluorescent lamps, the electronics can also contain harmful substances. Unfortunately, consumers are often not aware of this (particularly in the context of compact fluorescent lamps), so that the lamps are disposed of as normal garbage, and particularly the mercury pollutes the environment. However, that fact should not be exaggerated because only a tiny fraction of the mercury in household and commercial wastes is from lamps. Also, coal-powered electricity plants emit substantial amounts of mercury; in fact they cause by far the largest contribution to the global mercury emissions. For example, the operation of 100-W incandescent lamps over 5000 hours, causing an electricity consumption of 500 kWh, can easily be associated with mercury emissions of 10–20 mg, even if the electricity is not from coal-powered plants only. This is comparable to the amount of mercury in a 25-W fluorescent lamp, which produces a similar amount of light during its lifetime. Therefore, even without any lamp recycling, fluorescent lamps do not necessarily cause higher overall mercury emissions, and with recycling the situation can be substantially better.

Warm-up Time

Fluorescent lamps typically need some warm-up time before reaching the full brightness, essentially because sufficient mercury vapor pressure (around 3 mbar) needs to be built up. The warm-up time can be as short as a few seconds, but can be a full minute in other cases. For operation outside buildings, a fluorescent lamp may in winter not reach its optimum temperature, and thus have a reduced brightness.

fluorescent lamp

Figure 3: A typical fluorescent lamp with a long tube. Consuming 18 W of electrical power, it generates a similar amount of light as a incandescent lamp with 100 W.

Concerns About Electrosmog

Some consumers are concerned about the emission of electrosmog by fluorescent lamps. This is based on the fact that particularly electronic ballasts in compact fluorescent lamps exhibit significant electric and magnetic fields at higher frequencies in their immediate surroundings. On the other hand, efficient fluorescent lamps cause smaller currents in the wires, and thus smaller magnetic fields around the household wiring. Also, they are just one of many types of devices in a household which generate electric and magnetic fields.

The key question in this context is whether such electromagnetic emissions are associated with health risks, and how serious these may be. This question is very difficult to answer due to a lack of conclusive data. There appears to be some evidence that low-frequency electromagnetic fields have some biological effects, but despite intensive investigations there is no scientific proof that there are significant health risks. In that situation, it appears reasonable to reduce exposure to the fields where it is easily possible (e.g. by avoiding long-term presence very close to compact fluorescent tubes), but not to give up the well-proven beneficial effects of the reduced electricity consumption of fluorescent lamps, as far as replacement with LED-based lamps is not feasible.

Other Health Concerns

There are many other health concerns against fluorescent lamps, which are however poorly founded:

Special Types of Fluorescent Lamps

There are electrodeless fluorescent induction lamps, where an electromagnetic field transfers the electric energy to the gas discharge, so that electrodes are no longer needed. Resulting benefits are a further increased bulb lifetime and a wider range of usable light-emitting substances.

Competing Technologies

Frequently Asked Questions

How does a fluorescent lamp produce light?

An electric discharge excites mercury vapor inside a glass tube, causing it to emit ultraviolet (UV) light. A phosphor coating on the tube's inner surface absorbs this UV light and converts it into visible white light through the process of fluorescence.

What is the purpose of the ballast in a fluorescent lamp?

The ballast is an essential electrical component that stabilizes the electric current flowing through the gas discharge. It also typically provides the high-voltage spike required to start the lamp.

Are fluorescent lamps energy-efficient?

Yes, fluorescent lamps are several times more energy-efficient than traditional incandescent lamps, reaching a luminous efficacy of around 100 lumens per watt. However, modern LED-based lamps can be even more efficient.

What is a compact fluorescent lamp (CFL)?

A compact fluorescent lamp is a smaller version designed to replace incandescent light bulbs in standard fixtures. It features a folded or spiral tube and an integrated electronic ballast to achieve a compact size.

Why is mercury used in fluorescent lamps?

Mercury is essential for the lamp's operation. When excited by an electric current, mercury atoms efficiently emit the ultraviolet radiation that is needed to stimulate the phosphor coating and make it produce visible light.

What determines the color of light from a fluorescent lamp?

The color is determined by the composition of the phosphor coating. By mixing different fluorescent substances, manufacturers can produce various color tones, such as 'warm' or 'cold' white, and optimize the lamp's color rendering capabilities.

What is the color rendering index (CRI)?

The color rendering index (CRI) is a measure of how accurately a light source reveals the true colors of objects. Modern fluorescent lamps with triphosphor mixtures can achieve a very high CRI, making colors appear more natural.

Why do fluorescent lamps need time to warm up?

This is because sufficient mercury vapor pressure must build up inside the tube for efficient operation.

Suppliers

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.