Light Emitting Diode

Light emitting diodes or LEDs are miniscule devices that produce light when electrons pass through semiconductor crystals.

Some basic knowlegde of physics

is necessary to understand how an LED works. Detailed explanations can be found here. Video tutorials can also be helpful.

The following is a brief overview of the basic principles of LED, in no way complete. If the explanations feel too complex, skip through to general information below.


A light emitting diode is made up of four components:

An LED chip is the fundamental component of an LED. The chip is soldered into a metal reflector which contacts a negative pole (cathode). This dissipates heat, protects the chip from overheating, and reflects the emerging light. Sometimes an optic is built into the tub to enhance the lighting effect.

A bonding wire made of gold is the contact to the positive pole (anode). It is as thin as possible, so that enough space remains on the small chip to emit light.

A plastic lens (usually made of epoxy resin, transparent or coloured) fixes and connects the other components. It also protects the chip and improves radiation.

LEDs are technically semiconductor diodes. A semiconductor diode is an electronic device that allows current to flow in one direction only - it works like a one-way street, so to speak. Its basic principle is the rectifier effect: alternating current is converted into direct current. This effect was discovered in 1874 by Ferdinand Braun (co-founder of "Telefunken") and is also expressed in the graphic symbol for "Diode".

The LED chip itself is made of doped semiconductor crystals:


A semiconductor is a material that are neither pure conductors (e.g. copper, silver, aluminium) nor insulators (e.g. plastic, glass) but have an electrical conductivity somewhere in between. A semiconductor conducts better with increasing temperature - the heat conditions in and around the semiconductor are therefore of particular importance. Known semiconductors are silicon and germanium, for the luminous effect of the LED two or more elements are combined, for example gallium phosphide (GaP) and indium gallium nitride.


By doping a semiconductor, two separate types of semiconductors can be made from the same crystal and increase electrical conductivity. The positive or negative doping defines what properties a particular semiconductor will have, and as in the case of light emitting diodes, determines the colour of the light produced.

LED chips consist of two-lead, one positively and one negatively doped semiconductors. It is a p–n junction diode that emits light when activated. When a suitable voltage is applied to the leads, electrons are able to recombine, releasing energy in the form of photons, flashing light.

Interesting facts

Semiconductor technology is considered the key technology of the 20th century, found in devices such as transistor radios to sophisticated medical devices: Nothing works without semiconductors. Even though their basic electrotechnical phenomena was discovered 150 years ago, it was not until the development of quantum physics that the theory could be described in detail. These explanations are complex and require a lot of specialized knowledge; these descriptions are merely brief summaries.

In the 1930s, the potential of semiconductor transistors revealed itself paving the road for the success of the computer and modern technology.

LED chips are one of the smallest available light sources with an edge length of about one millimetre. They emit a punctiform light. Reflectors and lenses, combined with several chips enables the diode light to spread. However, due to their basic technical conditions, light from LEDs is concentrated in one direction, whereas incandescent bulbs and fluorescent tubes radiate to all sides, making them less suitable for uniform illumination of surfaces. (see the difference between ‘Lumen’ and ‘Candela’ in Terminology).


The light emitting diode will emit light of a different wavelength, i.e. colour, depending on which material and which doping is used to produce the semiconductor. Each colour stems from a specific semiconductor crystal. Important: LED light is always monochrome; each small diode can produce exactly only one colour which means it is not possible to produce white light directly.


As early as 1907, an English physicist observed the so-called electroluminescence for the first time: inorganic material (crystals) emitted light when they were charged with electricity. However, it took almost a hundred years for this discovery to be transformed into a light source suitable for everyday use (see Artificial Light)

In 1957, intensive research on the small, colourful diodes began leading to a breakthrough in 1990 with the production of white light, through the successful combination of energy-efficient blue light-emitting diodes with an orange fluorescent layer.


The direct generation of white light from light-emitting diodes is technically not possible, because diodes light - due to the combination of their output elements and the targeted doping with foreign atoms - always monochrome, so in a single, very specific colour. To get white light, you have to combine it. There are two different technical methods:

Three Colours to White

First method: red, green and blue diodes (RGB mode) are combined to create white light. However, this method requires a precise tuning of the three starter colours whose semiconductor crystals have different electrical properties. As LEDs age and their luminosity changes, mixing colours is difficult to control.

Luminescence Conversion

The second method turns to ‘luminescence conversion’, similar to fluorescent bulbs: a thin layer of orange fluorescent or conversion dyes, consisting of oxides or sulphides and rare earths, is vapor-deposited directly above the blue-emitting LED chip. This transforms some of the blue light so that it appears white. This method is currently used in almost all household LEDs. Depending on the lamp, the orange layer is often clearly visible.

Interesting facts

The thickness of the fluorescent coating on the LED chip determines the quality of the white: cold white, neutral white and warm white. The thicker the layer, the warmer the light appears. However, when layer is thicker, more energy is required to penetrate the luminescent layer, therefore the bulb is less energy efficient.

Instead of the light-emitting diodes described above, which are connected via wires (cathode/anode) to power, SMD chips are usually installed nowadays, in which the diode is built on a millimetre-sized board, which then directly can be soldered.


SMD stands for ‘Surface Mounted Device’. These small flat chips, from which you can see the orange surface in many LED bulbs, are easy to handle and found in almost all LED bulbs.

LED bulbs that have not been retrofitted (i.e. fitted with E27 or E14 fixtures) need luminaires with matching fixtures. LED versions of most bulbs used in the past are now readily available. This diagram shows a retrofit LED, which can be used instead of incandescent bulbs. Other models can be found on the pages of the major electronics retailers, as well as in specialized trade sites (e.g. for stage and photography).

The LED is connected to the power through the socket. An electronic ballast regulates the current and provides sufficient voltage to start the lamps. The diodes are sensitive to current fluctuations and without a ballast, the lamp would uncontrollably increase its current draw. The quality of the ballast affects how much an LED flickers.

The LED chips are arranged on a plate to spread their light more evenly to have a similar lighting effect as an incandescent bulb.

Although an LED does not produce light through heat like an incandescent bulb, they do create a considerable amount of heat that must be dissipated with a suitable cooling or thermal management system (as with a computer). A cooling element made of aluminium or ceramic dissipates this heat from the bulb into the room. For LED spots, a cooling system is installed at the top of the light around the reflector; with LED retrofits the cooling elements are usually under the round (plexi) glass bulb around the ballast. Permanent LEDs have air slots to let out heat.

After looking into the technical aspects of LED lighting technology, the question of health and ecological effects arises. LEDs are marketed as a clean, environmentally friendly and risk-free light source. Are they really? Can these claims stand up to closer scrutiny?



Light flicker or light source modulation (not to be confused with the irregular flickering of a lamp caused by a voltage drop) is the change of the luminous flux of a light source due to the technology of the bulb itself. In the moment, these fluctuations are invisible or imperceptible because the eye and brain initially compensate for such small irregularities. In the long term, a constant flicker of light can affect the entire nervous system.

In principle, all bulbs operating on household alternating current flicker. However, with incandescent and halogen lamps the flicker is virtually non-existent because the filament does not cool down significantly during a cycle of AC current.

Fluorescent and LEDs are different. For technical reasons, they respond immediately to voltage change. The ballasts counter this reaction by providing DC power to avoid flickering. Ballast quality determines how much a bulb flickers (higher quality means higher price).

There are various methods by which the flicker effect of a light source can be measured exactly, but so far, no normative standard with clear limits have been established - therefore no corresponding references on bulb packages. When purchasing a bulb, it is not clear whether a particular bulb is flicker-free or flickers a lot.

Another problem arises with dimming: As a rule, dimmable LEDs flicker significantly due to the technology and should not be used sensitive people.


Investigate Further:

Insights from a light sensitive (photophobic) person (in German):

Glare and Thermal Effects

From a medical perspective, the strong, concentrated, high-luminance light emitted by LEDs (in contrast to incandescent and fluorescent) cause glare which has both physiological and psychological reactions. This is most evident in car headlights; glare from headlights is a recognized cause of roadway accidents and LEDs do nothing to decrease this danger.

Glare can also be problem in the workplace, particularly if there is too much light in one room along with too many different lighting conditions in various areas. When the eye constantly has to compensate and adapt to such lighting challenges, this can result in health impairments which reduce and disrupt the workflow.

Glare produced by direct light into the eye can also be dangerous for other reasons: strong blue or cool white light can trigger photochemical reactions in the eye that attack the retina. This applies to all lamps that emit a proportion of blue light (see below: blue light hazard), but far more to LEDs given the increased blue content. In household standard LED lamps, the brightly lit chips are usually covered by opaque discs or lamp body, so there is no direct risk to the eyes. In the workplace, however, extended exposure accumulates and must be taken into account when choosing lights for work spaces.

As a result, the Federal Institute for Occupational Safety and Health has classified some makes of LED lamps in risk group 2; the glare of these lights is so unpleasant and disturbing, that people automatically turn away when looking into such a light. Extended exposure, even seconds, can cause damage to the retina.

LED flashlights must also be used cautiously and not shone directly into people’s eyes.

Blue Light Hazards and its Effects on the Circadian System

Blue light has a measurable effect on the human eye: high-energy visible light (HEV) or blue light hazard is responsible, among other problems, for damage to the retina (macular degeneration) which has increased significantly in recent years, the blue/violet content of LEDs are partially responsible.

Further reading (in German):

Blue lights affect the inner rhythms of a whole human being: the circadian rhythm, the inner biological timing, which ensures for example, a healthy waking and sleeping cycle, and is considerably disrupted by exposure to blue light. Simply put, if artificial light contains a lot of blue, it can cause disturbances in the sleep rhythm. Blue light from LEDs, including PC screens and smartphones, has significant levels of blue.

Further readning (in German):


LED Comparison:
Above a neutral white LED - the high proportion of blue light is clearly visible.
Below a warm whitw LED - here is still arealtively high blue value visible


Electrosmog is the invisible electromagnetic radiation resulting from all bulbs with transformers (energy-saving lamps, fluorescent lamps, low-voltage lamps), all screens, and LEDs with ballasts. Electrosmog is can have detrimental effects on our nervous and endocrine systems, causing conditions known as electrosensitivity (ES) or electro hypersensitivity (EHS).

The Swedish Confederation of Professional Employees (TCO) developed a standard for identifying and measuring levels of radiation on screens and displays. Investigations by the Ökotest magazine in 2011 and 2017 confirmed that many energy-saving bulbs and LEDs are significantly higher than these standards.

Continue reading, in German: (The article is subject to a charge)


Light Pollution

The easy and availability of LED technology and its simple application have led to the installation of more lighting, especially outdoors. The increased illumination of the industrial cities around the world has negative consequences for humans and nature, many of which are connected to LED use (see Electric Light and Consequences).

Further reading:

Rare Earth

Rare earth is a misleading term for substances (rare earth element) that are indispensable in almost all modern technological applications. These are metals that are not uncommon, but only in minute amounts, sometimes only as an admixture of other minerals. Elaborate chemical extraction methods are used to mine them, which are classified as environmentally problematic (toxic sludge, radioactive residues). China dominates the world market for rare earths.

Bulbs deplete 8% of the earth’s rare earth annually. LED chips contain a small portion of (such as cerium, lutetium or europium) and there are currently no effective recycling possibilities for extracting and reusing them.

Life Cycle Assessments

The present life cycle assessments (LCA) of the LEDs are based on calculations made by light manufacturers reporting to the environmental institute Munich. In this case, the manufacturing process (technical process, long transport routes, etc.) in relation to bulb life is considered low priority.

Independent life-cycle assessments of LEDs are not available to our knowledge.

An important factor here is also the interchangeability of the light source: since LED chips are small and easy to handle, more and more are being installed in new lights. If the bulb breaks, so the whole light must be disposed of. The German Environment Agency states that this procedure should be fundamentally brought into question.

Even with new cars, there are surprises: LED chips in headlights are glued directly to the housing, so the entire headlight is one inseparable unit. If there is a problem, the entire headlight must be replaced, an expensive repair.

Further reading (in German):


LEDs are advertised with very long lifetimes, manufacturers claim between 10.000 and 50.000 hours. These statistics have only been confirmed in laboratory tests. LEDs have not been on the market long enough, nor have all aspects of the bulbs been tested. Consumer feedback suggests that this information should be considered with caution. For one reason, life span not only depends on the light-emitting chip, the electronic ballast also affects the quality and lifespan of an LED.

Furthermore, individual lamps can be rejected from these claims, as EU regulations allow for an early failure rate of 5-10%.

LED chips themselves usually do not break, but slowly lose their luminosity. The life of an LED is considered to have ended when the luminosity has dropped to 70%.

Further reading (in German):

Technical Data

Technical Data
Power: 8 to 13 watts = 60 W incandescent, depending on the bulb
Luminous flux: 800 lumens at 10 watts = 60 W incandescent
Luminous efficacy: 56 - 105 lm/W at 800 Lumen
Energy efficiency: A+ to A++ (from A ++ to E, best value in terms of efficiency)
Colour temperature: 2600 K to 4000 K (warm white to neutral white) for household lamps, higher colour temperatures also possible
Colour rendering: 80-95 CRI (of 100, fluctuation value varies greatly)
Lifespan: Manufacturers claim 10.000 to 50.000 hours; manufacturer's warranties between 2 (statutory sales warranty) and 5 years



LEDs are extremely energy efficient; they consume the least amount of electricity of all light bulbs and are very durable. LEDs can be produced in almost all colours, although white light is only possible as a mixed colour.

LEDs do not contain mercury. There are no known health risks if they break and are not considered hazardous waste. They do contain electronic components, so must be disposed of at a local recycling centre for electronic waste.

Operating Life: frequent switching on and off has no influence on the service life in domestic use of LEDs.

LEDs are available in many shapes and sizes and offer a lot of design freedom in room lighting.


LEDs lose luminosity over time, and with warm white LEDs, the blue component increases. LED colour rendering is on average significantly worse than with thermal radiators.

LEDs almost always emit electrosmog, so they should only be used at a distance of 1-2 meters to the consumer. For more information about health-related aspects see previous text.

LEDs are only dimmable if so stated on the packaging, and even then, they are not compatible with conventional dimmers. Dimming often produces (invisible or visible) flicker.

According to a study conducted by the French Ministry of Health, LEDs are not recommended for use in children’s bedrooms or play spaces.

LED technology is new and in constant development – with the pros and cons that change offers. Many claims and statements are from the manufacturers and should be viewed with healthy scepticism.