Lighting Knowledge

Incandescent lamp

The first incandescent lamp was registered for patent in 1840. The version developed by Thomas Alva Edison gained worldwide use in the 1880s.

The principle of the incandescent lamp is very simple: electrical current flows through a thin tungsten filament usually situated in the centre of a glass bulb. The filament heats up so much that it begins to glow.


Incandescent lamps are very inexpensive to manufacture and their service life consists of around 1,000 hours. This means that they must be replaced each year with a new one assuming an average burning time of 2.7 hours daily.

The major disadvantage of incandescent lamps: they become very hot and only 5 to 10 percent of the energy is transformed into visible light. This means that most of the energy is wasted as unused heat. This fact ultimately resulted in the European ban of incandescent lamps


Halogen lamp

Halogen lamps are a further development of the incandescent lamp. Both are classified as heat emitters as their light is created by heating a tungsten filament. They emit a brilliant, pleasantly warm white light with very good colour rendering.


  • Incandescent lamps have a light output of 12 lm/W, and halogen lamps around double that with 25 lm/W.
  • Incandescent lamps have a service life of 1,000 hours, and halogen lamps up to 3,000 hours.

Energy-saving lamp

The principle of the energy-saving lamp, properly known as the 'compact fluorescent lamp' – CFL for short – was actually discovered around 1850 by the physicist Heinrich Geißler. But not more than one hundred years later were fluorescent tubes, also known as neon tubes, used mainly for industrial purposes.

Between 1980 and 1990 these developed into the first compact CFL fluorescent lamps, or energy-saving lamps. Compared to incandescent lamps, they reduce electricity consumption by 70 to 80 percent.


The light from energy-saving lamps is created by low pressure gas discharge, whereby mercury is firstly heated in the lamp and then vaporized, which is the reason why the first energy-saving lamps need some time until they achieve full output. Depending upon the ambient temperature, this process can last between one and five minutes.

In order to improve starting behaviour there are also 'pre-heated' energy-saving lamps that react very sensitively however to switching on and off. This has the major disadvantage that the lamps age rapidly. This means that the average service life of energy-saving lamps is significantly influenced by use. The fluctuation margin lies between 3,000 and 15,000 hours.

As well as mercury and slow start behaviour, their cool light is another factor not in their favour. This coolness however is not derived from a differing colour temperature, but rather their discontinuous colour spectrum. This means that according to lamp type and specification, specific wave lengths are missing, and therefore also the colours of visible light. Hence objects appear unnatural or cool.


LED Lamp

The abbreviation LED stands for “Light Emitting Diode”. The
light emitting diode is an electronic semiconductor component as found in almost every electronic device today.

The first light diodes were used in the 1960's as signallers. Their recipe for success even then was a long service life of 100,000 hours or more.


Not all colours can be created by LEDs. This is the reason why this technology was ignored over a long period of time for the creation of white light and therefore as a suitable replacement for lamps. Only in 1995 did it become possible to create white light from blue LEDs with the addition of a special phosphor mixture. This was the beginning of the rapidly inclining road to success for LEDs.

Similar to incandescent lamps and energy-saving lamps, LED lamps also age, but in other ways. The average service life of an LED within a lamp is around to 50,000 hours or more.

Further aspects for a positive ecological balance: LED lamps can be switched on and off without wear, contain no mercury and consume less electricity: in comparison to energy-saving lamps up to 30% less, and compared to incandescent lamps up to 85% less


Color Temperature (Kelvin)

You are familiar with the unit Kelvin [K] from temperature readings on thermometers, and it is also used as a unit of measure for the colour temperature of light. The lower the value, the 'warmer' the light appears. LED lamps from GREENLUMI have a pleasant warm white appearance, similar to incandescent lamps.

A few key figures for quick orientation with colour temperatures:

  • Candlelight: 1,500 Kelvin
  • 60 watt incandescent lamp: 2,680 Kelvin
  • Halogen lamp: 3,000 Kelvin
  • Morning/afternoon sun: 5,500 Kelvin


Colour rendering [CRI]

In order to specify the colour rendering of light sources, the so-called Colour Rendering Index (CRI) was developed. The principle is based on measuring light according to pre-defined wavelengths or colours (pastel colour tones). Although the complete spectrum of light is not considered, the colour rendering index is still a good guiding principle for the quality of light.

Quality criteria of CRI values:

  • Incandescent lamps serve as a basis for the scale. They have a CRI value of 100.
  • Light sources with a colour rendering index in excess of 80 indicate good lighting quality.
  • GREENLUMI LED lamps achieve CRI values greater than 90.

Luminous flux [lm]

The luminous flux specifies the quantity of light in lumens [lm] and is measured independently of distribution direction. The luminous flux of a 40 watt incandescent lamp consists of approximately 400 lumens. But because the lamp emits its light in all directions, a large component of the light is lost to the luminaire or lamp shade; only a small component of the luminous flux is used for actual illumination.

  • Luminous flux should not be compared one to one with the brightness of a luminaire and can vary according to application.
  • The brightness of a lamp is defined via the resulting illuminance value.

Luminous intensity [cd]

The luminous intensity describes that part of the luminous flux emitted in a specific direction. The luminous intensity is therefore decisively influenced by the light controlling elements. A typical example is the 'lamp shade' of a luminaire that causes the light from the lamp to only radiate downwards. The unit for luminous intensity is the candela [cd].

Typical luminous intensities:

  • A standard candle has a luminous intensity of 1 cd.
  • An incandescent lamp with 100 watts achieves 1,100 cd.

Illuminance [lx]

The illuminance is a measure of the luminous flux falling onto a defined surface. It is measured in lux [lx] and is calculated from the luminous flux per square metre [lm/m²].

Illuminance represents the brightness of a luminaire. In many applications, LED lamps can take advantage of the benefits of directed light. This explains the high performance effect of GREENLUMI LED lamps: a GREENLUMI LED lamp with 6 watts installed in a suspended luminaire achieves a higher illuminance than a 60 watt incandescent lamp.

Typical illuminance values:

  • Cloudless summer day: > 100,000 lx
  • Cloudy summer day: 20,000 lx
  • Dusk: 400 lx
  • Office: 500 to 1,500 lx
  • Starry night: 0.2 lx

Luminance [cd/m²]

Luminance is a measure of the impression of brightness of a surface. All surfaces absorb a part of the luminous flux and reflect the residual part. Colour and surface finish of the illuminated surface define the quantity of light absorption and light reflection. Luminance describes the impression of brightness perceived by the eye via the reflected component. As such, the light from a lamp in a room with a black floor appears significantly darker than in a room with a white floor.

  • The luminance is measured in candelas per surface unit [cd/m²].
  • A balanced and harmonious distribution of brightness makes a room pleasant and visually interesting.

Luminous efficacy [lm/W]

All watt specifications on electrical devices (and therefore also on lamps) define the power input. As such only consumption is declared, not however the output. The efficiency of a luminaire is more precisely determined by the luminous flux as a ratio of power input. The indicator known as luminous efficacy or efficiency is specified in lumen/watt [lm/W].

  • The higher the luminous efficacy, the more efficient the lamp.
  • GREENLUMI LED lamps achieve over 80 lm/W.
  • Incandescent lamps achieve only 12 lm/W, and halogen lamps 20 lm/W.
  • Most energy-saving lamps achieve between 40 and 50 lm/W.