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What is light

Luminous flux in Lumen (lm)

Illuminance in Lux (lx)

Efficiency in Lumen/Watt (lm/W)

Correlated Color Temperature (CCT in Kelvin (K)

Colour Rendering Index (CRI)

Unified Glare Rating (UGR)

Power factor


What is light?

Starting with this basic simple question looks easy but no single answer to this question “What is Light?” satisies the many contects in which light is experienced, explored or exploited.
The easiest answer could be:
Light is everything that the human eye can see, or Light is that part of the electromagnetic spectrun that is perceived by your eyes!
The wavelength range for humen eyes is between 380 and 780nm

Visable Light Spectrum

Luminous flux in Lumen (lm)

Luminous flux expresses to the total amount of light, in all directions and emitted by a lighting element. The unit of the luminous flux is lumen (lm). We are all familiar with the traditional fluorescent tube (18W,36W and 58W) or with an 250W or 400W HPS or the Halogen 1000W and 2000W and how much light these light sources gave us. We were not dealing with lumens. 
See the table below to get an idea about the light source and the Luminous flux these gave us

Light Source Luminous flux (lumens) Efficiency
Candle flame 12,57 lm - Lm/W
18W T8 fluorescent  1.350 lm 75lm/W
36W T8 fluorescent 3.350 lm 93lm/W
58W T8 fluorescent 5.240 lm 90lm/W
250W High Pressure Soduim
28.000 lm 112lm/W
400W High Pressure Soduim
48.000 lm 120lm/W
250W Metal Halide (HPI-T) 21.500 lm 86 lm/W
400W Metal-Halide (HPI-T) 32.000 lm 80 lm/W
1000W Halogen 13.000 lm 13lm/W
2000W Halogen 26.000 lm 13 lm/W

However, the power of LED technology is evolving. This caused us all to give place for different power and different lumen value’s. In other words, a brand ’A’ 10W LED bulb may giving less light than brand ‘B’ 8W LED bulb. Therefore, we need to pay attention of the value of lumens!


Illuminance in Lux (lx)

The unit to measure the illuminance is Lux. A light source with a luminous flux of one lumen which uniformly illuminates an area of one square metre has an illuminance of one lux.
This is comparable to the brightness of a candle flame, which also reaches an illuminance of around one lux at a distance of one metre.


Efficiency in Lumen/Watt (lm/W)

This is a measure of how much total effective lumens (light in all directions) is produced per Watt of electrical power consumed.

It’s important to compare overall effeciency of lamps including the losses in the associated circuitry. Luminous effeciency is typically quoted by LED manufacturers at the terminals of the LED rather than the input of the power supply, where the consumption will be considerably higher.


Correlated Color Temperature (CCT in Kelvin (K)

Correlated Color Temperature or CCT is a term used to describe the appearance of white light. Kelvins is the measuring unit by CCT. a light source measuring 2700k to 3000k would be considered warm in apearance, whereas 4000k+ would be considered cool. 

Light Source CCT Colour
Candle flame 1800K Ultra warm
High Pressure Sodium / Son-T 1950K ~ 2250K
Tungsten Halogen 2700K Extra warm white
Warm fluorescent 2500K ~ 3000K Warm white
Halogen 2700K ~ 3000K
Mercury vapour 3450K ~ 3750K Cool White
Metal halide 4000K ~ 4600K
Cool fluorescent 4000K ~ 5000K
Daylight fluorescent 6500K Daylight

Colour Rendering Index (CRI)

Each type of lighting has its place on what’s known as the Color Rendering Index (CRI) chart. This chart measures the ability of a light source to accurately reproduce the colors of the illuminated object, such as the sun. This means the CRI is determined by the light soures’s spectrum. 

Here’s an example showing the deffence between excellend, good and poor color rendering:


CRI 100


CRI 80


CRI 60



Unified Glare Rating (UGR)

With an increasing awareness of the effects poor lighting can have on a working environment, there has been an upsurge in the demand for UGR compliant light fittings. UGR is a metric that is used to calculate level of glare in certain applications. lighting standards, like EN12464-2, define for


Power factor

The power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit. The power factor can get values between 0 to 1. When all power is real power with no resitive load, the power factor is 1. 


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International Lighting Experts B.V.


  Elwin Mulderij
 +31 619 102 901

  Jaap Jan de Jong
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