Lux , Lumen and Candela

09-02-2015 Imbema Rhiwa Carrosserietechniek


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Lighting systems including LEDs are often referred to in terms of Lux and Lumen to indicate their strength. These units are frequently mixed up with each other, so there is often confusion about the quality and application of a lamp. This also makes good comparison difficult. We are happy to explain the difference between these units, and what you have to pay attention to when acquiring new lamps.

Lux and Lumen – both are units of light intensity. In a nutshell,

"Lumen indicates the amount of light that radiates from a light source, while Lux indicates the strength of light on a given surface, like a desk or workspace."

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Lumen 
The Lumen (symbol: lm) is a unit of measure of total light emitted by a source. It measures the total quantity of light that a light source radiates in all directions. Light emission takes into account the sensitivity of the eye to the visible spectrum of electromagnetic radiation. This makes Lumen the unit for measuring the brilliance of a visible light source independently of the direction of the light beam.

Light emission is a measurement of the total quantity of light in a light beam. Brightness is a measure of the light density. If you only see a part of a light beam, that part will have a smaller emission (in Lumen), but (theoretically) the same brightness (in Candela) as the entire beam. The light emission depends on the power of the source and the colour of the light. An incandescent bulb has a light emission of approximately 12 lm/W. The smaller the emission angle at which the light is transmitted, the higher the brightness (given a constant light emission). 

Lux
The Lux (symbool lx), from the Latin word for light, is a unit of illumination strength: 1 Lux is the illumination strength produced by 1 Candela on a surface perpendicular to the light rays at a distance of 1 m from the source.

This means the Lux corresponds to the light strength you would see if every square metre of the surface being viewed is receiving a light emission of one Lumen. The number of Lux is therefore calculated as the quotient of the total received light emission, expressed in Lumen, and the size of the lighted surface expressed in square metres: 1 Lux = 1 Lumen/m².

Lux can be measured at various distances. Take care when you are making a comparison between two lamps. A lamp that produces the same number of Lux at a distance of 3 m as a lamp measured at one metre will be much more powerful than the one measured at one metre.

You can use a light meter to measure the Lux of any surface or area. When measuring, be sure to place the light meter at the working height. As the distance from the light source to the object being lighted increases, the light strength decreases in inverse proportion to the square of the distance to the light source.

Candela
Light intensity is expressed by the Candela (cd). The light intensity indicates how much light is found in any section of a light beam. The word Candela is Latin for candle. One Candela is approximately the same as the light intensity of an ordinary candle.

The Candela is related to the units Lumen and Lux. The Lumen is the use for the total light emission in a light beam. A light beam with the intensity of 1 Candela and a solid angle of 1 steradian (for a round light beam, this corresponds to an angle of opening of 65.5°) will have a total light emission of 1 Lumen. This means that 1 Candela is equal to 1 Lumen per steradian. If a light beam with a strength of 1 Lumen shines on the surface of 1 square metre, it produces a light strength of 1 Lux. This means that 1 Lux is equal to 1 Lumen per square metre.

Regulations

The lighting standard NEN – EN 12464-1 defines the minimum lighting strength, in Lux, for various places, activities and equipment. The standard sets lighting requirements for workspaces indoors in order to meet the needs of visual comfort and visual performance. All normal visual tasks are taken into account in the standard, including working with computer screens.

The effective strength of the lighting must be increased if:

  • the visual task is critical 
  • rectification of errors is time-consuming or costly 
  • precision or high productivity are important 
  • small details or low contrast are involved 
  • the task must be performed for an unusually long time 
  • the vision of the person working is poorer than normal 

The practical lighting strength may be decreased if:
  • details are unusually large or have a very high contrast 
  • the task is to be performed for an unusually short time

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The right choice
When choosing a lamp, take a look at the big picture. Ask yourself the questions: where will it be hanging? What kind of work is going to be performed there? Don't just look at the number of Lumen of a lamp, but try to provide the right light strength for the job in question. Also, don't compare apples and oranges.

Still not sure of the best solution? 
Contact us by phoning +31 (0)180 33 12 00. We are happy to tell you about the possibilities!


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