The difference between the lux and thelumenis that the lux takes into account the area over which the luminous flux is spread. A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. However, the same 1,000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. In this way, you can consider lux (or footcandles) to be the amount of useful light our eyes are sensitive to.
In America, we still use the figure of merit of footcandle, which is a value similar to lux but is lumens per square foot. That old metric vs. imperial system still plagues the system. In any case, there is a simple relationship, not suprisingly, to convert from a square foot in inch terms to a square meter in inch terms, gives us a factor of 10.764. This factor is used to convert from fc to lux; 1 fc = 10.764 lux. I think that understanding this relationship, goes a long way in getting comfortable with these lighting terms.
To convert from Lux (lux) to Lumen (lm) insert the value in lux, distance between the light source and work surface as well as the viewangle of the source lamp. Click on "Lux to Lumen ".
To convert from Lumen (lm) to Lux (lux) Insert the value in lumen(s)*, the distance between the light source and the surface and the viewangle. Click on "Lumen to Lux ".
* It is interesting to note that the plural of lumen is lumena, yet to avoid confusion, I have opted to use the more modern "lumens".
Illuminance is the total luminous flux which falls on a surface and it shows the intensity of the incident light. The value is affected by the wavelength of the emitted light and the distance between the light source and the illuminated area.
The human eye is most sensitive to light that has a wavelength of around 555nm (green) and thus an green light source will have more lux than a blue one (for example) when using a biologically weighted figure. This is called the luminosity function.
The larger the distance between the light source and the illuminated area the lower the illuminance will be. Below we show some examples of optimal illuminace for day to day activities:
Illumination on task surface(lux)
Full moon at tropical latitudes
Public areas with dark surroundings
20 - 50
Typical family room setting
50 - 100
Overcast day or short visit work area (storage rooms)
100 - 150
Light Office Work, Classroom
Heavy Office Work, CAD Work, Library, Shops, Laboratories
Supermarkets, Mechanical Workshops
Normal Drawing Work, Studio Work, Quality control
Detailed and Precision Type Work
1500 - 2000
Performance of visual tasks of low contrast and very small size for prolonged periods of time
2000 - 5000
Performance of very prolonged and exacting visual tasks
5000 - 10000
Performance of very special visual tasks of extremely low contrast and small size