Original Post: Don't Confuse Lumens with Lux: Essential Understanding for Task Lighting Design
I know, we're still getting the basics down when it comes to shopping for new LED light bulbs-- we now base our judgment of how bright a bulb is by its lumen output, not its Wattage. This is fairly simple, straight forward, makes things seem pretty easy. But if you are aiming to combine task lighting with ambient lighting, than you are going to need to understand another measurement of light: Lux.
1 lx = 1 lm/m2
Lux (lx) is equal to one lumen (lm) per square meter-- it is a measure of luminous intensity as seen by the human eye, of light that hits or passes through a surface.
Where ambient lighting aims to create a general spread of light that is comforting to the eyes, task lighting aims to generate a focalized illumination of the surface you may be working on, or wanting to highlight. The distance the light is traveling, in addition to lumen output from the light source, must be weighed to achieve desirable lux levels.
Consider the kitchen: we put task lights-- hanging pendants and spotlights-- over kitchen islands and under kitchen cabinets, so we can see better when cutting vegetables and reading recipes. Likewise, in the office, we usually have a desk lamp that hovers over our work space, in addition to overhead lights and natural lighting from windows, so we can read and write with an alertness and high functioning capacity. If we were to hang these same task lights from the ceiling high above us, the lux of these lamps would be lower, thereby not providing sufficient lighting for the tasks at hand.
Information like lumen output, size and distance of surfaces, and beam angle is needed to determine lux levels.
The beam angle of a bulb is also a major factor in achieving desired lux levels, with higher beam angles producing lower lux levels than narrow beam angles. A luminous flux of 1000 lumens, concentrated into an area of one square meter, lights up that square meter with an illuminance of 1000 lux. However, the same 1000 lumens, spread out over ten square meters, produces a dimmer illuminance of only 100 lux.
The same lamp light hitting different surfaces at different heights have different lux levels. This is because light-- as it travels through space-- is dispersed and diffused the further it needs to go. Euri Lighting's DLC-3000e downlights series have a beam angle of 90 degrees, and a lumen output of 1400 lm. When installed in a ceiling of an average 7.87 ft (2.4m), the downlights produce a floor level illuminance of 132.08 lx. On a three-foot high kitchen counter top, the same recessed downlight produces an illuminance of 939.2 lx.
An excellent way to increase lux levels is to layer lighting. Including a chandelier or pendant over your counter or table top will increase your lux levels on those surfaces, allowing you to create the perfect ambiance for your room.
Lighting Design aims to achieve target lux levels on key surfaces, and there are recommended lux levels for different rooms in the home. These light levels are flexible and only meant as a suggestion for the preference of the home owner or lighting designer. To accurately measure lux levels in your home, consult a light meter.