Lumens, Lux, brightness and hype

The Lumen is the basic unit of light output and, as an example, a 50W (or 35W high efficiency) low voltage halogen downlight produces about 1000 of them (near enough, for the purposes of this explanation).

Now, if the lamp has a 10 degree spot beam it would project a circle of light 0.88 meter in diameter at a distance of 2.5 metres. (In actual fact, there is light outside of this 10 degree cone - but for now, let's assume that there is a perfect cutoff between light and dark.) This is an area of about 0.61 square metres of illumination area and therefore the intensity of the light is about 1640 Lux or Lumens per Square Metre.

Imagine the same amount of light (Lumens), but with a 24 degree beam. Now, at the same distance the circle of light is 2.22 metres in diameter, an area of 3.87square metres. Therefore the intensity of the light is about 258 Lux.

With the same amount of light (Lumens), but with a 38 degree beam, at the same distance the circle of light is 3.9 metres in diameter, an area of 11.95 square metres. Therefore the intensity of the light is about 84 Lux.

Replacement lamp manufacturers play on this relationship to make their replacement lamps seem brighter (while still in the packaging, before you pay for them and get them home!) They will claim 400 Lux, but forget to mention is all in a very small spot.

The eye is not terribly good at measuring light in absolute terms. Without an immediate point of reference, your eyes get used to the available light and your could think that anywhere from 100 to 1000 lux was a perfectly good level of illumination. It's not until you go outside, or walk into a brigher area that you realise the difference.

 10 Lux Candle at 30 cm 50 Lux Family living room 80 Lux Hallway 400 Lux Brightly lit office 1000 Lux TV Studio / well lit kitchen. 32000 - 100000 Lux Sunny Day

As a practical example, imagine you wanted to have a general level of illumination in a kitchen that was 500 Lux (Lumens per Square Metre). You would first need to work out the area to be lluminated. Let's assume its 3 x 4 metres or 12 square meters. Therefore you need 6000 Lumens, evenly distributed (12 metres at 500 Lux/metre = 6000). That equates to 6, 50W low voltage halogen lamps.

Ideally, you'd want the cones to overlap at the highest point in the kitchen that should be evenly illuminated - the worksurface for example, which we will assume is 1.5 metres from the ceiling. With a 38 degree beam lamp, the circle of illumination is 2.35metres diameter at 1.5 metres distance. Therefore place the lamps withing 2.35 metres of each other. The cones are not perfect light/dark boundaries, and so the overlapping cones will create an evenly illuminated effect, which added together would make 500 Lux. Using 24 degree lamps in the same place would produce pools of light, separated by darker areas. 10 degree lamps would create an even more dramatic pattern of light and dark patches.

This general level of illumination would then be supplemented by task lighting, over work surfaces for example. These might have more focused beams to create 1000 Lux over the required area.