Lumen vs lux: Differences and relevance when planning lighting

Lumen and lux are two terms that crop up again and again when planning lighting, but they often cause confusion. What exactly is behind these units and how are they connected?

This article explains the basics of lumen and lux, their significance when purchasing luminaires and planning lighting at workplaces.

What does the unit lumen mean?

Lumen (lm) is the unit of measurement for luminous flux: the amount of visible light that a light source emits in all directions. The higher the number of lumens, the more light a luminaire emits or the brighter it is.

You can use the lumen unit to compare the brightness of different light sources: for example, an LED luminaire with a power consumption of 8 watts emits around 800 lm, similar to a 60-watt incandescent lamp. This comparison shows just how energy-efficient modern LED technology is.

The lumen value of a lamp or luminaire can usually be found on the packaging or in the technical information. This information has been required by law in the EU since 2013.

What does the unit lux mean?

Lux (lx) is the unit of measurement for illuminance: the amount of light or luminous flux that hits a certain surface. Lux therefore indicates how brightly a surface is illuminated. The higher the value, the brighter the surface appears.

The illuminance depends not only on the light output (lumen) of the light source, but also on the distance to the illuminated surface and the angle at which the light hits it. The illuminance therefore decreases as distance from the light source increases, even if the luminous flux (lumen) remains the same. If the distance between the light source and the illuminated surface doubles, the illuminance is reduced to a quarter of the original value.

In practice, illuminance is measured using a luxmeter. The light sensor of the measuring device is placed on the surface to be tested, for example on the work surface or the floor. The luxmeter records the incident light and displays the illuminance in lux.

Lumen vs lux: Relevance for planning lighting

Lumen and lux are often confused or misinterpreted. They say different things and have different areas of application. The differences are highlighted here once again:

Lumens indicate the total amount of luminous flux emitted by a light source. They are, so to speak, the potential or rated power of a lamp or luminaire. Two luminaires with the same lumen value can nevertheless illuminate an area with different levels of brightness, depending on their design and position.

When purchasing luminaires, the unit lumen is an important factor. It helps to determine the right type and number of luminaires to ensure adequate basic lighting.

However, the indication of lumens alone is not sufficient for assessing the lighting quality at a specific workplace. This is where the lux unit comes into play.

By contrast, lux describes how much of this light actually reaches a surface: the dose of light that is available to the person working there for their task. As a rule: the further away the surface is from the light source and the flatter the light falls, the lower the illuminance in lux.

One analogy illustrates the difference: a vehicle has a technical maximum speed that is specified in the technical data (corresponding to the luminous flux in lumens). However, how fast the vehicle actually drives in a given situation depends, among other things, on the gradient, type of road and total weight (corresponds to the illuminance in lux). The technical maximum speed can practically only be achieved under ideal conditions.

Illuminance in lux is therefore the decisive value for workplace lighting. To assess whether there is sufficient light at a workplace, the actual illuminance on the surfaces in question must be measured in lux.

Minimum illuminance requirements in various work areas

Standards and guidelines specify minimum values for illuminance at workplaces in the unit lux. The Technical Rules for Workplaces ASR A3.4 give the following examples:

Conversion from lumen to lux/from lux to lumen

Lux and lumen cannot be converted directly into each other. Nevertheless, a formula can be used to establish a relationship between the two units.

The illuminance in lux is the ratio of luminous flux in lumens to the illuminated area in square metres:

Illuminance (lx) = luminous flux (lm) / area (m²)

For example, if you want to know the illuminance produced by a luminaire with 1000 lm on an area of 5 m², enter the values in the formula:

Illuminance (lx) = 1000 lm / 5 m² = 200 lx

To convert lux to lumen, you have to solve the formula for luminous flux:

Luminous flux (lm) = illuminance (lx) x area (m²)

One example: you want to illuminate a work surface of 3 m² with an illuminance of 500 lx. How many lumens does the luminaire need to have as a minimum?

Luminous flux (lm) = 500 lx × 3 m² = 1500 lm

However, these calculated values must be understood as approximations. The formula assumes uniform illumination of the area and does not take the entire situation into account.

In reality, illuminance is unevenly distributed because it depends on the distance to the light source, the beam angle and the light scatter of the luminaire as well as other objects in the room (which reflect or cast shadows).

For precise lighting planning, it is advisable to carry out professional lighting calculations or use lighting planning software. Here, the exact position and alignment of the luminaires as well as the characteristics of the room and surfaces can be taken into account in order to achieve standard-compliant and efficient lighting.

Other units for planning lighting

Watts

Watts (W) is the unit of measurement for the electrical power consumed by a light source. It indicates how much energy the luminaire consumes, but not how much light it emits.

It used to be possible to deduce the brightness of a light bulb from its wattage – a 75 W bulb was brighter than a 60 W bulb. With the introduction of energy-efficient light sources such as LEDs or energy-saving lamps, this relationship no longer applies.

The luminous efficacy in lumens per watt (lm/W), which indicates how efficient a light source is, is therefore of interest when planning lighting.

Candela

In turn, candela (cd) is the unit of measurement for luminous intensity or light output: the intensity of light in a particular direction. It is mainly used to calculate illuminance as a function of distance.

The formula for the what is known as ‘photometric distance law’ is as follows: Illuminance (lx) = luminous intensity (cd) / distance (m)²

This states that the illuminance decreases with the square of the distance. If a luminaire with 1000 cd illuminates a surface at a distance of 1 m, the illuminance is 1000 lx. At a distance of 2 m it is only 250 lx, while at a distance of 3 m only around 111 lx.

This formula is also only an approximation: It assumes a point light source and a surface perpendicular to the light propagation.

Luminance

Luminance (L) describes the brightness perceived by the human eye of a surface that emits light as a light source or through transmission or reflection. Luminance is defined here as the ratio of luminous intensity (I) to the area projected perpendicular to the direction of observation (Ap). The photometric unit is candela per square metre (cd/m2).