Luminous Flux (lm) Explained: What Lumens Tell You About Brightness
What Is Luminous Flux?
Luminous flux describes the total amount of light perceived by the human eye that a light source emits in all directions. It is expressed in the unit lumen (lm). The higher the lumen value, the more light the light source produces – and the brighter it is.
Luminous flux is one of the most important photometric metrics and is stated on every light source and on the EU energy label. It answers the question: "How much light does this lamp emit in total?" – regardless of which direction the light is projected.
Why Lumens Instead of Watts?
With incandescent lamps, the wattage was a reliable indicator of brightness: a 60-watt incandescent lamp was brighter than a 40-watt one. This worked because all incandescent lamps had a similar luminous efficacy – they converted roughly the same proportion of energy into light.
With LEDs, the situation is fundamentally different. A 7-watt LED produces the same luminous flux as a 60-watt incandescent lamp – at a fraction of the energy consumption. For LEDs, the wattage no longer says anything about brightness, only about electricity consumption. This is why luminous flux in lumens is now the definitive comparison value for brightness.
Conversion Table: Incandescent Lamp to LED
| Incandescent Lamp | Luminous Flux (approx.) | LED (approx.) | Application Example |
|---|---|---|---|
| 25 W | 220–250 lm | 3–4 W | Bedside luminaire, mood lighting |
| 40 W | 410–470 lm | 5 W | Desk luminaire, decorative lighting |
| 60 W | 700–810 lm | 7–8 W | Hallway, bathroom, dining table |
| 75 W | 920–1,060 lm | 9–10 W | Bedroom, children's room |
| 100 W | 1,300–1,530 lm | 12–13 W | Living room, kitchen, study |
How Many Lumens Do I Need for Which Room?
The required amount of light depends on the use of the room. A useful reference is the illuminance in lux (lx), which describes how much light arrives on a surface. As a guide for the total lumen count per room, the following approximate values apply:
- Living Room: approx. 100–150 lumens per m² – a total of approx. 2,000–3,000 lm for a 20 m² room
- Kitchen: approx. 250–300 lumens per m² – for good visibility on the work surface
- Bathroom: approx. 200–250 lumens per m² – brighter for mirror and grooming areas
- Bedroom: approx. 100–150 lumens per m² – cosy, supplemented by reading lamps
- Office / Study: approx. 300–500 lumens per m² – at least 500 lux on the work surface in accordance with DIN EN 12464-1
- Hallway: approx. 100–150 lumens per m² – orientation is sufficient
These values relate to ambient lighting. Additional task and accent lighting increases the total lumen count actually needed in the room.
Luminous Flux, Luminous Intensity, Illuminance – What Is the Difference?
In lighting technology, there are several related metrics that are frequently confused. The relationship:
- Luminous Flux (Lumen, lm): The total amount of light that a light source emits in all directions. Answers: "How much light does the lamp produce in total?"
- Luminous Intensity (Candela, cd): The luminous flux emitted in a specific direction – per solid angle (steradian). Answers: "How brightly does the lamp shine in a given direction?"
- Illuminance (Lux, lx): The luminous flux arriving on a surface. 1 lux = 1 lumen per square metre. Answers: "How bright is it on the table surface / at the workstation?"
- Luminous Efficacy (lm/W): The luminous flux in relation to the electrical power consumed. Answers: "How efficiently does the light source convert electricity into light?"
For purchasing light sources, luminous flux (lumens) is the most important specification. For lighting design, illuminance (lux) is decisive – it takes into account how much light actually reaches the surface to be illuminated.
Why Lumens Are Not Always Equal
Luminous flux alone is not sufficient to fully assess the perceived brightness of a luminaire. Several factors influence how bright the light actually appears in the room:
- Beam Angle: A spot with 400 lm and a 24° beam angle produces a noticeably brighter light cone than a lamp radiating omnidirectionally with the same 400 lm and a 300° beam angle. The same lumen value, a completely different lighting effect.
- Colour Temperature: Warm white LEDs (2,700 K) are subjectively perceived as slightly dimmer than neutral white LEDs (4,000 K) at identical luminous flux. This is due to the differing spectral sensitivity of the eye.
- Luminaire Efficiency: Part of the luminous flux is lost within the luminaire itself – through reflectors, diffusers, optics and the housing. The luminous flux stated on the packaging refers to the light source, not the luminaire. For luminaires with integrated LEDs, the luminaire luminous flux is increasingly being specified – this is the more accurate value.
- Room Surfaces: Light-coloured walls and ceilings reflect light and make the room appear brighter. Dark surfaces absorb light – the same luminous flux produces a darker spatial effect.
Luminous Flux and Luminous Efficacy: Efficiency Compared
Luminous efficacy (lumens per watt, lm/W) describes how efficiently a light source converts electrical energy into visible light. It is the key measure of energy efficiency and varies considerably depending on the technology:
- Incandescent Lamp: approx. 10–15 lm/W
- Halogen Lamp: approx. 15–25 lm/W
- Compact Fluorescent Lamp (CFL): approx. 50–80 lm/W
- LED Retrofit: approx. 80–150 lm/W
- High-Power LED Modules: up to approx. 200 lm/W
An LED with 100 lm/W therefore produces 1,000 lumens from 10 watts – an incandescent lamp would need approximately 75 watts for the same output. This corresponds to an energy saving of over 85 %.
Luminous Flux and Lifespan: What Do L70 and L80 Mean?
The luminous flux of an LED gradually decreases over the course of its operating life – the LED slowly becomes slightly dimmer. This process is called degradation. The lifespan specification for LEDs therefore refers to the point at which the luminous flux has declined to a defined percentage of its initial value:
- L70/50,000 h: After 50,000 operating hours, the LED still delivers at least 70 % of its original luminous flux.
- L80/30,000 h: After 30,000 hours, at least 80 % remains.
An LED with 1,000 lumens and L70/50,000 h therefore still emits approximately 700 lumens after 50,000 hours – in practice, a barely noticeable difference.
Frequently Asked Questions (FAQ)
How many lumens replace a 60-watt incandescent lamp?
A 60-watt incandescent lamp produces approximately 700–810 lumens. An LED light source with approximately 800 lumens delivers comparable brightness – at only 7–8 watts of power consumption.
Are more lumens always better?
Not necessarily. The required lumen count depends on the room, the activity and the desired ambience. In a bedroom, 3,000 lumens is too bright – 1,500–2,000 lumens is sufficient for ambient lighting. Dimmable luminaires offer the greatest flexibility.
What is the difference between lumens and lux?
Lumens (lm) describe the total amount of light from a light source. Lux (lx) describes how much of that light arrives on a surface. 1 lux = 1 lumen per square metre. Lumens are a property of the light source; lux is the measurable brightness at the workplace.
Why does the lumen value on some luminaires differ from that on the light source?
The lumen value on the light source is the lamp luminous flux – measured under laboratory conditions on the bare light source. The luminaire luminous flux takes into account the losses caused by the housing, reflectors and optics and is therefore lower. For luminaires with integrated LEDs, the luminaire luminous flux is increasingly being specified.
How does colour temperature affect perceived brightness?
Warm white LEDs (2,700 K) appear subjectively slightly dimmer than neutral white LEDs (4,000 K) at identical luminous flux. This is due to the sensitivity curve of the human eye, which is highest in the green-yellow range. When selecting warm white light sources, it is therefore advisable to choose a slightly higher lumen value.
In Summary:
Luminous flux in lumens is the key metric for the brightness of a light source and has replaced the wattage as a comparison value. When choosing the right light source, beam angle, colour temperature and luminous efficacy should also be considered alongside the lumen value. At Leuchtenland.com you will find the luminous flux in the product details for every light source and luminaire – allowing you to select precisely the right brightness for your room.