Design
How much light for indirect lighting
The same lamp gives 300 lux as direct light and 100 as indirect. Here we work out why, and tabulate how much strip is needed once you know the room, the ceiling and the target.
Why indirect light needs 2-3 times the power
With direct lighting, the light leaving the lamp lands almost entirely where it is aimed. With indirect lighting it first hits the ceiling, where part is absorbed, and the rest spreads through the whole room – onto the walls, the furniture and only last onto the floor. Every stage takes its share. This is not a fault of the luminaire but physics, and it must be included in the sizing.
In practice: the same 10,000 lumens as direct and indirect light, a 20 m² room
Direct ceiling luminaire: 10,000 lm straight down from the luminaire. Loss in the luminaire optics about 15 %, the light lands on the floor and furniture. About 8,500 lm / 20 m² on the floor = 425 lx.
Indirect, white matt ceiling: 10,000 lm from the strip to the ceiling. The ceiling reflects 82 % → 8,200 lm leaves the ceiling. It spreads in every direction: the walls absorb some, the furniture some, and about 55 % reaches the floor → 4,500 lm / 20 m² = 225 lx. Half of direct.
Indirect, pine-panel ceiling: the ceiling reflects 48 % → 4,800 lm, 55 % to the floor → 2,640 lm / 20 m² = 130 lx. A third of direct.
That is why 14.4 W/m is needed where direct light manages with 7.2 W/m – and on a timber ceiling 17 W/m is not enough for general light.
| Factor | Value | What it means |
|---|---|---|
| Ceiling reflectance ρ | White matt 0.82 · light grey 0.65 · pine panel 0.48 · dark 0.30 | The share of light that leaves the ceiling back into the room. The rest is absorbed by the ceiling as heat. |
| Room efficiency η | 2.4-2.8 m: 0.55 · 2.9-3.5 m: 0.45 | The share of light leaving the ceiling that reaches the floor. In a tall room the walls take more. Dark walls and lots of furniture lower the figure. |
| Total | White, low room 0.45 · timber panel, tall room 0.22 | The factor by which the strip’s lumens become lux on the floor. A timber ceiling in a tall room loses 78 %. |
The ceiling is the luminaire – not the strip
A dark ceiling does not reflect enough at any strip power. If the ceiling is dark or dark timber, there are three options: paint the ceiling light, accept that indirect light is mood light and add accent lights, or choose a structure where the light reflects off the wall rather than the ceiling.
Recommended lux by room type
Lux is the amount of light on a surface – the floor, a table or a face. The recommendations are design practice and, in workplaces, the EN 12464-1 standard. Indirect light economically delivers 100-300 lx; above that, direct light is needed as well.
| Room | General light | Task light | Note |
|---|---|---|---|
| Living room | 150-250 lx | Reading 300 lx with an accent light | Indirect general light alone is enough for relaxing |
| Bedroom | 50-150 lx | Reading light 300 lx | The only room where less is better |
| Kitchen | 300 lx | Worktop 500 lx direct | Indirect = general light, task light separate |
| Dining area | 150-200 lx | Table 300 lx with a pendant | Dimmable |
| Bathroom | 200-300 lx | Mirror 300-500 lx on the face | Mirror light as its own group |
| WC | 100-200 lx | – | Motion sensor, night light 20 lx |
| Hallway, corridor | 100-200 lx | Mirror 300 lx | Passage |
| Staircase | 100-150 lx on the step | – | Uniformity matters more than quantity |
| Utility room | 300 lx | Worktop 500 lx | Workspace – 4000 K |
| Sauna | 20-50 lx | – | Mood, only under the benches |
| Office, screen work | 300 lx | Workstation 500 lx, UGR ≤ 19 | EN 12464-1 |
| Meeting room | 300-400 lx on faces | Table 500 lx | Camera: light from the front |
| Shop | 300-500 lx | Product 750-1000 lx | A 1:3 contrast guides the eye |
| Restaurant, dinner | 50-150 lx | Table 200-300 lx | Lunch 300-500 lx – dimming essential |
How much strip – lumens per metre
The table below gives the strip’s luminous flux per metre (lm/m) needed to reach the target lux on the floor when the strip runs around the room’s perimeter. It assumes a normal room shape with about 0.9 m of perimeter for every square metre of floor – for example a 4 × 5 m room, 18 m of perimeter, 20 m².
The calculation changes with the number of walls: here it assumes light on all walls. Leaving out a wall means adding 25-30 % more light on the other walls.
The strip’s lm/m is in the product data. As a guide: 7.2 W/m ≈ 700 lm/m, 14.4 W/m ≈ 1,400 lm/m, 17 W/m ≈ 1,600 lm/m. CCT strips give 20-30 % fewer lumens for the same actual power draw.
Room height 2.4-2.8 m
| Ceiling material | 100 lx | 200 lx | 300 lx | 500 lx |
|---|---|---|---|---|
| White matt | 250 lm/m | 500 lm/m | 750 lm/m | 1200 lm/m |
| Light grey / beige | 300 lm/m | 600 lm/m | 950 lm/m | 1600 lm/m |
| Pine panel, light timber | 400 lm/m | 850 lm/m | 1300 lm/m | 2100 lm/m |
| Dark timber, concrete | 650 lm/m | 1300 lm/m | 2000 lm/m | not realistic |
Room height 2.9-3.5 m
| Ceiling material | 100 lx | 200 lx | 300 lx | 500 lx |
|---|---|---|---|---|
| White matt | 300 lm/m | 600 lm/m | 900 lm/m | 1500 lm/m |
| Light grey / beige | 400 lm/m | 750 lm/m | 1100 lm/m | 1900 lm/m |
| Pine panel, light timber | 500 lm/m | 1000 lm/m | 1500 lm/m | 2600 lm/m |
| Dark timber, concrete | 800 lm/m | 1600 lm/m | 2500 lm/m | not realistic |
More than 2,600 lm/m means two strips side by side or another special strip – it is no longer economical. The right solution then is to lighten the ceiling or add direct light.
In practice: reading the table – living room 4 × 5 m, white ceiling, height 2.6 m, target 200 lx
Row ”White matt”, column ”200 lx”, upper table → 500 lm/m. That is a 7.2 W/m strip (700 lm/m) – not enough – or 14.4 W/m (1,400 lm/m), which is ample and is dimmed to 40 %.
The same room with a pine-panel ceiling: 850 lm/m → a 17 W/m strip at full power. The same room 3.2 m tall with a timber ceiling: 1,000 lm/m → two strips side by side, or paint the ceiling.
The three most common sizing mistakes
- Sizing by power (W/m), not lumens. Two different 14.4 W/m strips can give 1,100 and 1,600 lm/m. A CCT strip gives 25 % less at the same actual power draw. Check the lm/m in the product data.
- Forgetting the ceiling colour. The designer calculates for a white ceiling or wall, but the client chooses a dark panel or paint. The result is half of what was expected. Decide the surface material before the strip.
- Mood power as general light. 7.2 W/m at the ceiling edge gives 30-80 lx – enough for mood, not for cleaning. If you need to see in the room, 14.4 W/m and a dimmer.
Strip metres, driver size and voltage drop are calculated with the sizing calculator – it takes the ceiling and height into account automatically. The driver 80 % rule and feeding long runs: Driver sizing.
Strips with lumen data → LedStore
Frequently asked questions
How many lux does indirect light give?
From a white matt ceiling, a 14.4 W/m strip gives about 200 lux on the floor in a normal room. From timber panelling the same strip gives 120 lx. Reading needs 300 lx with an accent light.
Why does indirect light need more power than direct light?
The light first hits the ceiling, which absorbs 18-70 % depending on the material, and the rest spreads through the whole room – about 55 % reaches the floor. In total 22-45 % of the strip’s light reaches the floor, whereas a direct luminaire gives 85 %.
How many lumens per metre for indirect light?
200 lx on a white ceiling: about 500 lm/m, i.e. a 7.2-14.4 W/m strip. The same target on timber panelling: 850 lm/m, i.e. 17 W/m. The table on the page covers 100-500 lx and four ceiling materials.
Can indirect light be the only light source?
In living spaces yes, if the ceiling is light and the power 14.4 W/m. In workplaces no – 500 lx indirectly would need three times the power. An accent light alongside also adds depth.
How does the ceiling colour affect the amount of light?
Decisively. White matt reflects 82 %, light grey 65 %, pine panel 48 %, dark under 30 %. A timber ceiling needs almost double the power, a dark ceiling does not work for general light at all.
What is the difference between lux and lumen?
A lumen is the amount of light leaving the source. A lux is the amount of light landing on a surface: lumens divided by square metres. You buy a strip in lumens, set the target in lux, and between them are the ceiling and the room.