For a room that feels like daylight, light it well past office level, 500 lux or more, from overhead. In a 20 m² (215 sq ft) room that takes about 25,000 lumens once you allow for the light lost to walls and ageing. Sunday Light gives 34,500 lumens from one ceiling fitting, and 5,000 lux and more sitting under it.
A single ordinary ceiling light gives a 20 m² room about 30 lux. The rest of this page is the arithmetic, so you can run it for your own room.
Lumens and lux
Lumens measure the light a fitting gives out. Lux measures the light landing on a surface: one lux is one lumen per square metre. A 1,000-lumen light spread evenly over 1 m² gives 1,000 lux; spread over a 20 m² floor it gives 50, before any losses.
Most homes sit at 100-300 lux. A well-lit office is about 500 lux, the level the European standard EN 12464-1 sets for reading, writing and typing. We spend around 90% of our time indoors, and indoors is 50 to 100 times dimmer than being outdoors.
How much light counts as daylight indoors?
In 2022 an international group of sleep and circadian scientists recommended at least 250 lux melanopic EDI at the eye throughout the day, measured vertically at seated eye height, about 1.2 m (Brown et al., PLOS Biology). They advise using daylight first where it's available.
Melanopic EDI is the amount of standard daylight that would stimulate the eye's melanopsin cells, the ones that set your body clock, as much as the light you have. For daylight the two numbers match. For ordinary white LEDs, melanopic EDI is lower: the paper uses a conservative ratio of 0.5 for bright white light, which puts 250 melanopic EDI at about 500 lux arriving at the eye. Cooler light does better. A 6500K LED in the paper's example has a ratio of 0.83, while 3000-4000K lighting is below 0.6.
The light reaching your eyes is lower than the light on the desk, and the paper notes that offices lit to the 500 lux standard typically give under 200 melanopic EDI at the eye. So with ordinary white light, clearing 250 takes more than office levels on the desk, and cooler light gets there sooner.
The lumen method
Lighting designers estimate a room with the lumen method:
Average lux = lumens × utilisation factor × maintenance factor ÷ floor area (m²)
The utilisation factor is the share of a fitting's light that reaches desk height after losses in the fitting and on the walls and ceiling. For a pale room, design tables in Kelly and O'Connell's Interior Lighting Design run from about 0.5 to 0.75, and their worked examples use 0.5. The maintenance factor allows for dirt and ageing: Zumtobel's Lighting Handbook gives 0.80 for very clean rooms and 0.67 for offices. We use 0.5 and 0.8, so four in every ten rated lumens end up as average lux on the working plane.
Turned round, lumens needed = lux × floor area ÷ 0.4. A 1,450-lumen ceiling light in a 20 m² room gives 1,450 × 0.4 ÷ 20, or about 29 lux, which is why a room lit by one ordinary ceiling light feels dim by day.
| Floor area | 300 lux (bright home) | 500 lux (office) | 1,000 lux |
|---|---|---|---|
| 10 m² (108 sq ft) | 7,500 lm | 12,500 lm | 25,000 lm |
| 15 m² (161 sq ft) | 11,250 lm | 18,750 lm | 37,500 lm |
| 20 m² (215 sq ft) | 15,000 lm | 25,000 lm | 50,000 lm |
| 30 m² (323 sq ft) | 22,500 lm | 37,500 lm | 75,000 lm |
Dark walls and floors push the utilisation factor down and the lumens needed up; white walls and a pale floor do the opposite. Spread over ordinary fittings, these totals are hard to reach: 25,000 lumens is seventeen 1,450-lumen ceiling lights.
The lumen method gives a floor average for light spread evenly from many fittings. It doesn't describe what you get under one large, very bright source, where the light is concentrated on the people sitting beneath it.
Is there a light that can do it?
Sunday Light, the artificial skylight we make in London, gives 34,500 lumens from one ceiling fitting. It lights a large area with 5,000 lux and more, reaching 10,000 lux at 60 cm from the sheet. Sitting under it, that's ten times a well-lit office.
The light comes from a sheet 120 cm across, overhead, so it reaches you wherever you sit. Professor Russell Foster, who directs the Sleep and Circadian Neuroscience Institute at Oxford and advises us, put it this way: "Critically, you do not have to sit next to the device to get the levels of light that will deliver physiological benefits."
Set a schedule once and Sunday follows the sun: cool 6000K daylight in the morning, then warmer and dimmer through the afternoon. The CRI is 97+. Water flows through the arm's tubes to cool the LED, which is what lets it run at this brightness all day. More on how it compares with other bright lights is in the brightest light fittings you can put in a home.
Brighter's floor lamp is rated higher still, up to 60,000 lumens at full power, from a 6 ft 3 in pole that stands in the room. Sky panels such as Philips' Skylight and Govee's Sky Ceiling Light give 3,800-7,000 lumens, the output of a good ceiling light, with the look of a sky.
Sunday Light, our sunlight fixture, is £5,900 in the UK, $6,900 in the US and €6,900 in Europe, with free worldwide delivery. The indoor sunlight ceiling light page has the full specifications, and the light glossary defines every term on this page.
