How bright is daylight? Lux outdoors and indoors

What does lux measure?

Lux is the SI unit of illuminance: one lumen per square meter (BIPM, 2019). Put simply, it tells you how much light arrives on each square meter of a surface.

The same room can give very different readings. How much light reaches you depends strongly on distance, the direction you look in, the shape of the room and how much daylight comes in (Piper et al., 2026). Expert light recommendations therefore say exactly where to measure: at the eye, in the vertical plane, about 1.2 m above the floor, which is eye level when seated (Brown et al., 2022).

Wearing an Apple Watch? What its Time in Daylight value tells you.

Daylight lux outdoors: sun, rain and dusk

At midday, sunlight outdoors can go beyond 100,000 lux. When it rains, it stays above 3,000 lux. At the start of civil twilight, it is still about 1,000 lux outdoors (Fernandez, 2022).

In the evening, civil twilight ends when the center of the sun is 6 degrees below the horizon. After that you normally need artificial light for ordinary outdoor activities (USNO).

Never look directly at the sun.

Get outdoor daylight on a morning walk: timing and minutes.

What color temperature is daylight in kelvin?

Color temperature describes the color of light, given in kelvin (K). It is the temperature of a Planckian radiator whose radiation has the same chromaticity as the light in question (CIE). When a light does not match such a radiator exactly, the standard uses the correlated color temperature (CCT): the temperature of the Planckian radiator with the nearest chromaticity (CIE).

The reference for standard daylight is CIE standard illuminant D65. It is intended to represent average daylight with a correlated color temperature of approximately 6,500 K (CIE). The same standard defines illuminant D50 as daylight at approximately 5,000 K and illuminant A, for typical tungsten-filament lighting, at approximately 2,856 K (CIE).

Real daylight changes all the time. Natural outdoor light goes through large changes in its correlated color temperature every day (Hernández-Andrés et al., 1999). In nearly 7,000 measured daylight and skylight spectra, the values spanned roughly 3,000 to 1,000,000 K (Hernández-Andrés et al., 1999). Two years of measurements in Granada, Spain, covering all sky states, found that 5,700 K best typified the local daylight (Hernández-Andrés et al., 2001). The standard also notes that the spectrum of daylight varies with season, time of day and location (CIE).

The kelvin number does not tell you how strongly a light acts on your inner clock. A 2026 policy brief states that lamp labels such as correlated color temperature do not reliably indicate the non-visual stimulus at the eye (Piper et al., 2026). For that, experts use melanopic EDI, which compares a light with a standard 6,500 K (D65) daylight spectrum (Brown et al., 2022). Two lamps with the same kelvin value can differ: an off-the-shelf 6,500 K white LED had a melanopic efficacy of 0.83 relative to D65 at the same lux, while well-designed 4,000 K sources can reach up to 1.4 (Brown et al., 2022).

How much daylight does your weekday get? Take the light check.

How bright is it indoors at home and at the office?

Many people spend most of the day indoors, where light levels are often much lower than outdoors (Piper et al., 2026). Existing lighting guidelines usually keep indoor light at least ten times below the natural light outdoors (Brown et al., 2022).

At work, the European guideline sets a minimum of 500 lux on the desk for writing, typing and reading (Brown et al., 2022). At home in the evening, measurements in everyday life commonly report around 30 lux; some rooms such as kitchens can be brighter (Brown et al., 2022).

Sitting by the window: how much daylight gets inside.

How much light experts recommend, and what melanopic EDI means

An international expert panel gives these values for indoor light (Brown et al., 2022). Throughout the daytime, a minimum melanopic EDI of 250 lux at the eye, measured in the vertical plane at about 1.2 m height. In the evening, starting at least 3 hours before bedtime, a maximum melanopic EDI of 10 lux, measured the same way. In the sleep environment, a maximum melanopic EDI of 1 lux. If daylight is available, it should be used first to reach the daytime level.

Melanopic EDI stands for melanopic equivalent daylight illuminance. Some nerve cells in the retina are light-sensitive themselves and contain melanopsin. Melanopic EDI tells you how strongly a light acts on them, given as the lux of standard daylight that would act just as strongly (Brown et al., 2022).

So melanopic EDI is not the number a normal lux meter shows. Common electric lamps act much less on these cells than daylight with the same lux. A desk lit to 500 lux with ordinary lighting typically gives less than 200 lux melanopic EDI at the eye (Brown et al., 2022). The recommendations apply to healthy adults aged 18 to 55 with regular daytime schedules.

Can you measure lux with your phone?

Phones have an ambient light sensor, and apps can read it. In one study, the light sensors of Android phones gave a reasonable measure of illuminance. They are unlikely to match a calibrated lux meter, but are probably good enough to see large differences in light (Gardasevic et al., 2022). On iPhones, apps have restricted access to this sensor (Gardasevic et al., 2022).

Keep two limits in mind. A phone measures plain illuminance, not melanopic EDI. And it measures in the direction the phone points: while you look at the screen, that is roughly the opposite of your direction of view (Gardasevic et al., 2022). Use the reading as a rough estimate, for example to compare a spot by the window with the middle of the room.

The LightOtter light check does not measure lux. It asks six short questions about your usual weekday and gives an estimate from your answers and the position of the sun, not a measurement.

Take the 1-minute light check for an estimate of your day.

Lux or minutes?

Research does not name one number of minutes of bright light that fits everyone. A 2026 policy brief recommends daylight each day, especially within the first few hours after waking, while following local sun safety advice (Piper et al., 2026).

LightOtter does not ask you to measure lux. It estimates from the clouds and the height of the sun where you are how long to stay outside in the morning: about 10 minutes when it is clear, 15 when partly cloudy, 20 to 25 when overcast and up to 30 with a low sun. Those minutes are a rule of thumb, estimated, not measured.

Check today’s minutes outside for your city.

What this is not

The lux values on this page describe typical light levels from the sources below. They are not personal targets and not medical advice, and LightOtter is not a medical device. If you feel tired for a long time, talk to a doctor.