Light Meter Online: Measure Lux With Your Phone

How the light meter works

Press Start light meter, allow camera access and point the back camera at the area you want to measure. The page reads the light in one of three ways and shows which in the Source box.

• Light sensor. Phones have an ambient light sensor near the front camera that sets screen brightness automatically. A few browsers let websites read it, and when they do, the reading is a true lux value. Most browsers keep it switched off, so this is rare.
• Camera. On many Android phones, the browser shares the camera's exposure time and ISO. Combined with how bright the image comes out, this gives a sound estimate of the light level, typically within a factor of two of a real lux meter, which is plenty to tell a dim room from a well-lit office.
• Camera, relative. Safari on iPhone, and some other browsers, hide the exposure settings. The camera then adjusts itself to every scene, so the page cannot work out lux. It shows a relative brightness from 0 to 100 instead and says so, rather than inventing a number.

Getting a good reading

To measure the light falling on a desk, a page or a plant, lay a sheet of plain white paper there and point the camera at the paper from about 20 cm away. Keep your head and phone from casting a shadow on it. To measure a light source itself, such as a lamp or a window, point the camera straight at it, but expect higher numbers. Hold still for a couple of seconds while the reading settles, and watch the Min and Max values if the light flickers.

Calibrating against a real meter

Every phone camera is slightly different. If you can borrow a lux meter, or you know the light level of a space, adjust the Calibrate slider until the readings match. Each step of 1 EV doubles or halves the reading. The setting is saved on your device. Without calibration, treat the number as a good guide rather than a certified measurement.

How much light do you need?

Lux measures how much light falls on a surface. One lux is one lumen spread over one square metre. Workplace guidance in the UK and Europe suggests around 300 lux for general office work, 500 lux for reading and computer work, and 750 lux or more for detailed tasks such as drawing or fine assembly. Corridors and stairs need only 100 to 150 lux. Outdoors, an overcast day gives roughly 1,000 to 10,000 lux and direct sun over 30,000 lux, which is why indoor rooms feel bright to us but dark to a camera or a plant.

Light for houseplants

Most houseplants labelled low light survive from about 500 lux, bright indirect light means roughly 2,000 to 10,000 lux, and sun-loving plants such as cacti and herbs want more than 10,000 lux for several hours a day. A spot two metres from a window can get a tenth of the light right next to the glass. Measure at the time of day the plant gets its best light, and again in winter.

Lux and foot-candles

Foot-candles are the older imperial unit, still used in the United States. One foot-candle is one lumen per square foot, about 10.76 lux. The meter shows both.