Light Meter (Lux Meter)
Measure how bright a room, desk or plant spot is, in lux and foot-candles. The meter uses your phone's ambient light sensor when the browser allows it, or estimates the light from your camera's exposure, and tells you whether the level suits the room or task you choose.
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For the best estimate, place a sheet of white paper where you want to measure and fill the view with it, or cover the lens with thin white paper as a diffuser and point it at the light.
Compare once with a real lux meter (or a phone with a reliable sensor) under the same light, enter its value and tap Calibrate. The factor is remembered on this device.
| Room or task | Recommended lux | Foot-candles | Your reading |
|---|
Typical maintained illuminance on the work surface, based on IES Lighting Handbook and EN 12464-1 guidance; plant ranges follow common horticulture guidance in foot-candles.
How to use the Light Meter (Lux Meter)
- Choose a source: Auto tries the phone's light sensor first, then the back camera. You can also pick the front or back camera directly.
- Tap Start light meter and allow camera access if asked.
- Measure at the spot that matters: hold the phone at desk, counter or leaf height. With a camera, fill the view with a sheet of white paper lying at that spot.
- Read lux and foot-candles, and use Hold reading to freeze the value.
- Pick a room or task to see whether the light is too dim, good or brighter than needed, and compare against the full table.
- Calibrate once against a real lux meter for more accurate camera readings.
How the light meter calculates lux
Illuminance is the amount of light falling on a surface, measured in lux (lumens per square metre) or foot-candles (lumens per square foot). 1 fc = 10.764 lx. This tool shows the same value three ways: lux, foot-candles and the photographic exposure value EV at ISO 100, using the incident-light meter relation EV = log2(E / 2.5).
- Ambient light sensor: the phone's own light sensor (next to the front camera) reports lux directly. Chrome rounds these values for privacy, so readings may move in steps (about 50 lux in current versions).
- Camera estimate: the camera image is reduced to 64 × 48 pixels, converted from sRGB to linear brightness M (Rec. 709 luminance), and combined with the exposure time t (seconds), ISO S and an assumed aperture of f/1.8. Pointing at white paper (about 90% reflectance, ρ = 0.9), the estimate is E ≈ π · K · N² · M / (0.18 · ρ · t · S) with the standard meter constant K = 12.5. Where the browser allows manual exposure, the page runs its own auto-exposure so the image stays unclipped.
The correction factor multiplies every reading. Measure once under steady light next to a reliable lux meter, enter its value and tap Calibrate to known; the camera and lens of your phone then give much more consistent results.
Typical light levels
| Situation | Approximate illuminance | Foot-candles |
|---|---|---|
| Clear night, full moon | 0.05–0.3 lx | 0.005–0.03 fc |
| Deep twilight | about 1–10 lx | 0.1–1 fc |
| Living room in the evening | about 50–150 lx | 5–14 fc |
| Office or classroom | 300–500 lx | 28–46 fc |
| Sunrise or sunset, overcast day | about 400–1,000 lx | 37–93 fc |
| Full daylight, not in direct sun | 10,000–25,000 lx | 930–2,300 fc |
| Direct sunlight | 32,000–100,000 lx | 3,000–9,300 fc |
Indoors, light falls off quickly with distance: from a ceiling light it drops with the square of the distance, and a desk 3 m from a window can receive a tenth of the light of the windowsill.
Tips for reliable readings
- Measure where the light is used: on the desk, worktop, page or leaves, not at face height in the middle of the room.
- Keep your shadow out: hold the phone at arm's length and don't stand between the light and the sensor.
- Light sensor: point the top front edge of the phone (near the selfie camera) at the main light source and keep the screen brightness steady.
- Camera: lay plain white paper on the surface and fill the camera view with it; avoid shiny or coloured surfaces, which bias the estimate.
- Take several readings across the area and use the average and minimum: good lighting is even, so the minimum matters as much as the peak.
- Coloured LED and grow lights can fool both phone sensors and cameras, because lux is weighted by human eye sensitivity and not by what plants use (PAR).
Frequently asked questions
How accurate is a phone light meter?
Phone ambient light sensors often agree reasonably well with a real lux meter under ordinary white light, but they vary between models, sit behind the screen glass and are tuned for screen brightness, not measurement. Camera estimates depend on the lens aperture and image processing, so treat them as rough estimates until you calibrate against a known reference. Use the readings to compare spaces and spot clearly too-dim areas rather than for certified measurements.
Why does the light meter use my camera instead of the light sensor?
Web pages can only read the ambient light sensor through the Ambient Light Sensor API, which Chrome on Android offers behind chrome://flags/#enable-generic-sensor-extra-classes. Safari, Firefox and desktop browsers do not provide it, so the tool falls back to estimating light from the camera's exposure settings.
How many lux do I need for reading or working at a desk?
About 300–500 lux on the desk surface is the common recommendation for office and computer work, and 500–750 lux for reading, writing and detailed paperwork. Older eyes often need more light for the same task.
How do I convert lux to foot-candles?
One foot-candle is one lumen per square foot, which equals 10.764 lux. Divide lux by 10.764 to get foot-candles, or multiply foot-candles by 10.764 to get lux. For example, 500 lux is about 46 foot-candles.
How much light do houseplants need?
As a rule of thumb, low-light plants need about 50–250 foot-candles (roughly 540–2,700 lux), medium and bright-indirect plants 250–1,000 foot-candles (2,700–10,800 lux) and high-light plants more than 1,000 foot-candles. Light drops quickly with distance from a window, so measure at leaf height at different times of day.
Why does my reading show only a percentage?
Some cameras, especially on iPhone and many laptops, do not tell the browser their exposure time and ISO. Without those values the brightness of the picture cannot be turned into lux, because auto exposure keeps the image near mid-grey in both dim and bright light. The percentage is then only the relative brightness of the image.
Does the light meter record or upload my camera?
No. Frames are analysed in your browser only, one tiny 64 × 48 pixel copy at a time, and nothing is saved or sent anywhere. The camera turns off when you press Stop or leave the page.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.