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What Is a Lux Meter and When Is It Useful?

A lux meter reads illuminance: the light arriving at a surface, weighted the way eyes see it. It answers whether a desk, a classroom or a windowsill is actually lit to the level a standard or a plant requires.

By the Akermin Editorial Team · 3 min read

A lux meter measures illuminance: the amount of visible light falling on a surface, in lux (lumens per square meter). Its sensor is filtered to respond like the human eye, so the reading tracks how bright a surface looks rather than raw energy. It is the tool for checking whether a workplace, classroom, warehouse or plant shelf meets a lighting requirement, for balancing lighting between areas, and for measuring daylight.

Lux, lumens, candela and foot-candles

Unit Measures Relationship
Lumen (lm) Total light output of a source On the bulb box
Lux (lx) Light arriving per square meter 1 lx = 1 lm/m²
Foot-candle (fc) Light arriving per square foot 1 fc = 10.76 lx
Candela (cd) Intensity in a direction 1 cd at 1 m gives 1 lx

Illuminance falls with the square of distance from a point source: a lamp giving 400 lux at 1 m gives 100 lux at 2 m.

How the meter works

A silicon photodiode behind a filter that matches the eye’s photopic sensitivity curve, under a white diffusing dome that gives cosine correction so light arriving at an angle is weighted correctly. The dome is important: without it, a meter reads low for light from the side and cannot measure a room’s general illuminance properly. Better meters state their deviation from the ideal curve (f1′) and cosine error (f2); Class B and Class C meters under DIN 5032 define acceptable limits.

Typical illuminance levels

Situation Approximate lux
Moonlight 0.1 to 0.3
Street lighting 5 to 20
Living room evening 50 to 150
Corridor, warehouse aisle 100 to 200
Office desk (common guidance) 300 to 500
Detailed work, drafting 750 to 1,500
Overcast daylight outdoors 1,000 to 10,000
Direct sunlight 30,000 to 100,000

When it is useful

  • Workplace and school lighting checks against guidance such as 500 lux for offices, with uniformity across the room.
  • Warehouses and car parks where minimum lux at floor level is a safety requirement.
  • Retail and display where product lighting is specified.
  • Horticulture, with the caveat that plants respond to a different spectrum; a lux reading is a rough proxy and a PAR meter is the proper tool for grow lights.
  • Photography and film for scene exposure and balancing.
  • Daylight studies comparing rooms and orientations.
  • Emergency lighting verification, which often needs a low-range meter reading down to 0.1 lux.

LEDs and spectrum

A meter’s accuracy depends on how closely its filter matches the eye curve across the spectrum. Incandescent light is broad and forgiving; LEDs have spiky spectra that expose filter errors. Inexpensive meters can misread LEDs by tens of percent. Meters marketed as LED-capable, or those stating a low f1′ figure, handle them properly. Extech’s LT45 is an example of a meter designed for LED and colored light.

Measuring properly

  1. Hold the sensor flat on the working plane (desk height for offices, floor for corridors), dome up.
  2. Keep your body and shadow away from the sensor.
  3. Take readings on a grid and report the average and the minimum-to-average ratio for uniformity.
  4. Let lamps warm up; some discharge and LED fixtures change output over the first minutes.
  5. Note daylight contribution or measure after dark for the artificial lighting alone.

Light meters sit alongside sound and airflow instruments in the environmental monitoring category; multifunction meters combine all three.

Extech LT300 Light Meter, Remote Sensor, Peak and Relative Modes

Extech LT300 Light Meter, Remote Sensor, Peak and Relative Modes

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Dr.meter Digital Illuminance Light Meter 0-200,000 Lux Range Lux Meter

Dr.meter Digital Illuminance Light Meter 0-200,000 Lux Range Lux Meter

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

What is the difference between lux and lumens?

Lumens measure the total light a source emits. Lux measures the light arriving on a surface: one lux is one lumen spread over one square meter. A 1,000-lumen lamp gives 1,000 lux over 1 m² or 100 lux over 10 m².

How many lux do I need for a desk?

Office guidance is commonly 300 to 500 lux on the working plane; detailed tasks 750 to 1,000 lux. Corridors and general areas 100 to 200 lux.

Can a lux meter measure LED lighting accurately?

Only if its sensor is filtered to the eye's response curve (photopic V(λ)). Cheap meters calibrated for incandescent light can misread LEDs by 10 to 30 percent; meters marketed for LEDs use better filters.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.