What Is a Lux Meter? Types and Selection Criteria

A lux meter is a handheld light meter that measures the amount of light falling on a surface, the illuminance, in lux (lx).

A lux meter (light meter) measures the luminous flux falling on a unit area of a surface, that is, the illuminance, in lux (lx); 1 lx is 1 lumen falling on one square metre. It is used in offices, workshops, warehouses, classrooms and car parks to check whether the lighting meets the design value and to document the level before and after a luminaire replacement. A lux meter does not measure a lamp's total luminous flux (lumens) or its colour temperature (K).

Types

• Mini (pocket) lux meter: The sensor sits on top of the housing; for quick checks and service work. In the catalogue: UNI-T UT383 and UT383S.
• Bluetooth lux meter: Sends readings to a phone app, which makes recording multi-point measurements easier. In the catalogue: UNI-T UT383BT (iENV app).
• Wide-range, high-resolution lux meter: Reads very low and very high levels; suitable for emergency lighting and outdoor measurements. In the catalogue: UNI-T UT381A.
• Lux meter with a separate sensor: The sensor head is connected to the body by a cable, so the person measuring does not shade the reading. Check the product pages for whether a catalog model has this design.

UV radiation and plant-growth light (PAR) need different sensors; a lux meter weights light according to the sensitivity of the human eye.

Selection criteria

1. Measuring range: Indoor lighting is mostly between a few hundred and a few thousand lux; direct sunlight can exceed 100,000 lx. The UT383 series measures up to about 200,000 lx and the UT381A up to 400,000 lx.
2. Resolution: Emergency lighting and night-time outdoor measurements involve values below 1 lx; the UT381A resolves 0.01 lx below 100 lx.
3. Accuracy and light source: Lux meters are usually calibrated with 2856 K incandescent light; under LED and discharge lamps the spectrum differs and the error grows. Unit (UNI-T) states ±3% for the UT381A with that source and ±6% with other sources; the UT383 series is specified at about ±(4% + 8 digits).
4. Cosine correction: Keeps the reading correct when light reaches the sensor at an angle; important in large areas with many luminaires.
5. Functions and logging: MAX/MIN, data hold and auto range are common; Bluetooth or memory speeds up a measurement report.
6. Calibration certificate: Measurements for an official report need a valid calibration certificate for the instrument.

How to measure correctly

• Hold the sensor parallel to the working plane; for a desk, the sensor should lie on the desk surface.
• Keep your own shadow and reflections from light-coloured clothing away from the sensor.
• After switching the lamps on, wait until the light level settles.
• Measure at points spread over the area, not at a single point; assess the average together with the ratio of the lowest value to the average (uniformity).
• When measuring artificial lighting, block out daylight or measure in the evening.

Common mistakes

• Confusing lux with lumens: Lumens are the total light a lamp emits, lux is the amount reaching a surface; the same lamp gives fewer lux on a distant surface.
• Ignoring the calibration-source difference when measuring LED lighting.
• Treating a single-point reading as the value for the whole room.
• Preparing a report with a phone app: A phone's light sensor is made for screen brightness and is not calibrated.

Related standards

• EN 12464-1: Required illuminance and uniformity for indoor work places.
• EN 12464-2: Lighting of outdoor work places.
• EN 1838: Illuminance on escape routes and open areas for emergency lighting.

Models in the catalogue

The Lux Meters (Light Meters) category has 5 active products from Unit. Four of them are lux meters: UT381A, UT383, UT383S and the Bluetooth UT383BT. The fifth, the UT385, is a laser power meter that measures the power of a laser beam (0–39.99 mW); it does not measure illuminance. For sound, air speed and vibration measurement, see the Environmental Measuring Instruments category.

Frequently asked questions

How many lux should an office have?

EN 12464-1 requires 500 lx on the working surface for writing, reading and computer work. Areas such as corridors and storerooms need less; the exact value for a room is given in the lighting design.

Can a lux meter measure a lamp's lumen output?

No. A lux meter measures the light falling on a surface; a lamp's total lumen output is measured with laboratory set-ups such as an integrating sphere.

Does a lux meter measure LED lighting accurately?

It does, but the error margin is larger than under incandescent light. Check the accuracy the manufacturer gives for other light sources; for the UT381A it is ±6%.

What is the difference between a mini lux meter and a Bluetooth model?

The measuring function is the same; the UT383BT sends readings to a phone, which makes recording multi-point measurements and preparing a report easier.