What is a lux meter?
A lux meter, or illuminance meter, measures how much visible light falls on a surface, expressed in lux (lumens per square metre). It is the core instrument of every lux test. A good meter does not simply count light: it is engineered to respond to light the way the human eye does and to weigh light arriving from different angles correctly. Those two corrections are what separate a measuring instrument from a gadget.
How does a lux meter work?
At its heart is a photodiode, usually silicon, which produces a small current proportional to the light reaching it. Electronics convert that current into a lux reading. On its own, though, a bare silicon photodiode is a poor judge of illuminance, for two reasons.
- It is sensitive to wavelengths the eye barely sees, including near-infrared, and its sensitivity across the visible spectrum does not match the eye.
- It responds to light from oblique angles differently from how light actually lands on a flat surface.
A quality lux meter corrects both problems with an optical filter and a carefully shaped diffuser.
V(λ) correction: matching the human eye
The eye is most sensitive to yellow-green light around 555 nm and much less sensitive to deep blue and red. This response is defined by the CIE photopic luminous efficiency function, written V(λ). A lux meter uses a filter so that its spectral response follows that curve as closely as possible.
The quality of this match is expressed as the f1′ value: the lower the percentage, the closer the meter is to V(λ). This matters more than ever with LED lighting. White LEDs have a strong blue peak and a spectrum quite unlike the tungsten lamps used to calibrate many meters, so a meter with poor spectral correction can misread LED installations by a significant margin, while reading tungsten correctly.
Cosine correction: light from every angle
Illuminance on a surface follows the cosine law: light arriving at a low angle delivers less illuminance than the same light arriving head-on. In a real room, light comes from many luminaires and reflecting surfaces at every angle. A cosine-corrected sensor uses a diffuser dome or disc so that its response falls off with angle in the same way. The deviation is expressed as f2. Poor cosine correction typically causes under-reading in rooms where much of the light arrives obliquely, such as wide open-plan offices or warehouses with luminaires spread across the ceiling.
Lux meter accuracy classes
German standard DIN 5032-7 groups illuminance meters into four classes based on their individual error characteristics. The figures below are the commonly cited class limits for spectral mismatch and cosine error.
| DIN 5032-7 class | Typical use | f1′ (V(λ) match) | f2 (cosine) |
|---|---|---|---|
| L | Laboratory reference | 1.5% | – |
| A | High-precision field measurement | 3% | 1.5% |
| B | General workplace measurement | 6% | 3% |
| C | Rough, indicative readings only | 9% | 6% |
Classes A and B are the ones appropriate for workplace compliance surveys. Class C instruments are only suitable for orientation readings.
BS 667
The British standard BS 667:2005 Illuminance meters – Requirements and test methods defines two types of meter: Type L for laboratory instruments and Type F for field instruments used in surveys. Its requirements for Type L meters align with BS EN 13032-1. In practice, when you commission a survey, ask which standard and class the meter meets and when it was last calibrated.
Calibration
All photodiodes and filters drift over time. A meter used for compliance work should hold a current calibration certificate, typically renewed annually, traceable to national standards. Calibration laboratories commonly use a reference tungsten source at a colour temperature of around 2856 K (CIE illuminant A). Because that differs from LED spectra, a meter with good f1′ performance is still essential even when the calibration is current.
Good practice on site includes checking the zero reading with the sensor covered, letting the meter and lamps stabilise, and keeping the operator’s body and shadow away from the sensor. These details are covered in how is lux measured?
Why phone apps are not compliance evidence
Smartphone light-meter apps read the ambient light sensor built into the phone. That sensor exists to adjust screen brightness, not to measure illuminance, and it has several weaknesses:
- No V(λ) correction to a recognised class. Sensor spectral responses vary by manufacturer and model, so LED lighting can be misread.
- Poor or no cosine correction. The sensor sits behind a small window in the bezel, often recessed, which restricts and distorts the angle of acceptance.
- No calibration. There is no traceable certificate, and two phones in the same spot can disagree widely.
- Unknown processing. Operating systems filter and scale sensor values in ways that are not documented for measurement use.
An app can tell you that one room is noticeably darker than another. It cannot show that a space meets 500 lux at a uniformity of 0.60, and it would not stand up to an insurer, an enforcing officer or a dispute with a landlord.
Should you buy your own lux meter?
A Class B meter is a reasonable purchase for organisations that want to spot-check between formal surveys. Remember that the meter is only part of the result: a defensible survey also depends on the correct grid, working plane height, daylight exclusion and interpretation against the right entry in BS EN 12464-1:2021. Our average lux levels guide explains how to read the numbers you get.
Get a calibrated survey
Polarity Lux uses calibrated, cosine and V(λ) corrected illuminance meters for every survey across London and the South East. Each report states the instrument used and its calibration status. Book a lux survey or check our fixed prices.
Frequently asked questions
What class of lux meter is needed for a workplace survey?
A Class A or Class B meter to DIN 5032-7, or a field-type meter to BS 667, with a current calibration certificate. Class C meters are only suitable for rough indicative readings.
How often should a lux meter be calibrated?
Annual calibration traceable to national standards is the usual practice for meters used in compliance work. The certificate date should be stated in the survey report.
Are phone lux meter apps accurate?
Not for compliance purposes. Phone ambient light sensors are not calibrated, have limited cosine and spectral correction, and different phones can give very different readings in the same spot.
Why do some lux meters misread LED lighting?
White LEDs have a spectrum very different from the tungsten lamps used for calibration. A meter whose spectral response does not closely follow the V(λ) eye-response curve, shown by a high f1′ value, can over- or under-read LED light.
What is cosine correction on a lux meter?
It is the shaping of the sensor's diffuser so it responds correctly to light arriving at oblique angles, matching the way illuminance actually falls on a flat surface. Without it, meters tend to under-read in rooms where light arrives from many angles.