How to measure lux properly
Measuring lux sounds simple: hold a meter up and read the number. In practice a single reading tells you almost nothing about whether a space is adequately lit. Illuminance varies from point to point, changes with daylight and declines as an installation ages. A meaningful lux measurement uses a calibrated meter, a defined grid, the correct measurement height and controlled conditions. This is the method we follow on every lux test.
Step 1: Use a suitable, calibrated meter
The meter should be cosine and V(λ) corrected and within its calibration period, ideally Class A or B to DIN 5032-7. Phone apps are not suitable. See what is a lux meter? for the detail.
Step 2: Define the areas to be measured
BS EN 12464-1:2021 sets requirements for the task area, its immediate surrounding area and the wider background area. For a practical survey we divide the premises into zones by activity – open-plan desks, meeting rooms, corridors, stairs, storage aisles, packing benches – because each has a different target. A warehouse may have racking aisles at 100 lux and a packing area at 300 lux under the same roof.
Step 3: Set out the lux measurement grid
The grid method in BS EN 12464-1:2021 ensures enough points to capture variation without wasting time on unnecessary readings. The maximum grid spacing is given by:
p = 0.2 × 5^(log10 d)
where p is the maximum spacing between points in metres (never more than 10 m) and d is the longer dimension of the area in metres. If the longer side is twice the shorter side or more, d is the shorter dimension instead. The number of points along each side is the nearest whole number to d divided by p, and the grid cells should be close to square, with a length-to-width ratio between 0.5 and 2. Readings are taken at the centre of each cell.
| Area dimension d | Maximum grid spacing p | Typical example |
|---|---|---|
| 1 m | 0.2 m | Individual desk task area |
| 5 m | about 0.6 m | Small office or meeting room |
| 10 m | 1 m | Medium open-plan office |
| 50 m | about 3 m | Warehouse floor or sports hall |
| 100 m | 5 m | Large distribution centre |
The 0.5 m perimeter band
A band of 0.5 m from the walls is normally excluded from the calculation area, because light falls off at room edges where nobody works. The exception is where a task area sits within that band, such as a desk or workbench against a wall, in which case it is included.
Irregular and narrow areas
For L-shaped or irregular rooms, the grid is set out across an enclosing rectangle and only points inside the actual area are used. For narrow strips such as corridors, the grid spacing should be no more than half the width of the strip, so a meaningful line of readings runs along its length.
Step 4: Measure at the correct working plane height
Illuminance is measured on the plane where the visual task happens. Measuring at the wrong height is one of the most common errors in DIY surveys, because readings change noticeably between floor and desk.
- Desk-based work: on the working plane at desk height, commonly around 0.75 m above floor level.
- Standing tasks and benches: at the height of the actual worktop or bench.
- Corridors, stairs and circulation areas: at floor level, or on the treads for stairs.
- Emergency escape lighting: at floor level along the centre line of escape routes and across open areas, as required by BS EN 1838:2024.
- Vertical tasks: on the vertical plane where relevant, for example racking faces, shelving or whiteboards.
The meter sensor should be level, and the surveyor should stand clear so their body does not shadow the sensor or reflect extra light onto it.
Step 5: Exclude or account for daylight
BS EN 12464-1:2021 requirements apply to the lighting installation, and daylight changes minute by minute. To assess the electric lighting fairly we:
- survey after dark where practical, which is often the most reliable approach for offices and schools;
- close blinds and shut out daylight in rooms where that is achievable; or
- take paired readings with lights on and off at each point and subtract the daylight contribution, when daylight is steady.
Some clients also want a daytime “as used” picture. That is useful context, but it should be reported separately from the compliance assessment.
Step 6: Allow lamps to stabilise
Discharge lamps such as fluorescent and metal halide need time to reach full output after switching on, and output varies with temperature. LEDs stabilise faster but still change slightly as they warm. Lights should run long enough to stabilise before measurement, and any dimming or presence detection controls set to give full output.
Step 7: Calculate average, minimum and uniformity
From the grid readings for each area we calculate:
- Average illuminance (Ē) – the mean of all grid readings;
- Minimum illuminance (Emin) – the lowest single reading; and
- Uniformity (Uo) – Emin divided by Ē.
These are compared with the required values, for example 500 lux and Uo 0.60 for office writing and reading tasks. Our average lux levels guide explains how the three figures work together.
Maintained versus initial illuminance
The values in BS EN 12464-1:2021 are maintained illuminance: the level below which the average illuminance should not fall at any point in the life of the installation, right up to the time of scheduled maintenance. Light output drops over time as lamps age, luminaires get dirty and room surfaces darken. Designers allow for this using a maintenance factor.
So a brand new installation should measure noticeably above the maintained target. If a new office is designed with a maintenance factor of 0.8 for 500 lux maintained, the initial average would be expected to be around 625 lux. A new installation that measures exactly 500 lux on day one is likely to fall below target as it ages. Conversely, an older installation measured just before cleaning or relamping should still meet the maintained figure. We note the age and condition of the installation in the report so results can be read in context. This is particularly important after an LED upgrade.
Step 8: Record and report
A good report includes floor plans or grid diagrams with individual readings, the meter and calibration details, measurement height, daylight conditions, the standard and table entry used for each area, and a clear status for each zone. See lux survey report explained.
Have your lux levels measured
Polarity Lux measures lux to the BS EN 12464-1:2021 grid method for offices, warehouses, schools and other premises across London and the South East. Book a survey and we will confirm a fixed price before we visit.
Frequently asked questions
How many lux readings are needed in a room?
It depends on the room size. BS EN 12464-1:2021 sets a maximum grid spacing of 0.2 × 5^(log10 d), so a room about 10 m long needs points roughly every 1 m, while a 5 m room needs them about every 0.6 m.
At what height should lux be measured?
On the plane where the task happens. That is typically desk height, around 0.75 m, for office work, the actual bench height for standing tasks, and floor level for corridors, stairs and emergency escape routes.
Should lux be measured with daylight?
For a compliance assessment of the electric lighting, daylight should be excluded, either by surveying after dark, blacking out the room, or subtracting daylight readings taken with the lights off.
What is the difference between maintained and initial illuminance?
Initial illuminance is what a new, clean installation delivers. Maintained illuminance is the minimum average level the installation must still achieve at the end of its maintenance cycle, so a new installation should measure higher than the maintained target.
Why is a 0.5 m band at the walls excluded?
Light naturally falls off at room edges where people do not normally work, so BS EN 12464-1:2021 excludes a 0.5 m perimeter band unless a task area, such as a desk against the wall, lies within it.