An employee complains about the light at their workstation. The safety manager takes a lux meter out of the cupboard, places the cell on the work surface at eleven o’clock on a bright sunny day, reads a comfortable value and closes the file. Three weeks later, the same complaint comes back, this time backed by the occupational doctor.
The measurement was not wrong: it simply did not measure what was needed. An illuminance survey is carried out according to a protocol, otherwise the figures obtained stand up in front of no one.
What a lux meter measures and what it does not say
A lux meter measures an illuminance, that is, a luminous flux received per unit of area, on the plane where the cell is placed and in the orientation in which it is placed. It says nothing about perceived glare, colour rendering, colour temperature, contrasts or flicker.
The quality of the instrument matters as much as the method. A proper lux meter corrects its response to follow the sensitivity of the human eye and corrects for light striking the cell at an oblique angle, two characteristics that entry-level instruments and phone apps do not handle.
Finally, a survey intended to be put before a third party requires an instrument whose calibration is traceable and dated. Without this certificate, the discussion ends before it even turns to the values.
Choosing the grid of measuring points
Measuring three points by guesswork does not characterise a room. The EN 12464-1 standard describes a calculation and measurement grid whose spacing depends on the size of the area studied, with meshes close to square and a limited ratio between their two sides.
Two practical rules avoid the most common biases. The grid is not aligned with the luminaires, otherwise it systematically measures the maxima or systematically the minima, and the first row of points is kept away from the walls, where illuminance drops for obvious geometric reasons.
The task area is treated with its own finer grid, separate from that of the rest of the room. That is where compliance at the workstation is decided, not in the middle of the aisle.
Height of the measuring plane and orientation of the cell
The measuring plane is the one where the task is carried out. For a seated workstation, the measurement is taken at the height of the work surface; for a standing workstation, at the height of the handling plane; for a circulation area or a car park, at floor level.
The cell is laid flat, perfectly horizontal, and above all never held at arm’s length above the area. The operator stands so as not to block any luminaire and avoids light-coloured clothing, which reflects light towards the cell and inflates the value read.
Neutralising the contribution of daylight
This is the most frequent mistake and the one with the heaviest consequences. A large window can on its own bring several times the illuminance of the artificial installation in the middle of the day, and completely mask a failing installation.
Two methods work. The first consists of measuring at night or with blackouts closed, which directly gives the contribution of the artificial lighting. The second consists of taking two series of measurements at the same place, luminaires on then off, and keeping the difference.
In all cases, the time, date, weather and state of the blinds are recorded. This information is part of the result, just like the values.
Stabilising the luminaires before the reading
An LED luminaire switched on two minutes ago does not emit the same flux as a luminaire in a stable thermal state: the rise in junction temperature brings the flux down before it stabilises. Measuring too early overestimates the installation.
The lux meter itself needs a warm-up time, and a cell that has just moved from a dark room to a very bright area takes a moment to stabilise. A reasonable waiting time before the first measurement, then a regular route through the room, are enough to rule out this bias.
Horizontal, vertical and cylindrical illuminance
Horizontal illuminance is not enough to describe every work situation. In front of a rack, on a loading dock or facing a control panel, it is vertical illuminance that determines legibility, and it is measured with the cell upright, facing along the line of sight.
The 2021 version of the EN 12464-1 standard also strengthened the requirements for cylindrical illuminance, which characterises the lighting of volumes and faces, with one measuring height defined for seated people and another for standing people. This quantity explains why a room can show a compliant horizontal average while giving an impression of greyness.
Common pitfall | Effect on the result | Remedy |
|---|---|---|
Daytime measurement near a window | Value greatly overestimated | Measure at night or subtract the daylight contribution |
Grid aligned with the luminaires | Biased average, false uniformity | Grid independent of the layout |
Cell held by hand | Variable tilt, scattered values | Cell laid flat or on a tripod |
Reading straight after switch-on | Flux overestimated on LED luminaires | Wait for thermal stabilisation |
Comparison with the value when new | Installation wrongly declared compliant | Compare with the maintained illuminance |
Recording the measurements so that they can be relied on
A usable survey includes the plan of the room with the numbered points, the values recorded point by point, the reference and serial number of the instrument, the date of its last calibration, the date and time of the survey, the state of the blackouts, the operating mode of the installation and the name of the operator.
The comparison is then made with the maintained illuminance expected for the task, and not with a value when new or a value found in sales documentation. A gap is documented, located on the plan and linked to a cause: dirt, a luminaire out of service, an unsuitable layout, a change in the use of the room.
This survey forms the basis of an on-site lighting audit and the reference document for establishing a gap with the applicable compliance requirements. Compared afterwards with a photometric study of the room, it makes it possible to tell what comes from ageing from what comes from an original design error. For the workstations concerned, the ranges of luminaires for workspaces cover the levels expected by the standard.
To have an independent survey carried out with a calibrated instrument and a usable report, request a visit to your site.
