Why LED upgrades change lux levels
Swapping fluorescent, high-pressure sodium or metal halide fittings for LED is one of the most common building improvements in the UK. The energy and maintenance savings are real. What is less often appreciated is that an LED retrofit is a lighting redesign, whether anyone treats it as one or not. The new luminaires have a different light output, a different beam distribution and a different surface brightness, so the illuminance on desks, floors and rack faces will change.
Sometimes it goes up, sometimes it goes down, and very often it goes up in some places and down in others. Without measurements nobody knows which.
What the standards expect after a retrofit
The benchmarks do not change because the light source has. For most workplaces the reference is BS EN 12464-1:2021, which sets maintained illuminance, uniformity, glare (UGR) and colour rendering values per task. HSE guidance HSG38 sets lower minimum values and regulation 8 of the Workplace (Health, Safety and Welfare) Regulations 1992 requires lighting to be suitable and sufficient. Typical targets include:
| Area | Maintained illuminance | Uniformity (Uo) | UGR limit |
|---|---|---|---|
| Office writing, reading, data processing | 500 lx | 0.60 | 19 |
| Corridors and circulation | 100 lx | 0.40 | 28 |
| Warehouse store rooms | 100 lx | 0.40 | 25 |
| Warehouse manned rack gangways | 150 lx | 0.40 | 22 |
These are the figures most UK designers and insurers work to. If you need a specific room type, our required lux lookup tool lists the values for common tasks.
The LED retrofit lighting survey: before and after
The baseline survey
Measuring before the work starts gives you a record of what the old installation delivered. It identifies areas that were already under-lit (so the specification can fix them rather than copy them), and it protects you in any later dispute with a contractor about whether the new lighting is worse. A baseline also lets you size the new fittings properly instead of relying on a “like-for-like” wattage swap.
The commissioning survey
Once the LEDs are installed and any controls are set up, a second survey checks the result against the targets. We measure on the same grid as the baseline so the two sets of figures can be compared directly, and we record:
- average maintained illuminance on each task area and its surround;
- uniformity (minimum divided by average);
- luminaire type, mounting height and optic, so UGR can be assessed;
- colour temperature and colour rendering as specified;
- visible flicker or stroboscopic effects, particularly near rotating machinery;
- daylight and presence-detection behaviour where controls are fitted.
Our guide to how lux is measured explains the grid method in more detail.
Common problems after an LED upgrade
Over-lighting
Modern LED panels are efficient enough that a one-for-one swap can push a 500 lx office to 800 lx or more. That wastes much of the energy saving and often brings glare complaints. Dimming the luminaires to the target level, and allowing for a maintenance factor, is usually the answer.
Under-lighting and dark patches
Narrow-beam LED high-bays in warehouses, or downlights in place of wide-distribution fluorescent fittings, can produce bright pools with dark gaps between. The average may look acceptable while the uniformity fails.
Glare
LED chips are small and intensely bright. Fittings without good diffusers or optics can exceed the UGR limit even at the correct lux level. This is the single biggest reason a retrofit makes a space measurably less compliant; see can an LED upgrade reduce compliance?
Emergency lighting overlooked
Where emergency fittings are replaced or converted as part of the project, the escape route levels in BS EN 1838:2024 (at least 1 lx on the centre line of escape routes and 0.5 lx in open areas) need re-checking, along with the testing and log-book requirements of BS 5266-1:2025.
Maintenance factor and the long term
All lighting loses output over time through ageing of the light source and dirt on the luminaires and room surfaces. The standards work in maintained illuminance, meaning the level must still be met at the end of the maintenance cycle. A new LED installation should therefore read above target on day one. If the commissioning survey only just meets the target, it may fall below it within a few years. Ask your designer what maintenance factor was assumed and compare it with the measured figures.
Practical tips for facilities managers
- Specify outcomes (lux, Uo, UGR, colour rendering) in the contract, not just fitting counts.
- Commission a baseline lux survey before any fittings come down.
- Make the final payment conditional on a passing post-installation survey.
- Keep both reports with the building’s O&M records; insurers and landlords increasingly ask for them.
- Re-survey after major layout changes such as new racking or partitioning.
Book an LED retrofit lighting survey
Polarity Lux carries out before-and-after LED retrofit surveys across London and the South East, from single offices to large distribution warehouses. A single-floor office or retail survey starts from £175 and a warehouse or industrial survey from £320, with prices confirmed before booking. Book your survey and get independent evidence that your LED upgrade delivers the right light, not just a lower bill.
Frequently asked questions
Do LED upgrades always improve lux levels?
No. A one-for-one LED swap can raise lux in some areas and lower it in others, and it often changes glare and uniformity. Only a measured survey shows the actual result.
Should I have a lux survey before an LED retrofit?
Yes. A baseline survey records what the existing lighting delivers, highlights areas that were already under-lit and gives you a reference for checking the new installation.
What lux level should an office achieve after an LED upgrade?
BS EN 12464-1:2021 recommends a maintained average of 500 lx for writing, reading and screen work, with uniformity of 0.60 and a UGR limit of 19.
Is it a problem if my new LEDs are much brighter than required?
It is not a safety failure in itself, but over-lighting wastes energy and often causes glare complaints. Dimming the fittings to the design level usually resolves both.