Office lighting: lux, UGR and continuous light lines to the standard

What the standard requires on the working plane

The requirements for office lighting are set out in EN 12464-1, the European standard for the lighting of indoor workplaces. For writing, typing, reading and screen work it asks for 500 lux on the working plane: the desktop, taken in the calculation as a horizontal plane 0.75 m above the floor. This is a maintained value, meaning the level that must still be reached after ageing and soiling.

Lower values apply around the task area: at least 300 lux in the immediate surroundings and 100 lux in the background. Other office functions have their own requirements:

• Meeting room: 500 lux, UGR 19
• Reception: 300 lux, UGR 22
• Archive: 200 lux, UGR 25
• Corridors: 100 lux, UGR 28

The standard also asks for a uniformity Uo of at least 0.60 on the working plane (lowest value divided by the average), a colour rendering index Ra of at least 80 and light on walls and ceiling, in the order of 50 to 75 lux on the walls and 30 to 50 lux on the ceiling, depending on the edition of the standard. An office that only lights the desks feels gloomy, even when the 500 lux is there.

UGR 19: controlling glare through the layout

UGR (Unified Glare Rating) expresses how much direct glare an installation produces; for desk work the limit is UGR 19. The UGR on a datasheet is a tabular value for a reference room. What you achieve in your project depends on the room dimensions, the mounting height, the reflectances and the viewing direction of the staff.

Three factors make the difference in practice:

• Viewing direction: lines running parallel to the line of sight give a lower UGR than lines running across the desk.
• Luminance above 65 degrees: for screen work the standard limits the brightness of the luminaire at shallow viewing angles so that it does not reflect in the screen. An opal or microprismatic diffuser keeps that brightness low.
• Background: a light ceiling and light walls reduce the contrast and with it the UGR.

The LINEA linear luminaires have UGR<21 as their tabular value. In an actual office with a light ceiling and lines parallel to the viewing direction the calculated value is often lower, but whether you reach 19 must be shown by the lighting calculation.

Linking light lines: from single luminaire to continuous run

A light line consists of modules that you link mechanically and electrically into one continuous run: no visible joint and one supply point per line instead of per luminaire. That saves installation time and cable. The LINEA modules link seamlessly and are available in white and black.

When setting out the lines, keep in mind:

• Line spacing: at a mounting height of 2.7 to 3 m, 2 to 3 m between the lines is usually right for 500 lux and Uo 0.60.
• Position: the line beside or between the desks, not directly above the screen, parallel to the viewing direction.
• Facade: the first line 1 to 1.5 m from the window so that the daylight zone is not lit twice.
• Ordering: count per room the modules per line, the number of lines, the colour and the uplight modules.

Pendant or surface mounted, and the indirect share

Surface mounting is the choice for ceiling heights up to about 2.7 m and for suspended ceilings with no free space above. The luminaire sits against the ceiling and all the light goes downwards. Pendant mounting becomes interesting from about 2.8 m: you suspend the line 0.3 to 0.6 m below the ceiling, closer to the working plane.

The big advantage of pendant mounting is the indirect share. An uplight module sends part of the light to the ceiling, which reflects it diffusely. The ceiling itself becomes a light source: less contrast, lower UGR, softer shadows on the desk, and the wall and ceiling values from the standard are met more easily. For LINEA a separate indirect uplight module is available that you combine per module on the line.

Indirect light only works with a light ceiling: reckon with a reflectance of 0.7 for white. A dark or open ceiling with exposed services swallows most of it. In that case choose surface mounting or a pendant without uplight.

Selectable wattage: 24, 32 or 40 W per module

The LINEA linear luminaire has a selectable wattage: 24, 32 or 40 W, set with a switch on the driver. At 120 lm/W that is roughly 2,900, 3,800 or 4,800 lumens per module. You order one article for the whole office and set the wattage per room during installation:

• Open-plan office: 40 W for 500 lux.
• Corridor or kitchenette: 24 W, where 100 to 200 lux is enough.
• Higher ceiling: one setting higher for the same lux on the working plane.
• After measuring: too much light? One setting lower and you save energy straight away.

Set the wattage before installation, with the supply isolated, and record the setting per room in the handover file.

Worked example: open-plan office of 48 m²

An open-plan office of 8 by 6 m, ceiling height 2.8 m, white ceiling, light walls, eight workstations in two rows. Required on the working plane: 48 m² × 500 lux = 24,000 lumens. Not all the light reaches the working plane and the installation ages; reckon with a room utilisation of about 0.65 and a maintenance factor of 0.8. That gives roughly 24,000 / (0.65 × 0.8) ≈ 46,000 lumens of installed luminous flux.

Two lines of five modules at 40 W give 48,000 lumens at 400 W, or 8.3 W/m². Three lines of four modules at 32 W give 46,000 lumens at 384 W and better uniformity, because the line spacing drops from 3 to 2 m. The second option needs more modules but meets Uo 0.60 more easily and leaves room to go one setting higher later. Which option fits your ceiling and what the UGR per workstation will be is worked out by a lighting calculation for 350 euros excl. VAT.

Frequently asked questions