Lumen, lux and watt: the difference and how to calculate with them

Three units, three questions

They sit side by side on every datasheet, yet they answer three different questions. Watt (W) states what the luminaire draws from the mains: the power consumed, driver included. Lumen (lm) states how much light the luminaire emits in total: the luminous flux. Lux (lx) states how much of that light lands on a given surface: the illuminance, defined as lumen per square metre.

For you as an installer, lux is what counts: the client wants 200 lux on the warehouse floor or 500 lux on the desk, not a number of watts on the ceiling. Watt matters for the energy bill and the circuit design; lumen is the intermediate step from luminaire to illuminance.

Lumen per watt: the efficacy

Divide luminous flux by power and you have the efficacy in lumen per watt (lm/W). A 100 W high bay with 15,000 lm achieves 150 lm/W; the 100 W version with 18,000 lm from the ATRIA range reaches 180 lm/W. That is 20 per cent more light for the same consumption, or the same light with 20 per cent fewer luminaires.

For comparison: halogen manages at most 20 to 25 lm/W, fluorescent tubes and metal halide sit around 80 to 100 lm/W. On top of that, a conventional fitting loses part of the lamp output in its reflector and ballast, whereas an LED module emits directionally. Hence the rule of thumb that a 400 W metal halide high bay is replaced by an LED high bay of 150 to 200 W.

Always compare luminaire lumens (light leaving the fitting) with luminaire lumens and not with lamp lumens of the LED chip, and check whether the stated wattage includes the driver.

From lumen to lux: area and height

The basic formula: lux = installed lumens × utilisation factor × maintenance factor ÷ area in m². The utilisation factor is the share of light that actually reaches the working plane and depends on mounting height, beam angle and the reflectance of walls and ceiling; for LED high bays in a hall usually 0.6 to 0.8. The maintenance factor (often 0.8) allows for dirt and ageing, so the installation still meets the standard years later.

Worked example: a warehouse of 40 by 25 metres (1,000 m²) needs 200 lux on the floor. Required on the floor: 200 × 1,000 = 200,000 lm. Divided by 0.7 (utilisation) and 0.8 (maintenance) that is 357,000 lm installed. With an 18,000 lm UFO high bay that means 20 luminaires, at 100 W each 2 kW of connected load. The same 20 units at 15,000 lm would give roughly 170 lux: just short.

Mounting height follows the inverse square law: hang the luminaire twice as high and you get a quarter of the lux directly beneath it. So in a taller hall choose not just more lumens but also a narrower beam: a UFO with 120° up to about 7 metres, 90° above that, and over narrow racking aisles a linear high bay with a 30 by 90° beam, so the light falls into the aisle rather than onto the top pallet.

Rules of thumb per application

The European standard for workplace lighting gives a minimum maintained average illuminance on the working plane for each task. The values you will need most often:

• Warehouse and storage: 150 lux in aisles with forklift traffic, 300 lux in picking and packing zones
• Production hall: 300 lux for rough work, 500 lux for assembly and inspection, 750 lux or more for fine work
• Office: 500 lux on the desk, with UGR<21 to limit glare on screens
• Sports hall: 300 lux for training and recreation, 500 lux for competition
• Covered car park: 75 lux on lanes and bays, more at the entrance and exit
• Outdoor car park: 5 to 20 lux, depending on traffic intensity
• Residential street and cycle path: 3 to 7.5 lux average, depending on the lighting class
• Tennis and padel court: 300 lux at club level, 500 lux for competition

A 25 W street luminaire with 1,500 lm from the VIA range on a 4 metre TURRIS column is intended for exactly such a residential street or cycle path; the VIA luminaires are made for a 60 mm spigot and fit these steel columns. For a car park you move to 50 to 100 W on 6 metres; the column spacing follows from the lighting calculation.

Common mistakes

• Comparing wattage instead of luminous flux: 100 W means nothing until you know the lumens
• Measuring lux at the wrong height: in an office the working plane is at desk height, roughly 0.75 to 0.85 m above the floor, in a warehouse it is the floor itself
• Forgetting the maintenance factor: an installation that just reaches 200 lux when new drops below the standard after a few years
• Looking only at average lux: the standard also sets uniformity requirements, usually 0.4 to 0.6 between lowest and average value
• Ignoring glare: in offices and sports halls UGR is as decisive as the lux level

Colour temperature (2700 K to 5000 K) changes nothing about the lux, but a great deal about perception: 4000 K for halls and offices, 2700 to 3000 K for residential streets and parks. The IP rating plays no part in the lux calculation but does in the maintenance factor: a sealed luminaire in IP65 or IP66 stays clean inside.

Calculate yourself or have it calculated

For a rectangular hall with one luminaire type this formula gets you well within tolerance. As soon as racking, varying heights, daylight or outdoor areas with columns come into play, a lighting calculation pays off: it calculates point by point, checks uniformity and glare and delivers a report for the client. Lichtplein prepares such a calculation for 350 euro excl. VAT based on your floor plan and the required lux levels, for businesses only.

Frequently asked questions