Lux vs lumens: how illuminance is measured

Lumens describe how much light a lamp gives out; lux describes how much of it lands on a surface. Knowing the difference helps you size lighting correctly.

The box in the shop says "806 lm", while an office lighting guideline asks for "500 lux on the desk". The two numbers look as if they describe the same thing, but they measure quite different aspects of light. Mix them up and you end up either with a gloomy room or with a lamp far stronger than you needed.

Lumens: how much light leaves the lamp

The lumen (lm) is the unit of luminous flux. It is the total visible light a lamp sends out in every direction, weighted by how sensitive the human eye is to different wavelengths. In other words, lumens belong to the lamp itself: the same bulb delivers the same number of lumens in a tiny bathroom and in a large hall.

Lumens say nothing about where the light goes. An 800-lumen lamp can spread its output all around like a globe or squeeze it into a narrow beam aimed at one spot. That is why the lumen figure alone cannot tell you how bright your desk will be.

Lux: light arriving at a surface

Lux (lx) is the unit of illuminance, the luminous flux falling on one square metre: 1 lux = 1 lumen per square metre. This is no longer a property of the lamp but of a particular point. The desktop, the middle of the floor and the corner of the room can all receive different lux levels under the same lamp.

The main things that change lux are:

• the total lumen output of the lamp;
• the beam angle, i.e. how large an area the flux is spread over;
• the distance between lamp and surface;
• the colour of walls and ceiling and how much light they reflect;
• the share absorbed by shades, diffusers or dusty glass.

Why distance matters so much

For a small light source, illuminance falls with the square of the distance. Move the lamp twice as far from the desk and the same light covers four times the area, so the lux in the centre drops to roughly a quarter. Light bouncing around the room softens this a little, but the principle holds.

The practical consequence is simple: in a room with a high ceiling, the same lamp delivers noticeably less light to the floor or table. Lowering a pendant by a few tens of centimetres is sometimes more effective than buying a stronger bulb.

A worked example

Say you want an average of 150 lux at floor level in a 10 m² room. On paper that is 10 × 150 = 1,500 lumens. In a real room, though, part of the light is absorbed by the walls, lost inside the fitting or spilled where it is not needed. So in practice you add a margin, and a room with dark walls needs a much bigger margin than a light-coloured one.

The rule also works in reverse: if you know a lamp's lumens and the room's area, you can estimate the average illuminance. A 1,200-lumen lamp in a 12 m² room gives 100 lux on paper, while the real result will be lower because of losses.

This is a rough guide, not a design calculation. On serious projects engineers use dedicated software and the photometric data file of the specific luminaire.

Typical levels for different places

For offices and workplaces, the European standard EN 12464-1 specifies 500 lux on the task area for writing and reading. Corridors and circulation areas need far less. Homes have no mandatory standard, but the same logic applies:

• hallways and passages manage with modest general light;
• a living room has moderate general light, with stronger local light in the reading corner;
• precise work surfaces such as a kitchen worktop, a writing desk or a sewing table need the most.

For a sense of scale: outdoors at a clear midday the illuminance runs into tens of thousands of lux, while moonlight is below one lux. The eye adapts to this range, but working on fine detail in too little light quickly leads to fatigue.

What to check when buying

Lux is not printed on the box, because it depends on the room rather than the lamp. You look at lumens and create the lux yourself through room size, ceiling height and where the lamp is placed. With spotlights, also check the beam angle: the same lumens give high lux on one spot with a narrow beam, or even but weaker light with a wide one.

Zaferoğlu Elektrik produces LED lamps of different wattages at its plant in Ganja; when choosing one for a room, comparing the lumen figure on the box with the simple calculation above is the most reliable place to start.