Lux Level: The Number That Does Not Describe Good Lighting

Why it matters on a project

Lighting is usually specified with a single number, and that number is lux. It is the easiest thing to measure, the easiest to write into a brief and the easiest to verify with a handheld meter on handover day.

It is also the reason offices get signed off as compliant and still feel badly lit.

What lux does not tell you

Criterion	What it covers	Described by lux?
Illuminance	How much light lands on the surface	Yes
Uniformity	How evenly it is distributed	No
Glare	Discomfort from contrast and reflections	No
Colour temperature	Whether light reads warm or cool	No
Colour rendering	Whether finishes look right under it	No
Control	Whether output responds to daylight and occupancy	No

A specification stating only a lux figure has left five of those six to chance.

The uniformity trap

The cheapest way to hit an average illuminance is fewer, brighter fittings spaced further apart.

The meter confirms the average. The occupants experience bright pools under each fitting and dim zones between them, and the eye adapts continuously as they move or turn. It is a measurable pass and a poor room.

Specifying uniformity alongside illuminance closes this, and it costs nothing at design stage.

Colour is not a detail

Two aspects matter and both are invisible on a lux meter:

• Colour temperature sets whether the space reads warm or cool. Mixing temperatures across one floor — often the result of replacing failed fittings later with whatever is available — looks conspicuously wrong.
• Colour rendering determines whether finishes, materials and skin tones look natural. It matters most where the interior was chosen for its materials, which is to say in exactly the spaces a client cares about.

Recommended values by task are set out in IS 3646 and EN 12464-1, with power and control requirements in ECBC 2017.

Common mistakes

• Specifying lux alone. Five other criteria are then unmanaged.
• Applying one figure across a whole floor. Tasks differ; requirements differ.
• Ignoring uniformity. The cheapest way to pass is the worst way to live.
• Forgetting screens. Reflected glare on monitors is the most common lighting complaint in offices and is a positioning problem.
• Mixing colour temperatures over time. Replacement fittings should match the original specification, which means recording it.
• No daylight-linked control. A well-daylit perimeter running at full output wastes the daylight and the energy.

Standards referenced

Interior illumination practice in IS 3646; workplace lighting in EN 12464-1; lighting power and control requirements in ECBC 2017. Design illuminance, uniformity and glare criteria for a specific space must be established by the project's lighting consultant.