Lux Level: The Number That Does Not Describe Good Lighting

Lux is illuminance — the amount of light falling on a surface, measured in lumens per square metre. It is the headline figure in any lighting specification, and on its own it describes almost nothing about whether a space is comfortable to work in.

By Dhruv Agarwal · · 2 min read

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

CriterionWhat it coversDescribed by lux?
IlluminanceHow much light lands on the surfaceYes
UniformityHow evenly it is distributedNo
GlareDiscomfort from contrast and reflectionsNo
Colour temperatureWhether light reads warm or coolNo
Colour renderingWhether finishes look right under itNo
ControlWhether output responds to daylight and occupancyNo

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.

Standards referenced

  • IS 3646 — Code of practice for interior illumination (Bureau of Indian Standards)
  • ECBC 2017 — Lighting and controls (Bureau of Energy Efficiency)
  • EN 12464-1 — Light and lighting — workplaces indoors (CEN-CENELEC)

Frequently asked

Lumens describe the total light a source emits. Lux describes how much of it lands on a surface, which depends on distance, spacing, reflectance and the fitting's distribution. The same lamp produces very different lux on a desk depending on how high it is mounted and how the light is spread, which is why a fitting's lumen output tells you little on its own.

Not necessarily, and this is the central point. A space can hit its average illuminance target and still be unpleasant — bright patches under fittings with dark gaps between them, glare on screens, a colour of light that makes finishes look wrong, or no control so the lights run at full output all day. Lux is one criterion out of several.

Uniformity describes how evenly light is distributed across the working area, usually as the ratio of the minimum to the average. Poor uniformity means the eye constantly readjusts as people move or look across the space, which is tiring even when the average figure is correct. Widely spaced fittings chosen to hit an average cheaply are the usual cause.

Glare is discomfort or loss of visibility caused by excessive contrast, typically from a bright fitting in the field of view or a reflection on a screen. It is assessed separately from illuminance, and a space can be simultaneously well lit by the numbers and uncomfortable to sit in. Fitting selection, mounting position and shielding control it, not the lux figure.

Yes. Different activities need different illuminance — detailed work, general office work, circulation and storage are not the same requirement, and standards such as IS 3646 and EN 12464-1 set out recommended values by task. What applies to a specific space must be established by the project's lighting consultant rather than applied as one number across a whole floor.

Through installed power rather than illuminance. ECBC 2017 addresses lighting power and control requirements for commercial buildings, so the design has to deliver the required light using no more than the permitted power, with appropriate controls. That combination — enough light, efficiently, with control — is the real design problem.

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