Warm white or neutral white: choosing the colour temperature

Two boxes of LED panels arrive on site with the same rated power and two different shades. Installed in the same office floor, the difference shows as soon as the power is switched on and the ceiling has to be redone row by row. Colour temperature is decided when the order is placed, never up in the cherry picker.

What colour temperature measures

Colour temperature is expressed in kelvins. It describes the shade of the white emitted, from orange-tinted white to slightly bluish white. The scale runs counter to common intuition: the higher the value, the cooler the light appears.

This value says nothing about the quantity of light produced. Two luminaires of different shades can emit the same flux in lumens and give the same illuminance on the working plane. Shade is a matter of perception, flux is a matter of sizing, and the two are read separately on the data sheet.

In a professional catalogue, the same reference usually exists in several shades. The choice is therefore made item by item and it commits the whole room. That is reason enough to state the shade on the purchase order, and not only on the quotation.

Warm white, neutral white, cool white: for which use

The professional market groups shades into three families. Warm white softens the atmosphere and enhances wood, leather and warm materials. Neutral white stays legible without colouring surfaces: it is the default working shade. Cool white heightens contrast and the feeling of cleanliness, at the cost of a more clinical atmosphere.

| Family | Order of magnitude | Spaces where it is commonly used | Point to watch |

| --- | --- | --- | --- |

| Warm white | around 2700 to 3000 K | reception, hotels, restaurants, clothing shops, outdoor lighting | can appear dim at the same illuminance |

| Neutral white | around 4000 K | offices, workshops, warehouses, healthcare, circulation areas | default working shade, poorly suited to subdued atmospheres |

| Cool white | from 5000 K | laboratories, visual inspection stations, food processing | prohibited for outdoor lighting, tiring over long periods |

These limits are usage benchmarks, not obligations. A single room accepts two neighbouring shades without visible discomfort, provided the choice is held from one end of the installation to the other.

Offices, workshops, shops: market practice

The EN 12464-1 standard governs the lighting of indoor workplaces through maintained illuminance, discomfort glare UGR, uniformity U0 and colour rendering index. It does not prescribe any colour temperature. For an office workstation, it requires 500 lux on the task area, a UGR of 19 or less and an Ra of at least 80: the shade remains the operator's choice.

In offices and meeting rooms, neutral white dominates. It holds up all day, does not yellow printed documents and blends well with daylight coming through the windows. In workshops and warehouses, the same shade prevails for another reason: labels, colour codes and cables must be read accurately.

In retail, the choice depends on the products sold. Warm white flatters textiles, bakery and leather goods. Neutral white suits DIY, household appliances and supermarket aisles. Adjustable spots and tracks also make it possible to combine two shades deliberately between general lighting and accent lighting, provided the separation is clear and intentional.

Outdoors and residential areas: why the shade is regulated

Outdoors, shade is no longer simply a matter of comfort. The French order of 27 December 2018 on the prevention, reduction and limitation of light pollution caps the colour temperature of outdoor lighting installations at 3000 K in mainland France. The threshold drops to 2700 K near astronomical observation sites and to 2400 K in protected natural areas.

The justification is biological: the blue component of light disrupts the orientation of insects, bats and migratory birds. The limit applies to new installations as well as renovations, which rules out cool white from the outset for a car park, a path or a façade.

This check is made before ordering. An outdoor luminaire at 4000 K, very common in general catalogues, is not suitable. The shade must appear on the order and on the acceptance report, otherwise the non-compliance is discovered at the first complaint.

Colour rendering, a separate criterion not to be confused

Colour temperature says which shade of white lights the scene. The colour rendering index, written Ra or CRI, says how faithfully the colours of objects appear under that light. These are two independent data.

A 4000 K light can have an Ra of 70 or an Ra of 90, with very different results on a colour chart, fresh produce or a textile. EN 12464-1 requires at least Ra 80 in most permanently occupied rooms, and more at visual inspection, sorting or retouching stations.

The R9 value, which reflects the rendering of saturated reds, is worth requesting whenever meat, textiles, paint or cosmetics are involved. It does not always appear by default on the product sheet.

Mixing two shades in the same room: the risks

Two shades sit badly together in the same field of view. The eye compensates for the dominant shade and perceives the second as a defect, often greenish or pinkish. The problem appears mainly during extensions and one-off replacements, when a new luminaire takes the place of an old one.

Even with the same declared shade, two production batches can differ slightly. This deviation is measured in MacAdam ellipses, written SDCM on data sheets: the lower the value, the more uniform the luminaires of a batch. Demanding specifications cap this value and require continuity of reference for the duration of the contract.

• Record the exact shade and full reference of the installed luminaires in the maintenance log.
• Keep a few spare luminaires from the same batch for one-off replacements.
• Treat a whole room rather than a single row when the original reference is no longer available.
• Check the shade before adding LED panels to a ceiling that is already equipped.

Luminaires with adjustable shade: when it is justified

Two product families exist. The shade selector, set once at installation by a switch on the luminaire, is mainly used to limit the number of references in stock. Controlled tunable white, often over DALI, varies the shade during operation.

Controlled shade is justified in rooms whose use changes during the day: multi-purpose hall, reception area, rest room, care unit. It adds equipment and commissioning costs, and it requires simple controls, otherwise users leave it on a single setting. In a standard office floor, a well-chosen fixed shade is enough.

When several zones relate to each other in the same building, a selection of lighting solutions by type of room makes it possible to fix the shades before costing rather than after delivery.

This week, record the shade of the luminaires in place in each room, enter the value on the operating plan, then order only in that value.

Compare the shades offered for each reference in the catalogue of professional LED luminaires.