Downlight spot: balancing energy performance and visual comfort

The design of a downlight spot lighting plan rests on four interdependent technical variables.

• Spacing: the rule of thumb sets the distance between light points at 1.5 times the ceiling height, i.e. 3.75 m for a standard 2.50 m ceiling. Adding more spots improves uniformity but increases consumption and installation cost.
• Unit power: 12 W is enough for circulation areas, 18 to 24 W meets the needs of a workstation, to reach the 500 lux required at an office workstation versus 100 to 200 lux in transit zones.
• Colour temperature: 3000 K (warm) suits relaxation spaces, 4000 K (neutral) supports concentration and screen reading.
• Beam angle: 38° for homogeneous general lighting, 24° for targeted accentuation.

These parameters offset one another: a more powerful spot allows wider spacing, which reduces the number of points and the cost of the electrical structural works, but concentrates consumption. The final trade-off depends on the usable floor area per workstation, which ranges from 8 to 12 m² in open-plan spaces in common office configurations.

Kytom’s position, which departs from lighting design dogma: the profession still largely recommends homogeneous lighting at 300-400 lux across the whole floor, for the sake of flexibility. Our reading differs: task zones and surrounding zones call for distinct levels, and real flexibility is achieved through dimming, not through initial oversizing. Differentiated zoning makes it possible to significantly reduce the number of installed luminaires without degrading perceived visual comfort.

When the downlight spot is not the right answer: under a ceiling height below 2.40 m, the recessed downlight creates marked shadow cones and a UGR difficult to keep below 19; in that case prefer a diffusing pendant or indirect lighting via an LED line. Likewise, above 4.00 m in height (lobbies, atriums), the efficiency per downlight drops and the W/m² ratio exceeds 30; switch to projectors with engineered optics or high-mounted pendants.

For the architect or lighting designer: three design biases to neutralise in the preliminary design phase

Three mistakes degrade the real performance of office lighting, regularly observed during preliminary audits. For the designer, each translates into a forced rework in the execution or as-built phase, with an impact on the RE2020 compliance certificate and the achievement of objectives.

1. Systematic oversizing: applying 300 to 400 lux everywhere, whereas the reference standards for indoor lighting of workplaces set 100 to 200 lux as sufficient in circulation areas and 500 lux only at workstations. The associated overconsumption can be significant on projects designed without a prior lighting audit. Over the building life cycle, it also means an equivalent share of luminaires to replace and recycle (WEEE).
2. Maintenance ignored: installing inaccessible spots (under structural grid, behind HVAC network) generates replacement extra costs via cherry picker or scaffolding. Good practice is to verify accessibility as soon as the layout grid is fixed, even if it means shifting points by 30 to 50 cm. The as-built file must record this accessibility, failing which the future operator will bill it to the project owner.
3. Direct glare: a spot placed in the visual axis of a workstation creates a UGR discomfort above 19, the maximum threshold allowed for offices according to the glare assessment rules applicable to indoor lighting of workplaces. For the architect, complying with the UGR is also a defence argument in the event of a workplace-quality-of-life lighting dispute.

The prior behavioural audit, integrated in the preliminary design phase alongside the photometric note, maps the dominant visual trajectories and avoids these conflicts while preserving lighting efficiency.

Limit where the audit method does not apply: on floors of less than 150 m² with fewer than 10 workstations, the cost of a full behavioural audit is generally not amortised by the savings on luminaires; a simplified survey on plan is sufficient. Conversely, on flex office floors with an occupancy rate below 50%, it is automatic dimming via detection that takes precedence over the initial calibration.

Kytom methodology: audit, simulation, integration, automation

The lighting design applied by the Kytom teams articulates use and performance in four sequential steps, which can be combined with the NF P03-001 phases (preliminary design, detailed design, technical studies, execution).

1. Behavioural audit: map actual uses by zone (fixed workstation, transit, meeting room, phone-box) to define useful lighting levels rather than theoretical ones. A ratio of 7 to 12 m² per workstation in open-plan spaces serves as the basis for zoning.
2. Photometric modelling: simulate 3 to 5 configurations using Dialux or Relux software, to identify the optimal performance/consumption ratio for each functional zone. This photometric note is included in the tender file and recorded in the as-built file.
3. Technical integration: coordinate the layout with the ventilation, fire detection and suspended ceiling networks, in order to avoid costly reworks during the construction phase. BIM synthesis in detailed design makes it possible to fix the grid before the architect finalises the ceiling layout.
4. Automation configuration: program presence detection and dimming according to natural light contributions, in compliance with the requirements of the tertiary decree (target of -40% in final energy consumption by 2030 compared with 2010).

Projects integrating detection and dimming enable significant reductions in consumption, with an automation ROI generally between 3 and 5 years depending on the electricity tariff applied. This design and build approach, deployed across the 11 Kytom agencies in France and Spain, allows adjustment according to constraints revealed during execution.

When automation is not justified: below 200 m² of lit area or for occupancy periods of less than 4 h/day, the ROI exceeds 7 years and a simple zone-by-zone switch remains more relevant than a lighting building management system.