Circadian lighting: calibrating intensity and spectrum by use case

Sizing a circadian installation relies on four interdependent variables, each measurable and adjustable during the design phase.

• Light intensity by zone: 300 lux minimum in circulation areas, 500 lux at a standard workstation, 750 lux in spaces of prolonged concentration, in accordance with the NF EN 12464-1:2021 thresholds measured at eye level rather than at floor level.
• Dynamic spectrum: amplitude from 2700 K in an evening ambiance to 6500 K in morning daylight, with a continuous gradient rather than discrete steps.
• Programmed timing: three phases over 14 hours, morning activation from 6 a.m. to 9 a.m., daytime maintenance from 9 a.m. to 5 p.m., gradual decline from 5 p.m. to 8 p.m.
• Integration of natural lighting: external light sensors controlling the automatic adjustment of artificial flux according to facade orientation.

Faulty calibration generates significant overconsumption with no measurable physiological benefit. Conversely, a properly configured system noticeably reduces consumption compared with static lighting of equivalent average lux, thanks to hourly modulation. The observed ratio of 7 to 12 m² per workstation in open-plan offices directly drives luminaire density.

For the architect and lighting designer: the target value is measured at eye level, not at floor level

The decisive methodological shift plays out on the measurement plane. The standard applicable to offices requires reading illuminance on the working plane, that is at eye level for circadian stimulation, and not at floor level as some photometric calculation notes inherited from circulation lighting practice still do. This 75 cm gap represents up to 30% of illuminance lost at the height of use, and explains why installations that are compliant on paper remain below the biological threshold of 300 lux at eye level.

Contrary to common practice in design offices, Kytom requires Dialux modelling at 1.20 m from the floor for seated workstations and 1.55 m for standing workstations, with a commissioning verdict using a lux meter under the same conditions. This requirement adds 4 to 6 hours of modelling per floor plate, but avoids post-delivery rework whose additional cost can represent a significant share of the electrical package. For the architect, the trade-off shifts: the issue is not the number of luminaires per m² but the layout grid validated at eye level. For the lighting designer, the uniformity coefficient (Emin/Eavg) must remain above 0.6 on the working plane, not on the theoretical plane.

Three design errors compromise installations delivered without method

Three technical pitfalls recur regularly on projects audited by Kytom during the remediation phase.

Daytime undersizing. Reducing daytime intensity under the guise of energy savings cancels out the intended biological effect. Below 300 lux measured at eye level, the stimulation of retinal ganglion cells becomes insufficient. This threshold is moreover the floor adopted for common visual tasks in the prevailing standards.

Abrupt spectral transitions. Programming a direct shift from 3000 K to 6000 K in 30 minutes creates visual discomfort reported by occupants during the POE phase. Kytom favours gradual transitions over 2 to 3 hours, aligned with natural physiological rhythms.

Neglected spatial uniformity. A uniformity coefficient, the ratio of minimum illuminance to average illuminance, below 0.6 generates shadow zones perceived as uncomfortable. The Kytom teams systematically calculate this coefficient during the design phase and size the number of luminaires accordingly.

Points to watch: when circadian lighting is not justified. Dynamic circadian lighting loses its economic value on small areas with few workstations occupied full time: the return on investment lengthens noticeably, and a quality static 4000 K LED then covers functional needs. Circadian lighting also proves counterproductive in spaces with discontinuous occupancy of less than 4 hours per day, such as some occasional meeting rooms, where the hourly programming does not match actual use. Finally, on buildings whose electrical renovation is not planned within 5 years, the circadian investment should be deferred: early removal of the luminaires destroys profitability.

The design and build approach makes it possible to arbitrate these parameters from the design stage, rather than enduring adjustments during the construction phase.

The Kytom methodology unfolds in five steps validated at commissioning

The calibration process follows a structured sequence of 5 phases over 8 to 12 weeks, applicable to tertiary projects of any scale.

1. Audit of uses by zone: mapping of presence time, dominant activities and workstation orientation. This behavioural analysis defines the target lighting profiles, distinct between creative spaces, administrative areas and meeting rooms.
2. 3D photometric modelling: simulation incorporating natural light contributions according to facade orientation. This step reveals potential underlit zones and guides luminaire placement.
3. Technology choice: trade-off between tunable white LED, whose 2700 K to 6500 K range suits the majority of tertiary uses, and RGB+W systems with extended colour rendering, whose additional investment cost must be weighed against the expected benefits in comfort and productivity.
4. Sequence programming: transition curves customised by use typology, with scenarios differentiated according to the occupants’ professions.
5. Commissioning and on-site measurements: lux meter verification of effective thresholds under occupancy conditions, adjustment of transition curves and a photometric acceptance report signed by both parties.