Adjustable color temperature in public lighting – when it makes sense and what to include in the call for tenders

Adjustable color temperature in public lighting – when it makes sense and what to include in the call for tenders

Discussions about the color temperature of outdoor lighting usually boil down to a single figure. A choice is made between 3000K and 4000K, it is recorded in the request for tenders, and the process moves on. That same figure then dictates the lighting across all thoroughfares, around the clock and throughout the year—even when the site includes an ecological corridor, when it is three in the morning and the street is deserted, or when the zoning plan changes five years after installation.

Adjustable color temperature resolves this impasse. However, its added value does not come automatically. If specified incorrectly, you end up paying for a feature that is never actually used. This article examines when adjustability makes sense for street, road, park, and industrial lighting—and how to draft requirements that ensure the benefits remain relevant ten years down the line.

Two different techniques that are constantly confused with each other

A request for proposals sometimes simply states “adjustable color temperature”. This can refer to two very different things, and the distinction determines which problem the solution can actually address.

White tone adjustment. The luminaire features warm and cool white LED channels, and by adjusting the ratio between them, the color temperature can be varied, for example, between 3000K and 5000K. The proportion of blue wavelengths decreases at the warm end of the adjustment range—though not dramatically—meaning the blue component remains present throughout.

Multi-channel spectral control. In addition to white LEDs, the luminaire features separate colored LED strings—typically orange and/or red—that are controlled independently. This allows for significantly lower color temperatures: a golden-brown hue of approximately 1800K (with a very low proportion of short-wavelength blue light), a soothing yellow at 2500 K, and the standard street-lighting tones of 3000–4000 K.

The distinction is significant. If the goal is to reduce disturbance to insects, bats, or migratory birds, two-channel white light tuning is insufficient; a channel producing light in the appropriate wavelength range is required. However, if the objective is simply to manage the evening and nighttime atmosphere and the perceived level of nuisance in a residential area, white light tuning may be a sufficient and more cost-effective solution.

A practical takeaway for procurement: do not simply specify the Kelvin range in the request for proposals. Instead, state the required number of channels, the desired colors, and whether they are to be controlled independently. Otherwise, you will end up comparing two completely different products side by side.

Where adjustability yields measurable benefits

Nature sites and ecological corridors. This is the clearest case. Areas along waterways, spaces beneath bridges, forest edges, bat foraging grounds, and bird migration routes. In Central Europe, nature conservation regulations have driven the adaptation of street lighting for years, and the solutions developed there are already in use in municipal and state infrastructure projects. In Finland, similar pressure typically arises from the proximity of nature conservation areas, statements from Centres for Economic Development, Transport and the Environment (ELY Centres), zoning regulations, and environmental impact assessment (EIA) processes. In these contexts, the value of adjustability is twofold: it meets current requirements while providing flexibility should those requirements become stricter.

Residential areas and parks. Evening and the dead of night present different situations. In the evening, people are moving about in the area, making facial recognition and a sense of security the deciding factors. At three in the morning, maintaining that same level of lighting is a waste of energy and—for many residents—a source of disturbance visible through bedroom windows. Lowering the color temperature in conjunction with dimming is far more acceptable than simply dimming the lights heavily, as warmer light is perceived as more subdued rather than merely “too dark.”

Road sections with varying traffic volumes. Adaptive lighting is already an established concept in road maintenance: the lighting class is adjusted based on traffic volume, time of day, and conditions. Color temperature is a natural additional dimension to this when integrated into the same control logic—rather than being treated as a separate gimmick.

Industrial yard and site lighting. Lighting at production facilities, ports, terminals, and storage yards is often the single largest source of light pollution in the area and the most visible nuisance for immediate neighbors. Here, adjustability resolves the conflict between the need for full illumination levels and specific operating conditions during shifts, and the lack of such requirements between shifts.

And when it’s not worth it. If the site is a motorway or an industrial area—devoid of nearby natural value or residential settlements, and with no anticipated changes in requirements—a correctly selected fixed color temperature and a proper dimming profile will suffice. It makes no sense to pay for a feature that will never be adjusted. This is a candid conclusion, and one that holds true more often than lighting suppliers admit.

Three recurring mistakes in procurement

1. Lighting calculations are performed using one setting, while the system is operated using another.

This is the most common and costly mistake. The calculation is performed in DIALux using a 4000K light distribution and full luminous flux—meeting the SFS-EN 13201 standard class on paper—but the profile implemented operates at 2200K and 40% power during the night. The result is that the installation fails to meet the class for which it was approved—and no one notices until the first accident occurs or a complaint is filed.

The correct approach: a calculation for every setting used in the profile, and for adaptive lighting a documented justification regarding the conditions under which the class is calculated. The standard provides the tools for this – one simply needs to use them.

2. Control is tied to a single supplier.

If color temperature control relies on a specific manufacturer’s proprietary controller and cloud service, the municipality or property owner effectively commits to a dependency lasting at least ten years. Demand a standardized control interface — D4i is currently the most common standard for street lighting. Replacing the control system should not require replacing the luminaires themselves.

3. No one owns the profiles.

The light fixtures are installed, the contractor sets up the profiles, and the project concludes. Three years later, no one in the client’s organization has system credentials or knows why the settings are the way they are. This is not a technology problem but a contractual one, and it is resolved in the request for tenders — not after the fact.

Safety: the aspect where there can be no cutting corners.

Very warm light with a narrow spectral distribution — specifically narrowband orange or red light — impairs color recognition. It affects the perception of traffic signs, road markings, and vehicle lights, as well as the ability to distinguish details in the periphery of the visual field.

A practical approach that stands up to scrutiny:

• Very low color temperatures are suitable for park paths, ecological corridors, the edges of natural areas, and low-traffic residential streets.
• On main routes and at junctions, changes are made only based on lighting calculations and in a manner agreed upon with the road authority.
• Crosswalks, junctions, and crossings for pedestrians and cyclists are treated separately. These are locations where lowering the color temperature is not a compromise, but a risk.

Technical points worth requiring

1. Number of channels, channel colors, and independent channel-specific controllability
2. Color temperature range and how it is produced – via white tone adjustment or colored channels
3. Luminous flux and luminous efficacy (lm/W) for each setting used in the profile, not just the best one
4. Spectral data, or at least color rendering, for the settings used on traffic routes
5. Standardized control interface (DALI-2 / D4i) and open control interface
6. Control system reporting: energy consumption per luminaire, fault alarms, predictive maintenance
7. Lifetime based on LM-80 and TM-21, stated at the operating temperature, as L and B values
8. Light distribution files (LDT) made available for calculations prior to the purchasing decision
9. Component interchangeability and spare part availability – target of at least 10 years
10. Scope of the warranty: does it cover the luminaire, the control gear, and the control components
11. Handover documentation: profiles recorded and justified, accounts held by the client, designated person in charge

Lifecycle: how a feature remains in use

• Open the profiles. Not merely a system manual, but a justification: which regulation, at what time, on what grounds, and by whose decision.
• Designate a person in charge and ensure access to the system immediately upon handover.
• Agree on seasonal changes in advance. The bats’ active season, migration period, and dark winter are not the same situations.
• Use data. When the system reports consumption and fault data, check after a year whether the calculated savings were realized. If not, the reason can usually be found in the profiles.
• Schedule a fine-tuning round after the first year of use. It is the most cost-effective way to turn resident feedback into improvements.

Summary

Adjustable color temperature is most valuable in public outdoor lighting when a site simultaneously faces requirements for traffic safety and the needs of the environment or residents — and these demands do not align. It represents a wasted investment when the feature is purchased without a control plan, without calculations based on the intended settings, or without anyone assigned to manage the profiles after handover.

The difference between the two is not determined by the choice of luminaire. It is determined by how the requirements are written.