What Don’t We Know?

CIE 191:2010 describes mesopic photometry and how spectrum and adaptation luminance affect photometric quantities at low light levels. A higher S/P ratio may benefit some peripheral visual tasks, but CCT alone does not determine S/P and no single source is best for every visual and ecological goal. See mesopic photometry and night vision.

No universal CCT optimum has been established that balances road safety, ecological impact, human health and energy use across all sites. The trade-off has to be assessed for the actual location.

A CCT requirement should therefore be justified by the site’s visual tasks and sensitivities, rather than presented as a general safety guarantee.

IES TM-30-24 evaluates colour rendition with 99 colour samples and reports, among other measures, fidelity (Rf) and gamut (Rg). It describes more of a light source’s colour properties than the general Ra value alone. The metric does not itself demonstrate a road-safety benefit.

The contribution of colour rendition to detection, orientation and perceived safety in a particular road or pedestrian setting needs to be assessed alongside luminance, uniformity and glare.

Determines whether it is justified to specify higher colour rendition requirements in procurement — or whether the premium cost of high CRI/Rf in road environments lacks a functional basis.

Changing an LED source’s spectrum can change colour fidelity, gamut and luminous efficacy. Laboratory or supplier results need to be linked to a defined task and compared at equivalent lighting levels.

How laboratory figures translate to safety, object detection, and actual energy savings in real road installations. Independent field studies with control groups are limited. Purchasing decisions based solely on manufacturer white-paper data should be made with awareness of this gap.

Separates a real functional benefit from a laboratory result that does not materialise in the field — and determines whether the premium for specialist spectrum is justified in road procurement.

Field studies show that street lighting can affect local insect abundance; effects vary with species, spectrum, light level and habitat. A Science Advances field study found fewer moth caterpillars near lit than unlit sites. The evidence is therefore not limited to laboratory attraction tests.

How these effects develop over years, across ecosystems and among specific LED spectra remains uncertain. Neither CCT alone nor a laboratory attraction result predicts population outcomes.

Either underpins or nuances the argument for low-CCT sources in ecologically sensitive areas. Procurers should weigh actual ecological risk against available data — not against projected laboratory results.

ISO 9227:2022 specifies standardised procedures for neutral salt spray (NSS), acetic acid salt spray (AASS) and copper-accelerated acetic acid salt spray (CASS). The methods can be used, among other purposes, to detect discontinuities and check that the quality of a specified material or corrosion-protection system is maintained. They are not intended to rank different materials by corrosion resistance or predict long-term corrosion resistance in service.

An ISO 9227 result alone does not establish how the tested product or material will perform over time in a particular Nordic installation. The relationship must be assessed with regard to the specimen and configuration, material, pre-treatment, coating system, any scribes or damage, evaluation criteria and actual exposure. Cyclic tests or field data may supplement the evidence, but their relevance must also be demonstrated for the specific case.

A requirement for a specified number of NSS hours therefore needs to define the method, specimen and configuration, pre-treatment and coating system, any scribes, exposure period, evaluation method, acceptance criteria and applicable product or test specification. A higher hour count does not by itself demonstrate a better product or longer service life in the field.

Greater mounting height can reduce the number of poles across large areas, but the outcome depends on geometry, optics, uniformity requirements and installation costs.

Whether high masts produce the best overall result must be assessed for the site, including glare, spill light, maintenance access and life-cycle cost. Mounting height alone cannot settle the choice.

Informs the choice of entire installation concept — pole height, optics, number of luminaires, maintenance strategy — and thereby both TCO and environmental footprint.

Snow changes road-surface reflectance and can alter the luminance seen by a driver. A calculated M-class result for a dry road cannot automatically be transferred to a snow-covered surface because reflection data and contrast change.

The net balance when glare against a white background (washout), contrast loss against bright objects, and snow accumulation on optics and housings are included. Quantitative field studies of net effect are limited. R-tables in EN 13201-3 do not describe snow surface reflectance properties.

Nordic projects should document local winter conditions and road-surface assumptions separately from the standard calculation. Higher luminance from snow does not, by itself, establish better visibility.

CIE 191:2010 defines a mesopic photometry system for relevant adaptation levels. CIE 257:2026 gives further guidance on its use in outdoor lighting. S/P ratio and adaptation luminance must be considered together; a high S/P ratio is not a general energy-saving claim. See mesopic photometry and night vision.

The practical effect on visual tasks and energy use in a project depends on the application and design method. A calculated mesopic advantage is not the same as a measured road-safety benefit.

The method can add useful evidence where applicable, but it does not replace current requirements or assessment of glare, colour rendition, health and ecology.

Demand-based dimming can save energy, but the result depends on the baseline, operating hours and chosen light levels. Sweden’s VGU 2024 addresses demand-responsive control for installations within its scope; the control strategy must reflect the site’s use and safety requirements.

There is no universal percentage threshold for safe night dimming. The minimum level must account for traffic, visual tasks, obstacle detection and project requirements.

Sets the lower limit for how aggressively municipalities and system integrators should recommend night dimming. An energy optimisation that compromises visual performance is not a saving.

IES LM-80 measures light-output maintenance of LED light sources, and IES TM-21 projects that maintenance from the measured data. Reporting limits depend on test duration and sample size. Neither method tests drivers, seals or the complete luminaire’s function. See MTBF is not lifespan — the bathtub curve explained.

How well a component projection reflects a particular luminaire’s service life depends on its other components, temperature, installation and maintenance. An L70 value alone is not a system-life estimate.

Procurers should request separate evidence for lumen maintenance and for system design, warranty and replaceability.