What Don’t We Know?

Mesopic photometry (CIE 191:2010) shows that a higher S/P ratio — typically associated with higher CCT — improves peripheral detection at low luminance levels. Ecological research and dark-sky considerations favour lower CCT. There is therefore a genuine tension. See mesopic photometry and night vision.

Where the net optimum sits when road safety, ecological impact, circadian health, and energy efficiency are weighted together in real traffic conditions. No independent field study has quantified such a combined optimum — there are too many variables and too few controlled experiments.

It directly affects which CCT requirement municipalities should specify in procurement. Today the choice is driven mainly by energy targets and aesthetics — not by an evidence-based optimum for visual performance.

TM-30 (IES TM-30-20) is scientifically more complete than Ra (CRI): it uses 99 colour samples instead of 8 and distinguishes fidelity (Rf) from gamut change (Rg). EN 13201 requires neither — CRI < 80 is permitted on roads.

Whether colour rendition is a meaningful safety or comfort parameter on roads at all, where driving speeds and illuminance levels make other factors more decisive. No robust field study has demonstrated a measurable safety or comfort benefit from TM-30 optimisation in road applications.

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.

Spectrally optimised profiles — with expanded gamut and tuned spectral path — show positive results in laboratory tests using standardised colour recognition tasks and higher reported colour saturation. These data originate from manufacturers, not from independent field studies.

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.

Laboratory experiments demonstrate clear attraction in a range of insect species to near-UV and short-wavelength blue light. Light sources with a high UV component are well-documented insect attractors. Low-CCT sources (amber, 1,900–2,200 K) reduce attraction in laboratory tests.

Population and ecosystem effects over time in actual environments. Laboratory attraction data is not equivalent to measurable ecological harm or benefit at population level. Long-term studies comparing LED CCT and insect populations in controlled field environments are lacking.

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.

NSS (neutral salt spray, ISO 9227) delivers reproducible and comparable test hours between products. That is the standard’s strength. It is also its limit: the test uses constant-humidity salt spray, not Nordic variable environmental conditions.

How well constant salt spray correlates with real Nordic exposure — where freeze/thaw cycles, road salt, UV variation, and wet/dry cycling act simultaneously. Cyclic corrosion tests (e.g. ISO 11997, VDA 233) may have higher field relevance but are not yet standardised in European lighting requirements.

Procurers specifying high NSS hours may have a false sense of security if the test’s field relevance is unknown. Supplementary cyclic tests or documented field installations are more informative for Nordic coastal environments.

High masts with few luminaires can reduce installation cost and cabling on large open areas. The photometric engineering is well understood; optic selection for specific mounting heights is a solved design problem.

Whether high masts are preferable when glare, skyglow (upward light ratio), peripheral detection, and maintenance cost (crane requirement, road closure) are all factored together. Life-cycle field comparisons are absent from the literature.

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

Fresh snow has high albedo and can substantially increase reflected luminance from the road surface. In theory, an installation may meet an M-class on a snow-covered winter road that it does not meet on bare asphalt. Snow is in practice the normal winter surface reflection condition in Nordic street lighting — but is not regulated.

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 street lighting is dimensioned for winter conditions in practice, but EN 13201 offers no tools for this. It is a relevant gap for Swedish municipalities dimensioning for year-round visual performance.

CIE 191:2010 formalises the mesopic adjustment factor M(S/P, L_a) and enables a more accurate description of visual performance at low luminance levels. A high S/P ratio delivers more lux-equivalent night vision per watt under mesopic conditions. See mesopic photometry and night vision.

How large the actual road safety benefit is when mesopic calculations replace lux-based requirements in practice, and why the method is rarely used in procurement despite the standard existing since 2010. Transition costs, software support, and awareness all limit adoption.

If the gain is meaningful, today’s lux/luminance-based requirements are systematically suboptimal for night-time visual performance. That is a significant driver in the long-term development of lighting standards.

Adaptive dimming and presence- or traffic-responsive controls deliver documented energy savings of 30–70% compared with fixed output. EN 13201 enables lighting-class switching and dynamic adaptation. The technology works.

Where the lower dimming threshold lies before detection acuity or perceived safety is negatively affected. Accident studies at deep night dimming (<50%) are limited; safety perception studies at low luminance levels show variable results.

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.

TM-21 extrapolates LED module lumen maintenance based on LM-80 measurements, with a 6x rule for extrapolation limit. The method measures the module, not the luminaire as a system. Drivers, gaskets, and solder joints are not part of the measurement. See MTBF is not lifespan — the bathtub curve explained.

How well an L70 claim correlates with actual luminaire lifespan in the field when drivers, gaskets, and solder joints are included. Systematic comparisons between stated L70 and actual field data over 15+ years are absent at scale.

The entire industry’s lifespan communication rests on a module metric that does not measure the system. Procurers comparing L70 figures are comparing module claims — which is an incomplete picture of what they are actually buying.