Mesopic Photometry — How the Eye Sees at Night

Is lux the right measure in darkness?

Lumens and lux use the photopic sensitivity function V(λ). They provide standardised, comparable photometric values for night-time lighting too. EN 13201 uses luminance for M classes and illuminance for C and P classes. At low adaptation levels, some visual tasks may require an additional assessment of the spectrum.

This is not a meaningful objection to using lux as a measure — lux is practical, well-established, and comparable. But there is a specific situation where the difference matters: when two light sources with the same measured lux but different spectral compositions are compared for visual performance in darkness. There, the comparison can become systematically misleading.

In short

Equal photopic illuminance does not describe every aspect of night vision. Assess luminance, adaptation and spectrum for the visual task.

Three vision regimes

The eye’s adaptation level depends on the luminance distribution in the visual field, not only on average road luminance. The table shows the CIE system’s application ranges for off-axis visual tasks. CIE e-ILV

Three vision regimes
Vision regime	Luminance range	Dominant receptors	Peak sensitivity
Photopic	> 5 cd/m²	Cones	555 nm (green)
Mesopic	0.005–5 cd/m²	Both cones and rods	507–555 nm (level-dependent)
Scotopic	Below ~0.005 cd/m²	Rods	507 nm (blue-green)

M-class maintained average luminance requirements range from 0.3 to 2.0 cd/m². These values lie within the mesopic range, but do not alone determine adaptation: surrounding light and headlights can also contribute. This does not invalidate the photopic class requirements.

The Purkinje shift

When the ambient luminance falls into the mesopic and scotopic range, the eye's sensitivity shifts toward shorter wavelengths — toward blue. This is called the Purkinje shift, described by Jan Evangelista Purkyně in the 19th century. Physiologically it occurs because rods, whose sensitivity peaks at approximately 507 nm, take over from cones, whose peak is at 555 nm.

The effect is observable in everyday life: a red object that appears bright in daylight becomes nearly dark at dusk, while a blue object remains relatively more visible. The same applies to road markings, vehicles, pedestrians, and obstacles on a night-lit street.

The S/P ratio — what it measures

The S/P ratio (scotopic/photopic ratio) is the numerical measure of this spectral difference. It is defined as the ratio of a source's scotopic lumens to its photopic lumens — both measured from the same source but weighted with different sensitivity curves (V′(λ) and V(λ) respectively).

Calculate the S/P ratio from the source’s spectral power distribution. Sources with the same CCT can have different S/P ratios; filtered white LEDs and narrow-band amber are not the same spectrum. CCT alone cannot establish a generic S/P value. CIE TN 005:2016

CIE 191:2010 — the mesopic system

CIE 191:2010 describes a normalised, adaptation-dependent combination of photopic and scotopic spectral sensitivity. The calculation requires S/P and adaptation luminance; it is not a general conversion from lux to road safety. CIE 191:2010

At the same photopic adaptation luminance, different S/P ratios can give different mesopic luminance. Interpret the result for the visual tasks and adaptation conditions covered by the model.

CIE 257:2026 develops practical application, including the adaptation field and the visual tasks for which the system is appropriate. Continue to report EN 13201 photopic requirements separately; a mesopic assessment does not automatically permit a lower specified class. CIE 257:2026

Peripheral vision and motion detection

Rods are heavily concentrated in the periphery of the retina. Central (foveal) vision consists almost entirely of cones; peripheral vision in darkness is driven almost exclusively by rods. This is why in very low light, faint objects are sometimes more visible when looked at slightly off-centre — a phenomenon astronomers call off-axis viewing.

In traffic environments, peripheral vision is critical for detecting movement at the edge of the visual field: the pedestrian stepping off the kerb, the cyclist crossing from the left. Laboratory and simulator studies suggest that sources with higher S/P ratios can produce faster peripheral detection at the same photopic luminance in mesopic conditions. The magnitude of this effect in real traffic is difficult to quantify; field data is limited and laboratory results do not translate automatically into traffic safety metrics.

The honest conflict: traffic safety versus ecology

An unresolved trade-off

A spectrum that assists one peripheral visual task is not automatically best for every traffic situation or the local environment. Assess visual requirements and environmental effects together.

Spectrum is one of several factors affecting nature and the night sky. Light level, direction, duration and location also matter, and species respond differently. DarkSky and IES recommend useful, targeted light at the lowest necessary level, controlled in time and with short-wavelength content limited. DarkSky / IES

CIE S 026 melanopic metrics describe stimulation of human photoreceptors. They are not general animal-response metrics or direct predictions of health effects. Spectrum, light at the eye and exposure duration must be distinguished from the road’s photopic lighting requirements. CIE S 026/E:2018

There is currently no consensus-based answer to where the net optimum lies when traffic safety, ecology, and energy efficiency are weighed together. In practice, the decision is made locally: is the installation in an urban core or in an ecologically sensitive area? What M-class requirement applies? Is there ecological vulnerability in the vicinity? Those answers drive the CCT choice more than any universal technical optimum.

The ageing eye

The eye’s spectral transmission changes with age. CIE describes how lens ageing reduces transmission, particularly at shorter wavelengths. A source’s S/P ratio therefore does not by itself describe an individual driver’s visual conditions. CIE TN 003:2015

Consider age and individual differences alongside contrast, glare and the visual task. Ageing alone does not justify a universal increase in blue content or lighting level. CIE 203:2012

What this means for specification

Start the specification with the required lighting class. Add spectral data and a mesopic assessment where the visual task justifies it.

Lux comparisons are not spectrally neutral. Two luminaires with identical lux but different CCT can deliver different peripheral visual performance in mesopic conditions. Comparing "at the same lux" without accounting for the S/P difference compares photopic lumens, not actual visual performance.
An HPS-to-LED replacement must meet the specified photopic requirements. A higher S/P ratio alone is not grounds for lowering the class.
CCT choice in sensitive environments: In nature-sensitive areas, at coastlines and nature reserves, the ecological argument for low CCT or amber competes directly with the mesopic argument for higher S/P. This is a policy and environmental judgement, not a purely technical optimum.
CIE 257:2026 develops practical application, including the adaptation field and the visual tasks for which the system is appropriate. Continue to report EN 13201 photopic requirements separately; a mesopic assessment does not automatically permit a lower specified class.

Next level of understanding

Mesopic theory assumes lux is measured correctly. But the calculation model has its own assumptions that break down when road surface properties change.

DIALux evo can calculate luminance using a selected road-surface model. A dry-road calculation does not automatically describe a wet evening; check the calculation case and reflection data. DIALux evo

Lighting fundamentals

What is a lumen?

How lux, lumens, and candela are defined — and why all three are photopically weighted.

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Frequently asked questions

Should S/P ratio be specified in procurement?

Request S/P where the project needs a mesopic assessment. Ask for spectral data for the selected configuration and identify the visual task and adaptation conditions. S/P does not replace EN 13201 photopic class requirements.

Which colour temperature is safest for road environments?

CCT alone cannot establish which installation is safest. Check class requirements, uniformity, glare, contrast and relevant spectral data. In sensitive habitats, also assess light level, direction, operating hours and local species.

Does mesopic photometry also apply to pedestrian and cycle paths?

Yes. CIE 257:2026 covers drivers and pedestrians. Relevance depends on the visual task and adaptation, rather than a general ranking of walking and driving. P-class requirements must still be met.

VALDUR and colour temperature

VALDUR has standard white options at 3,000 and 4,000 K. The reference Amber configuration uses a 2,200 K LED source and gives approximately 1,900 K after the optic. LED-source CCT and outgoing luminaire colour temperature are distinct values; specify the final value and any other CCT option for the ordered configuration.

Summary

Photopic requirements remain the basis for class verification. Mesopic photometry can complement the assessment of some visual tasks at low adaptation levels.

Request spectrum and S/P for the actual configuration. Neither CCT nor average road luminance alone describes the conditions for mesopic vision.

Assess the visual task together with the site’s environmental requirements. Consider spectrum, light level, direction and operating hours together.

Mesopic photometry — how the eye sees at night