Short answer
Low-pressure sodium lamps emitted almost only the yellow sodium lines near 589 nm, so streets looked monochrome; high-pressure sodium broadened that into an orange glow in which blues and greens looked dark. Much of the world has been replacing both with broad-spectrum white LEDs, which render colours again but emit far more blue, changing both the look of streets and the colour of the night sky.
For decades many roads were lit by low-pressure sodium: extremely efficient, but emitting almost entirely in the sodium D lines at about 589.0 and 589.6 nm. Under a single wavelength, surfaces differ only in how much of that yellow they reflect, so a red car and a green one of similar lightness look the same and road markings lose their colour. High-pressure sodium raised the vapour pressure, broadening the emission across yellow, orange and red; hue partly returned, but blue and green surfaces still received little light and looked dark or grey. The characteristic orange glow of cities and of clouds above them came from these lamps.
Many countries have been converting street lighting from narrow-spectrum sodium to broad white LED, and across Europe this has produced a pronounced whitening of artificial light at night over roughly the past decade. White LED street lights are usually phosphor-converted with a blue pump, so they emit a full visible spectrum with a strong blue component that sodium lacked. Colours of cars, clothing and signs become identifiable again at night. The increase in blue emission is also why ecologists study the change closely, since many biological processes are sensitive to short wavelengths; warmer-white LEDs reduce the blue share.
Street lighting sits at low light levels where vision is often mesopic, with rods contributing alongside cones. Rods are most sensitive near 507 nm and insensitive to red, so under white LED, blues and greens look relatively brighter and reds relatively darker than they would at daytime levels. Under sodium, whose light sat near the long-wavelength side, rods contributed little. That is part of why white LED streets can feel brighter than sodium streets at the same measured illuminance, and why colour at night is still not colour by day.
Anyone choosing a car, a front door or a sign that must work at night is now designing for white LED light in most new schemes, and for a mix of sodium and LED where conversion is partial. Lighter and more saturated colours stay identifiable; dark blues and dark greens still merge with black at street light levels. In a partly converted street the same parked car can change colour from one lamp to the next — the most visible everyday demonstration of how completely the light decides the colour.
| Light | Spectrum | Effect on colour |
|---|---|---|
| Low-pressure sodium | Almost only 589.0 and 589.6 nm | No hue at all; everything a shade of yellow |
| High-pressure sodium | Broadened sodium emission across yellow to red | Reds and yellows survive; blues and greens dark |
| White LED | Blue pump plus broad phosphor emission | Most colours identifiable; more blue emitted |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence· Global, with European trend data
Many parts of the world have been transitioning street lighting from narrow-spectrum high-pressure sodium lamps to broad white LED lamps, increasing emissions in the blue part of the spectrum; across Europe this has produced a pronounced whitening of artificial light at night over about the past ten years.
Source: Mitigating the impacts of street lighting on biodiversity and ecosystem functioning
FactStrong evidence
The sodium D lines lie at about 589.0 and 589.6 nm, which is why low-pressure sodium light is effectively monochromatic.
Source: NIST Handbook of Basic Atomic Spectroscopic Data: strong lines of neutral sodium
FactStrong evidence
Scotopic (rod) sensitivity peaks near 507 nm and is very low for red, so at low light levels blues and greens gain relative brightness over reds.
Source: HyperPhysics (optics, atmospheric optics and vision pages); CIE spectral luminous efficiency for scotopic vision, V′(λ) (CIE 018:2019 dataset)
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.