For decades, low-pressure sodium (LPS) lighting was widely used for highways, industrial areas, and street lighting due to its exceptional efficiency and unique spectral properties.
Today, many municipalities have transitioned to LED systems. While LEDs offer advantages in control, color rendering, and energy management, this transition has also introduced new considerations related to spectral composition, glare, and long-term visual comfort.
This article compares low-pressure sodium vs LED street lighting in terms of spectrum, efficiency, skyglow, and visual experience.
Spectral Characteristics
The defining feature of low-pressure sodium lighting is its nearly monochromatic spectrum. LPS lamps emit almost all visible light at approximately 589 nm, producing the characteristic amber glow.
Because of this narrow spectrum, LPS lighting contains virtually no blue light. In contrast, LED lighting—especially cool white LEDs—includes significant short-wavelength blue light (~450 nm), which scatters more strongly in the atmosphere due to Rayleigh scattering.
Efficiency and Performance
| Parameter | Low-Pressure Sodium | Typical LED |
|---|---|---|
| Luminous efficacy | 100–200 lm/W | 120–180 lm/W |
| Spectrum | Narrow (~589 nm) | Broad (includes blue) |
| Color rendering | Very low (CRI ~0) | Moderate to high |
Skyglow and Light Pollution
Short-wavelength light (blue light) scatters more strongly in the atmosphere, contributing to skyglow. Because LPS lighting emits almost no blue light, it has historically been preferred in areas where reducing light pollution is critical.
Additionally, the narrow spectral output of LPS allows astronomers to filter it out more easily, making it compatible with observatory environments.
Visual Comfort and Glare
Many modern outdoor lighting discussions focus on glare and visual comfort. Blue-rich light sources can increase discomfort due to stronger scattering both in the atmosphere and within the human eye.
By contrast, LPS lighting often appears softer and less glaring in nighttime outdoor environments, particularly in applications where uniform, low-glare illumination is preferred.
Light Sensitivity and Migraine Considerations
Light sensitivity (photophobia) is a common symptom among individuals who experience migraines. Research suggests that blue and white light can increase discomfort in some individuals, while longer wavelengths may be less aggravating.
Because LPS lighting emits almost no blue light, some individuals report that amber lighting appears less visually harsh compared with high-intensity white LED lighting. Individual responses may vary, and research in this area is ongoing.
Ecological Impact
Lighting spectrum also affects ecological systems. Many insects are attracted to ultraviolet and blue wavelengths. Since LPS lamps emit almost none of these wavelengths, they tend to attract fewer insects compared with broad-spectrum lighting technologies.
This property has historically made sodium lighting suitable for wildlife-sensitive areas, coastal regions, and observatory buffer zones.
Why 2200K LED Is Not the Same as LPS
Modern “warm” LEDs (such as 2200K) are often used to reduce blue light. However, even warm LEDs still produce a broad spectrum of light.
In contrast, LPS lighting emits light in a very narrow spectral band, resulting in fundamentally different optical behavior in terms of scattering, glare, and environmental impact.
Ongoing Use and Replacement Demand
Although many new installations use LED technology, low-pressure sodium lighting systems are still in operation in various regions worldwide. As a result, there remains ongoing demand for replacement SOX lamps used in existing infrastructure.
These applications include roadway maintenance, industrial facilities, and specialized environments where narrow-spectrum lighting characteristics are still preferred.
In such cases, maintaining compatibility with existing fixtures and ballasts remains an important consideration.
Conclusion
Low-pressure sodium lighting represents a distinct approach to outdoor illumination. Its narrow spectral output, high efficiency, and minimal blue-light emission offer advantages in applications where reducing skyglow and improving visual comfort are priorities.
While LED lighting has become the modern standard, the transition has involved trade-offs. Understanding these differences can help inform better decisions in lighting design, infrastructure maintenance, and environmental impact.
References
- Noseda, R. et al. (2016). A neural mechanism for exacerbation of headache by light. Brain.
- Martin, V. T. et al. (2016). Green light exposure and migraine sensitivity.
- Illuminating Engineering Society (IES), Lighting Handbook.
- International Dark-Sky Association publications.
- Eisenbeis, G. (2006). Artificial night lighting and insects.