What We Lost When We Replaced Low-Pressure Sodium with LED Streetlights

What We Lost When We Replaced Low-Pressure Sodium with LED Streetlights

Low Pressure Sodium (LPS/SOX) lighting was widely used for highways, industrial areas, and street lighting for decades because of its exceptional luminous efficacy and unique spectral characteristics. Its nearly monochromatic amber emission around 589 nm made it particularly useful in applications where energy efficiency, low skyglow, and specialized spectral performance were important.

Today, many municipalities have transitioned to LED systems. LEDs provide significant advantages in controllability, color rendering, dimming, connectivity, and system-level energy management. However, the transition from LPS to LED also introduces important differences in spectral composition, luminance, glare, atmospheric scattering, and nighttime visual experience.

This article compares Low Pressure Sodium and LED street lighting across spectrum, efficiency, skyglow, visual comfort, ecological impact, and practical replacement considerations.

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 their characteristic amber appearance.

Because its visible output is concentrated within an extremely narrow spectral region, LPS contains virtually no blue light. This is fundamentally different from most white LED systems, which produce a broad spectrum that typically includes a significant short-wavelength component.

Shorter wavelengths scatter more strongly in the atmosphere through Rayleigh scattering. As a result, the spectral composition of an outdoor lighting system can influence how much artificial light is scattered into the nighttime sky.

Low Pressure Sodium LPS and LED spectrum comparison

Efficiency and Performance

Low Pressure Sodium has historically been recognized as one of the most efficient conventional electric light sources. Its monochromatic emission is concentrated near the peak sensitivity of the human photopic visual response, allowing a large proportion of electrical energy to be converted into visually useful light.

Modern LEDs can also achieve very high luminous efficacy, and complete luminaire performance depends on factors such as optical efficiency, driver losses, thermal management, operating conditions, and control systems.

Parameter Low Pressure Sodium Typical LED
Luminous efficacy Very high; historically up to ~200 lm/W for the lamp Typically high; varies by LED and luminaire design
Spectrum Extremely narrow (~589 nm) Broad spectrum; varies by LED design
Blue light Virtually none Present in most white LEDs
Color rendering Extremely limited Low to very high depending on design
Dimming and controls Limited compared with LED Highly controllable

The comparison therefore depends on the objective. LEDs offer substantially greater flexibility, while LPS provides an exceptionally efficient and spectrally concentrated source of illumination.

Skyglow and Light Pollution

Artificial light contributes to skyglow when light is directed upward or scattered through the atmosphere. Shorter wavelengths generally experience stronger Rayleigh scattering than longer wavelengths, making spectral composition an important consideration in nighttime outdoor lighting.

Because LPS emits virtually no blue light, its contribution to short-wavelength atmospheric scattering is extremely limited. This characteristic has historically made LPS attractive for locations where controlling skyglow is a priority.

The narrow spectral output of LPS also provides a unique advantage for some astronomical applications. Because most of its visible emission is concentrated around the sodium D-lines, astronomers can use specialized filters to suppress LPS wavelengths while retaining much of the surrounding night sky spectrum.

LED systems can also be designed to reduce skyglow. Lower color-temperature LEDs, carefully selected spectra, full cutoff optics, appropriate mounting heights, and intelligent dimming can all reduce unnecessary light emissions. However, a warm LED remains a broad-spectrum source rather than a monochromatic source like LPS.

Visual Comfort and Glare

Energy efficiency alone does not determine how comfortable a nighttime lighting system appears. The luminance, optical distribution, mounting height, source size, and glare control of a luminaire all influence the visual experience.

Many modern LED luminaires use relatively small, high-intensity semiconductor emitters. When these sources are directly visible, their concentrated luminance can create bright points within the visual field.

LPS lamps use a long discharge tube as the emitting surface. This broad luminous surface distributes light over a much larger area and can produce a visually softer source compared with a concentrated point emitter.

Proper fixture design remains essential for both technologies. A poorly designed LPS luminaire can still produce unwanted glare, while a well-designed LED luminaire can provide excellent glare control.

Light Sensitivity and Human Visual Comfort

Human responses to artificial light vary considerably between individuals. Factors such as age, visual adaptation, contrast, glare, luminance, and personal sensitivity can all influence nighttime visual comfort.

Short-wavelength light can scatter more strongly within the human eye and atmosphere, which is one reason spectral composition is considered when evaluating nighttime lighting environments.

Because LPS produces virtually no blue light and has a relatively broad emitting surface, its visual appearance can be less harsh than some high-luminance white LED sources. However, individual responses vary, and lighting quality depends on the complete lighting system rather than spectrum alone.

For applications where visual comfort is a priority, lighting should therefore be evaluated using multiple factors, including source luminance, glare, uniformity, contrast, spectrum, and the viewing environment.

Ecological Impact

Artificial light at night can influence ecological systems in several ways. Different species respond differently to wavelength, intensity, timing, and direction of artificial illumination.

Many insects are particularly responsive to shorter wavelengths and ultraviolet radiation. Because LPS produces virtually no ultraviolet or blue emission, it has historically been used in wildlife-sensitive areas where reducing insect attraction and ecological disruption is important.

The same spectral characteristic can also be relevant to coastal environments and sea turtle conservation. Longer-wavelength amber lighting is generally considered less disruptive to sea turtle behavior than many shorter-wavelength white light sources.

Lighting spectrum should therefore be considered alongside shielding, intensity, mounting height, and operating hours when designing lighting systems near environmentally sensitive areas.

Why 2200K LED Is Not the Same as LPS

Modern warm-white LEDs, including products with color temperatures around 2200K, are increasingly used to reduce blue-light content and create a warmer nighttime appearance.

However, color temperature does not describe the complete spectral distribution of a light source. A 2200K LED still produces a broad spectrum containing multiple wavelengths.

In contrast, LPS produces light in an extremely narrow spectral band centered near 589 nm. The difference is therefore not simply whether the light appears warm or amber. The two technologies have fundamentally different spectral distributions.

This distinction becomes particularly important when lighting is evaluated for skyglow, astronomical observation, wildlife sensitivity, or applications where a highly controlled spectrum is desirable.

Color Rendering: The Major Trade-Off

The greatest limitation of LPS is also one of its defining characteristics: its extremely narrow spectrum provides almost no conventional color information.

Under LPS illumination, objects appear primarily in shades of amber because there are not enough different wavelengths available for the human visual system to distinguish their natural colors.

LED lighting offers a major advantage in this area. By combining multiple wavelengths, LEDs can provide significantly better color rendering and can be selected for different color temperatures and color-rendering requirements.

This makes LED the preferred technology for many urban environments where identifying colors, signage, vehicles, pedestrians, and architectural features is important.

LPS remains more appropriate for specialized applications where spectral efficiency, low blue light, low skyglow, or wildlife sensitivity are more important than full-color visibility.

Control, Dimming, and Smart Lighting

One of the strongest advantages of LED technology is its ability to integrate with modern lighting controls. LEDs can be dimmed, scheduled, networked, monitored, and dynamically controlled with relatively high precision.

These capabilities allow municipalities to reduce energy consumption by adjusting lighting levels according to traffic patterns, time of night, weather conditions, or other operational requirements.

Traditional LPS systems generally provide less flexibility for rapid dimming and dynamic control. This represents an important consideration when comparing technologies for new infrastructure projects.

However, for applications where sophisticated controls are unnecessary, the simplicity and efficiency of an LPS system can remain advantageous.

Ongoing Use and Replacement Demand

Although many new street lighting installations now use LED technology, Low Pressure Sodium systems remain in operation in various regions around the world.

Existing installations require replacement lamps, compatible ballasts, sockets, and maintenance components throughout their operating life. This creates continuing demand for replacement SOX lamps even as new installations increasingly adopt LED.

Applications can include roadway lighting, industrial facilities, tunnels, security areas, ecological zones, and other specialized environments where the narrow-spectrum characteristics of LPS remain useful.

For existing installations, maintaining compatibility with the original fixture and ballast is an important consideration. Replacing an LPS lamp is not always equivalent to replacing an entire lighting system with LED, particularly where existing infrastructure and optical performance are still satisfactory.

Choosing Between LPS and LED

Neither technology is universally superior. The appropriate choice depends on the objectives and constraints of the application.

LPS may be advantageous when:

  • Extremely high luminous efficacy is important.
  • Minimal blue-light emission is desired.
  • Reducing short-wavelength skyglow is a priority.
  • Wildlife-sensitive lighting is required.
  • Color recognition is not a critical requirement.
  • Existing LPS infrastructure is already in place.

LED may be advantageous when:

  • High color rendering is required.
  • Dimming and smart controls are important.
  • Multiple color temperatures or spectra are needed.
  • Precise optical control is required.
  • Dynamic lighting management is part of the system design.

The best lighting decision should therefore consider the complete system rather than comparing lamp technologies based on a single metric such as lumens or wattage.

Conclusion

Low Pressure Sodium and LED represent two fundamentally different approaches to outdoor lighting. LPS provides an exceptionally narrow 589 nm spectrum, virtually zero blue light, high luminous efficacy, and a large diffuse emitting surface. LED provides broad spectral flexibility, high color rendering, advanced controls, and extensive design options.

The transition from LPS to LED has delivered important benefits for modern lighting infrastructure, but it has also changed the spectral and visual characteristics of nighttime environments.

Understanding the differences between LPS and LED helps lighting designers evaluate more than energy consumption alone. Spectrum, skyglow, glare, visual comfort, ecological impact, color rendering, controls, and existing infrastructure can all influence which technology is most appropriate for a particular application.

Frequently Asked Questions

Is LPS more efficient than LED?

Modern LEDs can achieve very high luminous efficacy, and system efficiency depends on the complete luminaire design. LPS has historically been one of the most efficient conventional light sources, with lamp efficacy reaching approximately 200 lm/W in some high-efficiency systems.

Why does LPS produce almost no blue light?

LPS produces light primarily from sodium emission lines near 589 nm. Its visible spectrum is therefore extremely narrow compared with the broad spectrum produced by white LEDs.

Does 2200K LED have the same spectrum as LPS?

No. A 2200K LED produces a broad spectrum containing multiple wavelengths, while LPS produces an extremely narrow emission concentrated near 589 nm. Similar visual warmth does not mean similar spectral output.

Which is better for reducing skyglow, LPS or LED?

LPS has an inherent advantage in minimizing short-wavelength scattering because it produces virtually no blue light. LEDs can also be designed to reduce skyglow through warm spectra, low intensity, shielding, directional optics, and adaptive controls.

Why is LPS considered suitable for some wildlife-sensitive applications?

LPS produces virtually no blue or ultraviolet light and concentrates its output around 589 nm. This can reduce exposure to wavelengths that strongly affect some wildlife species. Lighting direction, intensity, shielding, and operating hours remain equally important.

Why was LPS replaced by LED in many cities?

LED systems provide advantages in color rendering, dimming, controls, optical flexibility, maintenance, and integration with smart-city infrastructure. These advantages have made LED attractive for many modern municipal lighting projects despite the unique spectral benefits of LPS.

Is LPS still used today?

Yes. Although many new installations use LED, existing LPS systems remain in service in various regions and specialized applications. Replacement SOX lamps continue to be required for maintaining compatible infrastructure.

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References

  • Illuminating Engineering Society (IES) – Lighting Handbook.
  • International Commission on Illumination (CIE) – Publications on outdoor lighting, glare, and visual performance.
  • International Dark-Sky Association – Resources on outdoor lighting and light pollution.
  • Eisenbeis, G. (2006) – Artificial night lighting and insects.
  • Noseda, R. et al. (2016) – Research on light and headache mechanisms, Brain.

Last Updated: August 2026

This article is part of the Low Pressure Sodium (LPS) Lighting Knowledge Hub, a technical resource covering the physics, engineering, efficiency, and specialized applications of Low Pressure Sodium (SOX) lighting systems.

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