Dark Sky Lighting - Beyond Up light: The Overlooked Role of Reflected Light
- Magnitech Lighting
- Apr 22
- 4 min read
Dark sky lighting has largely been built around a single idea: eliminate up light.
Shield the luminaire, keep emission below the horizontal, control glare—and you’ve done your job. These principles are well-established and essential. Direct up light is one of the main contributors to skyglow.
But this framework is incomplete.
Even in a perfectly shielded installation—0% up light from the light fitting—significant light can still reach the night sky. Reflection from illuminated surfaces is often over-looked and unaccounted for.
The Conventional Model: Up light as the Primary Offender
The mitigation of light pollution has historically focused on controlling direct emission above the horizontal plane. Classification systems such as BUG (Backlight, Up light, Glare) ratings reinforce this approach, with up light treated as the dominant contributor to skyglow.
This model is still valid and significant. Light emitted directly upward bypasses surface interactions and is efficiently scattered by atmospheric particles.
As a result, standards such as those from the International Dark-Sky Association (IDA) emphasise fully shielded luminaires, often requiring U0 classifications in sensitive areas.
However, this approach implicitly assumes that eliminating up light is sufficient to control skyglow. In many real-world installations, it is not.
The Missing Component: Reflected Light
When light strikes a surface, part of it is absorbed and the remainder is reflected. For most outdoor materials—concrete, asphalt, paving, vegetation—this reflection is predominantly diffuse, meaning it is scattered in many directions including upward.
In effect, every illuminated surface becomes a secondary emitter.
The magnitude of this reflected emission depends on two key factors:
Surface reflectance (ρ)
Incident illuminance (E)
Reflected Exitance ≈ E × ρ
For example:
If a concrete surface (ρ ≈ 0.3) is illuminated to 100 lux, it will re-emit approximately 30 lm/m² of light into the hemisphere above it.
This emission is broad and non-directional. A significant portion contributes to upward-directed flux.
Why Reflected Light Is Often Overlooked
1. Measurement Simplicity
Direct up light is straightforward to quantify using photometric data. Reflected light requires accounting for surface properties, geometry, and environmental variability, making it more complex to model.
2. Design Culture
Lighting design has traditionally been driven by illuminance targets. Meeting prescribed lux levels for safety and visibility often takes precedence over environmental impact. Once the check box has been ticked, over-illumination is ignored.
3. Regulatory Gaps
Most lighting ordinances and standards regulate luminaire characteristics—shielding, mounting height, and glare—but rarely impose strict limits on total site illuminance or luminance. As a result, installations can comply with dark sky criteria while still generating substantial reflected light (in essence defeating the entire objective).
From Reflection to Skyglow
Once reflected upward, light interacts with the atmosphere through the scattering process.
Reflected light is particularly effective at producing skyglow because:
It originates from large surface areas
It is emitted across a wide range of angles
It behaves as a distributed source rather than a point source
The result is a broad, uniform luminous dome over urban environments.
The Illuminance Paradox
This leads to a counterintuitive result:
Theoretically a lighting system can have zero up light—and still produce substantial skyglow.
We tend to treat skyglow as a fixture problem: light escaping upward from luminaires. But once up light is minimized, the dominant factor often becomes how much light is delivered to the ground.
In practical terms:
A high-output, fully shielded LED installation can produce more skyglow than a lower-output, partially shielded system.
This is because the ground itself becomes a much larger source of skyglow than a small point source.
Design Implications
Addressing reflected light requires shifting from fixture-centric design to system-level control.
1. Reduce Illuminance to Task-Appropriate Levels
Over lighting is one of the primary drivers of reflected skyglow. Designers should:
Re-assess the required lighting levels
Avoid unnecessary illumination over large unoccupied areas.
Implement adaptive lighting strategies.
2. Design for Luminance, Not Just Illuminance
Visual perception is governed by luminance and contrast. Designing for appropriate luminance distributions (where possible) can achieve visual performance with lower total light output.
3. Account for Surface Reflectance
Material selection influences reflected light:
High-reflectance surfaces amplify skyglow under high illumination.
4. Improve Spatial Control
Limit lighting to where it is needed:
Minimise spill light.
Use precise optical control.
Avoid illuminating non-critical areas.
5. Use Dimming and Curfews
Because there is no better solution to skyglow than no light at all, time and occupancy based light controls such as the following are highly effective:
Late-night dimming.
Occupancy-based lighting.
Zoned control strategies.
Case Study Scenario
Consider two parking area lighting schemes:
Required lighting levels: 20 lux
Scheme A:
50 lux average
Fully shielded luminaires (U0)
Scheme B:
20 lux average
Fully shielded luminaires (U0)
Assuming identical surface reflectance, Scheme A produces 2.5× more reflected light than Scheme B, while both are fully compliant.
Despite identical up light ratings and compliancy, Scheme A will generate significantly more skyglow.
Toward a More Complete Dark Sky Framework
A more effective approach to light pollution must include:
Maximum allowable illuminance thresholds
Integration of surface reflectance into design
Performance metrics based on skyglow contribution rather than luminaire classification alone
Light pollution is a system-level outcome, not just a fixture property.
Conclusion
Eliminating up light has been a necessary step in reducing light pollution—but it is not always sufficient.
Reflected light from illuminated surfaces represents a significant and often underestimated pathway for light to re-enter the night sky.
Even fully compliant dark sky installations can generate substantial skyglow if illuminance levels are excessive.
Effective dark sky design requires controlling not just where light is emitted, but how much light is used overall—and how it interacts with the environment.
Only by accounting for reflected light can we move toward genuinely sustainable nocturnal environments.
Magnitech — Lighting what matters, and making light matter.
