| Pedestrian Lighting | On the Lighter Side of the Sun | By Piotr Mikus
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What to demand in a pedestrian solar lighting proposal before you sign anything:
- Photometric submission includes vertical illuminance results at the minimum operating level, not just horizontal average at full output
- Uniformity ratios shown at the actual dimming floor, not at the output the system runs at for the first two hours of the night
- Optic type documented with confirmation it suits pedestrian-scale mounting heights and does not introduce eye-level glare at reduced output
- Dimming profile declared in writing: full output hours, transition points, minimum floor, and the RP-43-25 criteria that floor was derived from
- Energy budget modeled for worst-case winter conditions, with maintained pedestrian criteria confirmed at the minimum dim level after battery degradation is applied
- Commissioning plan that verifies orientation, tilt, mounting height, and control settings match the design before the project is accepted
A lighting designer who applies roadway logic to a pedestrian path has not done a simpler version of the same job. They have done a different job badly. The visual tasks are different, the criteria are different, and in solar lighting, the stakes of getting it wrong show up fast. Not at commissioning. At 11 p.m. on a January night when the battery is at 40 percent and the dimming floor kicks in.
What RP-43-25 is actually trying to protect
People do not walk at 80 kilometers per hour staring at a lane line. This matters more than most solar proposals acknowledge.
Roadway lighting is built around a driver at speed on a horizontal plane. RP-8-25 flows from that task: luminance on the road surface, uniformity across the lane, glare that does not impair someone moving at 50 or 80 kilometers per hour. Solar sizing in roadway applications follows the same logic. The criteria set the energy load, the energy load drives the battery and panel, and the system is built to meet that criteria floor every night of the year.
RP-43-25 is built around a different person with different needs. A pedestrian moves slowly, at ground level, in multiple directions. The visual tasks are orientation, wayfinding, surface recognition, and above all, identifying other people. That last task, face and body recognition, is what most solar pedestrian proposals fail to protect, because it lives in the vertical plane and the vertical plane is the first thing to collapse when the battery is running lean.
Vertical illuminance is a core metric in RP-43-25, not a bonus deliverable. It exists because pedestrians need to see faces and movement direction at close range. None of that information lives on the ground. A solar system that delivers strong horizontal numbers and ignores the vertical plane has spent its battery budget on a visual task pedestrians do not actually perform.
Uniformity matters differently here too. A driver at speed experiences an average over time. A pedestrian walking through a dark gap between two lit pools experiences the gap, not the average. In a solar system with cheap distribution optics and wide pole spacing chosen to reduce hardware cost, that gap is not a design oversight. It is a budget decision that shows up as a safety problem every night the system dims past its operating floor.
The solar-specific reality nobody puts in the headline
The dimming profile is not a lighting design decision in most solar pedestrian installations. It is an energy survival decision. Those are not the same thing.
A solar pedestrian lighting system does not run at a fixed output level. It runs at whatever the controller decides based on battery state of charge, time of night, season, and the logic programmed at commissioning. In a properly engineered system, that logic is explicitly tied to the RP-43-25 criteria. The minimum dim level is the output level at which the system still meets the maintained pedestrian lighting criteria for that specific installation. In most systems on the market today, the minimum dim level is whatever percentage the vendor set as a default, with no connection to any criteria at all.
Here is what that produces in the field. The system starts the night at full output. Two hours later it steps to 60 percent. By midnight it is at 40 percent, or lower if the battery state of charge is poor from a run of cloudy days. The specifier accepted this because the vendor described it as an adaptive lighting profile. What the specifier never checked is what 40 percent output actually delivers for vertical illuminance at the specific mounting height and pole spacing of that installation. In a significant number of cases, the answer is not enough. In cold climates where battery capacity drops 25 to 30 percent on a January night, the system may be hitting that floor two or three hours earlier than the sizing model assumed.
Cold weather compounds this at both ends. A LiFePO4 battery operating at minus 10 degrees Celsius has roughly 70 to 75 percent of its rated capacity available. A system sized for a full winter night at rated capacity may be running out of usable energy before dawn, stepping down output earlier and harder than the design intended. The pedestrian path does not get brighter when temperatures drop. It gets dimmer, at exactly the time of year when nights are longest and the lighting is needed most.
Cheap charge controllers make this worse. A controller without proper state-of-charge management does not know where the battery actually is in its charge window. It estimates. And when it estimates wrong on a cold January night, the system steps down output not because the lighting design calls for it but because the controller is protecting itself. That step-down has no relationship to RP-43-25 criteria. It has a relationship to the cost savings from not buying a better controller.
Where the disconnect usually lives
Roadway logic is comfortable. It has a clear metric, a clear table, and a clear compliance path. Pedestrian logic asks harder questions, and most solar proposals are not built to answer them.
The first failure point is the photometric submission. Most vendors submit horizontal illuminance calculations at full output. Some include a uniformity ratio. Very few include vertical illuminance results, and almost none show what the layout delivers at the minimum operating level. The submission looks complete because it has numbers. The numbers just do not describe what the pedestrian experiences at 1 a.m. in February after three cloudy days.
The second is the optic selection. Solar pedestrian projects are often specified with whatever fixture is available at the target price point. That fixture is frequently a roadway-style optic mounted on a short pole. Roadway optics push light forward along a travel surface. On a pedestrian pole at four or five meters, that distribution creates high-angle intensity at pedestrian eye level and delivers poor vertical plane performance. It also wastes battery energy sending light where pedestrians do not benefit from it. In a solar system where every watt-hour counts, bad optics are not just a visual quality problem. They are an energy efficiency problem.
The third is the dimming logic. When a roadway solar lighting system is specified correctly, the RP-8-25 criteria set the dimming floor. For collector and major streets, Section 6.10.2.2.2 says the system cannot dim below the lowest applicable criteria for the road classification. Pedestrian solar systems rarely have an equivalent requirement in the specification because the specifier did not write one. The result is that the controller dims to wherever the battery and the vendor default tell it to, with no criteria-based floor in place.
What to Require in a Specification (especially for solar)
If it is not in the specification, it will not appear in the submission. Nobody submits the numbers that expose the gap.
- Photometric calculations showing horizontal and vertical illuminance at the declared minimum operating level, for the actual optic, mounting height, and pole spacing of this installation
- Uniformity ratio for both horizontal and vertical results verified at the minimum dim floor, not at full output
- Optic type and distribution documented with confirmation the selection limits high-angle intensity at pedestrian eye level at the specified mounting height
- Dimming profile stated in writing: full output hours, step-down schedule, minimum floor as a percentage of full output, and the RP-43-25 criteria value that minimum floor was derived from and maintains
- Energy budget modeled for the worst-case winter month at the project location, with battery capacity derated for expected low temperature, and maintained pedestrian criteria confirmed at the minimum dim level under those conditions
- Battery management documentation: how state of charge is monitored, whether low-temperature charge cutoff is enabled, and whether the dimming profile can be adjusted remotely if real-world performance diverges from the sizing model
- Commissioning verification: mounting height, tilt, orientation, and control settings confirmed against design before acceptance
- Acceptance criterion tied to measured vertical illuminance at a representative location at the minimum operating level, not a visual walkthrough at dusk
Three Questions That Separate Pedestrian Designs from Roadway Logic
What vertical illuminance does this system deliver at the minimum operating level, and what RP-43-25 criteria value was that floor derived from? If the answer does not include a vertical illuminance result at the minimum dim level for this specific installation, the pedestrian visual task was never verified. The system was sized for energy survival, not for lighting performance.
What happens to the dimming profile on a night when the battery enters the installation at 70 percent state of charge after two overcast days, and what does the system deliver for vertical illuminance at the point where it hits its lowest output? If the vendor cannot answer this with numbers from the energy model for this project, the sizing did not account for real operating conditions. In a solar system in a northern climate, that scenario is not a worst case. It is a normal January.
Can the battery management parameters and dimming profile be adjusted remotely after commissioning, and how would you correct the system if real-world performance falls short of the criteria floor in the first winter? If the answer is that the settings are locked at commissioning, the system cannot respond when the field diverges from the model. And it will diverge. It always does.
Closing Thought
If people can’t recognize each other, the path is not lit. It is decorated. In solar, decorated costs battery cycles.
Pedestrian lighting is not a simpler problem than roadway lighting. It is a different problem with different criteria, a different set of visual tasks, and a pedestrian who cannot see oncoming faces when the battery has been running low since Tuesday. Write the criteria into the specification. Tie the dimming floor to a number derived from RP-43-25. Model the energy budget for January, not June. Then verify it at commissioning under actual operating conditions.
Sources and Where to Verify
ANSI/IES RP-43-25: Recommended Practice for Lighting Design for Outdoor Pedestrian Applications. Visual task analysis, horizontal and vertical illuminance criteria, uniformity requirements, glare guidance. ANSI/IES RP-8-25: Roadway and Area Lighting. Adaptive lighting floor requirements, Section 6.10.2.2.2, for context where pedestrian zones interface with roadway lighting and control strategies.
Quick FAQ
Q: Can I use a roadway luminaire on a pedestrian-scale pole if it meets the footcandle target? A: Meeting a horizontal average at full output does not make a roadway fixture appropriate for a pedestrian application. Roadway optics on short poles generate high-angle intensity at eye level and deliver poor vertical plane performance. They also waste battery energy sending light where it does not serve pedestrians. The optic has to match the mounting height, the visual task, and the energy budget.
Q: How much does battery degradation affect pedestrian lighting performance over the system’s life? A: Significantly, and it compounds. A battery at 80 percent of rated capacity after three years delivers less usable energy per night, which pushes the dimming profile earlier and harder. In a system with no criteria-based dimming floor and no remote adjustability, the pedestrian lighting criteria erode silently over the life of the installation with no mechanism to correct it.
Q: If the photometric submission passes review, is the system compliant? A: Only if the submission included vertical illuminance results at the minimum operating level for this installation’s specific optic, mounting height, and pole spacing, modeled against the worst-case winter energy budget. If the submission showed horizontal averages at full output, the review confirmed the system looks good on paper. Not that it performs for pedestrians at 2 a.m. in January after three cloudy days.
Piotr Mikus, MIES, is a roadway lighting designer and solar lighting specifier. He writes about solar street and area lighting standards, system sizing, and real-world performance at solarlightingnightshift.com.
Also in Pedestrian Lighting: Faces, Not Footcandles: Vertical Light After Dark | Crosswalks Don’t Dim Like Midblock: Solar Controls for Conflict Points | The Trip Hazard Is 10mm Tall. Your Solar Light Probably Can’t See It.
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