Lux Targets and Lighting Classes

Why the road, not the luminaire, sets the target

Illuminance is a property of the road surface, not of the equipment on the pole, so the design question is always how much light must arrive at the carriageway and how evenly, rather than how powerful the light should be. That reframing is what makes classes useful: a class is a performance contract at the surface, and any luminaire that satisfies it is acceptable whatever its wattage. Frameworks such as CIE 115 and ANSI/IES RP-8 organise roads into categories by function — motorway, main road, collector, local road, pedestrian way — and attach performance requirements to each.

M-class and P-class: what the letters mean

M-classes are the vehicle-traffic classes, graded from M1 at the busiest and fastest roads down to M6 for lightly used carriageways. The design criterion is mostly the driver’s ability to detect an obstacle in time to stop, so average illuminance, overall uniformity and longitudinal uniformity are all controlled and glare is limited. P-classes are the pedestrian classes, P1 to P7, covering footpaths, cycle tracks and residential lanes where traffic is slow and the user is on foot or on a bicycle. Average illuminance requirements are lower, but minimum illuminance and uniformity still matter because no part of the route should be dangerously dark. Two practical consequences follow for solar projects: a P-class route needs a far smaller energy budget than an M-class route of the same length, and a scheme that mixes road types can apply different classes pole by pole instead of one requirement for the whole site.

Indicative numbers, and how to treat them

Exact values must come from the standard your client uses, but the shape of the numbers is consistent enough to be useful when checking a proposal. Vehicle classes commonly sit in the range of roughly 30 lux average on a busy main road down to around 10 lux on a lightly used carriageway, with overall uniformity (Emin/Eavg) not worse than about 0.4 on the higher classes. Pedestrian classes typically range from around 15 lux average on a busy mixed-use path down to a few lux on a quiet residential lane, with minimum illuminance of about 3 to 5 lux where the class calls for it.

Uniformity, glare and maintained values

Average illuminance is the easiest number to hit and the least informative. A narrow, intense beam can produce a respectable average while leaving pools of light and deep shadows between poles, which is exactly the condition that hides a pothole or a pedestrian; uniformity ratios and minimum illuminance exist to prevent that. Glare is the third element, since on a fast road a bright source in the driver’s field of view degrades vision rather than improving it, so a class typically limits the threshold increment rather than the light output. The class is also expressed as a maintained value, meaning what the installation must still deliver at the end of the maintenance cycle, so the design starts higher and applies factors for luminaire depreciation — LED lumen maintenance at 25,000 or 50,000 hours, usually reported from IES LM-80 data extrapolated per TM-21 — and for dirt on the luminaire and cover glass. A proposal quoting an initial lux value and calling it a class is describing a different, easier requirement.

From class to luminaire: the calculation chain

The working method runs in one direction. Fix the class for the road type and confirm whether the standard is expressed as illuminance or luminance; set the geometry, including mounting height, spacing, overhang and whether poles are single-sided, staggered or opposite; choose a luminaire whose photometric file is available; run the calculation in the client’s own software using that file; then iterate on height, tilt and spacing until the class is met with margin. A spacing-to-height ratio somewhere between 3:1 and 4:1 is a common starting point for vehicle classes, though the calculation always governs, and a wider ratio chosen to reduce pole count raises the luminaire output and battery load at every pole. Photometric test data should be produced to the recognised measurement method, IES LM-79, by the testing laboratory. The geometry arithmetic is set out in pole height and spacing.

Putting the class into the specification

Write the class, the standard it comes from, the average maintained illuminance, the uniformity ratio, the minimum illuminance where required, the glare limit, the mounting height and spacing assumed, and the maintenance factors applied. Require the bidder to submit the calculation and the photometric file used, so the client’s own consultant can reproduce the result. Because a solar luminaire’s output must also be funded by a panel and a battery, the class and the energy system are one decision rather than two; the load that follows from the class feeds directly into battery autonomy sizing and into the components listed in the bill of materials.

How we can help

We are a China-based trading and project sourcing company, not a manufacturer. We review specifications and BOQs, match a product and configuration to your site conditions, source from qualified manufacturers, and manage quality through to pre-shipment inspection.

• Turn a road classification into a written lighting class requirement with the numbers spelled out.
• Check a bidder’s lighting calculation against the stated class, including uniformity and maintenance factors.
• Reconcile the resulting load with the panel and battery the bidder proposes.
• Ask the supplier to obtain the photometric files your consultant needs.

If you are preparing a tender or specification and want a second opinion, contact us.