How Far Does Outdoor Light Really Travel?

Why outdoor lighting doesn't behave like indoor lighting

Most people size outdoor lighting the same way they'd shop for a lamp — bigger wattage, more lumens, "brighter must be better." That instinct is carried over from indoor lighting, where rooms are enclosed, surfaces bounce light back at you, and the goal is usually even, generous illumination.

None of that applies outside. There are no walls or ceiling to reflect light back into the space. There's no fixed distance between the fitting and the surface — a path light might be 300mm from the ground it's lighting, while a façade wash could be throwing light 6 metres up a wall. And critically, the human eye behaves completely differently outdoors at night: it dark-adapts, and over-lighting actively works against that adaptation rather than helping it.

The result is that outdoor lighting projects sized using indoor instincts are almost always over-lit — and over-lit outdoor lighting doesn't just waste energy, it actively makes a space feel less comfortable and less safe, for reasons covered below.

The inverse square law, in plain terms

Light intensity falls off with the square of the distance from the source. Double the distance, and you don't get half the light — you get a quarter. Triple the distance, and you're down to a ninth. This is the single most important physical fact to understand before speccing any outdoor fitting.

How little light you actually need outdoors

Because of the fall-off above, and because the eye dark-adapts at night, outdoor lighting standards call for dramatically less illuminance than most people expect. AS/NZS 1158.3.1 — the Australian standard covering pedestrian and residential-scale path lighting — sets category levels far below typical indoor lighting:

Compare that against typical indoor targets — living areas 100–300 lux, task and reading areas 300–500+ lux — and the gap is enormous. A residential garden path sitting at 3 lux is doing its job correctly, even though that number looks tiny next to an indoor spec.

Glare — the real enemy, not dimness

The most common outdoor lighting failure isn't insufficient brightness — it's glare. Glare happens when there's a direct line of sight to the LED chip itself, typically from an unshielded fitting mounted too low or aimed carelessly. A path can be technically compliant on paper — correct lux level, correct spacing — and still be genuinely unpleasant and hard to walk if the fittings themselves are glary.

Glare and illuminance are measuring two different things. A fitting review that only checks "is it bright enough" and never checks "can you see the light source directly" will miss the problem entirely. Proper shielding — recessed lenses, hoods, baffles, or simply mounting height and aim angle — is what separates a comfortable, professional-feeling installation from one that looks technically fine in a spec sheet but is unpleasant to actually stand in.

Light trespass and obtrusive lighting

Light trespass is light spilling beyond the boundary of the property it's meant to be lighting — onto a neighbour's bedroom window, into the night sky, or as glare visible from a public road. AS/NZS 4282:2023 (Control of the Obtrusive Effects of Outdoor Lighting) governs this, covering spill light, sky glow, and glare as assessed at property boundaries and building façades.

This isn't a theoretical compliance detail — councils increasingly reference AS/NZS 4282 during planning approvals for larger residential and commercial landscape lighting schemes, particularly where a project borders residential neighbours. The practical takeaway is straightforward: the fix for light trespass is almost always aiming and shielding fittings correctly, not simply reducing the number of fittings or their wattage. A well-aimed, well-shielded low-output fitting causes far less trespass than a poorly-aimed high-output one.

Real fixture wattages — rules of thumb from the field

Once the lux targets and glare/trespass principles above are understood, actual fixture selection becomes far simpler than most guides make it sound. These are the wattages that consistently work well across typical residential garden lighting projects:

These figures assume good-quality LED fittings with reasonable optical control — a cheap fitting with poor beam shaping may need to run at higher wattage to achieve the same practical result, which is itself a good argument for spending on fitting quality rather than compensating with raw output.

Practical takeaway — how to plan without guessing

Outdoor lighting layout doesn't need to be guesswork if you work from the right starting point:

1. Start from the target lux level for the application — a residential path is PP3–PP5 territory (0.85–3 lux), not indoor room brightness.
2. Choose fixture wattage from proven ranges — the table above covers the common cases; don't default to the biggest fitting on the shelf.
3. Prioritise shielding and aim over raw output — a well-aimed 3W fitting beats a poorly-aimed 10W one on both comfort and compliance.
4. Check boundaries, not just the space itself — walk the property line at night before calling a design finished; that's where trespass problems show up.
5. Use the inverse square relationship to plan spacing — don't assume even coverage from widely-spaced fittings; check the actual fall-off at the midpoint between fixtures.