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Lighting and electronics

Reactive lighting

Also called adaptive lighting, Reactive Lighting

What is reactive lighting?

Reactive lighting uses a sensor to measure reflected light and adjusts brightness automatically, dimming when you look at a map and brightening when you look down the trail. It extends battery life and can feel unpredictable at speed.

Reactive lighting at a glance

Sensor
Measures reflected light
Saves
Battery, sometimes substantially
Divides
Runners

How it works

Petzl's Reactive Lighting is the best-known implementation, and the idea is straightforward: a photo sensor next to the emitter reads how much light is bouncing back. Close, reflective surfaces like a map or a tent wall bounce a lot, so the lamp dims. Open trail bounces very little, so it brightens.

What it saves

The battery saving is genuine and can be substantial over a long night, because most of the time you don't need full output and a manual lamp gives it to you anyway. For camp use and steady hiking it works well and you stop thinking about it.

Why runners disagree about it

Running is where opinions divide. The lamp responds to what's in front of it, which includes your own arms, a rock face beside the trail, and the back of the runner ahead of you, so brightness can shift at moments you'd rather it didn't. Some races have banned it for that reason. Every lamp that offers it also offers a manual mode, and plenty of experienced runners use manual on technical ground and reactive everywhere else.

What this means when you're choosing

  • Whether it can be switched off, because plenty of people want it off
  • How fast it reacts, since lag is what annoys runners
  • Battery figures quoted with it on and off

The common mistake

Leaving it on while running technical trail

The sensor responds to whatever is in front of it, which at running pace includes your own arms and any reflective marker. The lamp dims exactly when you needed it not to. It is excellent walking and in camp, and contentious at speed.

Common questions

Is reactive lighting worth having?

For hiking and camp use, yes, since it saves real battery with no downside. For fast technical running some people find the automatic shifts distracting and switch to manual, which every reactive lamp supports.

Why does my reactive headlamp keep dimming?

Something reflective is in the beam: your arms, a nearby rock face, a pack in front of you, or fog. The sensor reads that as plenty of light and turns down. Manual mode overrides it.

Where this comes up

Guides on Tom's Hiking where reactive lighting changes which product is right for you.

More in lighting and electronics

Lumens

Lumens measure the total light a headlamp emits in all directions. It's the headline number on every box and it says nothing about where that light goes, so a 400 lumen lamp with a bad beam can be less useful than a 200 lumen one with a good one.

Beam distance

Beam distance is how far a headlamp usefully projects light, quoted in meters. The industry standard measures it to the point where illumination falls to 0.25 lux, roughly the brightness of a full moon, which is dimmer than most people picture.

Flood vs spot beam

A flood beam spreads light wide and close for camp tasks and slow ground. A spot beam concentrates it into a narrow cone that reaches further, for seeing ahead at speed. Most trail headlamps mix both, and the mix matters more than the lumen count.

Regulated output

A regulated headlamp holds a steady brightness as its battery drains, then drops off sharply near the end. An unregulated one fades continuously from the moment you switch it on, so it's dim long before the battery is actually empty.

Red light mode

Red light mode is a dim red setting on a headlamp that lets you see nearby things without spoiling your night vision. Your eyes take 20 to 40 minutes to adapt fully to the dark, and white light undoes that in seconds, while dim red light barely disturbs it.

IP rating

An IP rating states how well a device resists solids and water, under the IEC 60529 standard. The first digit covers dust and the second water, so IPX4 is splash resistant, IPX7 survives brief submersion, and IP67 does both.

Watt-hours (Wh)

Watt-hours measure how much energy a battery stores: voltage multiplied by capacity in amp-hours. Unlike mAh, which is quoted at the cell's own voltage, watt-hours let you compare any two batteries fairly, and they are the unit airlines use to set carry-on limits.

GPX file

A GPX file is the standard open format for storing GPS routes, tracks and waypoints as plain text. It's what you download from a trail site and load onto a watch or phone, and almost every mapping app and GPS device reads it.

Breadcrumb navigation

Breadcrumb navigation draws your loaded route as a line on a blank screen with an arrow for your position, without any underlying map. It tells you whether you're on the route, and nothing about what's around you.

ABC sensor

ABC stands for altimeter, barometer and compass, the three sensors in an outdoor watch. The altimeter and barometer are the same pressure sensor read two ways, and the compass is a magnetometer that gives you a heading when you're standing still.

Exit pupil

Exit pupil is the width of the beam of light a binocular sends to your eye, found by dividing the objective lens size by the magnification. An 8x25 has a 3.1mm exit pupil and an 8x32 has 4mm; the bigger number looks brighter in dim light.

Eye relief

Eye relief is how far behind a binocular's eyepiece your eye can sit and still see the whole field of view, measured in millimeters. Glasses hold your eye farther back, so hikers who wear them need about 15mm or more to avoid a view ringed in black.

ED glass

ED glass, short for extra-low dispersion, is lens glass that bends red, green and blue light by nearly the same amount. In binoculars it cuts the purple and green fringes around dark edges against a bright sky, so the image looks sharper and truer in color.

By Tom · Last reviewed . Back to the glossary