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Lighting and electronics

Beam distance

Also called throw, beam throw

What is 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.

Beam distance at a glance

Standard
ANSI FL1, measured to 0.25 lux
0.25 lux
Roughly full moonlight
Walking
About 40 m is ample

How beam distance is measured

The figure comes from the ANSI FL1 standard, which defines the end of the beam as the distance at which the lamp puts 0.25 lux on the ground. That's about what a full moon delivers on a clear night. You can see by it, but only just, so a lamp advertising 100 meters of throw is not lighting your path at 100 meters in any way you'd want to walk by.

Why it is comparative, not absolute

Treat the number as comparative rather than absolute. A lamp quoting 120 meters does genuinely throw further than one quoting 70. Both are measured the same way, so the comparison holds even though the absolute claim flatters both.

How far you need to see

How much you need is set by speed. Walking, you're reading the ground for the next few steps and 40 meters is plenty. Running downhill you need to see far enough ahead to plan a line before you arrive, and that's where longer throw matters. It's also why a pure flood beam feels unnerving at speed no matter how many lumens it produces.

What this means when you're choosing

  • The figure quoted on the mode you will actually run
  • Whether the lamp has a spot element at all, since flood-only lamps throw poorly
  • Beam distance and lumens together, since neither works alone

The common mistake

Reading it as how far you can see

The standard measures to 0.25 lux, roughly full-moon brightness, which is far dimmer than useful. A lamp quoting 100 meters gives you comfortable detail at maybe a third of that. Treat it as comparative between lamps, not as a promise.

Common questions

Is beam distance the same as how far I can see?

Not comfortably. The rating goes to the point where the light is about as bright as a full moon, so useful working distance is a fraction of the quoted number.

How much beam distance do I need for trail running?

Enough to pick a line before you get there, which for most people means a lamp with a real spot component rather than a specific meter figure. Longer throw matters most on fast descents.

Where this is defined

Where this comes up

Guides on Tom's Hiking where beam distance 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.

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.

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.

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