What is a Lumen?

The area light covers increases exponentially with distance

The total visible light emitted from a light source is measured in lumens.

As the distance increases from the light source the area the light illuminates increases exponentially. The area the light covers at one foot would quadruple at 2 feet. At just four feet the light will cover sixteen times the area at 1 foot. This is true for a narrowly focused beam and a wide angle beam.

The illumination over any given area is measured in lux. In general the higher the illumination or lux the easier it is to see. The relative illumination or lux decreases exponentially with distance from the light source. This is because the area of illumination increases exponentially with distance. The doubling of distance from the light source decreases lux or illumination by 75%.

Lux = Lumen / Area

 Lumen = Lux times Area

Recommended Lux Values

Recommended Lux Values

Illuminance or lux is the metric that is used to measure the light intensity on any given area. 100 lumens falling on a square meter is equal to 100 lux. Different tasks require different levels of illumination or lux. In general the more detailed the task the more illumination or lux is required. Detailed drawing work can require 1,000 lux while navigating a stairway at night only requires 15 lux. Measuring the lux or the amount of light that falls on a given surface makes it possible to evaluate if there is sufficient light to perform a given task.

Lux Value Measured in the Office

The measured illuminance at the office showed a lux of 233.9. This is a bit low for the illuminance of an office.

Approximate Lux Values for a 100 Lumen Light

In the above chart the approximate illumination area and lux values are shown at various distances from a 100 lumen headlamp. As the distance from the light source increases the area the light illuminates increases exponentially. As the illuminated area increases the illumination or lux decreases exponentially.

When the distance doubles from the light source the illumination or lux drops 75%.

When hiking you need the right illumination for the terrain. 

The minimum amount of illumination most people need for hiking at night is 5 lux. However, some trail conditions will require more illumination.

Remember lux is not equal to lumens. Lux equals lumens divided by lighted area. Therefore, the larger the area the light illuminates the lower the lux. Illumination or lux decreases 75% when the distance is doubled.

A 100 lumen headlamp that puts out about 1,400 lux one foot away, will put out about 350 lux two feet away. This means when looking about 16 feet away the light will be about 5 lux or the minimum illumination for hiking a groomed trail at night. In most cases you would want to use a more powerful headlamp with 200 lumens or more to hike at night.

It is important to test your headlamp in safe conditions before your hike to see if your headlamp is effective for  your personal needs.

Summary

The total visible light emitted from a light source is measured in lumens. This is commonly stated on most headlamp packaging. Generally the higher lumens the better illumination. However, if the headlamp has a very wide beam the illumination at a distance could actually be poor.

What you should focus on is the illumination or lux you need for your desired tasks. If you are planning on hiking at night you will need a headlamp that can provide at least 5 lux at a distance of about 16 feet. If you are navigating more difficult terrain or moving fast you will likely need more illumination.

If you select a headlamp that fails to provide the illumination you need be ready to move a bit slower at night.