When buying or choosing a headlamp to use on the next trip a hiker typically considers attributes such as light output, battery type, battery life, weight and the feature set (multiple light settings, strobe capability, multiple LED colors, etc.). In this blog I will be focusing on light output. Subsequent blogs will dive into the other details such that you can be armed with all the information required to make an informed decision.
Headlamp manufacturers typically specify light output in the unit of lumens (lm). A lumen is a unit of luminous flux which is a measurement of the amount of visible light emitted from a light source. The key word here is “visible”. A lumen is a measure of light that reflects the human eye’s sensitivity to different wavelengths of light. For this discussion, visible light refers the spectrum of wavelengths from 390 nm to 700 nm (i.e. what colors we see). As a comparison, infrared light (we cannot see) encompasses wavelengths of about 740 nm to 1,000 nm. In other words, headlamp manufacturers are doing the right thing. They are providing us with a specification that relates to the product’s use.
People tend to get hung up on the lumen specification. I have heard many times people saying that headlamp X is better than headlamp Y because it produces 10 more lumens of light. In reality, that 10 lm is not noticeable. This is because perceived light intensity is proportional to the square root of lumen output. In other words, perceived brightness = sqrt(lm). This means that in order to double the perceived brightness the luminous flux needs to increase by a factor of 4X. As such, do not put too much emphasis on small differences in lumens.
The next question that comes up frequently is how many lumens of light do I need? The truth is there is no right answer. The answer will vary between users and their respective needs. In general, I would recommend targeting a peak output around 30 – 50 lm (i.e. when the battery is fully charged). For me, the real issue is how much the light output decreases over time. A headlamp that provides 50 lm for 1 minute and 5 lm for the rest of the time is pretty much useless. The rest of this blog will explain how you can quantify or compare the light output of a headlamp(s) over time.
For an absolute measurement, a hiker would want to measure the lumen output of the headlamp at discrete time intervals. The problem is that measuring luminous flux requires fairly expensive equipment. I came up with a quick and dirty method that is inexpensive, but don’t worry I won’t put you to sleep with those details. Basically, I just wanted to give you a sense of what intensity data might look like.
The figure below shows the plots for the 2 headlamps I compared. I stopped my analysis at five and a half hours. Ideally, you would continue until the batteries are dead.
In my case, the 2 headlamps are pretty comparable from an intensity standpoint. Both start at about the same level for fresh batteries and as expected the intensity decreases with time. The Petzl lamp seems to remain at a slightly higher level for a little longer. If I continued the experiment, the data suggests that the Princeton Tec would reach the no light output mark sooner as its intensity is dropping more rapidly than the Petzl at the 5.5 hour mark.
As mentioned earlier, there are other factors that a hiker needs to consider for a headlamp. A future blog will investigate one of these factors, beam profile. Maybe this is where the difference between the two headlamps rests. For now, both are still in the running. Stay tuned!