LUMENS, LUX, CANDELA–WHAT’S THE DIFFERENCE?

Here’s everything you need to know about light output measurements and how they can help you choose the safest solution for your aircraft’s mission.

If you’ve found yourself here, there’s a good chance you’ve been researching the extensive amount of LED options available for your aircraft, and you’ve probably seen the words “Lumens,” “Lux,” and “Candela” highlighted in product descriptions. They’re all units of measurement for light output–aren’t they interchangeable? Unfortunately, that would be far too simple. For years, manufacturers have most commonly advertised raw Lumen ratings as benchmarks for their LED’s performance, possibly while overlooking the LED’s Lux and Candela measurements. We’re here to help you unpack the difference between the three measurements in order to find the safest lighting solution for your aircraft.

LIGHTS WITH THE MOST LUMENS ARE THE BEST, RIGHT?

Not always. Lumens are “a unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source of one candle intensity.”[2] Lumens tell us how much light a light source should emit in the immediate area around it. The issue with selecting an LED based only on its Lumen count is that the measurement only paints a theoretical picture of the LED’s performance. Lumens refer to the potential amount of light emitted from the source area in an ideal world, based on each individual LED chip's rating multiplied by the total number of LED chips in the light assembly.[4] As a result, Lumens do not tell you the actual, real-world effective output of the light assembly–especially when factoring in the assembly’s electrical efficiency, lens clarity, distance from the runway, presence or absence of daylight, and so on. This issue is why we prefer to highlight (no pun intended) Lux and Candela.

LUX

Lux tells us how much light actually falls on a surface from a given distance. By definition, Lux is “a unit of illumination equal to the direct illumination on a surface that is everywhere one meter from a uniform point source of one candle intensity or equal to one lumen per square meter.”[3] The closer the surface to the light source, the higher the lux reading.[6] Imagine your final approach to the runway at night. As you descend and get closer to the runway, the runway gets brighter, and your landing light’s lux reading increases. The Lumen measurement does not change, of course. A landing light might have an insanely high Lumen count, but that doesn’t mean it will have high Lux levels. Lux factors in the assembly’s efficiency at distributing light to a specified area–in this case, the runway. For example, landing light A has a low lux measurement. Landing light B has a high lux measurement. If these lights were on airplanes landing on parallel runways at night at the same time and position in glide slope, landing light B would be able to light up the runway well before landing light A. Landing light B would allow the pilot to see further down the runway and possibly avoid a deer strike. The kicker? Both of these landing lights could have the same number of Lumens.

CANDELA

Candela places an emphasis on visibility from a distance while factoring in the total volume of light within a certain beam angle and direction.[4] By definition, Candela (also known as “candlepower”) is “the base unit of luminous intensity in the International System of Units that is equal to the luminous intensity in a given direction of a source which emits monochromatic radiation of frequency 540 × 1012 hertz and has a radiant intensity in that direction of ¹/₆₈₃ watt per unit solid angle.”[1] Candela tells us how far away we can be from a light source while still being able to see it. Think about the Batman Signal Lamp. It would have low Lumen rating because it’s not great at dispersing light to the immediate area around the light source, but it would have an exceptionally high Candela measurement since it focuses in a single direction–and Batman can see its light from great distances. It’s also important to note higher Candela measurements mean the light source is more situationally effective during daylight hours. Pilots will have a much easier time spotting airplanes decorated with high-Candela anti-collision lights while the sun is still up. Candela is likely the most mission-critical measurement when choosing lights for your aircraft because its entire purpose serves to communicate how far you’ll be able to see downrange on takeoff and landing, and how far away other aircraft can be in order to see you in the sky before it’s too late.

TO SUM IT UP...

If you buy your LEDs based only on Lumens, you may be missing the point that Lumens don’t equate to actual situational effectiveness–a.k.a. whether or not a faraway aircraft will be able to see you–or whether or not you’ll be able to see that pesky deer downrange on the runway at night. Lux and Candela measure the LED’s actual intensity, volume, and strength of light from a specific distance.[4] Lumens tell you what each assembly of little diodes “should” emit derived from a calculation, but Lux and Candela tell you how powerful the assembly of little diodes actually is by real measurements factoring in distance. When combined with Lumens, the measurements of Lux and Candela paint a more accurate picture of the LED’s real-world performance. If you’re looking for a light to illuminate the ambient area around its location–like a lamp in a dark living room–pay attention to Lumens. If you’re looking for a light to project onto an area far away–like onto a runway at night or into the eyes of a faraway pilot during the daytime–pay attention to the Lux and Candela.[5] The sheer quantity of Lumens may look enticing, but don’t let them overshadow the quality of the Lux and Candela measurements. 

We offer a huge assortment of lights here at Whelen Aerospace Technologies, and despite what might seem overwhelming at first, our goal is to give you the options you need to complete your mission safely–whether you’re operating helicopters over the North Sea, landing 737s at night, taking your family out to breakfast in your Bonanza, or putting your bush wheels to the test in the backcountry. The “perfect light” for one mission may not be the “perfect light” for another. We’re here to help you figure out the perfect light for yours. 

REFERENCES

1. “Candela.” Merriam-Webster, Merriam-Webster, www.merriam-webster.com/dictionary/candela.
2. “Lumen.” Merriam-Webster, Merriam-Webster, www.merriam-webster.com/dictionary/lumen.
3. “Lux.” Merriam-Webster, Merriam-Webster, www.merriam-webster.com/dictionary/lux.
4. Staff, KC HiLites. “How to Choose Off-Road Lights.” KC HiLites, 29 Apr. 2018, www.kchilites.com/blog/how-to-choose-off-road-lights.
5. Staff, Safety Lights and Signals. “How the Brightness of Safety Lights Is Measured.” Safety Lights & Signals, www.safetylightsandsignals.com/p-1339-how-the-brightness-of-safety-lights-is-measured.aspx.
6. Staff, Waveform. “What Is the Difference between Lux and Lumens?” Waveform Lighting, Waveform Lighting, 19 Jan. 2018, www.waveformlighting.com/home-residential/what-is-the-difference-between-lux-and-lumens.