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The main difference Between Lux and Lumen: What's Brightness?

When searching for new headlights, for example LED headlight bulbs, we all want to understand “What’s the brightest headlight bulb?” Well to reply to that question and to truly obtain a better picture of what “brightness” is, we need to get scientific and define several terms. But not to worry! I’m going to make this short article as easy as easy to wrap your head around. So stick to me for a moment and you’ll be a pro with regards to figuring out and understand true headlight brightness.

The two main words to define once we talk about headlight brightness are lumen and lux.

Lumen: One of measurement for luminous flux that is a way of measuring the entire quantity of visible light emitted with a source.

Lux: A unit of measurement for illuminance which is a measure of just how much luminous flux is disseminate over a given area.

“It's possible to think of luminous flux (measured in lumens) like a measure of the total “amount” of visible light present, and also the illuminance (measured in lux) like a measure of the concentration of illumination on a surface at a particular distance from the source.”

So let’s think about these definitions as we see how lumen and lux amounts are measured and just how that process pertains to headlight bulbs.

Integrating sphere accustomed to measure lumen.

Luminous flux, which I’ll just describe as lumen, is measured in a device called an integrating sphere. A bulb is placed within the sphere and the light given off by the bulb is scattered by the interior from the integrating sphere and evenly distributed total angles. The entire lumen quantity of a light source could be measured accurately since light could be captured all angles from the sphere. Although this method of measuring the brightness of a bulb can be useful for lighting applications where 360 degrees of light is required, (such as a table lamp) the lumen amount is just half of the storyline while testing the brightness of headlight bulbs. For this reason illuminance and lux are extremely important.

Lux is measured by installing the bulb inside a headlight housing and testing to see how well the lumen amounts are projected or reflected out from the housing. In cases like this, we measured the beam pattern made from the headlight housing and bulb combination on a wall 25 feet away. From this distance and this position, we could begin to see the lumen amounts from the bulb which are truly being utilized and turned into a usable beam pattern through the headlight housing. When measuring lux by doing this, we are able to take into account multiple factors playing into the brightness of the bulb and find a clearer picture of true, usable light. The lumen amount in the bulb, the connection between bulb and headlight housing, the resulting beam pattern, and distance are factors when calculating lux by doing this.

A digital light meter accustomed to measure lux. Within this application, lux has been measured inside a beam pattern developed by a LED bulb inside a headlight housing.

So you’re probably thinking, “Well, doesn’t that mean a headlight bulb with the highest lumen amount would be the brightest?” Maybe. Remember, lumen amounts are only one piece of the puzzle when determining usable light brightness.

“Confirmed quantity of light will illuminate a surface more dimly if it is spread on the larger area, so illuminance (lux) is inversely proportional for an area when the luminous flux (lumens) is held constant.”

It is possible for any bulb with high lumen comes down to focus poorly once placed inside a headlight housing because of poor engineering and design. The result would be an unfocused beam pattern with light reflected or projected poorly. In that case, a poorly focused beam pattern from a bulb by having an initially high lumen amount would have low lux measurements since the light is disseminate or unfocused. A headlight like this would be “bright” on paper, although not actually usable inside a real-world scenario. Below we see an example such as this scenario in a 2007-2019 Toyota Tundra headlight housing. Two LED bulbs were tested here along with the stock bulb, but check out the beam patterns and lux numbers.

Original equipment halogen bulbs typically emit 900-1,000 lumen, and this stock bulb was measured at 530 lux at 25 feet with this headlight housing. The GTR Lighting GEN 2 LED bulb emits 3,600 lumen and was measured at 360 lux at 25 feet inside this headlight housing. That LED bulb in that headlight housing actually produced less lux compared to stock bulb even though it has 3-times the lumen. The main difference is visible when comparing the two beam patterns. The additional lumen amounts are not focused as precisely within this Tundra headlight housing as the stock bulbs are. The result is a beam pattern less concentrated and arguably less bright.

And this increase of just one,000 lux is achieved by merely a 100 lumen increase in light…

Now compare the 2 LED bulbs. The GTR Lighting GEN 2 bulb emits 3,600 lumen and was measured at 360 lux at 25 feet inside this headlight housing. Whereas the GTR Lighting GEN 3 bulb emits 3,700 lumen (only 100 more) and was measured at 1,360 lux at 25 feet inside this headlight housing. This bulb is emitting high lumen amounts and lux amounts inside this headlight housing. Take a look at the beam patterns and you’ll see why. This LED bulb is capable of creating a beam pattern that is more concentrated in comparison to the other LED bulb, and much more closely matches the stock bulb beam pattern. And this increase of 1,000 lux is achieved by merely a 100 lumen rise in light output. This is done through a better-designed bulb that interacts with the headlight housing similar to a stock halogen bulb would.

All that to state, when you compare the brightness of headlight bulbs, be sure to look at the main issue. With LED headlight bulbs especially, make sure you take into consideration the beam pattern the LED bulb creates when placed inside a headlight housing. Take a look at more than just the advertised lumen amounts–if the manufacturer gives lux numbers look for those too. Also, observe how the bulb itself is designed to check out bulbs that more closely fit the profile and dimensions of a regular halogen bulb.