Lighting plans for photography and videography can be complicated. The terminology used to measure the light in lighting fixtures can make things even more confusing. When shopping online, most lighting technical specifications will list “lumens” or “lux,” although “lumens” may also be written as “luminous flux.” You might also get tripped up by the difference between brightness and illuminance. There are a lot of them. So let’s break it all down.
One word of warning: You’ve probably seen light bulbs whose output is measured in watts. You’ll often see “60W equivalent” on LED bulbs. However, watts are a measurement of the power a bulb consumes, not the amount of light it gives off. This measurement is a relic from the days when incandescent light bulbs were common and used much more energy than LEDs do today. However, as more energy-efficient lighting becomes more common, using watts as shorthand to describe the amount of light a bulb gives off is no longer useful (it never was very useful anyway). That’s why you’ll see terms like lumens and lux on professional lighting fixtures.
What is a Lumen?
The first term to know is Lumens. The amount of visible light emitted by a light source is called luminous flux, and Lumens is the unit of measurement for that raw output. You can think of this as “distance” referring to the distance between two places, and “kilometers” being the unit that measures that distance. In this analogy, Lumens per Kilometer.
This can get a bit confusing as companies will list “Luminous Flux” without specifying the units of measurement. For example, the Godox SL-60W, one of the most popular lights for professional videography, lists its luminous flux as 4,500 in its description. Using our analogy above, this is like saying “Distance: 4,500” but not mentioning what units that number refers to.
That said, they’re not interchangeable, but if you see “luminous output” listed on a product’s specs, it’s probably referring to lumens, but if you’re buying lights from different manufacturers, it’s important to double-check that the numbers are comparable.
This metric is sometimes also called “brightness,” which is another, less common way of describing the output of a light source. And like “luminous flux,” this is a general term for the concept, not a unit of measurement. However, it is IlluminanceThis refers to reflected light, not light that comes directly from a light source. Admittedly, this is confusing.
What is Lux?
The amount of light a source puts out is only one part of the picture. After all, the sun puts out enough energy to basically melt anything. Luckily, the sun is good at understanding personal space and is far enough away from Earth that it won’t destroy us all. For similar, but less dire reasons, taking into account how far away your source is placed changes the type of light you need.
Lux is defined as 1 lumen per square meter, but the calculations can get complicated since you’re dealing with a three-dimensional surface area. Don’t worry, you don’t need to worry too much. Most professional lighting lists its output in lux (and lumens) and specifies the distance. For example, the Aputure Amaran P60X is rated at 5,070 lux at 1 meter.
This means that a subject one meter away from the light source will effectively perceive just over 5,000 lumens of light. If the subject is further away than that, less light from the source will hit the subject and the subject will appear darker. This is helpful when buying lights, because you may not need the brightest light in the world; you just need to make sure your subject is a reasonable distance from the light source.
Inverse Square Law (and Other Mathematics)
Figuring out the appropriate distance for a light source is non-intuitive due to two big factors. The first is one of those weird quirks of the universe: the inverse square law. Very simply (please don’t yell at me, mathematicians): double Depending on the distance between the subject and the light source, quartet How much light would it take to give the same amount of illumination?
In other words, every time you are twice as far away from a source, only a quarter of the light from that source will hit your subject. So, using the Amaran P60X example above, if it’s rated for 5,070 lux at 1 meter, your subject will experience approximately 1,267 lux at 2 meters. Some lights specify lux at shorter distances, such as 0.5 meters, so when looking at lights from different manufacturers, it’s important to make sure you’re comparing like specs.
