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Planning lighting for a photo or video shoot can be complicated, and the terminology used to measure light on lighting equipment can make things even more confusing. When shopping online, most lights include “lumens” or “lux” among their technical specifications, although sometimes “lumens” is written as “luminous flux.” You could even make a mistake between luminance and illuminance. Its alot. So let’s break it all down.
One thing to keep in mind: you’ve probably seen light bulbs whose wattage is measured in watts; LED bulbs usually say something like “60W equivalent.” However, watts are a measure of how much energy a bulb uses, not how much light it emits. This metric is a holdover from when incandescent lights were common and consumed much more energy than today’s LEDs. However, as energy-efficient lights have gained popularity, it is no longer useful to use watts as an abbreviation for the amount of light a bulb emits (it wasn’t very useful to begin with). That’s why you’ll see terms like lumens or lux on professional lighting equipment.
What are lumens?
The first term you should know is lumens. The amount of visible light emitted by a source is known as luminous flux and the lumen is a unit of measurement for that gross output. You can think of this as “distance” referring to how far apart two places are, while “kilometers” is the unit used to measure that distance. Lumens in kilometers in that analogy.
This can be a little confusing because companies will list “luminous flux” without naming the unit of measurement. For example, one of our favorite lights for shooting professional videos, the Godox SL-60W, lists its luminous flux in its description as 4,500. Using our metaphor above, this is like saying “Distance: 4500” without indicating what unit that number refers to.
That said, while they’re not interchangeable, if you see “luminous flux” on a product’s spec sheet, it’s probably referring to lumens. However, this makes it important to check that you are comparing comparable numbers when purchasing lights from different manufacturers.
You could also see this metric as “luminance,” which is another, less common way to refer to the output of a light source. And, like “luminous flux,” it is a general term for the concept, not a unit of measurement. However, this is different from illuminance, which refers to reflected light, not light coming directly from a source. And yes, it is confusing.
What is Lux?
The amount of light a source emits is only part of the story. After all, the sun emits enough energy to melt… basically everything. Fortunately, the sun has a good sense of personal space and stays far enough from Earth not to destroy us all. For similar, less catastrophic reasons, considering how far away your light source is affects the type of light you’ll need.
Lux is defined as one lumen per square meter, although the math can get complicated because we are dealing with surface areas in three dimensions. Don’t worry, you don’t need to worry too much. Most professional lighting will simply list their output in lux (as well as lumens) and specify a distance. For example, him Aputure Amaran P60X It has a nominal power of 5,070 lux at 1 meter.
This means that a subject 1 meter from the light source will effectively perceive just over 5,000 lumens of light. If they are further away, less light will reach them from the source and therefore the subject will appear dimmer. This is helpful when purchasing a light because you may not need the brightest lights in the world. You just need to make sure your subject is the right distance from the light source.
The inverse square law (and other mathematics)
Calculating the correct distance for a light source is not exactly intuitive due to two main factors: the first is the inverse square law, one of those strange quirks of the universe that we have to deal with. Simply put (mathematicians, please don’t yell at me), every time double the distance between the subject and the light source, you quadruple How much light is needed to illuminate them in the same way.
In other words, it means that every time you move twice as far away from a light source, only a quarter of the light from that source will reach your subject. So, using the Amaran P60X As an example above, if it is rated at 5070 lux at 1 meter, then the subject will experience around 1267 lux at 2 meters. Some lights will specify their lux at shorter distances, such as 0.5 meters, making it crucial to ensure you compare equivalent specifications when looking at lights from different manufacturers.