Continuous Light Output: Lux, Lumens, and F-Stops – A Guide to Converting Different Units of Measurement
“How bright is it?” This question sounds pretty simple until you actually try to answer it. “Very”, “Sorta”, or “not really” don’t work very well. And yet, a common descriptive term that manufacturers still use for lighting equipment is “high output”. That tells us nothing. The fact is, measuring the brightness of a light is actually a pretty complicated task simply because there are several variables to factor in. In addition, there are several different systems of measurement in use today. What we’ve ended up with is a bunch of people speaking different “languages” trying to measure different things with different rulers! Today, I want to try to simplify all of that, and provide a method for you to compare lighting fixtures in a way that works.
Problem 1: Units of Measure
Different industries use different jargon. Even different people in the same industries use different jargon with each other! Most of those people have a difficult time translating each others terminology into their own. Here are some common terms for measuring lighting:
- Watts (halogen)
- Watts (HMI)
- Watts (LED)
None of these units of measure are remotely the same. For example, it takes far less wattage to make an LED with equal lumen output to an HMI. It takes far less wattage to make and HMI with equal lumen output to a halogen. So you can see the problem when someone is asking for information on an LED fixture, and their point of reference is a 2000 watt halogen fixture. There’s a bit of conversion to do to answer that question.
To complicate matters further, people in the photo industry will refer to brightness using words like “stops”. If you’re unfamiliar, stops is a relative term that is used in conjunction with other camera settings like ISO (lumen sensitivity). We’ll talk about this a bit more below. But just think of stops like squinting your eyes when looking at a light. The more you squint, the less light hits your eye.
Problem 2: Distance and Angles
If you turn on a flash light and point it at your eyes from a distance of one foot, the light itensity will be painful. If you point that same flashlight at your eyes from 300 meters away it will look pretty dim. That’s because of two factors working together: distance and beam angle.
A perfectly focused light source is a laser. Lasers can travel over great distances without losing any intensity at all. Other light sources aren’t as sophisticated, and the further the light has to travel, the more it spreads out. As light spreads out over greater distances, it will appear less bright since fewer lumens are hitting any given point. So, when we’re measuring light, we need to factor in how far the light has to travel, and what the angle of the beam is as it’s leaving the light source.
Problem 3: Equipment
As if the problem with the light itself wasn’t enough – we also need to factor in the equipment we’re using to capture. This is crucial when measuring light for film or photography. At the end of the day, you need to have enough light intensity at a given distance to be able to use your equipment at desired settings.
Before we get to solving our problem, let’s recap what we need to accomplish: We need a standard system to measure lumens that takes into account beam angle and distance that can then be applied to relative systems of measurement (i.e. stops) for capture equipment. Let’s go!
Solution Part 1: Correct Language!
We already know that measuring light intensity by using a measurement of energy (watts) doesn’t work. Different fixtures will be more or less energy efficient than others, and wattage does not take into account the beam angle or the distance of the light from the subject. So let’s take another, more detailed look at our list of terms:
- Watts (halogen) – A measure of energy consumed
- Watts (HMI) – A measure of energy consumed
- Watts (LED) – A measure of energy consumed
- Lux – Lumens per square meter
- Footcandles – Lumens per square foot
- Stops – Size of lens aperture
- EV – Exposure Value
- Lumens – Measure of amount of visible light
Looking again at this list, you can see that there are two units of measure that account for beam angle and distance: Lux and Footcandles. Both of these units of measure are taken from the subject point rather than the source point, so angle and distance can be accounted for. The difference between the two is that one is metric and one is imperial measurement.
To get a better idea of how this works imagine drawing a 1×1 meter square on a wall and then shining a light on the wall from 10 feet away. The total amount of lumens in that square will tell you the lux. Similarly, the same process in a 1×1 foot square will tell you the measurement in footcandles. Lux is a much more commonly used measuring tool, so I’ll focus on lux going forward, but converting from lux to footcandles is very straightforward.
These days, most lighting manufacturers will have lux ratings at various distances in the spec sheets of their fixtures. This will help you compare output between two different fixtures. However, that still doesn’t tell you what lux means when it comes to using your equipment.
Solution 2: Translating Lux
In the world of cameras and lenses, there are three variable for controlling light capture: ISO, shutter speed, and aperture (f-stops). Remember before that I said that these are relative units of measurement. But – if we control for one of these variables, we can use the other two to create an absolute unit of measurement (EV) that we can then translate to lux. Here is a conversion chart that shows EV to Lux.
Solution 3: Finding EV
The last part of our solution is finding the proper exposure value for your equipment. There are plenty of EV charts out there that typically control for ISO 100. I’ve included one below that includes multiple ISO settings.
Putting it all together.
So what does all of this tell us? It’s simple:
- Measure light output in lux to control for distance and angle variables.
- Convert lux into EV to control for equipment variables.
- Use EV to determine correct equipment settings.
- Get amazing results!
For example, let’s say you have a 60 watt LED light fixture (like the Dracast LED1000 Pro Series) that emits 2100 lux at 2m (6 ft). Using the conversion table above, you now know that 2100 lux converts to about 9.5 – 10 EV. Knowing this, you can use that number to find the correct settings for your capture equipment when using that fixture at that distance. Everything from shutter speeds, to aperture (f-stop) setting to ISO can be determined and then tweaked so you get the perfect image.
Working in this industry, I hear confusion on this topic all the time, and not just from beginners. I often hear industry veterans ask questions like, “So, is it kinda like a Joker Bug 200?” Well, a wide angle Joker Bug 200 at 2m emits 2690 lux. You tell me.