How They Can Help and Hurt When Buying Lights
So you’ve probably heard and seen Lumens, Lux and Footcandles, maybe even Candelas. Lumens and lux are metric, candela’s and ft candles imperial. Lumens and candelas refer to the total light output of a source. Lux and foot candles the light falling on an area some distance from the light source, that is the light illuminating our subject. Lux and foot candles are more useful to us in the film and video business and what you’ll see in the specs for industry lights.
They’ll be listed with a distance, usually 1 meter or three feet which is a meter. Without that distance, the spec is useless, as the light from any light source falls off the farther away from the light you are. You gotta have that distance for it to mean something.
How to keep Lumens and Lux, Candela's and Foot Candles straight.
Converting Lux to Foot Candles
Now if a spec gives Lux but you’re used to foot candles, no problem, there’s an easy conversion. There are 10.76 lux to 1 foot-candle. So if the spec reads 8000 lux at 1 meter, just divide by 10.76, or heck keep it simple and round it to 10 (800 ft candles). If the spec is in foot candles, multiple by 10 to give you the lux equivalent.
Example of specs given in Lux and Foot Candles measured at 3 ft (roughly 1 meter). Specs are from B&H Photo.
Are Lumens a Useful Spec?
Now some specs don’t list lux or ft candles at all, especially when you’re looking at commercial lights like CFL’s, fluorescent tubes, and LED bulbs. This is when you run into Lumens. Not very useful to us but very useful to architects and industrial lighting designers. They have complex formulas and tables that tell them how many total lumens they need to light an office or a warehouse. Pretty important when you need to order thousands or 10’s of thousands of fixtures and bulbs to light every floor of a skyscraper. You don’t want to get your calculations wrong there.
This CFL is rated at 1600 Lumens, but how much of that total lumen output is falling on our subject?
But for us, here’s the issue. The CFL in the image above, it’s rated at 1600 lumens. But it’s outputting light in all directions, 360 degrees. At three feet away, how much of that total lumen output will fall on your subject? The only way to really know is to test it. Here I’m getting 40 ft candles at 3 ft.
Lux and Foot Candles to the Rescue
If I use two of those bulbs, or even 4, now I know what I’m going to get, double or quadruple the light output. If I put them into a softbox with a reflective back to harness more of that 360 light output I’ll still need to take another reading to get me my own specs.
Measuring the light output at 3 ft with the flat disc on my light meter. This gives me my own specs for my lights I can use when planning a video shoot.
What about this 4’ fluorescent tube? It’s rated at 2850 lumens. The fixture I use takes two bulbs so that’s 5700 lumens. Sounds pretty good. At three feet I get 80 ft candles, 800 lux. You would think it would be higher, I mean 5700 lumens, but a 4’ fluorescent is a much wider source than this CFL, which means it’s outputting out all those lumens over a wider area.
So, when you’re on non-industry sites, like Amazon, even when looking at film lights, note that the specs you get there are posted by the seller and may only list lumens or worse, list a lumen and lux rating together with no distance. I’ve seen a spec listing lumens: 600 lux/meter. What the hell does that mean?
Lux listed with Lumens is confusing. And worse, a Lux spec without the distance it was measured at is useless.
Ultimately, nothing beats using and testing lights yourself to determine how much light you’ll get at what distance. For more expensive units, that may mean renting them to test them out if lux/ft candle specs aren’t given.
For some additional info check the description below for links to a great article by Sareesh Sudhakaran on wolfcrow.com and by Andy Shipsides on HDvideopro.com. These guys are great and even get into some hardcore math at times if you’re into that thing.
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"How to put together a Lighting Kit for Video (Part One): Quantity of Light" by Sareesh Sudhakaran
http://wolfcrow.com/blog/how-to-put-together-a-lighting-kit-for-video-part-one-q/
"Watt is your Lumen count?" by Andy Shipsides
HDvideoPro.com https://www.hdvideopro.com/columns/help-desk/watt-is-your-lumen-count/
Arri Photometric Calculator
http://calc.arri.de/calculator
Office Lighting Design Calculations
http://ecmweb.com/lighting-amp-control/quick-and-clean-look-lighting-design
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