Use the inverse square law

I wanted to light my videos with a colored-light video background as I’ve seen on my favorite youtube channels like GeraldUndone and SenpaiGaming. When I tried, it was washed out, and the color didn’t show up. This is how I solved it.

To get a nice colored-light video background, I moved further from the background and placed my key light closer to my face. Then, when my face is properly exposed by the camera, my background looks relatively much darker because of the light fall-off (inverse square law). That way, when I then light the background separately with my colored LEDs, the color shows up better. 

Lighting is all about the amount of light falling on the subject per square inch (or foot, or meter). With a small light source like a light bulb, the light spreads out as it gets further from the bulb, becoming more diffuse. For any given subject, as you move it further away from the light source, less of the light emitted by the bulb hits the subject. Most of it misses the subject and heads off to light other things or gets lost in the darkness. 

A note on light units. 

Two common units of light are lux and lumens. Lux measures the amount of light falling on a surface. Lumens measure the amount of light emitted by the source. So, 1 lux = 1 lumen per square meter.

Imagine you have a small light bulb producing 1600 lumens (that’s pretty bright, and you can usually find the light output of a light bulb on the packaging). That light spreads out in all directions as it leaves the source. 

If you measure a constant radius in all directions, you’re describing the inside of a sphere for which, in every direction, the light has traveled the same distance. That means, as the light produced by the bulb moves further and further from its source, it’s having to illuminate a larger and larger sphere.

Think about the math for a minute. 

I promise it won’t be much.

The surface area of a sphere is A = 4πr²

For any distance from the bulb r, the surface area increases proportionally to r².

Here’s an example. 

At a distance of 1 meter, the light intensity from our imaginary light bulb is 1600/(4*3.14*1²) = 127.4 lumens/square meter, or 127.4 lux.

At a distance of 2 meters, the light intensity from our imaginary light bulb is 1600/(4*3.14*2²) = 31.8 lumens/square meter, or 31.8 lux.

To make the math easier, I’m going to round those values to 128 lux and 32 lux, respectively. Notice that, when I doubled the distance, the light intensity dropped to one quarter.

If I raise the distance to 4 meters, the light drops to 1/16th or 8 lux.

We don’t need to know the actual lux values for a given scene, but the simple idea that illuminance drops by the square of the distance is really useful in understanding how to light your scene.

How does it help, you ask?

Here’s how it helps

If you’d like your background to be illuminated with colored lights, but you want your face illuminated with white light, you have a conflict. If too much of the light illuminating you also falls on the background it will wash out the color.

You can use the inverse square law to solve this. Imagine you’re in a room with about 12 feet of depth to work with. If you place yourself against a wall with your lights and camera in the middle of the room, then both you and the wall behind you will be at about the same illumination.

Move yourself to the opposite wall, though, and things change. If your light is backed up to one wall, and you’re in the middle of the room, the wall behind you is twice as far from the light as you are, so it’s ¼ as bright (2 exposure stops darker).

If you then move closer to the light, the wall stays the same, but you get relatively brighter. If you’re 3 feet from the light, and the wall is still 12 feet back, that background is now 1/16th as bright as your face (4 stops darker).

A little counter-intuitively, you can make this even more extreme by bringing the light closer to you (and the back wall). Try this on.

Leave the camera against the wall, and keep yourself at the same 3-foot distance. Now bring the light closer, say 18 inches, and just out of the frame of the camera. Now, the distance from the light to you is 1.5 feet, and the distance from the light to the back wall is 10.5 feet. The wall is now 1/49th as bright as you (about 5.5 stops darker), and looking pretty much dark.

Now we can finally get that colored-light video background up with nicely saturated colored lights. No more will your background color be washed out by your key lights.

Give it a try!