The Colors of Light

Most of us don't pay too much attention to lighting. Beyond how a light fixture looks, usually in the daytime, there isn't too much thought given to how well it actually does with what it is supposed to do - provide illumination.

There are several important factors that should always be considered for both interior and exterior lighting beyond just aesthetics. Certainly the total amount of light and where that light shines are important considerations, especially outdoors.  Using too much light, especially light that is poorly directed, can lead to excessive light pollution. It turns out that the color (Well, the spectrum really. More on that below.) of light is equally important in preserving the night sky.

It may surprise some to know that the color of light is important for interior lighting too.   

Here's a light fixture with two different CFL bulbs in it. There's a pretty dramatic color difference between the two of them. The one on the left is blue-white in color, while the one on the right has a warmer hue to it.

Most light sources are not pure, single colors, but rather are a mix of different colors. Recall that ordinary sunlight is a mix of colors too. Nature splits them into their components forming a rainbow.
By passing the light of the bulbs above through a spectroscope we can analyze the colors in greater detail:

That's the spectrum of the blueish looking CFL bulb on the left and the warmer one on the right. The graphs show how much of each color there is shining out from each bulb. Blue, green and yellow-orange dominate the mix of the bulb that looks blue-white, while green and yellow-orange dominate from the warmer blub on the right.

Okay, "so what?", you might ask. First off, most people have preference for warmer light sources like the one on the right. Recall (if you're old enough), that when CFL bulbs were first introduced many people complained about how cold and harsh they looked. Those early CFLs were much like the bulb on the left. But it's more than that. In 2010 the International Dark-Sky Association (full disclosure: I work for them) published a paper providing warnings about blue-rich light and how it can cause problems for both interior and exterior use (See also IDA's "Seeing Blue" article here).

The physics of the situation says that any blue light that ends up in the sky, from poorly directed light or even reflection off of the ground, disproportionately brightens the night sky more than any other color. But for interior use, like the CFL bulbs shown here, there are problems too.

It turns out that blue light is an important marker for our internal clocks. Why would that be? Nature has given us an important indicator that tells our bodies when it is daytime - the blue sky.

Here's the spectrum of the daytime sky. No surprise here, it is really blue. Evolution has given us photoreceptors beyond the more familiar cones and rods of the eye that don't contribute to our vision. Instead, they are tuned to blue light, giving our bodies the cues to keep our internal clocks in synch.

That's why it is important to limit the amount of blue light you are exposed to at night. Blue light from CFL bulbs, screens from TVs, smartphones and more can help throw our systems out of whack, leading to sleep disorders and even long term medical problems.

When choosing bulbs be sure to look at the color temperature (CCT) on the packaging. It will use words like "warm white" or "cool white," but you'll also see a number in the Kelvin temperature scale. For the CFL bulbs here, the blue one has a CCT of 5000K and the warm one is 2500K. Generally speaking, lower numbers have less blue light in them.

To help limit my blue light exposure at home I have low CCT bulbs in the house. I also I use the program f.lux for my laptop. It adjusts the color temperature based on the time of day, dropping my display down to 2700K at night.

The Times, They are a Changin'
We're going through a lighting revolution right now as new technologies are appearing. LEDs (Light Emitting Diodes) are becoming affordable and are showing up for inside and outside applications. The spectrum of an LED light source bears little resemblance to that of a CFL, which isn't too surprising as the light is created differently.

Here are the spectra of two different LED lamps. The CCT of the one on the left is 5800K while the one on the right is 2800K. Notice that both of them have a spike of blue light, but it is much smaller in the 2800K light source. For LEDs used inside it is really important to go with the warmer (lower number CCT) sources so as to not disrupt your internal clock.

It is also important to go with warmer choices for outside lighting, but for different reasons.

More and more cities are making the conversion to LED streetlights. It is a concern because, while generally speaking these new LED streetlights are shielded (downward pointing), the cities sometimes end up choosing lights with excessive amounts of blue and, as I said above, blue light disproportionately brightens the night sky more than the other colors do. So if not done properly, a conversion to LED could lead to brighter skies - something those of us interested in astronomy will want to avoid. 

I'll be blogging more about LEDs in a future post.