Why Moonlight Is Blue.
Ever notice how when you’re watching a movie and a scene takes place at night, the moonlight almost always looks blue? Of course you did. It’s pretty glaring. But did you ever wonder why?
Well, prepare to stop wondering.
There’s an important aspect to light called Color Temperature. The hotter something gets when it’s burning, the more the light it puts out tends toward white. Or to put it another way, white hot is hotter than red hot.
Since the sun burns hotter than a light bulb, daylight is whiter than what you get when you turn on a lamp.
(Where it gets weird is the vocabulary around color temperature. Daylight is said to be “cooler” and tungsten light is called “warmer,” even though the “cooler” light is actually burning at a temperature that’s something like 2,400 degrees hotter than the “warmer” light. Whatever.)
Has anyone told you that you have amazing eyes and no, I’m not changing the subject because you really do. Your eyes can adjust to whatever color temperature you’re being illuminated by. So whether you’re reading a book by candlelight (“warm” –– meaning not so hot) or by daylight (“cool” –– meaning pretty super hot), you perceive the pages as white.
The tricky part is when you mix light sources of different color temperatures. Since moonlight is really just sunlight that took a 475,000 mile detour to get to you, it’s pretty much the same color temperature as regular daylight. But the the moon only reflects between three and 12 percent of the light that hits it, so there’s a lot less of it. When you turn on a lamp, the light from the lamp looks a lot redder than the light from the moon.
Those amazing eyes of yours kind of split the difference, so what’s lit by the lamp looks golden and what’s lit by the moon looks blue.
And now we sing.
There’s actually a little more to it than all that, if you really want to geek out on this stuff. Because of the surface of the moon and the effect of the atmosphere, the color temperature of moonlight is actually a bit lower than direct sunlight. Your eyes are also more sensitive to colors at the blue end of the spectrum when the light is dimmer, so the way you perceive relative colors and contrast changes at night, too.
All this only matters if you’re doing what’s called Day For Night shooting –– filming during the day and trying to make it look like night. I’ll get into that stuff in a different blog. I’d go into it here, but I don’t want to tire out those amazing eyes of yours.
Brian Belefant is a director who stays up late into the night, working his ass off under the blue light of the moon. Give him a call at (503) 715 2852 or send an email to belefant (at) me (dot) com.