Friday, October 1, 2010

Chromatic Shadows, Part 2

As we saw yesterday, adjacent colored light sources result in overlapping colored shadows.

Here’s another rule of thumb: If you have two light sources of different colors shining on the same form, the cast shadow from each light source will be the color of the other source.

That’s because each colored light shines into the shadow of the other. So when the amber light shines on the cylinder, it casts a shadow that is filled only by the blue light—hence the blue shadow.

Let’s make the scenario even more challenging: What happens to the shadows when you have three colored sources? Here’s the effect from blue, red, and green lights.

Note that the cast shadows behind my raised hand and my head are magenta, cyan, and yellow. The yellow shadow on the far right is cast by the blue light. It’s yellow because it is lit by a mixture of green and red. In the realm of light, as opposed to pigment, green and red mix to make yellow.

The bright cyan shadow is cast by the central red light. It’s cyan because it contains the blend of blue and green light. Blue and green light mix to make cyan.

If you’re intrigued by colored light interactions, I’d recommend going to the home improvement or theater lighting store and picking up red, green, and blue lightbulbs that you can put in three different light sockets. The best way to learn this stuff is to set them up and try all sorts of combinations. I took the photo in the New York Hall of Science, which has an interactive setup installed.
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Related GJ post:
Colored Light and Form

12 comments:

Joel said...

That's pretty cool. It makes perfect sense now.

Rebecca.Yanovskaya. said...

It would probably be more economical for someone just trying it out to buy coloured gels or coloured transparency papers and put them in front of regular flashlights.

Mary Bullock said...

Oh my goodness! As if trying to paint light and shadows wasn't difficult enough already - my brain hurts now.

James Gurney said...

Rebecca, you're right, especially if you're lighting a small subject. You can get free sample books of professional gels at lighting supply stores. The sample pieces are about the size of a business card, but you can tape them over an LED flashlight, and they work great for playing with these effects.

Mary, I know what you mean...it's kind of confusing when you're looking at images like this. When you actually try it out, it becomes clear right away.

António Araújo said...

You get the two-colour effect quite often without trying. In my sci illustration class I'd sit under the skylight, so I'd be getting diffuse blue reflection from the sky. Since I was working under an ordinary yellowish lamp, I'd always have a yellow and a blue shadow under my objects. Funny enough, people trained to see like the impressionists always noticed the blue shadow (always calling it purple) and never noticed the yellow one. :)

I get the same effect in my room. The angles are just right for it to be very clear on an electrical plug on the wall, under the light of the window and the one from the ceiling; and since I have a dimmer switch in my lights, its very easy to manipulate the effect in the afternoon. People finally seem to get it when they can make it change with the dimmer, but before that the concept seems to confuse them, don't really know why. Your drawing illustrates it really well.

António Araújo said...

In the same vein, that is also the reason why you cast a blue shadow in the snow. You are blocking the sun's light, so you get a shadow, but the shadow isn't black because it is still being lit by reflected light; most of all by the diffuse blue light from the sky. Hence, you get a blue shadow.

Electric Blue said...

That's amazing! I've never seen this before. Thanks for sharing.

Daniel Silberberg said...

Wow. I've seen this a million times, but never understood it this clearly. Revelatory!

P.J. Magalhães said...

So it is not a case of the shadow being the complimentary colour of the light source but taking on the properties of the light that surrounds the shadow side. Did i get that right? :)

James Gurney said...

P.J. I'm glad you brought that up. The shadow from a strongly colored source can take on a tinge of the complementary color of the source anyway, just because of the way our visual system works.

But here we're talking about a form that has separate cast shadows from separate colored sources. The shadow of the amber source is particularly blue because only blue light from the other source shines into it.

foobella said...

That's very interesting. I hope this isn't a dumb question, but can you explain why you look mostly red in the picture even though you have three colors shining on you? THAT I don't get.

James Gurney said...

Footella, it's a good question. The red light was closest and shining directly on me, while the other two were more off to the side and not aimed straight.