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Shedding Illumination on Lighting

The question for this issue is “Why do fixtures all tout CMY color systems?”

First let’s define the acronym, CMY, which stands for Cyan – Magenta – Yellow. If you read last issue’s column, you will remember that these are the secondary colors of light. In review, the primary colors are red, blue, and green; the combination of equal parts of these colors produces white light. A primary color is pure, that is to say it does not contain any other colors. The secondary colors each contain equal parts of 2 of the primaries.

Now if you take equal parts of the secondaries you will get black, effectively the combination of these 3 colors blocks all light. What we are trying to understand here is why the secondary colors are used in intelligent fixtures. An intelligent fixture, or moving light, has a single light source, which for this discussion is producing pure white light, something no real fixture can produce. White light is fine for some things, but most people also like to have some color from time to time. In standard fixtures you can place a sheet of gel in front of the light and have any color you want. You create another color by shining another light source with another color; you are mixing the 2 colors by addition to create a new color. Additive color mixing works great with multiple light sources, but let’s go back to our intelligent light. They have only one light source, yet we want to be able to have any color we want, and the manufacturers have delivered.

How can we get all those colors from this one light source? One way is to put in a color wheel and rotate a color in front of the light. The problem here is one of limitation, a color wheel can only hold a fixed amount of colors, no mixing allowed. There are a few fixtures out there with 2 color wheels that allow for a limited amount of mixing, not an easy way to mix. If you want to get an infinite variety of colors, you will have to use more sophisticated mixing. Here is where we switch from addition to subtraction.

Let’s back up here for a moment and discuss how filters work. A filter blocks certain wavelengths and passes the rest. So what we call a red “filter” is really a blue and green blocker.

In order to create any color, we have to subtract the wavelengths we don’t want from the pure, white source. If we try to make yellow with primary colors we would start with the knowledge that yellow requires equal parts red and green. So we place a red filter in front of the light source, then we mistakenly try to add a green filter. What we would get is not yellow, but “mud”. (We’ve now blocked blue, green, and red, all the primaries!) Another way to look at this is subtraction; we subtracted all the colors without creating anything. Now if we approach the problem starting with the secondary colors in our subtraction the equation turns out differently. If we want to create red we use a yellow and a magenta filter. The yellow filter has green and red, while the magenta has red and blue. The yellow blocks the blue in the magenta and the magenta blocks the green in the yellow which leaves only the red component in each to shine on thru. By varying the combinations of the secondary colors you can create any color you want, almost.

This leads to another aspect of creating colors we should talk about, saturation and value. Saturation deals with how much of a color is present, the more saturated the more pure. Value deals with the relative brightness or darkness of a color.

We need to not only mix the pure secondary colors but also control the saturation and value of them. In order to do this we somehow have to be able to introduce a variation of not only the color, but the amount of saturation of color. In order to control the value, we need to control how much white light we filter out. The intelligent light manufacturers use at least two methods to accomplish this feat. The original design by Vari Lite utilizes vanes of color that they can rotate into the light source. The vanes are wedges that create a complete disc when they are all flat. They placed a disc of each color in a different plane at the front of the fixture. When all 3 sets of vanes are closed, (no light output) remember the secondary colors all add up to black. Other manufacturers use a 3 plate layout that has the colors on them in a pattern similar to a hand. The tips of the fingers are the least saturated portion. The heel of the hand is the most saturated section. By rotating the plates into the light, you can vary the saturation of each color. Each plate is controlled by a separate motor so you have an infinite amount of variation.

One side note; most of these fixtures have the plates fully in when the DMX value is equal to zero, in other words, when you turn the fixture on there won’t be any light output. There’s a trick for getting around this. Set up the profile of the fixture to ‘flip’ the color channels in value so that the console outputs full light at a DMX value of zero.

I hope this has demystified the subject of Subtractive Color Mixing. If you have a limited selection of color filters available for your conventional fixtures, try experimenting with multiple filters and subtract to get the color you want.

I thank all of the readers who have responded with positive feedback and interesting lighting questions. I will faithfully research them and strive to provide information and feedback for the enlightenment of the church body. Keep communicating with us!

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