How to control the color and color mixing of LED lamps?

About Additive Color Mixing

LED fixtures use multiple light sources to achieve a variety of colors and intensities. Additive color mixing is a commonplace in the entertainment lighting industry. For many years, practitioners have used fixtures with color filters to project the same area on the cyclorama, which is not easy to control. The first intelligent fixture I used was a spotlight with three MR16 light sources, each with a red, green, and blue filter. In the early days, this type of fixture only had three DMX512 control channels and no independent intensity control channel. So it was difficult to keep the color unchanged during the dimming process. Usually, computer lighting programmers will also set a "light off color change" to easily turn off the fixture. Of course, there are better ways, which will not be listed here one by one.

Color Control and Definition

If the user does not use pure DMX values ​​to control intelligent lamps, but uses some abstract control method, a virtual intensity value can be used. Even if the manufacturer stipulates that the lamp uses 3 DMX channels, the abstract control method can also allocate 4 handles to control: intensity value and 3 color parameters.

I wrote "3 color parameters" here, not red, green, and blue, because RGB is just one way to describe color. Another way to describe it is hue, saturation, and luminance - HSL (some people call it intensity or lightness instead of brightness). Another way to describe it is hue, saturation, and value - HSV. Value (lightness) is also often called brightness, which is similar to brightness. However, HSL and HSV have very different definitions of saturation. For simplicity, I will define hue as color and saturation as the amount of color in this article. If "L" is set to 100%, it is white, and 0% is black, then 50% of L is a pure color with 100% saturation. For "V", 0% is black and 100% is a pure color, and the saturation value must make up the difference.

Another effective way to describe it is CMY, which are the three primary colors, using subtractive color mixing. If white light is emitted first, then red can be obtained by using two color filters: magenta and yellow; they remove the green and blue components of the white light respectively. Usually, LED color-changing lamps do not use subtractive color mixing, but it is still an effective way to describe color.

In theory, when controlling an LED, it should be possible to adjust the intensity and one of RGB, CMY, HSL or HSV (there are some differences between them).

About LED Color Mixing

The human eye can detect light with a wavelength of 390nm-700nm. The original LED lamps only used red (about 630nm), green (about 540nm) and blue (about 470nm) LEDs. These three colors cannot be mixed to produce every color that the human eye can see. Figure 1 is the assumed area of ​​the RGB model proposed based on the entire visible spectrum.

The three vertices of the triangle fall into the highly saturated red, green, and blue regions. By changing the power emitted by each LED chip, any color within the color gamut can be obtained, but this is only theoretical. In fact, the color mixing effect is affected by many factors. For example, the exact wavelengths of red, green, and blue vary from lamp to lamp, and there may be huge differences between them.

A color gamut describes not only hue, but also intensity and saturation. If you do a quick Google search for “colorgamut,” you’ll see circles, donuts, cubes, cones, and even fruit shapes, all of which attempt to demonstrate the three-dimensional relationships of HSL.

Adding More Colors

With the technological innovation and price reduction of LED, more and more manufacturers have entered this market. Lighting designers have higher and higher expectations for this new light source, and the requirements for the brightness and color consistency of lamps have also increased accordingly. New LED colors such as white, amber, cyan and violet have come out. At first, the most popular combination was RGBA, which added an amber chip. This makes the color gamut more like a rectangle than a triangle.

Another variant is RGBW, which has a wide spectrum of white LEDs. Newer lamps add white and amber to RGB (RGBAW).

With the continuous advancement of LED technology, chip manufacturers have also successfully produced deep red, cyan and royal blue LEDs. These colors have been applied to the 7-color system (deep red, red, amber, green, cyan, blue and royal blue), thus expanding the color gamut and providing designers with more colors.

Controlling so many chips can be laborious; there are multiple ways to combine the power of each chip to achieve the same color point in color space.

How to control these LEDs

As LED technology develops further, control becomes more and more complex. Fortunately, some modern control systems can drive any type of color system in a very simple way. In addition to the intensity, the user is provided with different color parameters: RGB, CMY, HSL and HSV.

The author examines these possibilities through a real-life example. For example, a designer is making a musical and is using mixed-color lamps to color the skylight.

A sunset scene needs to be created on the stage, and the designer wants the color to change from amber to pink. Using the RGB color space, cue 1 is amber (R=100%, G=60%, B=0%), and cue 2 is pink (R=100%, G=0%, B=60%).