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Color Control & LED Lights

Color Control & LED Lights

Using the Cognito Lighting Console to manage color mixing, intensity curves, resolution, and other lighting effects.

Getting what you want on stage can be a daunting task if you don't take the time to understand what the console and lights do and how they interact with each other.

With recent mass acceptance of solid-state LED lighting, it's time for an explanation of this technology's capabilities and complexities and the ways in which it can be tamed. Moreover, it warrants the ways in which one control panel, Cognito, enables various solutions.

The Cognito Lighting Console is control interface created by Pathway Connectivity, for which this author provides product development guidance. The console enables intuitive control and programming of incandescent dimmers, moving lights, and color-changing LEDs.

LED lights use multiple sources to achieve different colors and intensities. Additive color mixing is nothing new to our industry. We've done it for years on cycloramas with two or three gelled cyc lights hitting the same surface. An early "intelligent" light that incorporated additive color mixing was a spotlight that had three MR16 lamps, each fitted with dichroic glass one for each of red, green, and blue. In the early days, the DMX protocol for lights such as these was simply three control channels. Using early moving light desks, the drawback of just three channels was that these lights had no intensity control. For instance, if you had a mixed rig of LEDs and moving lights, the LEDs would stay on when you pulled down the Grand Master. It also meant that you could not roll down the brightness even if you liked the color. Typically, moving light programmers would also build a "Color Black" so they could easily turn off these lights.

With the advent of abstract control (marketed as "natural language control with Cognito"), we gave additive lights a virtual intensity control. So, even if the manufacturer defined the light to use three DMX channels, Cognito allocates four "handles" to control it intensity and three color parameters. Note: That's "three color parameters" and not "red, green, and blue" because RGB refers to hue, but not saturation and luminance (or intensity or value).

For simplicity, one can think of hue as color and saturation as the amount of color (versus lack of color definition).

In addition to RBG, another valid description of what comes out of a light can be described in the triplet CMY (cyan, magenta, and yellow). Those are the primaries used with subtractive color mixing. If you start with white light and you desire red, what you do is introduce two filters one is magenta (which removes the Green component of the white light) and one is yellow (which removes the Blue component of the white light). When controlling LEDs with Cognito, you have the choice of adjusting intensity and one of either RGB, CMY, HSL, or HSV.

Most human eyes can detect light in the range of 390 to 700 nanometers. The first LED luminaires used just three color dies. Those were red (about 630 nm), green (about 540 nm), and blue (about 470 nm).

It should be no surprise that mixing those three colors does not create every color your eye can see. Altering the power supplied to each die should enable you to reach any color inside of the locus we say "should" because, in reality, many factors will affect what really happens. Foremost, the exact location (or wavelength) of the R, G, and B can vary drastically from light to light. This is similar to how standard-colored glass varies from batch to batch. In the manufacturing world, we call the process of qualifying the parts "binning," and it describes the tolerance a factory may accept before installing the LEDs in a fixture. Also the control systems (the pulse width modulation rates and power supplies that control the dies) may introduce artifacts seen in sub-second time slices. So, how you view the image (either by camera or by naked eye) can affect what you see. Even moving your head quickly can emphasize some of these artifacts. Finally, who is to say that this hypothetical gamut is representative of the real world? You are likely viewing this document on a computer screen that only uses RGB emitters to render the images in any case.

As LED technology evolved, prices dropped, and various intellectual property lawsuits subsided, enabling more companies to enter the market. Lighting designers' appetites for this new light source grew and, along with that, came demand for brighter lights and more consistent color control.

To answer that call, binning became tighter and the number of sources increased. New LED colors became available, too white, amber, cyan, and violet, for example. Initially, the most popular integration was the RGBA in which manufactures added an amber chip onto the light. This made the locus that was sort of triangular look more like a rectangle (or, at least, a four-sided-ish thing).

The addition of amber offered a few more colors, but not necessarily more light. Brightness is purely a matter of efficacy and the number of watts you pump into the luminaire. If you want a brighter light, just add more or bigger LEDs.

Another variation on this theme (just to confuse the brightness vs. color debate) was the RGBW. The white LED grossly overlaps colors you could already reach by adding primaries together. In this case, whiter does mean brighter. Lights exist that use both white and amber with the three other colors (RGBAW). 

Cognito's Natural Language Control enables you to drive any type of color system discussed so far (additive RGB, RGBA, RGBW, RGBAW, and subtractive CMY) in very simplistic ways. Cognito has a general color picker, a series of popular gel colors, and simple three-slider mixer. If you use the advanced wheel modes to mix color with Cognito, apart from intensity you are offered three distinct color parameters. For the sake of repetition, those are RGB, CMY, HSL, and HSV.

As LED technology continued to advance, chip manufactures managed to make orange, cyan, and violet LEDs too. These have been used in "seven-color" systems (red, green, blue, amber, orange, cyan, and purple). This again distorts and widens the locus giving us potentially more colors.

Unfortunately, Cognito has not implemented an automatic method to control the orange, cyan, and purple chips. For any one color (at varying brightness), there are multiple combinations of power to each chip that can get you to that one point in space. Lucky for us, those who manufacture the seven-color systems have figured out the "best" way to get the "brightest" color light. It's important to note that only the light manufacturer can provide this solution properly. They present this to you using one of two systems that we all know and love: RGB or HSL.

The definition of what RGB brings you is theoretical and is unique to manufacturers' lights in their implementation. Likewise, an implementation of HSL is yet another way of getting to the same end point. When comparing RGB to HSL control, one is not better or different or faster than the other. On a properly designed fixture, each can achieve the exact number of colors as the other.

Regardless of your light's primary color system (RGB, RGBA, RGBW, RGBAW, CMY, etc.), Cognito can write cues and do transition in multiple color spaces (RGB, CMY, HSL, etc.). What Cognito cannot do at this time is use the six color spaces to control a seven-color LED systems (RGBAOCP) because, again, only the light manufacturer knows the best way to control those seven LEDs to obtain a certain color.

Luckily the light manufacturers have devised a method that uses just three control channels. But, to add to the confusion, some give you two choices: RGB and HSL. However, since HSL control of the DMX channels for a light are not standardized, Cognito can use only the RGB model to control the light. This is not a detriment because:

  • Using the light's native RGB protocol enables you to reach any color using all seven chips.
  • Whichever color the light produces for a given set of values of RGB, you can rest assured it is as bright as they can possibly make it.
  • Using Cognito's Natural Language Control enables you to define colors and write cues in six color spaces (RGB, CMY, HSL, HSL, hsv, hsv').

Another attribute some of these modern day LED lights offer is amber-drift or red-shift. When a tungsten light cools (approaching off), it becomes more red, just like 2800K appears more red than 5600K. In practice, we choose colors assuming the light is at full, but we don't often drive lights at full in the theater. We don't, therefore, often see 3200K on stage. It's more like 2800K and, as we approach a blackout, things do in fact get quite red. Even if you're using bluish gels, you can see this effect.

To help integrate theatrical LED lights into conventional rigs, manufacturers favor the red chip at the bottom end. This profiling makes the colors at very low light levels of conventional lights and solid-state lights match more closely this intelligence is in the light and, at this time, Cognito does not artificially emulate a red-shift when fading to black.

Solid-state response time is instantaneous. So, if you stop driving the chips, they just stop making light. This is problematic when dimming LEDs, as slow fades using low-resolution control (particularly at low levels) end up looking very choppy. Early-day LED lights did nothing to compensate for low-resolution control, but more recently, advanced LED drivers have added "shock absorbers" in their firmware to smooth out the bumps.

The DMX protocol we use to control almost all lighting in the theatre pushes out eight-bit data slots, or 256 different values in each control channel. For dimming a tungsten load, which inherently has thermal inertia, that is plenty enough resolution. To pan a light, however, 256 steps over 360 degrees (or more) is far too few. Using two data slots gives you 65,535 different values, and that is often enough. Cognito, by the way, can set and fade attributes with a precision of 1 in 4,294,967,296.

Using 16-bit control (two data slots together) is another way light manufacturers have solved the issue of choppy fades without having to add software shock absorbers. This puts the onus on the control desk to push lots of data, versus having to over-sample the data at the light and predict where the level is going. Doing this prediction can drastically delay the response time you get out of the LED lights which is a bad thing when you're trying to bump them in time to the music.

Some LED light manufacturers have incorporated intensity curves in their firmware. Dimmer manufacturers have often enabled users to tweak the curves that translate the incoming control level to the actual power output of the dimmer. Common curves are linear, inverted, and square-law.

LEDs' instantaneous response time can be used with great effect when you want a quick strobe, but it can look unnatural when you have a mixed rig and you hit the blackout button. The LEDs go off in a split second, but the conventional lights take almost a full second to cool off substantially enough that no perceived light hits the stage. Some manufacturers offer various curves on the LED lights such as quick, standard, linear, and Tungsten emulation modes. Again, at this point in time, Cognito has no control' options to emulate Tungsten-type curves, which essentially translate to a slower response time to the LED drive system.

When you are using 20 LED lights to wash a cyc, variations in manufacturing dates and/or factory binning can easily be seen. For this reason, high-end solid-state lights have built-in calibration channels. These channels enable you to dampen or boost the control level going to each chip. You can achieve the same results by tweaking up your color palettes for each light. The idea here is that if one light is consistently redder than the rest, you can pull down the red at a global level and hope things fall in line. If you have a picky designer, however, and they don't like the cue they see with their eyes, you have to keep tweaking it until they do. 

The International Commission on Illumination describes correlated color temperature as "the temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions." Non-scientists might just call it "white."

Correlated color temperature is typically quantified in Kelvin. To give you an idea of what the different values of Kelvin might represent when describing white light, a candle is about 1800K, a domestic light bulb is about 2800K, a theatrical spotlight is 3200K, daylight is 5600K, and TV screens can be upward of 10,000K.

Can intelligent lights with tunable color (blood red, bastard amber, no-color pink, pea green) define white, too? Sure they can, but it can be difficult given there are so many flavors of white. For that reason, Cognito allocates another parameter when dealing with color mixing lights called CCT. For any light, whether you choose to control it in RGB, HSL or HSV, you can also dial in a white in Kelvin.

This functionality is offered on many LED lights in Cognito's library, and some lights offer similar functionality by setting aside a dedicated control channel mapping 0100 percent to some Kelvin range the light can hit. For those that offer it natively, Cognito just outputs the value as published by the manufacturer and leaves the profiles of the other chips alone.

As you can tell, turning on an LED light is not as simple as switching on the reading light by your bed. If you take all the methods of control discussed here and do the permutations and combinations, it's likely there are well over one hundred different methods of getting any one particular color out of an LED light. Cognito goes a long way to simplifying some of these tasks. Even so, getting what you want on stage can be a daunting task if you don't take the time to understand what the console and lights do and how they interact with each other.

ROBERT BELL is manager of product development with Pathway Connectivity (pathwayconnect.com), a Calgary-based data communications company in the entertainment lighting industry.

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