Color Control of LED Luminaires by Robert Bell
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Color control of LED luminaires BY ROBERT BELL Why it is not as easy as you might think. Another description is by hue, saturation, not create every color your eye can see. Below A bit about additive and luminance, HSL. (Some say “intensity” is a hypothetical locus of an RGB system color mixing or “lightness” instead of “luminance.”) rendered on the entire visible light spectrum. WITH RECENT MASS ACCEPTANCE Equally valid is hue, saturation, and value, of solid-state LED lighting, it’s time HSV. Value is sometimes referred to as for an explanation of this technology’s brightness and is similar to luminance. complexities and ways in which it can be However, saturation in HSL and HSV differ tamed. LED luminaires use the output of dramatically. For simplicity, I define hue multiple sources to achieve different colors as color and saturation as the amount of and intensities. Additive color mixing is color. I also try to remember if “L” is set to nothing new to our industry. We’ve done 100%, that is white, 0% is black, and 50% it for years on cycloramas with gelled is pure color when saturation is 100%. As luminaires hitting the same surface, but for “V”, 0% is black and 100% is pure color, control can be tricky. The first intelligent and the saturation value has to make up the luminaire I used was a spotlight that had difference. That over simplifies it, but let’s three MR16 lamps, fitted with red, green, carry on, as we’re not done yet. and blue filters. In the early days, the control Another valid description of what comes Figure 1 of luminaires such as these was simply three out of a luminaire can be described by DMX512 control channels and no single CMY (cyan, magenta, yellow), which are The triangle’s vertices hover around deep intensity control channel. You could not the primaries used with subtractive color saturated reds, greens, and blues. Altering easily roll down the brightness and keep the mixing. If you start with white light, you the power supplied to each LED die should color. Typically, moving light programmers introduce two filters to get red: magenta to allow you to reach any color inside of the would also build a “color black” so they remove the green component of the white gamut area, but this is theoretical, and many could easily turn off these luminaires. There light and yellow to remove the blue. Color- factors affect what really happens. Foremost, are better ways. changing LED luminaires in general do not the exact wavelength of the R, G, and B can use subtractive color mixing, but this is still vary drastically from luminaire to luminaire. a valid way to describe a color. The locus of a color can just about Control and Ideally, when controlling LEDs, you describe hue but not intensity nor definition of color should have the choice of adjusting intensity saturation. If you do a quick Google search and one of RGB, CMY, HSL, or HSV. There on “color gamut,” you will see circles, If you don’t control your intelligent are some variations on these themes to donuts, cubes, cones, and even fruit, all luminaires using pure DMX levels but come, but let’s move on. attempting to show the three-dimensional instead use some form of abstract control, relationships of HSL. you can have a virtual intensity level. So, even if the manufacturer defined the Mixing color with LEDs luminaire to use three DMX channels, Our eyes can detect light from 390 to 700 Adding more abstract control allocates four handles nanometers. The first LED luminaires used to control it: intensity and three color just red (about 630 nm), green (about colors at the source parameters. Note I wrote “three color 540 nm), and blue (about 470 nm) LEDs. As LED technology evolved, prices dropped parameters” rather than red, green, and Contrary to what your grade school art and various intellectual property lawsuits blue. RGB is just one way to describe color. teacher said, mixing those three colors does subsided allowing more companies to enter SUMMER 2013 28 SUMMER 2013 the market. Lighting designers’ appetites Controlling this many chips can be imagine a transition from pink to green. In for this new light source grew and along tricky; for any one hue there are multiple this model, moving in a straight line takes with that came demand for brighter combinations of power to each chip that can you right through white. This may or may luminaires and more consistent color get you to that one point in the color space. not be desirable. control. New LED colors became available You may describe the two cues slightly too: white, amber, cyan, and violet. Initially, differently. In the example in Figure 5, the most popular integration was RGBA, What to do with the two end points are exactly the same: with an added amber chip. This made the all these LEDs? amber and pink. But in this case, the color gamut that was sort of triangular colors are represented using HSL. Cue 1 is This is becoming a lot more complex than more rectangular. amber with a hue of 10% (percent being turning a light on and off. Luckily, some an arbitrary unit for hue), and cue 2 is modern control systems allow you to drive pink with a hue of 90%. Note that if you any type of color system in very simple go counter-clockwise from amber, you ways. Apart from intensity you are offered reach red. just three distinct color parameters: RGB, CMY, HSL, and HSV. Using a real-world example, let’s examine the possibilities. Say we’re doing a musical and we’re lighting the cyc with color mixing luminaires. We’re doing a sunset scene, and the designer wants a transition from amber to pink. Using the RGB color space, cue 1 is amber (R=100% G=60%, B=0%) and cue 2 is pink Figure 2 (R=100% G=0%, B=60%). Another variation is RGBW, with broad- spectrum white LEDs. Luminaires exist that use white and amber with the three other Figure 5 colors (RGBAW). As LED technology continued to advance, The transition is from amber to pink, chip manufactures managed to make deep but in this case, you pass through a red red, cyan, and royal blue LEDs, too. These that has the same saturation as the two end have been used in seven-color systems (deep points because it is equidistant from the red, red, amber, green, cyan, blue, and royal center of the circle. We move in an arc, not blue). This enlarges the gamut, giving us a straight line. potentially more colors. What if the color space were defined differently? Say, it was mirrored so that when you were on amber and you moved Figure 4 counter-clockwise on the wheel, you would first reach yellow instead of red. In On any lighting console that defines color fact, you’d have to go a long way before as RGB, the transition you see in Figure 4 you reached red. In Figure 6, cue 1 is is exactly what you get. It is a straight line again in amber (hue' of 10%) and cue 2 from amber to pink going through a slightly is pink (hue' of 90%). These are the same less vivid color of red. I write “slightly less values; they just appear at a different place vivid” because, in this case, the distance in space. from the center of the wheel represents the saturation of the color and the mid-point on the line between the two end-points Figure 3 is closer to the center of the circle. Now SUMMER 2013 29 PROTOCOL Resolution Calibration Solid-state response time is instantaneous, When you are using 20 LED luminaires so if you stop driving the chips, they just to wash a cyc, variations in LED output stop making light. This is problematic when can easily be seen. For this reason, high- dimming LEDs: Slow fades using low- end solid-state luminaires have built-in resolution control, particularly at low levels, calibration channels. These channels allow end up looking choppy. Early-day LED you to dampen or boost the control level luminaires did nothing to compensate for going to each chip. You can achieve the low-resolution control, but more recently, same results by tweaking your color palettes advanced LED drivers have added shock for each luminaire; if one luminaire is absorbers in their firmware to smooth out consistently redder than the rest, you can the bumps. pull down the red at a global level and hope Using 16-bit control (two data slots things fall in line. I’ve found over the years, Figure 6 together) is another way luminaire if you have a picky designer that doesn’t like manufacturers have solved the issue of the look of a cue, you have to keep tweaking As these are purely hypothetical color choppy fades without having to add it. It is useless to argue saying it should be spaces (and we’re running out of letters), software shock absorbers. This puts the right because the desk said so. The designer we’ve dubbed this the prime version of HSL, onus on the control desk to push lots of always has the final say. HSL'. If you read carefully above, you will data, versus having to over-sample the data see that amber was defined having a hue' at the luminaire and predict where the of 10%.