in Feeding the Machines
LED Color Mixing
Most automated lighting fixtures manufactured in the last 30 years incorporated subtractive color mixing. This meant that the fixture starts with a white source, and the programmer used Cyan, Magenta, and Yellow dichroics to remove wavelengths of light and color the output. Now, LED-based fixtures instead utilize additive color mixing where the fixture starts with no output, and Red, Green and Blue sources are added to create a white output. Well, not really white, but a combination of the colors that is usually a white-ish lavender color.
Rather quickly in the development of professional LED fixtures, manufacturers improved the color abilities by adding additional LED colors. Now most LED fixtures are RGBW (Red, Green, Blue and White). A white LED channel allows a pure white output that can also be mixed with the Red, Green, and Blue to create pastels and other lighter colors. Some manufacturers also include Amber, Indigo, Cyan or other colors to allow users to achieve an even larger palette.
Most LED color-mixing fixtures provide the user with various color modes from which to mix color, regardless of the LEDs built into the unit. Typically, you can choose from RGB, CMY, Hue/Saturation/Intensity or preset colors that match common gels. Understanding how these modes will operate is an important task of the automated lighting programmer. From cue to cue, you may need to change color-mixing modes to achieve the desired color. Furthermore, your console may also offer the same or different color mixing models, without having to adjust the fixture’s control channel.
Colors and Binning
As our industry continues to move down the road toward LED-based lighting, higher light output quality is expected. In recent years, there has been a trend to “color-calibrate” LED fixtures at the factory. Manufacturers and designers noticed that, if they have a rig with a fair number of LED fixtures, the colors often didn’t match. Even if all the fixtures were the same, and were dialed to the exact same DMX value, the color output might vary. Significant differences in color was an issue affecting traditional automated fixtures, too, but most manufacturers have tightened up their tolerances of dichroic color mixing.
Today, most professional automated luminaires using dichroic color mixing have differences that are small enough to be considered acceptable by designers and customers — and the scrutiny on color differences has shifted to LED-based fixtures. You might think that LED sources would be more consistent, but the truth is that it is very difficult (and expensive) to get perfectly-matching LED light sources. To keep the pricing affordable, when a lighting fixture manufacturer purchases LEDs, they do not just order 10,000 red LEDs with a particular wavelength. Instead, they order 10,000 red LEDs from a certain “bin.”
The LED manufactures will categorize the color range of LEDs and then sell them in certain lots, known as “bins.” They actually test each LED as it is manufactured to determine the exact wavelength it produces. It is then labeled and stored with others near the same range of color. When a customer wants to purchase red LEDs, they sell them according to the size of a bin. Wider bins have a greater range of wavelengths and a lower the price. All the LEDs in a particular bin will fall within the acceptable wavelength range, but they will not all be identical. The selection of LED wavelengths is commonly known as binning.
Lighting fixture manufacturers will often purchase multiple or larger bins for a particular color, thus increasing the odds that the colors will not match from fixture to fixture. Remember that LED lighting fixtures typically contain from nine to 80 LEDs of each color. If each one is a slightly different wavelength, then the total combined output will be a mix of all the variants from the selected bin. The resulting lighting fixtures will all create a very similar red output, but they will not match perfectly. This is frequently very acceptable, and many designers are not unhappy with slight differences. However, some designers and applications require the fixtures on stage to perfectly match when a color is output. To accommodate that need, lighting fixture manufacturers began offering color calibration systems.
Color calibration systems are designed to ensure that all products coming off the production line will mix to the same set of colors, regardless of the LEDs installed and the binning selection. Manufacturer R&D departments do this typically by shining each LED lighting product into a test chamber, then they run a computer program to determine the entire color range of the fixture. Then an algorithm will be created, specific to that particular fixture, to restrict colors to a uniform range that every fixture can meet. That way, every fixture can mix to the exact same red, blue, lavender, or any other desired color. The unique “formula” for each fixture is loaded into the fixture’s memory. The output will then perfectly match between fixtures when the automated lighting programmer begins color mixing. (These same systems are also used with white LEDs to match color temperatures of LEDs.)
The downside is that some of the most saturated colors that a given LED fixture might be able to achieve will never be revealed when color calibration is turned on. Because the algorithms are restricting output for the sake of uniformity, they might also be blocking a range of unique wavelengths that might yield a deeper color. This is why it is essential that lighting programmers are aware of fixtures that have calibration features — and that they understand how to turn color calibration on or off.
I recently was programming a rock tour, and we had a large number of LED wash lights. They were all over the rig, and mostly pointed back at the audience.
When I first turned them on and was checking the rig, I noticed that the pure red did not seem very saturated. I discovered that the fixtures default to enabling the color calibration. As soon as I turned it off (from the console), I was able to achieve the deep red that I have come to expect from LED products. Given the size of the rig and the purpose of the show, the designer and I both agreed that we did not need the calibration on, and that the fixtures actually looked close enough for this application.
About two weeks later, I was programming a corporate event that included lots of live video. We had some LED wash lights scalloping up the walls behind the main podium, and we could clearly see that they did not match when dialed to the exact same DMX values (it was even more obvious on camera). I turned on the color calibration for these units, and suddenly they began matching perfectly. Sure, we could not get to some of the more saturated colors, but in this case, it was more important to ensure that the colors would match.
LEDs provide us with many benefits and incredible colors, effects, strobes and more. They are certainly an important part of the lighting industry, and their use will continue to grow. It is essential that lighting programmers understand the nuances of color mixing with these units. As lighting manufacturers continue to improve this technology, we must all understand the technology, color modes, calibration systems, and other new inventions. Take some time to read up on the features of LED products you get to program and look at them with the features both on and off. Only then can we ensure that we are providing designers with the best programming possible with the fixtures they select for each production.