In light of the convergence of Barco and High End Systems last month (pun intended), I thought it might be nice to review one of the key differences between the way the lighting and video guys look at color. These two visual mediums view the production of color in one of three main color spaces: CMY, RGB or YUV (SMPTE).
Cue the Lights
Lighting generally utilizes the CMY (or CMYK) color space, which works on the premise of SUBTRACTIVE color mixing. CMYK stands for Cyan, Magenta, Yellow and Key (Black). To achieve the desired color, CMY starts with white light as a background color, and then uses variable amounts of the three colors to mask out color wavelengths. For example, mixing yellow and magenta will net you Red. Black is achieved by turning off the light, and not by using all three colors to mix to black. When you try to subtract out all the colors, you generally get mush and not true black.
Adding Video to the Mix
Whereas the CMY model is subtractive, the RGB model (as used in video) is an ADDITIVE color model, which works as light is emitted directly from a source such as an LED or CRT. By combining the three primary colors (Red, Green and Blue) in varying amounts, you can achieve any color under the rainbow. The absence of all primary colors in this color space yields black, and all three primaries added together at full intensity will produce white light.
Unlike the CMY model, the RGB model is device dependant. Different technologies will reproduce the R, G, and B elements in a unique fashion. This accounts for color fluctuations between LED, DLP and LCD technologies. Each one produces the three primary colors in a different fashion, and thus each represents a similar image differently. To maintain a uniform color match between RGB devices, you must use some type of color management tool, like a color meter, to match the devices together. Literally, the ability to adjust color within the display device is what separates professional equipment from consumer level gear.
Lastly, video also uses the YUV / YCrCb color space. YUV is used for Analog signals, where YCrCb is used for digital signals. The Y stands for the Luma (or brightness) component, while the U and V represent the chrominance (or color) components. In essence, black and white television was just the Y or Luma value without any of the color values.
To achieve the correct color mix, the U and V components are subtracted from the Y value. For example, the U signal is created by subtracting Red from Y, which yields a mix of blue and green. Conversely, the V signal subtracts blue from Y, and yields a mix of red and green. By adding all three values together, we achieve the correct color mix for the image.
The reason we use YUV is to create a means to mask any transmission or compression errors that can occur during broadcasting. YUV also requires less bandwidth for transmission or storage.
Bringing It All Together
As the video and lighting convergence continues and strengthens, each side will need to adopt the color language of the other. By doing so, we can effectively mix technologies on the same stage and achieve the look that our clients are paying us to achieve. The challenge is clear, and getting to the same color from two departments will require tweaking, color management and a keen understanding of each color space.
For example, in the CMY world, achieving pure blue requires a non-equal mix of cyan and magenta, while in the RGB world, pure blue is a primary color and easily achieved by not adding in any red or green.
As lighting manufacturers adopt more LED and other traditional RGB devices in their lighting fixtures, and as video manufacturers create new moving head projectors that bring RGB color mixing and digital gobos into play, the line between the worlds will continue to blur.