Overview
Lime is an additional LED emitter color found in some stage lighting fixtures. It sits between green and amber in the spectrum, with a yellow-green appearance. Fixtures that include it usually add it alongside red, green, and blue, and sometimes alongside other emitters such as white, amber and UV.
Why a lime emitter is added
The green-to-red gap in color mixing
A fixture with only red, green, and blue emitters creates many colors by mixing them. The region between green and red is covered only by blending those emitters. Colors there, such as yellows and warm whites, can look less rich or less natural than the saturated primaries.
A lime emitter puts light directly into part of that region instead of relying on a mix.
Better white light and skin tones
White produced by mixing a few narrow-band emitters can have gaps in its spectrum. Gaps can make skin tones, fabrics, and scenery look off-color under the light. Adding lime fills in part of the green-yellow range, which can improve color rendering when the fixture mixes a white. The effect is most noticeable on performers and on materials with warm or yellow-green tones.
By its very definition, naturally occurring "white" light is the presence of all wavelengths of light. The more wavelengths that are added into the source of LED-produced "white" light, the better it replicates natural sources such as the sun for color rendering, and the more natural colors look to the eye.
Output in the green-yellow range
Green LEDs are generally less efficient than other LED colors, and lime emitters are often built differently. As a result, a fixture with lime can often produce more usable light in the green-yellow range than one that has to mix it from red and green.
The wavelength of lime LEDs is typically in the 550nm - 567nm range, depending on the specific lime chip used. The peak of the human photoptic curve, the wavelength at which our eyes are most sensitive, is around 555nm. Adding lime can therefore also boost the apparent output of a fixture when it is in use. This is the same reason many fire trucks and ambulances use lime paint stripes on the back of their vehicles to boost visibility.
More even pastels and mid-tones
Adding lime also tends to subtract saturation from primary colors. For instance, if a Blue LED is at full output and you start dialing in the Lime LED, you will notice an immediate drop in saturation. Doing this allows you to create pastel blues and even color correction (CTB) tints easily without much of a shift in hue.
To make a pastel version of a hue:
Set the hue with RGB.
Dial in Lime to get a more pastel version of that hue.
What to expect in use
Extra control channel
An additional emitter normally means an additional channel in DMX control.
Many fixtures offer several operating modes with different channel counts. In some modes, instead of giving you direct access to every color emitter in the fixture, the fixture presents color to you as CMY. The fixture then automatically translates that CMY value into the values the individual emitters need to output in order to reproduce the requested color.
Greater flexibility in premixed whites
Many fixtures containing a lime LED chip give you a White CCT (correlated color temperature) channel that premixes white for you, typically in the range of around 2400k - 8500k. Some fixtures are able to extend that range with other emitters such as cyan or mint.
Not a replacement for other emitters
Lime complements red, green, blue, and other emitters. It does not widen the range of saturated reds or blues.