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In The Lime Light

Posted on December 1, 2015

Screen shot 2015-11-24 at 10.51.15 PMThe apple of our eyes is lime. Look at a photopic curve and you’ll see that the color the human eye is most sensitive too is a band of lime-green. Ford Sellers, senior product manager of CHAUVET Professional explains how this principle was put to work in the new Ovation E-910FC to create an ellipsoidal that offers superior color rendering and brighter whites.

From a technical standpoint what is a lime LED?
“The Lime LED is an LED like any other. From a technical standpoint it works exactly the same way as any other LED that you’ll find in an entertainment lighting product. Energy is released by the diode, which excites a phosphor, which produces visible light. What’s special about the Lime LED is the recipe of the phosphor and the spectra that it produces. The Lime LED was originally developed by Lumileds as a part of the development of a LED lamp to replace the traditional incandescent 60w light bulb.”

Can you explain the idea behind this innovation?
“Sure, the idea is really quite simple — produce light in the area of the spectrum that our eyes are most sensitive to, then use a few red and blue LEDs to shift the overall color back to white. The area of color which our eye is most sensitive to is shown in the Photopic curve. As it happens the human eye is most sensitive to a band of lime-green light. Using this color in a lighting fixture has several benefits, including increasing apparent brightness and color rendition.”

Why is it important in the Ovation E-910FC?
“Any LED color mixing fixture struggles with the same engineering challenge: getting the best and widest possible range of colors from colored LEDs (narrow band emitters). This task is compounded when you are trying also to increase color rendering. When choosing colors for mixing, you need to take into account several factors among the most important of which are color saturation, and color rendition.”

Can you elaborate? What do you mean by color saturation?
“If you look at a CIE chart, it appears to be a parabola full of color. If you plot the colors of your LEDs (put a dot at the X and Y coordinates in this chart), you can mix any color which lands within the polygon defined by these points. But there’s a catch — the brightness of the light is a factor of the distance from the color point. So, if you want to be able to generate the widest possible palette of color, you will want to choose colors as far as possible toward the outside of the CIE chart. However, when mixing colors toward the center of the chart (pastels and whites) the intensity will be low. The challenge is to find a balance between saturation and brightness.”

How about color rendition?
“When someone is looking at an object (a painting, for instance) the colors that they see are not being generated by the paint in the picture; the color is being reflected by the paint. For instance if you are looking at Van Gogh’s Starry Night, you are seeing a lot of blues. This isn’t because the painting is making blue light, but because the paint is actually absorbing all of the colors except blue and reflecting the blue light back at your eye. Imagine if the light that you were viewing the painting under did not have blue light in it! There would be very little reflected light and the painting would look mostly dull-black. The issue with color-phosphor based lighting (like LED) is that the colors generated by the LEDs are very specific. Typically you see a spike around a certain nm of Red, a spike of Green, a spike of Blue, etc.

“In early LED lighting there were big gaps in the color spectrum generated by lighting fixtures. Also the light produced did not align very well with the Photopic curve. Some LEDs have wider spectral distribution. The Lime and Amber LEDs in the Ovation E-910FC fill in a large gaps in that chart, while also delivering a lot of punch in the wavelength of color that our eye is most sensitive to.

“So the bottom line is that the addition of the Lime LED to our RGBA color system, allows us to use LEDs closer to the outer edges of the CIE chart to mix a broad spectrum of colors without sacrificing intensity — and while increasing color rendition significantly.”

Can you explain how the addition of Lime results in better whites?
“If you look at a CIE chart, you’ll see a black line that runs through the center of the chart. This line, which is called the Black Body Radiator, corresponds to the color temperatures of white light. There is an LED color called Mint which lands closer to the black body radiator than Lime. However, we chose to go with Lime, because aside from assisting in mixing shades of white, Lime also aids in increasing the brightness and punch of more saturate colors.

“By using the Lime LED, we are able to produce light along the black body radiator with greater intensity and an overall wider spectrum than was previously possible with color mixing LED sources. This means that the colors of white light that we can produce have a high CRI (Color Rendering Index), which is a measure of the ability of light to portray colors properly. The end result of this is that the Lime LED allows us to make brighter, better whites with great CRI.”

From a practical standpoint what difference does this make to designers?
“The color selection in this fixture was specifically engineered with lighting, scenic and costume designers in mind. Each of these designers is an artist, and each carefully selects the design colors that will best communicate the message of a play, concert, or other project.  Being able to offer the widest selection of colors, with the most intensity, gives these designers a wider range of creative options.

“For instance, a production may have a color palette (in lighting, scenery, and costumes) which is full of pastels. With the Lime and Amber LEDs in the Ovation E-910FC, the designer is able to produce colors which will work very well with painted drops, scenery, and costumes with these colors in them. Likewise, if you are designing a show which uses a lot of saturated deep reds, and blues, the saturate colors of the red, green, and blue LEDs in this fixture allow you to really make your designs pop with color.”

Can you tell us about the fixture’s HD lens?
“When we were designing this fixture, one of the major challenges centered around blending the color from all 91 of the LEDs on our PCB to generate one, even field of color and intensity. We found that using a standard lens tube yielded good results, which would satisfy the majority of our customers — but with the Ovation line, we are developing products for the most selective customers. So, we went back to the drawing board to make a new lens, which would offer more output, and cleaner gobo projections and would be compatible with both the E-910FC, and the E-190WW. In addition to improving the brightness and clarity of projections, we added a couple of “ease of use” features. For example, each Lens Tube now includes a color coding which is visible from the ground (to help identify fixture type from a distance), and the color-coded section incorporates the standard drafting symbol for each beam angle.”

What other features stand out about the Ovation E-910FC?
“We designed the Ovation E-910FC to be a no-compromise fixture. In addition to the dimming that has made Ovation a stand-out among other theatrical fixtures, we offer full 16-bit control of all five colors, selectable PWM (for use with high speed cameras,), and we’ve added RDM support to all of our new fixtures in the Ovation line.

“Since color mixing with a 5-color system can be challenging, we complemented the full individual control in the Ovation E-910FC with a virtual color wheel which matches the color from the E910-FC to a fixture using an HPL lamp with the corresponding Rosco Gel color. We also print these “color recipes,” which are built around our specific selection of LEDs in our manuals. This can be a huge time save for designers and programmers, as building custom color palettes is one of the most time consuming aspects of designing with modern LED fixtures.

“In addition, to best take advantage of the Lime LED when producing white light we added a channel that can pre-mix white light with correlated color temperatures of 2800° k all the way up to 6500°k. The colors have been perfectly blended to offer the highest color rendition and land directly on the black body radiator. This allows you to use the fixture as a variable white unit (for television and film for instance) in addition to using it for its great color projection.

“Finally, we were also able to keep the size and weight of this unit very similar to a traditional ellipsoidal, making it easy to swap into a light plot. We spent a long time developing this unit to be user friendly. It was really created with the end user in-mind.”