John Selverian, research & development scientist, Solid State Lighting at Osram Sylvania, will present a technical presentation entitled “Color in Light: Understanding how spectrum, CRI, and color temperature relate to what the eye and camera sees” at LDI2010 on Saturday, October 23 from 1:30-3:00pm in meeting room N117 (free to all LDI attendees). Live Design gets an update from Selverian on the state of LEDs and color:

1) Please tell us a little about your background/training and how you became a "color in light" specialist?

I have been involved with various aspects of lighting for over 20 years and have always been interested in using computers and simulations to speed-up the design and development process. My general interests in lighting and computers led me to this area, and I saw the need for a sophisticated design tool to aid in the design of color mixing solutions.

2) How has LED lighting evolved in terms of color rendering in recent years?

For many years the lighting field was dominated by stable solutions and the area of color rendering for these technologies was more or less fully developed. Thanks to the ability of LEDs to generate very saturated colors, the study of color rendering for these new light sources needed to be revisited, as evidenced by the CIE's effort to find a new metric to replace CRI for LEDs. For instance, a LED can render blues much better than a halogen, which has a CRI of 100.

The first generation of LED lights typically had very poor color rendering, they typically consisted of a blue LED exciting a single yellow phosphor. With passing time new phosphors have been developed and new strategies such as the addition of red LEDs to a greenish-white LED for high CRI warm white. It is not uncommon for LED phosphors to consist of a blend of two or three individual phosphors to achieve the proper spectrum.

3) How does LED lighting compare to tungsten in terms of household lighting? Theatrical lighting? Will lighting designers be able to mix LED colors to match existing gel colors?

LEDs for household lighting are just starting to appear in the marketplace. Currently there are very few compelling products and most of these come at a high price. Aside from the price, the main drawback is the total lumens. Costs should come down significantly in the next year or two but total lumens will take a bit longer, due to the high heat load that needs to be dissipated.

Theatrical lighting on the other hand has different, distinct needs—most importantly, the true pigments in the scenic and costume elements of the production must be displayed accurately. Nothing upsets a costume designer more than seeing the orange fabric he or she painstakingly picked out turn blood red throughout the production due to the lighting. In order to achieve this with LEDs, you will need a solution with a full color spectrum, not just peaks in the RGB ranges. The ability to achieve these full spectrums lie in the phosphors used on (or in the case of remote phosphor solutions—above) the chip.

For lighting designers, I think LEDs will be able to mimic the colors you get from current theatrical gels with halogen light sources for one very important reason—when you dim a halogen lamp, the spectrum changes and the lamp puts out more wavelengths in the range of the ambers and reds, causing the gel color to shift. So if you have two ellipsoidal fixtures next to each other, with the same lamp and gel in them, and one is at full intensity and the other is at 50 percent, the color that comes out will be different. Since LEDs do not shift nearly as much when dimmed, they will remain true once you have that desired color programmed in, even when you lower the output. The Color Calculator program is a tool that can help you find that correct mix by matching that final result through additive mixing using the correct combination.

4) What does color stability mean?

To me there are two types of color stability—the first being how much does the spectrum of the light source vary over time; secondly, how much does the spectrum vary from fixture to fixture of the same product. There are many contributions to color stability. The most significant factors, like material degradation (e.g. reflectors, lens, filters, filaments) apply to all lighting technologies. Thermal effects are also common across many technologies but can be especially problematic for LEDs. The issue of wavelength binning is very significant and is unique to LEDs. This is a significant problem when trying to manufacture a large number of identical fixtures. LEDs suppliers will not sell a single wavelength of a color, for instance if you want to buy a blue LED with a wavelength of 450nm, LEDs manufactures will only sell you LEDs with a wavelength range of 445-455nm. If you design a light source based on a 450nm wavelength you will get a different spectrum with a 445nm or a 455nm wavelength. For very demanding applications there is currently no completely satisfactory way around this.

5) How does the "Color Calculator" work?

Initially, the Color Calculator was designed for internal use but we decided to release publicly to help spur the adoption of LEDs in the marketplace. Basically, the Color Calculator takes spectra, adds them together and calculates the photometry using standard equations. It can handle the effects of filters, perform various types of design optimization, make several types of plots and can do a type of calculation I call "combinations.” The user selects some LEDs and every possible combination is calculated; it's a brute force approach but very informative and, in many instances better than a standard optimization.

6) What is next in terms of LEDs and color?

The industry will begin to adopt standards and costs will come down significantly. In terms of white light, color points will continue to vary across the world but will trend towards warmer in NAFTA and cooler in Europe and Asia, as with traditional light sources. Color tunabiblity was initially a big selling point of LEDs but will be much less important for the non-entertainment market. I am personally very excited by the possibility for the economical production of highly customized color solutions - this will be especially important for the smaller markets which have traditionally been ignored and had to make due with a comprised solution.

Related article:

Osram Sylvania's Color In Light at LDI2010