Methods of Achieving High CRI with LEDs

The proper use of light is both a science and an art form. Lighting can be mere illumination or can create an effect, but new LED technologies can help to make dull spaces come alive. Everyone's color vision is different, therefore interpreting color can be very subjective, but specifications can provide a basis for objective comparison.

One major specification used in the lighting industry is the Color Rendering Index (CRI). "Color rendering" describes how an object appears to the human eye based upon an ideal or natural light source. An artist showing their paintings would most likely want their work lit so that rich, vibrant colors are noticed. A discussion using attributes of CRI can quickly get artist and gallery manager "on the same page" regarding lighting.

CRI is used to compare how "true" objects are rendered. Presently, this is the only recognized measure of color rendering in the lighting industry since being introduced in the early 1960s. Light sources cause subtle variations in how colors are rendered. CRI is rated on a scale from 0 to 100. Lighting with a CRI of 85 to 90 is typically "very good" at rendering the color of objects while light sources with a CRI of 90 or greater are "excellent" and are used for tasks that require very accurate color discrimination. With respect to LEDs, typically the lower the CRI value, the higher the luminous efficacy (lumens per watt.)

Figure 1

CRI is also an indicator of how "natural" an object's color appears when illuminated (Figure 1.) These are the highest achievable CRI values for common light sources. The lighting community tends to use CRI as an indicator of quality, or preference, even though it was not necessarily intended for this purpose. Light sources with dramatically different spectral power distributions can have identical color points yet render colors very differently.

At the centerpiece of the LED is a highly efficient semiconductor chip, which is affixed to a lead frame that has an Anode (positive side) and Cathode (negative side.) All are assembled in a thermally optimized IC-type package. Phosphor is either coated over the die, placed on top as a layer, or mixed within and encapsulated to protect the diode.

Several phosphors are used; the OSLON SSL LED family, for example, is available in four different phosphor blends based on the target application. Outdoor lighting may require cool or neutral color temperatures (4000-6500K) with a CRI of 70. For general interior lighting, a CRI of 82 may be sufficient for office related tasks. Retail, museum, and other color-critical applications may require yet another phosphor blend for high CRI requirements.

Producing white light with LED sources is a challenging task. Two methods are commonly used to create white light with LEDs. The first approach is through the combination of multiple colors, usually red, green, and blue, in multi-chip packages or LED clusters. This is the same principle used in backlighting LED LCD TVs: mixing the three colors in various proportions results in an entire spectrum of colors.

The second approach is to combine a semiconductor chip (blue or UV) with converter materials (phosphors) through luminescence conversion within a single package. A third approach involves an innovative combination of the best of both methods, called the "Brilliant Mix" Concept.

A phosphor-based high CRI approach is a method that includes an additional stable red phosphor. The latest developments using enhanced phosphors increase the CRI value to 95, but the trade-off is lower efficacy. LEDs are particularly efficient when color coordinates are closer to blue light, and the phosphor does not have to shift the blue coordinates quite so far. To achieve a warm white light, several luminescent substances must be combined, although this reduces the LED's efficacy. There is a limitation; with standard LEDs, it is not possible to realize LED luminaires and lamps with both high efficacies and high CRI.

However, with OSRAM's "Brilliant Mix" concept, both high CRI and high luminaire efficiencies can be effectively realized. This approach fuses the two existing methods — mixed color from monochromatic LEDs and phosphor converted LEDs — to create a "warm-white" LED light source with both high CRI (> 90) and high luminous efficacy. This new approach makes total luminous efficacies of over 110 lumens per watt (lm/W) possible, producing up to up to 30 percent more light than phosphor-converted warm-white LEDs with a comparable CRI, but with similar power consumption.

Brilliant Mix is the mixing of two colors in order to generate warm white light. This approach uses red or amber monochromatic LEDs, combined with special white LEDs which have been shifted into the green range (called "EQ-White"). EQ-White is produced like a "normal converted white LED" with a blue chip and green phosphor. The green phosphor has a very low conversion loss rate and makes for a very efficient light source in combination with the blue chip.

Advantages offered by the Brilliant Mix Concept include high CRI, very high LED efficacies (> 110 lm/W possible), and warm white coverage from 2,700 K to 4,000 K.

Figure 2

Luminous efficacies of the two light sources are maintained (Figure 2), thus the new approach makes for a high luminous efficacy. The red LED is combined with EQ white (a highly efficient source with majority of its energy in within the green region of the visible spectrum, where the eye is most sensitive.)

When these two sources combine in the correct ratio, we get a pleasant, warm white light with a CRI over 90, giving us the ability to identify differences in the shades of the purple, blue and pink flowers by increasing our color discrimination, or how "vivid" and easily distinguishable colors appear. The average CRI of "Brilliant Mix" is very high (Ra>90). Additionally, the highly compact OSLON SSL LED is the vehicle for both phosphor-based and Brilliant Mix solutions. The standard phosphor conversion method produces reduced efficacy, but implementation is easier. Brilliant Mix enables higher efficacy but is more complex to implement.

In the future, lighting could dynamically adjust for any lighting condition, accurately mimic lighting conditions outside, and be able to provide a fully customizable lighting environment for individuals. OSRAM's vision is to make this a reality. Research and development continues to improve color rendering and efficacy. OSRAM Opto Semiconductors leads the way in developing LEDs with superior, high color rendering for every application.

Marc Dyble

Marc Dyble is a Product Marketing Manager in the Solid State Lighting (SSL) group with OSRAM Opto Semiconductors, Inc., focused on the growth and development of architectural and outdoor lighting applications. Mr. Dyble is responsible for both product and marketing strategies for the North American SSL market. Prior to accepting his current position he was an Applications Engineer in the general illumination segment, supporting design development and customer applications.

Prior to joining OSRAM in 2006 Mr. Dyble held various positions in lighting engineering development with several architectural and entertainment lighting manufacturers, including Vari-Lite and Lighting Services Inc.

Mr. Dyble holds a Bachelor of Science Degree in Electrical Engineering from Southern Methodist University in Dallas, TX and a Masters of Science Degree in Lighting, specializing in Solid State Lighting, from the Lighting Research Center at Rensselaer Polytechnic Institute. Mr. Dyble also earned his lighting certification through the National Council on Qualifications for the Lighting Professions (NCQLP).