ANSI works to update the solid-state lighting standard for chromaticity (MAGAZINE) (UPDATED)

Industry trends such as warmer LED-based products below 2700K and the demand for tighter color uniformity are driving continuous improvements in the ANSI SSL chromaticity standard, explains JIANZHONG JIAO.

Work started on C78.377 in 2006 and ANSI published the first version in 2008. Similar to the chromaticity specifications for linear fluorescent lamps (FLs) and compact fluorescent lamps (CFLs), the intended purpose for this standard is to communicate between makers and users how indoor white color lighting is categorized, named, and color variation tolerances are defined.

LED versus FL sources

The primary difference in LED lighting, however, has always been that the light sources used for a lamp or luminaire are discrete LED packages, or LED dies, and many are often used in one lighting product. When developing the SSL chromaticity standards, lighting industry experts focused on how the color points of final products should be defined, how color variation tolerances should be specified, and what would be the optimal requirements to balance the consumers' acceptance relative to LED manufacturing costs.

LED dies are produced with distributions of color variations and spread on the chromaticity coordinates, or planes, in a continuous fashion. By changing the phosphor contents or compounds, LED packages can produce practically any color point in a white space.

Related article: DOE reports that LED PAR38s deliver superior lumen and color maintenance

LED chromaticity is fundamentally different from FLs where a target color point is chosen and the color variation is in a circular or elliptical fashion around the color point. At the time that FL color specifications were developed, targeted color points were not directly linked to consumer preferences but chosen by the manufacturers based on the technology and cost limits. In summary, the differences in chromaticity between LEDs and FLs are: 1) LED color points can be rather flexibly chosen; and 2) LED color variations can be continuous with a distribution between the chosen color points.

Flexible white point

Color point flexibility is a benefit of LED lighting. Ideally, if the industry could determine the preferred white color points for consumers, standards committees could put all these points in the specifications as the target points. Unfortunately, color preference is a subjective matter highly related to familiarity, cultural background, and other human factors. Based on human factor studies, there is currently no consensus on what should be the targeted center points or "white curve."

ANSI lighting experts have agreed that in the lower CCT range, most people prefer white color that is centered on or close to the black-body locus. In the higher CCT range, the preferred white color is close to illuminant D65, standard daylight with an approximate CCT of 6500K. This series of illuminants tries to portray standard open-air illumination conditions in different parts of the world.

Based on this agreement, it seems logical that connections of color center points should be a curved line connecting the black-body locus from 2700K to the daylight point at 6500K. Until there is sufficient evidence of universal color preference, ANSI's approach for defining a white curve has been the most logical. Recent studies published by the Lighting Research Center (LRC) of Rensselaer Polytechnic Institute and the National Institute of Standard Technology do not affect their definition because of the multiple resulting conclusions.

Once the white curve was defined, ANSI needed to choose the targeted CCTs. For fluorescent technologies, as mentioned earlier, target CCTs were not chosen for consumers' preferences but for the existing technology and cost limits. It is logical to utilize color flexibilities for LEDs and define nominal CCTs.

ANSI nominal CCTs

With a given CCT range for indoor lighting, 2700K to 6500K, ANSI specified two types of nominal CCTs - fixed CCTs and flexible CCTs. The white color space between 2700K and 6500K are divided into eight regions, each given a CCT tolerance. The eight regions' center points are the fixed nominal CCTs. In addition, if a manufacturer or specifier chooses a color center point other than these fixed nominal CCT points, ANSI specified flexible CCTs with 100K increment change within the white color range. This approach provides two advantages: 1) It gives the producers and users more flexibility to choose CCTs; and 2) it allows LED makers to use wide LED die or package distributions without sacrificing the product yields.

Related article: Metrics for solid-state lighting

Another element of the chromaticity specification is the color variation tolerance. The human eye is very sensitive to color variation. Statistically, from a defined color point, the standard deviation is described by MacAdam ellipses as the "steps." If a large sample of the population were used (which it wasn't), and a trained observer could reliably repeat his or her observation (which they can't), then the steps would translate to probabilities for the general population as three steps equal to three standard deviations, or 99.44%.

The lighting industry generally accepts and practices the approach to use MacAdam ellipses to gauge color variations. For four steps or more, it is assumed consumers are able to notice the color difference. Based on FLs' color variation tolerances, ANSI decided to use the same seven-step MacAdam ellipse as the tolerance for SSL products. With seven-step MacAdam ellipse and CCT variations, the ANSI standard gives a new but more practical tolerance type, namely quadrangles. For each nominal CCT, either fixed or flexible, it has an associated quadrangle (box) for the color variation tolerance.

As a result, the SSL standard has a continuous white color space rather than the discrete ellipses used for FLs. It provides much higher flexibilities to LED lighting producers and users, it has a logically determined white curve as the color center points, and it has seven-step MacAdam ellipse equivalent quadrangles for color variation tolerances.

Chromaticity standard updates

Three years into practice, ANSI decided to make improvements for this standard and published the newer version ANSI_ANSLG C78.377-2011. The 2011 version provides more accurate color center points by adding an additional decimal. Thus the color boundaries (upper and lower limits) became continuous curves without zigzag boundaries. That helps LED suppliers deliver LEDs with more precise binning. The updated version also provides detailed descriptions for the calculations of chromaticity values between corner points of each quadrangle.

In 2013, ANSI formed an ad hoc group again to update the standards. Two initiatives were put in the revision agenda. The first was to extend the CCT range to lower than 2700K. LED lamps or luminaires have recently provided lower than 2700K CCT values. There is a need for recognizing these lower-CCT SSL products and ANSI wants to standardize how they should be specified so that it is consistent between producers and users. With this intent, the ad hoc group proposed to add two seven-step MacAdam ellipse equivalent quadrangles centered at 2500K and 2200K. The second initiative came from specifiers, like the California Energy Commission (CEC) to have smaller color variation in an attempt to improve LED lighting product quality.

Color preference is a subjective matter for choosing preferred CCT values as well as a point along the Judd line for a given CCT value, above or below black-body locus (e.g., Duv value). Conversely, the color consistency is rather an objective matter. A larger than four-step MacAdam ellipse variation can be noticeable for most of the population.

The challenge is determining what color points in the current white color space should be used as the tighter tolerance center points. For simplicity, the ad hoc group recommends using the current nominal CCT center points to add a four-step tolerance. This includes both fixed CCTs and flexible CCTs.

Within the existing seven-step quadrangles, four-step quadrangles are specified using the same center point. There is no international standard for MacAdam ellipse calculations, which has created difficulty writing program codes for color measurement equipment. ANSI decided to adopt the approach published by CIE, TN 001 ("Chromaticity Difference Specification for Light Sources"). Instead of using ellipses, C78.377 provides a four-step description for CIE circles on a u'v' plane. For convenience, the standards also provide the conversion calculations for mapping four-step circles from u'v' plane circles to ellipses on an xy plane.

For international standard harmonization, the new version of ANSI lists the comparison of nominal CCTs with tolerances (quadrangles) and three-, five-, and seven-step MacAdam ellipse LED lamps in IEC. Readers will find ANSI standards are well harmonized with IEC standards. With the principle of continuous improvement, the ANSI SSL chromaticity standard has paved the road for fast adoption of SSL products. The novelty, flexibility, accuracy, consistency, and practicality are reflected in developing and updating C78.377. ANSI will continue to improve the standard. In particular, white preference will be further studied, and a specification for where the white color center points should be targeted below black-body locus or above will be developed.

DR. JIANZHONG JIAO, director of regulations and emerging technologies at OSRAM Opto Semiconductors, Inc., is an internationally recognized lighting expert. He has been actively involved in LED and SSL standard development activities. He serves as the past chairman of the SAE Lighting Committee, past chairman of NGLIA, past chairman of the NEMA SSL Technical Committee, active member of IESNA Testing Procedure Committee, Roadway Lighting Committee, and Computer Committee, ANSI SSL Working Groups, Standard Technical Panel of UL8750, standard committees in IEEE, CIE USA, SEMI, JEDEC and other organizations. He can be reached at [email protected] (osram-os.com).