Few topics have been more confusing than color rendering, and with the rapid advancement of power LEDs in lighting applications, it’s clear that there’s much work to be done and that the specification community should take care in approaching the subject.
In a technical report (CIE 177:2007), the International Commission on Illumination (CIE) concluded that the Color Rendering Index (CRI) it established in 1974 is not applicable in determining the color rendering capabilities of modern white LED light sources. This development has significant implications for the outdoor lighting industry, which has been attempting to hold LEDs to the same CRI standard as other light sources. The CIE bulletin recognizes that, while no other standard yet exists, using the existing Color Rendering Index to compare light emitting diodes to incandescent lamps makes for a proverbial apples-to-oranges comparison, and more work is needed to develop a new index or indices to arrive at a more accurate standard.
For a technical explanation of CRI the CIE Publication 13.3 provides full explanation on how it is calculated. Simply put, it is method to compare how different light sources with the same correlated color temperature reproduce eight pastel colors to a reference source, which is a black body source for below 5000K and an agreed upon daylight source for above. Theoretically, the higher the CRI rating the better.
However, in practical applications, according to the CIE, “CRI is generally not applicable to predict the color rendering rank order of a set of light sources when white LED light sources are involved in this set,” so while CRI can be useful in comparing the color rendering properties of conventional light sources, “the applicability of the current CIE color rendering index (CIE,
1995) is limited if applied to white LED light sources. In fact, it is possible, for example, for higher CRI conventional solutions to be visually ranked below a lower CRI white LED solution.” (CIE Technical Report Color Rendering of White LED Light Sources, CIE 177:2007.)
The advent of LEDs as a practical light source has thrown the CRI standard a curve. New research indicates that even LEDs with a low CRI can produce light that is visually appealing and that the premise that high CRI provides a better visual experience may not be true in all cases.
The Outdoor Lighting Environment
In typical outdoor lighting applications, the key objectives are safety and security. Municipal street lighting and parking lot lights are primarily intended to enhance visual acuity – to help people navigate otherwise dark areas, to feel safe as they transit, or to deter would-be vandals by illuminating their potential target areas.
The use of artificial light to mimic daylight or accentuate specific colors in outdoor pedestrian, parking and roadway applications is not usually the highest priority and, in fact, color temperatures closer to moonlight (~4100K) are generally preferred to daylight (6500K). Accurate color rendering is historically a secondary consideration and anyone who has experienced the amber glow of low pressure sodium lamps knows that color rendering in that location is not a priority.
Safety, security and cost objectives have led to the popularity of highpressure sodium, metal halide, and low/high pressure sodium lamps, which provide visual acuity at an affordable price, but in some cases are just about as low on the CRI scale as a lamp can get. Drivers may be able to find their own vehicles in a parking lot illuminated with high-pressure sodium lamps, but they won’t have identified it by color differentiation.
Despite the inconsistencies in CRI Measurement and actual visual experience when white LEDs are considered, urban planners, municipalities and utilities continue to specify high CRI for white LED based outdoor applications, possibly without full appreciation of potential effect of doing so. Without a visual understanding of differences in CRI in the intended application, it’s likely that cost effective and energy efficient solutions preferred by the ultimate end-users could be dismissed without due consideration.
Clearly, the CIE has recognized that while CRI is an important metric, it is not the most appropriate comparative standard in all cases, and this is especially true in outdoor applications.
Finding the appropriate balance between end-user applications Needs, ownership costs, and sustainability clearly requires additional work by all affected parties.
The U.S. Department of Energy (U.S. DOE) concurs, specifically recommending that:
If color appearance is more important than color fidelity, white LEDs should not be excluded purely on the basis of their low CRI – some with CRIs as low as 25 can still emit visually appealing white light.
LED systems – and all light sources for that matter – should be evaluated in person and on-site when color fidelity or color appearance are important.
CRI may be compared only for light sources of equal CCT – for any variety of light sources, not just LED – and that differences in CRI of less Than five points are insignificant.
If color fidelity is critically important to the visual tasks to be performed, CRI values may be a useful metric for rating LED products.
At present, outdoor lighting specifications created by the U.S. DOE for its ENERGY STAR program do not reference CRI.
In lieu of a more appropriate system, the CIE CRI remains the only internationally accepted standard to gauge the color rendering capabilities of artificial light sources.
While work continues to develop new and more appropriate measures, lighting designers, specification organizations and engineers should consider all the properties of prospective light sources before determining project specs – and there remains no substitute for a visual inspection.