Although the earlier logs regarding phosphors suggest that fluorescent and LED sources can be manipulated from relatively monochromatic emitters of photons into producers of white light, the quality of that white light must be quantified given the distinctive construction of its constituent color spectrum. Lacking the relatively smooth and continuous energy band of wavelengths associated with incandescent sources results in a performance difference as it relates to color rendering. A metric for this performance is the Color Rendering Index or CRI and ranges from 0 for a monochromatic source to 100 for incandescent sources (with typical values for LED and fluorescent light sources ranging from 80-90)¹. The efficacy of a light source can be significantly benefited by limiting its energy output to a finite number of wavelengths, allowing the light to appear white while only generating a fraction of the wavelengths associated with the incandescent spectrum. However, the capacity of such a light to produce acceptable color rendering is a trade-off which has to also be taken into consideration.

As documented in the first project log about color temperature, the color temperature of the light source defines the peak of the energy curve of the associated light spectrum. Lower color temperature light sources accentuate lower energy light wavelengths like reds and yellows (such as the light from an incandescent filament bulb). Higher color temperature ranges transition from white to blues with 5000 Kelvin to 6500 Kelvin range sources being most closely associated with balanced white to slightly blue daylight. Note that in these cases white is meant to suggest a balance of light energy as perceived for each wavelength. Lower temperatures (2000-3000 K) are typically described as warm, temperatures around 5000 Kelvin are labeled as daylight, and even higher temperatures (≥6500 K) are labeled as cool.

These two metrics are important to consider in unison since CRI only measures the conformance of a source’s color rendering performance to that of an incandescent source of the same color temperature. However, the different color temperatures have naturally different impacts upon the way colors appear to the human eye. The example obvious to most would be the warm light visible when the sun is at the horizon (typically rated at 2300 K) versus the sun in the noon sky (5000 K). The strong presence of red wavelengths emanating from a 2300 Kelvin source would suggest that a white (or any other) object would naturally appear more reddish regardless of the rated CRI. The CRI can identify the possibility of unintended color shifts for any given source at a specified color temperature when compared to a source of a different type at the same rated color temperature. However, the CRI does not preclude the presence of color shifts which are a function of a given color temperature. At the same time, personal preferences and historically established expectations may ensure demand for lighting options at every color temperature range.

If CRI is a primary distinction to be overcome between efficient lighting sources and the incandescent sources they replace what other drivers exist to maximize color performance? Well that will be a great subject to tackle in the next project log!

¹ Color Rendering Index (CRI) Explained.” Full Spectrum Solutions, Full Spectrum Solutions, Inc., 2 Aug. 2017, www.fullspectrumsolutions.com/cri_explained.htm.