When choosing an LED based product for illuminating people and objects, there are many factors one must consider. Output levels must balance or exceed other lighting and video elements, while the color quality of the white light is equally important. Prior to LED based fixtures, the color spectrum and color rendering were not a concern as incandescent and arc sourced fixtures tended to rate very well.
With the introduction of LED based fixtures, the color spectrum of the source became an important factor in contributing to the quality of light output from a fixture. Early LED fixtures tended to have a spectral distribution with spikes of color in certain wavelengths. This resulted in poor white and color qualities as the source was missing key colors of light. A fixture that has a poor spectrum will not only render colors poorly when projecting white light, but also will result in unsatisfactory color mixing colors.
The industry has matured, and we now see many LED fixtures with “full spectrum” sources that provide a good range of color through the visual spectrum of light. For instance, the Martin MAC Encore fixtures contain wavelengths throughout the spectrum without any gaps. This results in a superior white output and great color mixing abilities.
The white output of a fixture is generally scored in terms of color quality through a color rendering scale. Color rendering scores can help designers determine how well colors (sets, costumes, skin tones, etc.) will look when illuminated by a particular white light. These scales are important as they describe the quality of the light and how colors will appear when illuminated by a specific source.
Every color rendering system has the same goal: to describe how well a light source (or fixture) will render specific colors compared to a known source (typically pure daylight or pure tungsten). Usually, there is a defined set of colors and a score is given, based on how well each color looks when lit by the source compared to the reference. If a color on the chart does not look good under a specific source, then a lower score is given for that particular color. The measurements for all the chart’s colors are then averaged together to create an overall score (usually on a 0-100 scale). Note that a perfect score is only possible with the original source (pure daylight or pure tungsten).
Because it can be very difficult to actually look at a color reference chart and determine the rendering ability of a source, light meters are regularly used to measure color rendering. They will examine the light coming from a source and use complex mathematical formulas to calculate the color rendering score based on the source’s color temperature and spectrum.
CRI or Color Rendering Index is currently the most commonly provided metric for color rendering, which is a shame as it is also the poorest. Basically, it consists of eight* colors that are given a 0-100 score and simply averaged together. The eight colors primarily consist of skin and nature tones, but not any saturated hues. Because these eight values are simply averaged together, it is possible to get skewed results.
For instance, a source that scores high on seven of the values, but very low on one could still result in a high CRI value. Furthermore, manufacturers can adjust the CRI score simply by filtering certain wavelengths to get to a better average score (although not necessarily better light output).
*The full CRI specification consists of 14 colors, but typically only the first eight are used for calculation.
The Illuminating Engineering Society (IES) created the TM-30 metric to replace CRI. This scale uses 99 color references, based on real world color objects and again uses the root-mean-square calculation to determine the final score. This is known as the TM-30 Fidelity index (Rf). This standard also provides a Gamut index (Rg) which scores the saturation of the colors as over or under saturated. An Rg score of 100 means a perfect match, with greater than 100 being over saturated and below 100 being under saturated. Furthermore, TM-30 reports provide color vector graphics to visibly display which wavelengths are over or under saturated.
The Television Lighting Consistency Index is specifically designed to represent how colors will render on camera as opposed to the human eye like CRI or TM-30. Using 18 colors and the root-mean-square calculation method, TLCI results also provide data to help make appropriate corrections to the video image.
Judging the Quality
With so many different color rendering metrics to choose from, it can be confusing to determine how well a specific light will render colors. It is important when comparing fixtures to understand how scores are calculated and what they are based upon. CRI is the worst choice, while CQS and TM-30 are much better references. TLCI should always be used for television. Further detailed descriptions of each of these color rendering methods can be found on Mike Wood’s Consulting website.
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