Light Color Quality

While intensity, distribution and color temperature are all lighting characteristics that are given primary consideration during retrofits and new installations, color quality is too often overlooked.  With the extended lifespan of LEDs (7-20 years), property owners should ensure their lighting system has proper color accuracy.

Improving the color quality of light has been shown to:


• Increase productivity
• Enhance alertness
• Support health and well-being
• Improve emotional responses
• Encourage circadian system function

These benefits are multifaceted.  They can enable property owners and managers to create a human-centric work environment, which can improve employee health and productivity.  Retailers can achieve a more comfortable and color accurate shopping experience.  Art curators can ensure pieces are accurately displayed.  Caregivers can better enable patient recovery.


Assessing Color Quality in Light Sources


Two of the principal color characteristics of a light source are its color rendering ability (CRI), and its color appearance or CCT.

Color Rendering Index 


The CRI is a measure of how well a light source reveals the colors of an object compared to a reference light source (defined as having a CRI of 100) with the same colour temperature. The reference light source is either incandescent light or daylight, depending on the color temperature.  The original CRI measure is based on eight standardized test colors. A CRI of 70+ is generally required for most lighting applications.



Correlated Color Temperature


The correlated color temperature is expressed in degrees Kelvin (K˚) and represents the color of the light source to that of a black body radiator (ideal opaque object that absorbs all light and emits thermal radiation) heated to high temperatures. Lower temperatures below 3,000K are designated warm colors, while temperatures 5,000K+ are called cool colors.
 
Lighting characteristics are often dictated by setting, and the subjective nature of perceiving light quality makes it difficult to express color accuracy in any one metric.  While there is no universally applicable definition for quality light color, several measures are being considered by the broader illumination community.

Spectral Power Distribution


A spectral power distribution is a visual portrayal of the emitted power of a light source at visible wavelengths of the electromagnetic spectrum. The spectral composition of a light source determines its ability to ‘naturally’ render colors. [2]


Spectral distribution curves demonstrating the relative amounts of energy versus wavelength for the three most common sources of white light. [3]

Power distribution curves can compare the composition of different light sources, as their color properties are based on the wavelengths of light present. Daylight has an even power distribution across all wavelengths, while incandescent tungsten lighting has more red than blue light. Fluorescent lighting typically spikes with blue, green and yellow light but is low on red light.

Color Quality and Visual Acuity


Visual acuity is the sharpness of vision, and this resolving power is dependent on the physical size of the cone cells within the retina of the eye.  Sensitivity of the cone cells reduces in low light levels; therefore, acuity is better in bright light.  Acuity is also affected by pupil size, and a smaller pupil improves visual acuity by minimizing peripheral aberrations and by increasing the depth of focus. [4]

Many studies have shown that light with greater blue content (higher CCT) allows greater visual acuity compared to light with lower CCT [9].  In a study of school children, visual acuity was found to be significantly better with high CCT lighting, and the underlying mechanism was concluded to be the effect of spectral content on pupil size.  The dilation of the iris has been shown to be regulated primarily by exposure to blue light [10].  Higher illumination levels and higher color temperature also showed positive effects on concentration levels and reading fluency in a study of school children. [1]

Color vision sensitivity is directly correlated with illuminance level; thus observers can detect smaller color appearance differences at higher illuminance levels.  Greater consideration must be paid to color accuracy in brighter areas, and changes in color rendering can often be more important than changes in illuminance.  [11]

New Color Standards


While two light sources with the same CCT can look similar, deviations in color accuracy can create profoundly different effects on objects they illuminate.  Color metrics that attempt to measure a complex human perception with a single number are inherently limited, and so multiple metrics must be considered simultaneously.

Historical metrics such as the CRI do not fully describe how the human eye interprets color with LED lighting [12].  The growth of LED lighting and its unique spectral characteristics has led to increased demand for improved lighting metrics.  

One of the most promising measurement methods is TM-30-15.  The IES method for evaluating light source rendition creates three new measures to provide more information about color rendering with greater accuracy [13]:

1. Fidelity Index (Rf)


Used to measure the closeness of light color to a reference source - similar to CRI, but based on 99 color samples instead of just 8.  The color samples were chosen from real world objects and are categorized into 7 groups:  Nature, Skin Color, Textiles, Paints, Plastics, Printed Materials, and Color Systems.



2. Gamut Index (Rg)


This metric expresses the average color saturation (intensity) compared to a reference source, with a higher value meaning a greater saturation or intensity.  Color preference and discrimination are directly related to saturation, and increased saturation makes it easier to discriminate between colors.

3. Individual Color Distortion


Color vector and distortion graphics show exactly which colors are saturated or desaturated.


Color vector graphic showing hue and chroma shifts from a reference illumination source (black) and a comparison source (red). [15]

The color graphic above shows chroma shifts from a reference illumination source (black circle) to the source being compared (red line).  Wherever the red line extends beyond black, colors will appear more vivid and saturated.  Where the red line comes inside the boundaries of the reference source, colors will be faded and less saturated.  Intersection points represent hue shifts.

A significant advantage of TM-30-15 is the use of 99 color samples whose chromaticity are chosen from real-world objects.  TM-30-15 also offers sub-indexes that provide detailed information such as fidelity by hue, fidelity by sample, chroma shift by hue and skin fidelity.

Feeling of Contrast Index  


The FCI was developed to assess color rendering quality and incorporates a subjective color perception to calculate a quantifiable comparison value. The FCI can be used to assess color quality of all types of light sources and is particularly suited when objects are to be attractively presented and illuminated [16]. The FCI is determined by comparing the gamut (full spectrum range) of a test lamp with the gamut of a standard light source. Test measurements are carried out on a sample with four saturated color patches illuminated by the test lamp. [17]

Full-Spectrum Index


The FSI measures the deviation of a light source’s spectrum from a theoretical spectrum emitting equal power across all wavelengths. A light source emitting all visible wavelengths would have good color rendering ability, whereas sources that are deficient in parts of the spectrum would have lower FSI values.

The cumulative spectral power distribution for an equal energy spectrum (blue) compared to daylight at 5500 K. Source [19].

Color Quality Scale (CQS)


The CQS method, developed by the National Institute of Standards and Technology (NIST), addresses the limitations of CRI in describing the color quality of white light.  CQS uses a broader palette of 15 reference colors, compared to the traditional eight-colour scale [13].  The 15 colors are highly saturated and help to measure chromatic distinction and human preference.

Color Consistency


This is a measure of light quality for LED lighting and can be evaluated at several levels: over time, between products and batches, across light beams, and between color specifications and actual colors when in use.


Enlightenment


While no single metric can fully capture the multidimensional aspects of color rendering, the adoption of LEDs has brought much needed attention to the color accuracy of lighting systems.  Light color quality can lead to greater visual acuity and positively affect viewer perception to a greater extent than illuminance.  

The need for color accuracy has been well established, yet the measurement approaches remain varied.  Regardless, real estate owners would be wise to ensure color accuracy is a top priority for any LED upgrades they consider.


References

1. Shamsul, et al (2013). Effects of Light’s Colour Temperatures on Visual Comfort Level, Task Performances, and Alertness among Students. American Journal of Public Health Research, Vol. 1, No. 7, 159-165

2. NLPIP Lighting Answers. Rensselaer Polytechnic Institute, October 2004. http://www.lrc.rpi.edu/education/learning/terminology/spectralpowerdistribution.asp. Accessed 15/4/2016

3. Molecular Expressions TM Science, Optics & You Light and Colour. https://micro.magnet.fsu.edu/optics/lightandcolor/sources.html Accessed 04/5/2016

4. Physiology of visual acuity. http://www.acuity.de/physiology-of-visual-acuity/index.php. Accessed 12/4/2016

5. Altomonte, S. Lighting and Physiology: Artificial and natural lighting and its relation to human body. http://www.melbourne.vic.gov.au/.../Study3TechnicalPaper_updated.DOC. Accessed 04/4/2016

6. Boyce, P. et al. Lighting quality and office work: A field simulation study (PNNL-14506) 2003. http: //www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/b3214.1/B3214.1.pdf. Accessed 04/5/2016
7. Hameed, A. and Amjab, S. Impact of Office Design on Employees’ Productivity: A Case study of Banking Organizations of Abbottabad Pakistan. Journal of Public Affairs, Administration and Management, 3(1).2009.

8. Sanaz, A.S. The Influence of Light on Student’s Learning Performance in Learning Environments: A Knowledge Internalization Perspective. Journal of World Academy of Science, Engineering and Technology, 81. 2011.

9. Berman, et al. (2005). Children’s near acuity is better under high colour temperature lighting, CIE Midterm Meeting. http://www.naturalux.com/BermanNavvabChildrenNearAcuityStudy.pdf. Accessed 2/4/2016

10. Melatonin regulation spectral curve data from Action Spectrum for Melatonin Regulation in Humans (2001), Brainard, Hanifin, Greeson, Byrne, Glickman, Gerner & Rollag In http://www.illinoislighting.org/lightcolor.html. Accessed 11/4/2016

11. Papamichael et al (2015). High Color Rendering Can Enable Better Vision without Requiring More Power. LEUKOS, 12:27–38.

12. Seeing Beyond CRI: The Industry Faces a Colour Quality Challenge with the Advent of LEDs. http://www.usailighting.com/seeing-beyond-cri-the-industry-faces-a-colour-quality-challenge-with-the-advent-of-leds. Accessed 19/4/2016

13. Inside the new IES method for colour evaluation. http://www.lightnowblog.com/2015/09/inside-the-new-ies-method-for-color-evaluation/. Accessed 8/4/2016

14. Designing Light the companion web site to the book Designing with Light. The Art, Science, and Practice of Architectural Lighting Design. http://designinglight.com/?p=491#sthash.Qv2jxNMo.dpbs. Accessed 04/5/2016

15. Hutchinson, L. Will TM-30 Supplant CRI with Dual Metrics? http://www.lightshowwest.com/will-tm-30-supplant-cri-with-dual-metrics. Accessed 05/5/2016

16. Osram. Light is Energy. Outstanding quality of light and colour contrasts through optimized FCI. http://www.osram.com/media/resource/HIRES/642658/high-fci-shop-lighting-gb.pdf. Accessed 14/4/2016

17. Osram. Feeling of Contrast: the FCI as an additional index for the perception of color contrasts. http://www.osram.com/osram_com/news-and-knowledge/research-and-innovations/feeling-of-contrast-the-fci-as-an-additional-index-for-the-perception-of-color-contrasts/index.jsp Accessed 03/5/2016
18. NLPIP Lighting Answers. Rensselaer Polytechnic Institute, October 2004. http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/lightsources/whatisFullSpectrumIndex.asp. Accessed 15/4/2016
19. NLPIP Lighting Answers. Rensselaer Polytechnic Institute, September 2003 (revised March 2005). http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/fullspectrum/comparisons.asp. Accessed 05/5/2016

20. NLPIP Lighting Answers. Rensselaer Polytechnic Institute, October 2004. http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/lightsources/communicate.asp. Accessed 17/4/2016

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