Color Rendering Index (CRI)

Data: 2015-07-13
Color Rendering Index or CRI, light temperature, Stoke’s shift, are terms that are often used by manufacturers to promote their lighting products. There are CRI and Color Temperature wars waging in the cyberspace to defend incandescent bulbs against the onslaught of legislation and energy efficient LED Lights.
Have you ever wondered what all that jargon behind light quality means?
Is a Color Rendering Index (CRI) of 80 good enough or should you settle for nothing less than 100?
Is a Color Rendering Index (CRI) score of 100 perfect?
Are Light Temperature and Color Rendering Index related?
How will LED lighting technology impact parameters like CRI?
Understanding these technical terms will help you understand the issues related to the quality of the light offered by LED devices, CFLs and Bulbs and decide if you really should pay a premium for a better Color Rendering Index (CRI) product.
Understanding the Light Spectrum
Good quality light is made up of different colors of light. The multi colored rainbow is seen when light is split by small water drops acting as prisms. “VIBGYOR” is a short version of the different colors constituting white light. You can create your own rainbow by using a compact disc. When you hold a CD towards light the prismatic layer of the CD breaks up light into its constituent colors and you can see all the colors constituting light.

Violet, Indigo, Blue, Green, Yellow, Orange and Red are the visible colors of light. Red is the least energetic and violet light is the most energetic of the lot. A spectrum also contains invisible light. As their names suggest – Infra Red light comes before red light and Ultra Violet comes after violet.

A good source of light like LED bulbs produces full spectrum light i.e., light with all colors. Human beings do not need Infrared or UV lights as they cannot be seen by our eyes. In fact both Infrared and UV light may damage art objects and paintings in your house.

Figure: The spectrum of UV, visible and IR light radiation. For lighting purposes it is only the colored portion of the spectrum that is important. IR and UV lights can damage art objects. Besides art objects, UV light also harms the skin. Incandescent bulbs produce 95% of their output in the invisible infrared range making them highly energy inefficient.

How Do We See Colors?
White light has all the different light colors in almost equal proportions. When white light strikes an object, most colors of light are absorbed and a few are reflected. The color of an object is determined by the color of light it reflects. It is important that a light source emits light with all the colors of light in it.
If light from a source does not have a particular color of light, objects of that color cannot be seen in their natural colors. This is why sodium lights are so bad at reproducing colors. Sodium light is what is called a narrow spectrum light. It is made of primarily yellow light. When an object of a color other than yellow is placed under sodium lights different shades of yellow are the only color that is reflected and almost every object has a sickly hue. LED lights produce natural colors as the light is well balanced and contains an optimum spread of all light colors.
Lighting Devices and Light Spectrum
Warm yellow light produced by an incandescent bulb has light of all colors but is slightly poor in green, blue and violet lights. Therefore when green, blue or violet objects are seen under an incandescent bulb’s light the colors are not reproduced faithfully. LED's by contrast produce a complete, well balanced spectrum of light and natural colors of objects are reproduced.
Sodium lights have a very poor CRI of 25,
Mercury lights weigh in at 50 and
HID have a CRI of 72.
The CRI of CFLs and LED lights vary with light color but generally lie between 75 and 85.
It is pertinent to note here that the CRI of a light source is comparable across sources that work at the same temperature. LED lighting works at very low temperatures compared to an incandescent bulb. Thus the CRI of 85 under estimates their performance. Plans are afoot to take another look at the CRI standard to reflect the reality of the lighting world.
Light Spectrum and Color Rendering Index
The Color Rendering Index is a measure of how faithfully light from a source will reproduce colors. Incandescent bulbs have a CRI of 100 – which is taken as a perfect score.
Incandescent light is the light source with the best CRI. It serves an index of 100 because the spectrum provides all wavelengths. An incandescent light bulb can be compared to the sun, the sun is our natural light source having an CRI of 100 too. CFLs normally provide a CRI80.
That is the single most important reason that CFLs have not been able to replace the incandescent bulb despite heavy promotion. There are CFLs that produce warm light. But to produce a warm light the glass coating of a CFL has to be suitable doped with additives that essentially absorb light in one part of the spectrum and emit light in another part. This further reduces the lighting efficiency of CFLs.
LED lights are in a different league altogether. If you look at images produced by a LED projector, the vibrant colors reproduced and its superiority over two centuries old incandescent technology becomes apparent. LED light sources are among other light sources superior in lighting effciency and quality. If you look at the history of lighting, the first electrical and popular wide-spread lamp has been the incandescent one. Other light sources that followed took advantage of newer technology background as well as other possibilities of producing light. Each light source is different, with advantages as well as disadvantages and it is just natural that older light sources will phase out because others are better than the old technology. LEDs are anyway the best lighting source you can get right now for many applications.
Visual tests have shown that the definition of the CRI needs to be reviewed. Scientists asked observers to rate color schemes, which are used to define the CRI of a lamp, under different light sources. The comparisons of LED with lamps of a higher CRI have been surprising. The observers had the visual impression that both light sources would be equal in lighting quality, although the LED had per definition a worse CRI. One needs to know that the definition of the CRI has been taken part within the time of the invention of discharge lamps. Regarding the basis of the color rendering index, the testing method is taking into consideration 14 different standardized colors. But only the first eight colors of the reference color scheme are taken into the calculation of the CRI, surprisingly that are pale pastel colors. Now that standard itself is under scrutiny. There is a debate to change the standards to reflect the reality of today’s lighting technology.
Even if you consider CRI to be reliable because that is all that we have at present a CRI in the 80’s denotes excellent quality light. In user trials in United States, users were unable to distinguish between incandescent and LED lights. For the user the most important considerations are that the light should be of good quality and affordable. LED's work out to be much cheaper than incandescent bulbs due to their stupendous savings in utility bills. To know more, refer to our article “The Death of the Light Bulb.” They produce better light at a fraction of the cost of either incandescent bulbs or CFLs. The question that begs to be asked is “Why not choose energy efficient LED lighting today for better quality of life and help save the planet at the same time?” 

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