Color Rendering Index or CRI, light temperature, Stoke’s shift, are all terms used by manufacturers to promote their lighting products. There are CRI and Color Temperature wars waging to defend incandescent bulbs against legislation and energy efficient LED Lights.
Have you ever wondered what all that jargon behind light quality means?
What is Color Rending Index and why does it matter?
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?
These technical terms help understand issues related to light quality offered by LED , CFLs and Bulbs. Then you can decide to pay a premium for a better Color Rendering Index (CRI) product.
Understanding the Light Spectrum
Good quality light composed of different colors of light. The multi colored rainbow is visible 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 CD. When you hold it in light, the prismatic layer of the CD breaks up light into its constituent colors.
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 are not visible to our eyes. In fact both Infrared and UV light may damage art objects and paintings in your house.
The spectrum of UV, visible and IR light radiation. For lighting purposes it is only the colored 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. This makes them very 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 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 reflected.
Additionally, almost every object has a sickly hue. LEDs produce natural colors as the light is well balanced and contains an ideal 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 poor in green, blue and violet lights. When green, blue or violet objects are placed under an incandescent bulb’s light the colors are distorted. 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.
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 accurately 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 is comparable to sun light, the sun is our natural light source having an CRI of 100 too. CFLs typically provide a CRI 80.
This is a main reason why CFLs have not been able to replace the incandescent bulb. There are CFLs that produce warm light. To produce a warm light the glass coating of a CFL has to absorb light in one part of the spectrum and emit light in another part. This further reduces the lighting efficiency of CFLs.
LED products and systems are in a different league altogether. Looking at images produced by a LED projector, the colors produced are superioir to older technology. LEDs are an ideal lighting source for applications such as factories, schools, warehouses and used as LED shop lights used to light up work areas.
Visual tests have shown that the definition of 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 observer has the visual impression of equal lighting quality. One needs to know that the definition of CRI has been taken part within the time of the invention of discharge lamps. The color rendering index takes into consideration 14 standardized colors. Only the first 8 colors of the reference color scheme are taken into the calculation of the CRI. Surprisingly, these 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.
What is Color Temperature?
The term color temperature describes the temperature of objects by which they emit light. These are modeled through the black body radiator. When increasing the temperature of a “black body”, it starts to emit visible light in a continuous spectrum.
The filament of a 60W incandescent lamp heats up to about 3000°F, as a result the lamp emits light with a color temperature of 3000K. Many of our artificial light sources do not create light by heating up a material until it glows like an incandescent lamp. Instead of creating a continuous spectrum, they generate an assortment of color emission lines. A black body color temperature cannot be given to a non-blackbody type of emission.
An example is one generated by high intensity discharge and fluorescent lamps for example.
What is Correlated Color Temperature?
For those light sources, a correlated color temperature(CCT) is indicated based on the black body color temperature.
A cool white fluorescent lamp has a CCT of about 5000K. The lamps light spectrum is different from a black body one but has the same temperature.
The light color influences the atmosphere in a space. The more cozy the ambiance, the warmer the light color (lower color temperature) should be. The more dynamic the scene, the cooler the light source (higher color temperature). The CRI is the most important factor to watch when choosing light sources. How good is the light source representing faces, colors and contrasts? The easier the visual task and perception the lower the CRI can be. The color temperature needs to be watched too as it is supporting the space’s atmosphere.
What are color temperatures relative to LED Lighting?
|Lamp Color Name||Apparent Color Temperature (Kelvin)||Characteristics and Examples||Common Adjectives Used to Describe the Light||Best Location|
|Warm White||2700-3200K||Similar to incandescent bulb, yellowish light is best for accentuating skin tones and color of wooden objects||Friendly, warm, inviting, intimate, relaxing||Homes, boutiques, reception areas, Hotels.|
|Natural White||4000-4500K||Similar to early morning sunlight, Xenon lamp for automotive use||Neat and clean, Natural tone||Offices, School Lights, Outdoor Areas|
|Day White||5500-6000K||Typical day light, Flash light. Metal Halide||Crisp light, efficient, brightly lit, natural outdoor||Retail stores, Factories, Printing, Warehouses, Schools, Parking Lots, Outdoor Area Lighting|
|Cool White||7000-7500K||Best contrast but least flattering to the skin, may need mixing with light from a warm white lamp.||Bright light, bluish light||Special applications needing high light intensity and good color rendition like art Galleries, museums, showcases for precious stones and jewelry|
So how much CRI do you need? It depends who you are. Are you a retail store like the Gap or Bed Bath and Beyond? Are you a car dealership? Are you a hospital that wants to replace your parking lights? Are you a factory, a warehouse or a school? Different businesses and facilities all have different needs.
If you are a retail shop selling to consumers, high CRI is good. 80+ is preferred. 85 is great, even 90. If you are a not a retailer, then 70+ will be fine. But can you see the difference between 70 CRI and 90 CRI. Then answer is yes, if you know what to look for. At a recent lighting fair, we walked into a booth that displayed a series of glass bowls of colorful gum balls. Each bowl was under a LED light, and the CRI of the light was labeled above the bowl. We walked from the 70 CRI display all the way to the 95 CRI bowl and thought to ourselves ”We’re not seeing the difference”. So then an engineer walked up as we kept on walking from bowl to bowl and said “Look at the red gum balls”. And sure enough, the red gum balls became brighter and more vibrant as the CRI increased. The other colors seemed similar from one display to the next.
As a factory or a warehouse manager, do you care if the reds in your warehouse are not as vibrant under 70 CRI than under a 95 CRI light? Are you prepared to pay more for that difference? You are not. If you own a clothing store, then you want the best quality of light available.
Comparing LED to Metal Halide and High Pressure Sodium
This is many ways is not even a fair fight. High Pressure Sodium CRI is somewhere between 20 and 40. The color temperature is around 2300K. What you have is a very warm (yellow) light that tends to discolor everything. Did you notice how everything under a HPS light looks yellow? That in fact, is the perfect example of poor CRI. Good CRI makes colors look as they should appear, bad CRI discolors everything. Metal Halide fairs better. Its CRI is much better than HPS, but usually lower than LED. So the light quality of Metal Halide is not as good as LED.
In fact something else happens with Metal Halide bulbs. A brand new Metal Halide bulb is very bright right out of the package, but every one knows it loses it lumens. Within the first 6 months of use (assuming 12 hours a day), the lumens can drop as much as 20%. By the time the light is 50% though its 15,000 hour life, it can lose as much as 50% of its initial lumens. But what isn’t commonly known is it appears that Metal Halide loses its CRI effectiveness as lumens depreciate.
So not only is the quantity of light dropping, so is the quality of light. Its a double hit on the amount and usefulness of light. It appears Metal Halide bulbs are like fireworks. Bright to start, but fizzle out quickly. It make sense to replace metal halide with LED because of the significant savings available when you convert, but also due to the quality of light.
Color Rendering Index versus Color Temperature
Color Rendering Index is a measurement of the quality of light relative to how it appears compared to a reference light source.
Color Temperature is a measurement of the color of the light emitted.
A LED Bulb or fixture always has both. It has a specific color and its light is of a specific quality.