High CRI LED lighting

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The bulb on the left has a CRI of 80. The bulb on the right has a CRI of 92. An incandescent bulb has a CRI of 100.[1]

The color rendering index (CRI) of a light source is a quantitative measure of its ability to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. In general terms, CRI is a measure of a light source's ability to show object colors "realistically" or "naturally" compared to a familiar reference source, either incandescent light or daylight.[2] It is significant because it has been the most difficult metric for incandescent replacement light bulbs to match (while maintaining high efficiency) and therefore the most frequently ignored (the CRI spec value appears on only a very small percentage of LED product packaging)[citation needed]. For that reason, LED light bulbs with a high CRI can be worthy replacements for incandescent light bulbs. Most LED lights do not have a CRI above 90. For example, the top bulbs listed in the 2016 Consumer Review only have a CRI of 80[3][citation needed].

However, CRI is a poor indicator of the perception of light produced by LEDs, and scores as low as 25 can produce vivid-appearing white light, while high-scoring sources can still be very poor at rendering reds, including skin tones.[4] This is because CRI is not an indication of light colour, but a measurement of a light source's ability to accurately illuminate different colors compared to a radiating black body light source of the same light color, such as the sun or an incandescent light. The "warmer" light colors, such as a 2700K incandescent bulb or a 1700K candle light are more easily reproduced than more neutral white lights, such as 4800K direct sunlight, and thus usually have higher CRI ratings in alternative light sources such as CFL and LED bulbs; "warmer" light (more red) naturally renders colors less accurately. Think of how the world looks at sunset (2000K) compared to high noon (5600K).[5]

In 2008, the US Department of Energy created the L Prize to find an incandescent light bulb replacement that met efficiency metrics and had a CRI above 90.[6]

CRI is calculated from the differences in the chromaticities of eight CIE standard color samples (CIE 1995) when illuminated by a light source and by a reference illuminant of the same correlated color temperature (CCT), commonly measured in Kelvins, indicating the light color produced by a radiating black body at a certain temperature; the smaller the average difference in chromaticities, the higher the CRI. A CRI of 100 represents the maximum value. Lower CRI values indicate that some colors may appear unnatural when illuminated by the lamp. Incandescent lamps have a CRI above 95. Cool white fluorescent lamps have a CRI of 62, however fluorescent lamps containing rare-earth phosphors are available with CRI values of 80 and above.[2]

For CCTs less than 5000 K, the reference illuminants used in the CRI calculation procedure are the SPDs of blackbody radiators; for CCTs above 5000 K, imaginary SPDs calculated from a mathematical model of daylight are used. These reference sources were selected to approximate incandescent lamps and daylight, respectively.[2]

Glossary[edit]

Base is the type of connector on the bulb. E26 is the standard American screw base.
Color temp or color temperature is a measure of the frequency of light given off by the bulb. 2700 K is equivalent to a standard incandescent light. Sunlight's color temperature is 5780 K, but indoor lights with a color temperature this high will tend to look unnaturally blue to some. Generally any color temperature other than 2700 K will look unnatural in a home setting until people become used to the higher color temperatures.[citation needed]
CRI is a measure of how natural the light given off by the bulb looks in comparison to a broad spectrum light source.[7]
Dimmability Whether or not the bulb's brightness can be controlled by a dimmer. Most high-CRI LED bulbs are made with the circuitry to allow them to be dimmed with standard dimmer switches.[citation needed] Some are not able to be dimmed with a wall dimmer, but instead are controlled over a wireless network with a device like a smartphone.
Efficacy or lumens per-watt is a measure of the efficiency with which the bulb produces light. It is not a measure of the effectiveness of the light. For instance, a ceiling-mounted bulb that directs all light downward will be more effective than a bulb inside a lamp shade that absorbs a large portion of the light. The maximum possible value for any light source is 683. The maximum possible value for LED lighting is 300.
Form factor is the shape of the light bulb. A19 is the traditional light bulb shape.
Incandescent equivalent is the number of watts an equivalent incandescent bulb would consume.
Lifespan is an estimate of the amount of time the bulb can be on before it stops working. Another related metric is "lumen maintenance", which is a measure of the brightness of the bulb at the end of its life compared to its brightness at the beginning. It is not generally advertised but can range from 70% brightness to close to 100%.[citation needed]
Lumens is a measure of the light given off by the bulb. The direction in which light is emitted may change how much light is usable.
R9 is CRI for red color.
Watts is the amount of power the bulb consumes at full brightness.

See also[edit]

References[edit]

  1. ^ "LED 12.5W A19 Soft White 12.5W (60W) Dimmable A19". Energy-saving light bulbs. Philips. Retrieved 2014-04-18. 
  2. ^ a b c "What is color rendering index? | Light Sources and Color | Lighting Answers | NLPIP". Lighting Research Center. Rensselaer Polytechnic Institute. Retrieved 2017-01-10. 
  3. ^ "Lightbulb Buying Guide". Consumer Reports. Retrieved January 10, 2017. 
  4. ^ Baier, Simon. "Is Color Quality Scale (CQS) an improvement on CRI?". Lumenistics. Retrieved 2013-09-09. 
  5. ^ "Color Temperature & Color Rendering Index DeMystified". Lowel EDU. Tiffen. Retrieved 2017-01-10. 
  6. ^ "L Prize Competition Overview". L Prize. U.S. Department of Energy. November 7, 2012. Retrieved 2014-04-18. 
  7. ^ "What Is CRI?". Retrieved 11 August 2016.