Unlike incandescent and fluorescent lamps, LEDs are not inherently white light sources. Instead, LEDs emit nearly monochromatic light, making them highly efficient for coloured light applications such as traffic lights and exit signs. However, to be used as a general light source, white light is needed. White light can be achieved with LEDs in two main ways:
- Phosphor conversion, in which a phosphor is used on or near the LED to emit white light; and
- RGB systems, in which light from multiple monochromatic LEDs (red, green, and blue) is mixed, resulting in white light.
In terms of colour quality, the key properties are colour appearance (whether a white light appears more yellow/gold or more blue) and colour rendering (the ability of the light source to render colours, compared to incandescent and daylight reference sources).
- Colour appearance. Colour appearance is measured by correlated color temperature (CCT) on the Kelvin (K) scale. For most interior lighting applications, warm white (2700K to 3000K) and in some cases neutral white (3500K to 4000K) light is appropriate. Many products use cool-white LEDs with very high CCT (bluish in appearance) since they tend to offer higher efficacy at low cost, but an increasing number of LED products are available in warm-white or neutral-white. They are less efficient than cool white LEDs, but have improved significantly, to levels almost on par with CFLs.
- Colour rendering. The colour rendering index (CRI) measures the ability of light sources to render colours, compared to incandescent and daylight reference sources. The leading high-efficiency LED manufacturers now claim a CRI of 80 for phosphor-converted, warm-white devices. In general, a minimum CRI of 80 is recommended for interior lighting. The CRI has been found to be inaccurate for RGB (red, green, blue) LED systems. A new metric is under development, but in the meantime, colour rendering of LED products should be evaluated in person and in the intended application if possible.
- Colour quality. CCT and CRI have been used for many years in describing conventional lighting, but they are not adequate for SSL. Two LED light sources with identical CCTs can render object colours very differently due to the differences in spectra. While CCT provides an indication of whether a light source may appear yellowish or bluish in colour, Duv is needed as a supplemental metric to prevent excessively greenish or pinkish hues. To learn more about Duv, see Understanding Photometric Reports. Similarly, CRI value is poor at predicting the quality of the appearance of saturated red objects, and doesn’t correspond well to human perception of colour quality. The National Institute of Standards and Technology (NIST) has developed a Colour Quality Scale (CQS) that is intended to replace or supplement the current CRI. The developers believe CQS will better capture what most humans perceive as superior color rendering.