But what is the right light? Modern fluorescent and LED lighting provides too much blue-rich light exposure during the night, and too little blue-rich light exposure during the day. Therefore, it is important to design and specify effective circadian lighting which automatically switches between blue-rich daytime lighting to synchronize and strengthen circadian rhythms, and blue-free evening and nighttime lighting to prevent the harmful effects of light at night. This paper explains:
- The key metric for assessing circadian lighting is the blue content of the light entering the cornea of the eye, and not desktop illumination in lux or foot-candles or the color temperature (CCT) or CRI used in traditional lighting design.
- The specific wavelengths of blue light (“circadian blue”) that must be controlled to strengthen and protect circadian rhythms and good health are 438-493nm.
- The minimum threshold levels of blue content required for effective daytime lighting (sunrise to sunset) is 20 µW/cm2 of 438-493nm circadian blue light.
- The maximum safe threshold levels of blue content for evening and night lighting (sunset to sunrise) is 2 µW/cm2 of 438-493nm circadian blue light.
In practical terms, assuming typical IES lighting illumination standards of 300-500 lux at the tabletop, circadian lighting fixtures should emit less than 2% circadian blue content during the evening and night hours, and more than 20% circadian blue content during daytime hours. Lighting products which meet this standard for effective and safe circadian lighting at night can be recognized by the UL verification mark
