LED Dimming and Human Centric Lighting: Circadian Rhythms and Tunable White

For most of the history of electric lighting, the goal was simple: provide enough light to see by. Human centric lighting, often abbreviated HCL, expands that goal to include the non-visual effects of light on human physiology, particularly the circadian system. Light does more than illuminate; it regulates the body's internal clock, suppresses melatonin at the right times, and influences alertness, mood, and sleep quality. As research into these effects has matured, HCL has moved from a niche concept to a mainstream specification requirement in offices, healthcare facilities, schools, and senior living. Dimmable LED drivers are the hardware foundation on which HCL systems are built.

The circadian system is regulated by a subset of retinal ganglion cells that are intrinsically photosensitive, called ipRGCs. These cells contain a photopigment called melanopsin, most sensitive to short-wavelength blue light at approximately 480 nanometers. When light reaches the ipRGCs, the signal travels to the suprachiasmatic nucleus in the hypothalamus, the body's master clock, and to the pineal gland, where it suppresses production of melatonin, the hormone signaling the body to prepare for sleep. In the morning, bright light with a strong blue component suppresses melatonin and increases alertness. In the evening, dimmer, warmer light with less blue allows melatonin to rise, preparing the body for sleep.

The CIE addressed measurement of circadian light in its standard S 026/E:2018, which defines the alpha-opic irradiance system. Instead of using the traditional photopic luminous efficiency function, peaking at 555 nanometers and describing visual brightness, S 026 defines five action spectra corresponding to the five photoreceptor types in the human eye: three cone types, rods, and melanopsin-containing ipRGCs. The melanopic equivalent daylight illuminance, or melanopic EDI, is the metric most commonly used to quantify the circadian stimulus of a light source. A light source with high melanopic EDI, such as a 6500K cool white LED at high illuminance, is a strong circadian stimulus; a 2700K warm white LED at low illuminance is a weak one.

A human centric lighting system typically varies both intensity and color temperature over the course of the day, following a profile mimicking natural daylight. In the morning, the system ramps up to high illuminance, perhaps 500 to 1000 lux at the occupant's eye level, with a cool color temperature of 5000K to 6500K, delivering a strong circadian stimulus that suppresses melatonin and promotes alertness. Through the middle of the day, the system maintains moderate to high illuminance with a neutral color temperature. In the late afternoon and evening, the system dims to lower illuminance, perhaps 100 to 200 lux, and shifts to a warm color temperature of 2700K to 3000K, reducing circadian stimulus and allowing melatonin to rise.

Implementing a tunable white system requires two channels of LED control: one for a warm white LED array and one for a cool white LED array. By varying the relative intensity of the two channels, the system produces any color temperature between the warm and cool endpoints, while maintaining roughly constant total luminous flux. Each channel requires its own driver, or a dual-channel driver with two independent outputs. A single-channel constant-voltage driver like the PV100 can power a fixed-color-temperature LED module and dim it smoothly from full output to a low level, but it cannot vary color temperature because it has only one output channel.