Human Centric Lighting, a return to daylight basics

For most of history, humans lived, worked and slept under the governance of natural light. The flickering light of candles was a poor substitute and did little to change the rhythm of life for all but the wealthy few.

Life went on under a dynamic light ,with changing tone and brightness, as a day went by and seasons changed. Then, one day, electric light was invented and everything changed. From that point forward a "golden hour" or "afternoon" could be at any time of day, on any point on the planet or beyond, outside or inside, from our living room to a windowless office or deep underground.

An illustration of the changing in tone and brightness during a day

Daily life transformed radically with the theoretical freedom to work, relax or sleep at any time of the day or night. The architecture of buildings changed too, released from the necessity to have a natural light source (window) close by in every space.

This freedom is "theoretical" because a little over 130 years is not enough to change what millions of years of evolution shaped us to be. Research has shown we best sleep at night, in complete darkness and are at our optimum during the peak of day, under natural light. Doing it differently can have mild to serious adverse effects for health, well being and productivity.

Therefore, artificial light must adapt to our fundamental need for daylight and not the other way around, we to artificial light. 

The hue of daylight changes over the course of a day. The image above is an example. The latitude and time of year influence the color temperature values for a particular hour.

To emulate this effect with artificial light a combination of white light sources is used, usually warm white (2700K) and cold white (6500K). By adjusting the intensity of each, bright or dim white light with a hue between 2700K and 6500K can be obtained.

This new way to use and implement artificial light, in accordance with changes in natural light and research about effects on people, has become known as Human Centric Lighting.

We will focus on the the important aspects than make Human Centric Lighting an effective lighting solution.


Human Centric light sources must have a spectrum similar to daylight. It could seem as basic as saying that "a car must have wheels" but there are plenty of light sources on the market that do not.

A sample comparison of the spectrum of the best known lighting sources and daylight. Halogen and incandescent have the best spectrum but their light is stuck in perpetual "golden hour". Fluorescent has a spectrum of extremes. LED is currently the best candidate for Human Centric Lighting.

High color rendering index is a good measure of spectrum quality, with 90+ being optimum and 80+ acceptable. Below 80 colors will not look at all as they do under natural light.  

CRI is one of the many standards to quantify the spectrum quality of a light source, in comparison to daylight, equivalent to CRI 100. CRI has the highest popularity and the widest acceptance.

This is particularity useful when choosing LED lights, as they can have a CRI as low as 60 or as high as 95.


Human Centric Lighting should provide enough illumination for the task it serves. This is challenging due to the fact that a combination of warm white and cold white light sources with adjustable brightness has to be used. 

Usually, the Human Centric Lighting solution requires double the light sources compared to the static version. System cost, reliability, energy efficiency are very relevant and interconnected.

There are two principal methods to obtaining a dynamic white light, via interpolated or parallel light sources.

Interpolated light sources can only be LEDs for reason of size. There can be many alternatives to interpolate, from Warm White/ Cold White pairs to parallel lines or complex clusters. The combinations are placed on module which is made the core of a lighting fixture.

By using interpolated LEDs on a module, a lower system cost will result. Issues are insufficient brightness, lower reliability due to higher operating temperatures and problematic thermal management.

A Human Centric Light source with LEDs has white and warm white LEDs closely placed together. The module uses 70 LEDs of each and has dimensions similar to single color modules. However, the brightness for most color temperatures is equal with a 70 LED static module (right) and half of a 140 LED module (center). 

Even with this disadvantage, Human Centric luminaries such as desk or table lamps, tubes or spotlights can only be built with interpolated LED modules.

Using separate modules or lamps for each of the two white color temperatures will yield higher brightness, improved reliability but will increase the cost. LED, OLED or fluorescent lamps can be used. Such a combination is impractical in fixtures that have limited space, such as linear or slim luminaries and practical for cove lighting and luminous ceilings.

Typical linear LED modules can be used for Human Centric Lighting under the condition they are available in the required color temperatures.

Ease of use 

Natural light changes by itself and a Human Centric Light should do so too.

In spaces that are illuminated by daylight the lighting solution can have sensors that measure the color temperature and relative intensity of daylight and adjust the artificial lighting accordingly. The need for manual adjustment should be small or nonexistent, as very few people will have the desire to tune the light every few hours.

Different programs and manual programming must exist, for evening and night hours or even for the hole day. For residential use there should be a very easy app and also a fully programmed modes.

The transformers and control systems must talk to each other in a broadly used standard from the Internet of Things (IOT) universe. Zigbee, KNX, 0-10V, are relevant examples. The reasons are obvious, from an increased number of software solutions for control to long term guarantee that the light system can be upgrades or serviced.

KNX is the global standard for wired communication between control units of electronic devices, from lighting to garage doors and electronics. As long as KNX is used, the manufacturer of the product is not relevant in respect to interoperability.

ZigBee is the leading global standard for wireless communication between control units of electronic devices, from lighting to garage doors and electronics. As long as ZigBee is used, the manufacturer of the product is not relevant in respect to interoperability.


Only be having a high quality spectrum, sufficient brightness, good reliability of components and easy & future proof control can Human Centric Lighting eventually become the primary way to lighting our homes, shops and offices.