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We owe our very existence to light. Not just the visible part, but the entire electromagnetic spectrum from radio waves to gamma rays. These waves transmit energy and information about our surroundings to our bodies through our eyes. In the case of the visible spectrum, we value both. Direct sunlight deposits about 1000 joules of energy per square meter per second on a sunny day. That’s 1000W per second!

In the beginning, our main sources of light were daylight and much later, fire. These sources have vastly different quantities of light, but they have a couple of things in common – their spectral power distributions are continuous (all the colors of the rainbow) and change over time. The intensity of light is also highly variable. Fast forward a hundred thousand years or so and notice that industrialized societies get the majority of their light by converting electricity. In the 21st century, electrical light is often very different from daylight in composition. It’s typically a fixed, discontinuous spectrum.

The advent of multi-channel LED lighting about 10 years ago ushered in a new era in the supply of illumination. Not only in traditional engineering terms – solid state vs vacuum and less energy use – but also in terms of our ability to digitally control the spectrum. It’s now possible to accurately replicate subtle changes in spectral power distribution electronically – color tuning – and to compose illumination "content" for electrical sources of light.

The instruments

Before going further with the lighting hardware, let’s take a look at what nature is giving us. About 3 years ago Telelumen began traveling the world using a spectrometer to record daylight in many different situations. We now have about 2000 recordings from near the Arctic Circle to near the equator at different times of the year, different times of day, in different weather conditions, inside and outside of buildings. The variability of the results has surprised us.

The Daylight Data chart summarizes some of this data, plotted on a chromaticity diagram showing the average color of the sky when the measurement was taken. For reference, the black line shows the colors of thermal radiators, and the red line is the daylight locus. In general terms, the right end of the line is warm red and sunset and the left end is blue-sky, white hot. Colors above the line tend to have a greenish tinge and below the line a pinkish coloring. Beautiful sunrises and sunsets tend to be well below the line, and blue sky above the line. As anyone can observe from this plot, the variation of daylight as we experience it, is much greater than either calculated curve, let alone a single spectrum that defines a typical indoor light fixture.

Daylight Data chart

For electrical illumination purposes, the color of objects under electric lighting is even more important than the color of the light itself when viewed directly – at least until recently. For object color rendition we care more about the spectrum than the chromaticity.

Another graph shows the spectrum changes over the course of a day in Como, a beautiful place in northern Italy. The horizontal axis shows the wavelength (color) of the light: the right is deep red, in the middle is green, and the left side is violet. In other words, it shows all the colors of the visible rainbow. The vertical axis shows how much of each color is present at the time the recording was made. Each plotline on the graph references the time of day it was taken. We see that the continuous spectrum of daylight changes significantly throughout the day.

The spectrum changes over the course of a day in Como, Italy.

These spectrometer recordings are high-quality samples of daylight and can be reproduced (tuned) using multi-channel LED lighting programmed over time. Illumination compositions – sing "Morning Has Broken" in your head – can be played on LED luminaires in your home. A 10 min–long spectral recording of a sunrise over Como can be used to program the spectra that a light fixture emits. In a nutshell, it means to have fine control over ROYGBIV over time.

There is no need for a fixed white light per se, just control of the underlying rainbow of colors over time. The right combinations of all wavelengths produce the beautiful variegations of white light we know as daylight.

Tunable luminaires are a rapidly growing segment of the lighting market. But beyond any clinical implications of "circadian light" or biophilia, there is the sheer enjoyment we get when things are just right. Many of us enjoy an early morning sunrise, lunch under a blue sky, a glass of wine watching the sunset, or dinner by candlelight. Returning to the music analogy, we don’t need a medical or productivity study to tell us a song relaxes us, enlivens us, or puts us in a romantic mood. We just enjoy it or switch to another song.

Steve Paolini

About Steve Paolini

Steve Paolini founded Telelumen LLC, USA in 2007 and is currently the president. He earned a BSEE from Pennsylvania State University in 1981 and joined Hewlett Packard, Optoelectronics Division, where he held a variety of engineering and management positions in California, Japan, and Malaysia. In 2000 he joined Lumileds as a founding member and, later, became CTO at Lunera Lighting and NEXT Lighting. He speaks frequently on a variety of lighting-related topics, particularly daylight replication, and holds 25 issued patents.

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