Imitating sunlight with LED bulbs

Two master's students focus on the task

January 19, 2016
François Roy-Moisan and Gabriel Dupras, two master's students at ÉTS, created an LED light that is truer than life... almost.
Seeing things in their 'true' light is not as easy as it seems. The proof? This strange debate that sparked a controversy on social media several months ago, following the release of an ordinary picture of a striped dress. Was the dress blue and black, or white and gold? In one week, the world had a divided viewpoint, each resolutely defending their own vision of the image.

Actually, the dress was blue and black, but it didn't really matter much (the photo was over-exposed and some viewers unconsciously corrected for the mistake, relying more on their correction than their vision). What this story reminds us about is that colours are 'made up' and interpreted by the brain.

Our perception of color is relative and depends on interpreting the amount of light in a room or scene. Thus, a lemon illuminated with a red light will appear red; and black if lit up with a blue light. Our eyes are used to sunlight, a natural 'white light' source that is the result of the combination of radiation from different wavelengths, ranging from violet to red (the well-known colours of the rainbow). It is this light that allows us to better distinguish between various nuances of colour (it is said that it is an excellent colour rendering index).

Replicating the natural aspect of light: quite the challenge

Replicating this natural aspect has always been a challenge in the lighting industry. From sodium lamps to pale neon bulbs, it has never been easy to reproduce an agreeable light. Daylight provides a colour rendering index very close to 100. Incandescent light bulbs register a colour rendering index of approximately 90, while conventional LEDs come in around 78 or 80. LED lights cost less, but their colour rendering is not very natural.

Two master's students from ÉTS have found the solution... almost!

The blue-white light from LEDs has bothered them for quite a long time. But François Roy-Moisan and Gabriel Dupras, two master's students at ÉTS, might just have solved the problem, by producing an LED light that is truer than life... almost. "We managed to reach a colour rendering higher than 99," they confirm. Encouraged by this success, last January they created the start-up Sollum Technologies.

Their first objective is to propose their product to museums. "Our lighting imitates light from the sun, but without the harmful rays, that is, the ultraviolet and infrared rays that can damage works of art," explains François Roy-Moisan. Better yet, it can be modified to provide various colour nuances depending on the user's needs.

How did they accomplish this feat? It should be noted that each diode emits at a very specific wavelength. In fact, diodes are manufactured from the stacked layers of various semiconductor materials. By varying the materials and thicknesses of each layer, the colour of the light emitted can be modified. The two students analyzed the composition of the visible spectrum of sunlight (which contains more blue-green light radiation than red, for example) and then reproduced it.

"We aligned multiple different diodes in a very specific manner, and adjusted them in order to adjust the final colour," they explain, glad that they patented their technique. Their almost perfect replication of sunlight, which fits in a 10 cm by 10 cm luminaire, could be used in numerous applications. "Because we can modulate the light, the fixture could also be used in greenhouses, where it could reproduce light resembling springtime, summer, autumn or winter, rotating on a cycle, or even to mimic morning, noon or evening light modes," adds François Roy-Moisan.

This lighting could also be used in surgery or in transportation.
Pour information
Text taken from  "La recherche dans le réseau de l'Université du Québec", Québec-Science, December 2015.

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