MATERIALS SCIENCE


that it had the predicted qualities. Won Bin Im’s team in South Korea optimised the procedure for industrial application and built prototype LED lights to demonstrate its performance. They found that the new phosphor has a colour rendering index (CRI) above 90, which is at the high end of the range currently available commercially. Natural sunlight and incandescent bulbs have a CRI of 100, so one can read these measures as a percentage of perfection. Many commercial LED bulbs have a CRI in the 80s. Thanks to the computational methods used, the whole development process only took a few months. ‘Calculations are quick, scalable and cheap. Using computers, we can rapidly screen thousands of materials and predict candidates for new materials that have not yet been discovered,’ Ong said. However, the material isn’t quite perfect yet. ‘We have achieved excellent colour quality. Now we are working on optimising the material to improve quantum efficiency,’ Ong said.


Lighting solutions As researchers are coming closer to developing energy-saving lights mimicking the light our eyes evolved for, there remains room for diversification and adaptation to the requirements of different environments. Thus, a retail business may want lights that encourage people to linger and look at the exhibited ware favourably, whereas the light in an operating theatre has the vital task of revealing subtle details to the operating surgeon. Companies like Selectronic, based


in Witney, near Oxford, UK, specialise on building lighting solutions around the latest LED chips and phosphors. Working with partner company Honglitronic in Guangzhou, China, which produces the LED chips, Selectronic builds the electronics around the chip to ensure a smooth current and good disposal of excess heat. Although LED lighting produces a lot less waste heat than Edison’s light bulb, the small scale and sensitive nature of the electronics means that the dispersal of that heat is still an important consideration. As Selectronic technical manager, Xuesong Hu emphasises, these support structures enable the end users to get a long lifetime of up to


50,000 hours out of the lights. Selectronic works for large


customers like retail chains helping them to design a uniform lighting solution for their premises. Automotive lighting is another important field where the company tailors the lighting to the requirement of the environment. Its latest product is a light that


comes as close as possible to natural sunlight. ‘The option to create lighting products that replicate daylight opens a new era of what can be achieved in previously closed-off rooms or underground environments, not to forget the health benefits in dark winter periods from creating artificial daylight, particularly when linked with a time clock, to accurately reflect the various times of a bright sunny day,’ says Selectronic managing director Kevin Dry. ‘The Sunlight Spectrum 2835,


which produces perfect colour, measuring more than 95 in the range R1 to R15 and continuous saturation, is very close to the actual sunlight spectrum. There is much less blue light and it is a first choice for protecting our eyes.’ While Honglitronic keeps the chemical composition of its phosphors secret, the main difference to other commercial options is that the primary light produced by the LED is violet rather than blue. ‘This LED uses violet light to induce RGB [red, green and blue] lights,’ Xuesong Hu explains. ‘There are three types of phosphors inside this white LED. All the RGB lights come from these different types of phosphors. As a result, Honglitronic’s LEDs produce a spectrum closer to that of sunlight without a “blue light peak”’. Even a given environment or a


given person may require different qualities of light at different times of the day, and on different days in the year. This is again in line with our experience of nature, where the light changes in daily and seasonal cycles, to which our physiology responds. One of the lighting trends identified in a recent report from the German lighting company Osram, which celebrates its centenary in 2019, reflects this under the headline of ‘human centric lighting’. LED lighting can be programmed to follow the requirements of the human circadian rhythm or biological clock. A primary school in Denmark has


The Sunlight Spectrum 2835, which produces perfect colour, measuring more than 95 in the range R1 to R15 and continuous saturation, is very close to the actual sunlight spectrum. There is much less blue light and it is a first choice for protecting our eyes.


Kevin Dry MD, Selectronic


already adopted a time-variable lighting scheme shaped to fit the circadian rhythm in a bid to improve students’ attention and avoid fatigue. A hotel chain has also developed a time-specific lighting scheme to improve sleep quality for its customers. At the commercial end, an


increasing number of companies are competing in the field of making and selling LED lighting. The trade website LEDinside reported recently that, due to the drop in price and profits, ‘restructuring business focus to concentrate on higher margin operation has been a trend the LED industry.’ Companies including Osram, GE, Cree and Eaton have sought to spin off their LED operations in some form. However, there are many other


things for which we need light. For instance, there is a growing demand for LEDs in the UV-C range, between 200 and 280 nm. This radiation can kill microbes by damaging their DNA. The traditional lamps used for this were mercury based. They will be banned under the Minamata Convention on Mercury from 2021. Sterilising drinking water in the home is one of the areas where a 2018 report released by LEDinside predicts a large growth in future LED applications.4 Other areas where there is still


unexplored potential for LEDs include the lamps used for plants. Once a sufficiently versatile construction kit with LEDs and phosphors in various colours is at hand, there is no limit to what people can do with such lights.


Researchers have recently identified a promising new phosphor, Sr2


LiAlO4


, by


using high-throughput computational screening to explore as yet uncharted chemical space, namely the possible combinations of the elements strontium, lithium, aluminium and oxygen. Although each of these elements occurs in known phosphors, a combination of the four had not been tested before.


References 1 P. Pust et al., Nat. Mater., 2014, 13, 891.


2 P. Wagatha et al., Chem. Mater., 2018, 30, 7885.


3 Z. Wang et al., DOI: 10.1016/j. joule.2018.01.015


4 LEDinside, 2018 UV LED Application Market – Curing, Medical and Sterilization (ledinside.com/ newsletter/1971)


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