In America, the government banned incandescent light bulbs, convinced that if they banned them, the free market would replace it with something affordable, effective and efficient. In reality, since there was no good alternative, people just horded light bulbs. The automobile did not supplant the horse and buggy because horses were banned and banning regular light bulbs was not going to make compact fluorescent bulbs or light emitting diodes popular.
In the case of LEDs, the bluish, cold light has precluded their widespread use (and a desire not to wear a haz-mat suit if a bulb breaks precluded CFLs) for indoor lighting but, like early cars, research and development will improve that over time. University of Georgia scientists have fabricated what they claim is the world's first LED that emits a warm white light using a single light emitting material, or phosphor, with a single emitting center for illumination.
Two main variables are used to assess the quality of artificial light. Correlated color temperature measures the coolness or warmth of a light, and temperatures of less than 4,000 kelvins are ideal for indoor lighting. Correlated color temperatures above 5,000 kelvins, on the other hand, give off the bluish color that white LEDs are known for.
The other important measure, color rendition, is the ability of a light source to replicate natural light. A value of more than 80 is ideal for indoor lighting, with lower values resulting in colors that don't seem true to life. The material the researchers fabricated meets both thresholds, with a correlated color temperature of less than 4,000 kelvins and a color rendering index of 85.
Warm white light can commonly be achieved with a blue LED chip coated with light emitting materials, or phosphors, of different emitting colors to create what are called phosphor-based white LEDs, Pan said. Combining the source materials in an exact ratio can be difficult and costly, however, and the resulting color often varies because each of the source materials responds differently to temperature variations.
"The use of a single phosphor solves the problem of color stability because the color quality doesn't change with increasing temperatures," said lead author Xufan Li.
To create the new phosphor, they combined minute quantities of europium oxide with aluminum oxide, barium oxide and graphite powders. They then heat the powdered materials at 1,450 degrees Celsius (2,642 degrees Fahrenheit) in a tube furnace. The vacuum of the furnace pulls the vaporized materials onto a substrate, where they are deposited as a yellow luminescent compound. When the yellow luminescent compound is encapsulated in a bulb and illuminated by a blue LED chip, the result is a warm, white light.
Results are promising but there are still hurdles to overcome before the material is used to light homes, businesses and schools so here is hoping government does not run out and ban something new based on this. The efficiency of the new material is much lower than that of current LEDs. Scaling the production to an industrial scale will be challenging as well, since even slight variations in temperature and pressure in the phosphor synthesis process result in materials with different luminescent colors.
The new yellow phosphor also has a new lattice structure that has not been reported before. The researchers currently are working to discern how the ions in the compound are arranged in hopes that a better understanding of the compound at an atomic level will allow them to improve its efficiency.
Published in "Light: Science and Applications."