'Blackening' boosts bulb brightness

12 Jun 2009

Laser-fired filaments yield brighter, more efficient light bulbs.

Firing laser light onto the filament of a standard incandescent light bulb boosts the bulb's brightness and efficiency, according to researchers at the University of Rochester, New York, US.

The group has already used the process, known as "blackening", to transform shiny metal into a variety of different colours. Now the same approach has been shown to enhance the emission efficiency of tungsten bulbs to up to 100%.

"We've been experimenting with the way ultrafast lasers change metals and we wondered what would happen if we trained the laser on a filament," Chunlei Guo, associate professor of optics at the University of Rochester, told optics.org.

"We fired the laser beam right through the glass of the bulb and altered a small area on the filament," he continued. "When we lit the bulb, we could actually see that this one patch was clearly brighter than the rest of the filament, but there was no change in the bulb's energy usage."

The team's result follow Kirchhoff's law, which states that the absorptivity of a body (or surface) in thermal equilibrium must equal its emissivity. Since a black metal is good at absorbing light, Guo wanted to find out if the reverse was true.

His instincts were correct. "I call Kirchhoff's law the 'take more, give more' law governing the amount of light going in and coming out of a material," he explained. The Rochester lamp exhibits 60% enhanced emission efficiency and, according to Guo, the efficiency rose to 100% over the entire visible spectrum.

Blackening involves transforming a smooth metal surface to generate a variety of nano- and microscale structures, which enhance light coupling into the metal. The surface treatment was carried out using an 800 nm femtosecond source trained onto a section of a tungsten filament at a repetition rate of 1 kHz. The blackened filament was then inserted inside a bulb before monitoring its emission with a photomultiplier tube.

Not only was the efficiency of the bulb boosted, but so too the colour balance of the emission. The emission was also found to be partially polarized, which until now has been impossible to do without special filters that reduce the bulb's efficiency.

The team is now working to discover what other aspects of a common light bulb they might be able to control and has even used a similar femtosecond laser process to create a tiny metal micropump (see Metal pumps liquid uphill on optics.org).