A Firefly's Crooked Scales Showed LEDs How to Shine Brighter

On a warm summer evening at the edge of a meadow, small green-gold lights drift up out of the long grass, blink, and go dark again. Fireflies were lighting their own lanterns long before anyone invented a lamp. In 2013 a team of physicists looked very closely at one of those lanterns and came away with a way to get more light out of an LED, the tiny electric light inside modern bulbs and screens.

Where the light gets stuck

A firefly makes its light inside its abdomen, in a small organ called the lantern. Making the light is only half the job, though. It still has to get out. The glow must pass through the insect's outer shell, the cuticle, and then into the air, and light travels at a different speed in each. At a boundary like that, part of the light bounces back inside instead of crossing, and whatever bounces back is lost to the evening. An LED has exactly the same trouble: some of the light made inside it is reflected at the surface and never leaves.

The researchers, from the University of Namur in Belgium with colleagues in France and Canada, studied fireflies of the genus Photuris, which live in Latin America and the United States. Under powerful microscopes they found tiny ribs on the lantern's surface, far too small to see by eye. They also found something larger, and a good deal less tidy.

A roof of crooked tiles

The scales that cover the lantern do not fit together neatly. Each one overlaps the next with its edge sticking up at a slant, like tiles on a badly laid roof. The team called it a factory-roof shape. The jagged edges repeat roughly every 10 micrometres and stand about 3 micrometres high, a micrometre being one thousandth of a millimetre. Computer models showed that these crooked edges, and not the finer ribs, did the most to help light escape. The sharp slopes offer the light more angles at which it can get out.

• ≈ 10 µm between one jagged scale edge and the next on the firefly
• ≈ 3 µm the height of those edges on the firefly
• 5 µm the height and width of the copy made on the LED
• 55% the most extra light the coated LED let out

Then the team borrowed the idea. On top of a standard gallium nitride LED they laid a light-sensitive layer and shaped it with a laser into the same slanted profile, scaled up to 5 micrometres high and wide to suit the new material. The coated LED let out up to 55 per cent more light than the same LED without the coating. The work appeared as two papers in the journal Optics Express in January 2013. Helpfully, the layer goes on top of a finished LED, so the part underneath does not have to be redesigned.

What people often get wrong

It is easy to assume that a brighter lamp needs more power, or that a surface must be smooth and polished to let light through well. The firefly shows the opposite on both counts. Its lantern is rough on purpose, and the gain comes from wasting less of the light that was already being made. The number needs care as well: 55 per cent was the best result on a test LED in a laboratory, and it measures how much light gets out, so it is not a promise about the bulb in your kitchen. And one small correction to the name: a firefly is not a fly at all. It is a beetle, from the family Lampyridae.

None of this was what the firefly had in mind, of course. It was not trying to light a room or save on the electricity bill. It only wanted to be noticed across a dark meadow, and a slightly untidy roof turned out to be the best way to do it.