Pulse-driven LEDs have higher apparent brightness

May 5, 2008
Human perception studies by a Japanese university indicate that pulse-driven LEDs appear up to twice as bright.
According to an article by Naoshige Shimizu of Nikkei Electronics, a research group at Ehime University, Japan, has developed a pulse drive control method to make LEDs look twice as bright by leveraging the properties of how people perceive brightness.

When a short-cycle pulse voltage with a frequency of approximately 60Hz is applied to an LED at a duty ratio of about 5%, the LED looks about twice as bright to the human eye in comparison with an LED driven by a direct voltage, the research group said.

Based on an evaluation test using subjects, the group reported that a blue (464 nm) LED looks 1.5-1.9 times brighter while green (520 nm) and red (633 nm) LEDs look 2.0-2.2 and 1.0-1.3 times brighter, respectively.

The group was led by Masafumi Jinno, an associate professor of Dept of Electrical and Electronic Engineering at Graduate School of Science and Engineering of Ehime University.

"With this method, the brightness of LED with a luminance efficiency of 100lm/W can be simulated by using a 50lm/W LED," Jinno said.

The test result was unveiled at the "New Technology Presentation Meetings by Four Universities in Shikoku Region" sponsored by Japan Science and Technology Agency (JST).

There are two principles, the Broca-Sulzer effect and the Talbot-Plateau effect, involved in how human eyes perceive brightness. The Broca-Sulzer effect refers to a phenomenon in which light looks several times brighter to the eye than it actually is when exposed to a spark of light, such as a camera flash.

In addition, the Talbot-Plateau effect is a principle where human eyes repeatedly see flashes and sense the average brightness of the repeated lights. Until now, said Jinno, it has been believed that, due to the Talbot-Plateau effect, the brightness perceived by human eyes would not change even if an LED is pulse driven.

"The Talbot-Plateau effect is a principle derived in the days when fluorescent mercury lamps and other light sources driven by a power supply with a longer voltage cycle of about several hundred milliseconds were used," Jinno said.

The group decided to drive the LEDs using a power supply with a shorter voltage cycle of about several hundred microseconds. As a result, the group discovered that, when a pulse voltage with a frequency of approximately 60Hz is applied at a duty ratio of about 5%, the impact by the Broca-Sulzer effect becomes greater than that of the Talbot-Plateau effect, so that the light emitted from the LED looks brighter to human eyes.