A research group at Ehime University developed a pulse drive control method to make LEDs look twice as bright by leveraging the properties of how people perceive brightness.

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.

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 brighter to human eyes than that driven by a direct voltage, the research group said.

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

"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. Thus far, "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," Jinno said.

"The Talbot-Plateau effect is a principle found 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.

Thus, 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.

The LEDs in three colors used in the evaluation test were all manufactured by Nichia Corp. The model number of the 464nm blue LED is NSPB500S, the number of the 520nm green LED is NSPG510S, and that of the 633nm red LED is NSPR510CS.