Ergonomic Considerations May Save Power of Large LCD TVs

Mar 16, 2009
Naoki Tanaka, Tech-On!

Power consumption of large LCD TVs can be drastically cut simply by controlling their luminance to a level "agreeable" to viewers, said Satoru Kubota, a professor at Seikei University, in his speech at Ergonomics Symposium on Flat Panel Displays (FPD) 2009, which took place on March 6, 2009.

"An agreeable luminance is determined by four factors: display-viewing angle, viewer's age, average luminance level of images and lighting environment," he said.

He reached this conclusion after conducting onsite measurements of viewing spaces, an evaluation experiment with groups of young and elderly subjects, an HDTV signal analysis and 24-hour measurements of living room environments.

For example, the "agreeable luminance" for the younger viewers in their early twenties is determined by three parameters. They are the display-viewing angle, the average luminance level of images and the lighting environment. Kubota discovered a relation between these factors by conducting LCD TV viewing experiment with 24 student subjects.

When the average luminance level of images is 25%, and the screen illuminance and the viewing angle are set at 100lx and 33°, respectively, an agreeable luminance is 140cd/m2. By controlling an LCD TV to have an agreeable luminance, it is possible to save a large amount of energy, Kubota said.

When performing such a display luminance control, a standard luminance control based on the detection of incident illuminance is not always appropriate. Instead, it is required to control the luminance in accordance with field luminance, he said. It is because screen illuminance is not always equal to field luminance because natural light from windows during daytime and light from lighting equipment on the ceiling have different directionalities.

In addition, Kubota pointed out that the "dynamic mode" (so-called "storefront mode") is often utilized for no reason at home, resulting in unnecessary power consumption. He found this by measuring the luminance of TV screens in 82 households.

LCD TVs were used in 31 of the 82 households. And the peak white luminance (including the luminance of ambient light reflection) on the screen was 200cd/m2 or higher in 70-80% of the 31 households. In nearly 80% of such households, a luminance sensor function was disabled, and more than 80% used the TVs in the dynamic mode, Kubota said.

"An interface that prompts users to make an appropriate setting is required," he said.

He estimates that large LCD TVs can eventually save energy by 20-30% by taking ergonomic considerations.

Furthermore, Kubota said that environmental factors must be considered to promote wall-mounted TVs. A survey on living rooms in 83 households revealed that, in the great majority of Japanese houses, windows are facing south and TVs are facing north (so that TVs are not in direct sunlight from the windows). Thus, in Japan, there are extremely few rooms that enable to hung a wall-mounted TV facing north on a south wall.

"So, how should a TV be hung on the wall?" he asked in his lecture.

Kubota is promoting a project called "Development of Basic Technology for Next-Generation Energy-Saving Large-Screen Liquid Crystal Displays, Image Evaluation Based on Ergonomics" in collaboration with Hitachi Displays Ltd, Sharp Corp and Sony Corp. It is sponsored by Japan's New Energy and Industrial Technology Development Organization (NEDO). The achievement announced this time is part of the project, which is now in the second year of the five-year plan.

"Ultimately, we hope to make a guideline that is useful to both manufacturers and users," Kubota said.