The development of luminance uniformity measurement for CNT-BLU based on human visual perception

Abstract

Carbon nanotube backlight units (CNT-BLUs) offer advantages (including light weight and superior color performance) that cold cathode fluorescent lamp backlight units (CCFL-BLUs) cannot deliver. If these advantages find favor with liquid crystal display manufacturers, CNT-BLUs could possibly replace CCFL-BLUs. CNT-BLU light-emitting arrays are made by a thick-film screen printing process, which can leave dark obstructions within and between pixel areas. Thus, when the pixels light up, CNT-BLU arrays can display dark blotches within one or more pixels. Existing luminance uniformity measurements, such as Video Electronics Standards Association (VESA) or International Organization for Standardization (ISO) standards, are not designed to deal with this problem. These standards are based on the luminance of multiple non-adjacent points on a display; they cannot reflect the luminance change of adjacent pixels, which is important for measuring the uniformity of CNT-BLU. This situation is aggravated since CNT-BLU is still under development and the current luminance uniformity of CNT-BLU still cannot compete with that of CCFL. This study presents a new luminance uniformity measurement, line non-uniformity, for CNT-BLU. This method was compared with VESA and U Formula with respect to human perception. A set of CNT-BLU images with different levels of mottling was presented to 18 participants. The subjective acceptance thresholds for these images were then calculated. The uniformities using VESA, U Formula, and line non-uniformity approaches for these images were also calculated and fitted to the subjective acceptance threshold. The results showed that line non-uniformity was the best to fit the acceptance threshold; its R 2 ranged from 0.80 to 0.92 whereas the R 2 values of VESA and U Formula ranged from 0.00 to 0.49.