Time Course of Chromatic Adaptation to Outdoor LED Displays Mohamed Aboelazm, Mohamed Elnahas, Hassan Farahat, Ali Rashid Computer and Systems Engineering Department, Al Azhar University, Nasr City, Cairo, Egypt Abstract The chromatic adaptation process for the colors displayed on an outdoor LED Display follows two intermediate stages in its path to reach steady-state. In this paper, we investigate the tuning of system parameters in the MACCLED (Mixed Adaptive Color Correction) system in order to insure better color generation while the observer HVS is in transient state. The performance of the system is then analyzed through conducting a series of psychophysical visual experiments in different environmental ambient setup. The resulting z-score of each ambient condition is then compared to select the optimum system parameters adjusting. Keywords: LED display, MACCLED, color appearance, ambient lighting, chromatic adaptation, incomplete adaptation, mixed adaptation. 1. Introduction The perception of colors in photopic vision mode is a very complex process, requires millions of cells in both the human eye sensor and the nervous system to detect and sometimes predict the perceived color[2]. Such sophisticated process is called the chromatic adaptation. In running this process the Human Vision System (HVS) tracks the ambient white point as a reference used in colors predicting. This complicated process can be explained by the independent sensitivity adjusting or gain control of the three cone responses in order to eliminate the effect of the illumination color and to preserve the appearance of a seen object. The analysis of this process is even more complicated when we consider a softcopy image on a display device. Since the display itself a source of illumination, the HVS becomes affected by both ambient white point and the display white point. The result is then regenerated inside the HVS based on an adapted white point relative to the both luminance sources. Previous studies on self luminance displays made by N. Katoh, M.D. Fairchild and others show that this point is some where between 40% to 60% relative to the display white point [3][6]. These studies also showed that the adaptation process in this mixed adaptation situation is also incomplete, In which even if we watch a display in complete dark room the HVS will not be a 100% adapted to the display image. The resulting colors in such case suffer from great discrepancies when compared to the original colors intended to be displayed [3][6]. Moreover, the results also become more dramatic when the display is installed in outdoor environment. Figure 1 shows how the lighting conditions white point (measured in correlated color temperature) in outdoors keep changing continuously from one severe state to another. Fig. 1: Ambient white point in CCT through the day [1] In our previous research, we introduced the MACCLED adaptive color correction system for outdoor LED displays [1]. The system showed significant improvement on the displayed colors in our psychophysical experiments carried in the steady state of chromatic adaptation. In this paper, we continue to examine the system performance in transient chromatic adaptation state. The experiments represented here were conducted in several realistic ambient conditions that simulates outdoor environment in both ambient color and luminance. The time course of chromatic adaptation consists of three phases [2]. The first phase is very rapid and ends in less than 100ms. This IJCSI International Journal of Computer Science Issues, Vol. 10, Issue 3, No 1, May 2013 ISSN (Print): 1694-0814 | ISSN (Online): 1694-0784 www.IJCSI.org 305 Copyright (c) 2013 International Journal of Computer Science Issues. All Rights Reserved.