# Effects of Illuminance Level of Light Source on White Appearance of a Tablet Display
## 2 Methods
### Experimental Setup  
The experiment is conducted in a viewing booth, and the interior walls are painted using Munsell N7 spectrally neutral paint. The booth measures 60 cm in length, 60 cm in width, and 60 cm in height. A 9.7-inch e-reading device (6 th generation iPad with a black border) is positioned on a custom table, tilted at a 45-degree angle to the horizontal plane. This table is centrally located within the booth and is also coated with Munsell N7 neutral paint. A chin rest is mounted outside the booth, aligned with the center of the opening, allowing the observers to view the screen at a perpendicular angle. Figure 1 illustrates the experimental setup from the observer's point of view, with the viewing distance between the eye and the iPad being approximately 45 cm. After a 30 min warm-up period, the iPad's peak luminance reaches 549 cd/m\({}^{2}\), with a chromaticity of (x = 0.3133, y = 0.3299) in the CIE 1931 xy chromaticity diagram. An 11-channel spectrally adjustable LEDCube (Thouslite Ltd., Changzhou City, China) is used to generate uniform illumination within the viewing booth. The experimenter meticulously adjusted the 11 channels of the LED to create eight different light sources, which include four levels of illuminance (1200 lx, 600 lx, 300 lx, and 150 lx) and two CCT levels (3500 K and 6500 K). During the experiment, the space had no general lighting, with the tunable LEDCube being the sole light source. The relative spectral power distributions are displayed in Figure 2, and the characteristics of the eight light sources are listed in Table 1. These measurements are taken using a reflectance standard, a calibrated illuminance meter (CL200A, Konica Minolta, Inc., Tokyo, Japan), and a calibrated spectroradiometer (Specbos 1211 TM, JETI Technische Instrumente GmbH, Jena, Germany).  
Figure 1: Experimental setup (picture captured from the observer’s eye).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Illuminance (lx)** & **CCT (K)** & **CRI Ra** & \(\mathbf{D_{\text{w}}}\) \\ \hline
150 & 3535 & 95.62 & 0.00206 \\
300 & 3481 & 95.94 & 0.0026 \\
600 & 3496 & 96.49 & 0.00411 \\
1200 & 3481 & 96.66 & 0.00206 \\
150 & 6541 & 94.78 & 0.01233 \\
300 & 6550 & 96.51 & 0.00893 \\
600 & 6526 & 97.67 & 0.00893 \\
1200 & 6481 & 97.97 & 0.01233 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The colorimetric characteristics of the ambient lighting conditions.  
Figure 2: The relative spectral power distribution of the light source: (**a**) 3500 K (**b**) 6500 K.  
A total of 47 observers (26 males and 21 females) participated in this study, consisting of 20 young adults aged between 18 and 20 years (mean = 19.848, standard deviation = 0.942). However, one observer failed the Ishihara test, so their data were excluded from the analysis. As a result, this study is conducted using valid data from 46 observers with normal color vision.  
The iPad's RGB values are modified to create 76 stimuli, with their chromaticities evenly uniform in the CIE1976 UCS. These stimuli were also adjusted to achieve a horizontal luminance level of approximately 265 \(\pm\) 10 cd/m\({}^{2}\), measured using a JETI Specbos 1211 TM placed at the observer's eye level. Each observer rated the 76 stimuli on the iPad under two different illuminance levels with a single correlated color temperature (CCT). The chromaticity details for the 76 stimuli are shown in Figure 3 and Table 2.  
Figure 3: The chromaticity coordinates of the 76 stimuli presented on the iPad.