## 4 Discussion  
In experiments 1 and 2, user acceptance was measured to investigate the appropriate color attributes for human-centric office lighting. If subjects evaluated LED lights as being acceptable, the LED light color attributes were considered suitable for HCL in offices. The user acceptance questionnaire included the categories of preference, visual comfort, readability, satisfaction, and self-productivity for the task. The finding for self-productivity for experiment 2 was nearly the same as that for experiment 1. The experimental results showed that the interaction effects between the CRI and CCT for user acceptance were significant. The warmer the color temperature was, the lower the CRI values were that users would accept. The cooler the color temperature was, the higher the CRI values were that users would accept. These results demonstrate that the appropriate color attributes for human-centric office lighting involve lower CRI values for warm lighting and higher CRI values for cool lighting. For HCL in offices, the CRI < 80 and CRI \(\geq\) 80 conditions were used to compare the color rendition properties of LED lights. The CCT 3000 K and CCT 6500 K conditions were specifically selected to evaluate the color appearance of LED lights. The results from experiments 1 and 2 demonstrate that the existing criterion of CRI \(\geq\) 80 is not useful for HCL in offices. [PERSON] et al. [16] concluded that LED lights with a low CRI can produce higher user acceptance than LED lights with a high CRI in offices. However, the experimental results here demonstrated that LED lights with a high CRI were rated as more acceptable than LED lights with a low CRI at 6500 K. Lighting manufacturers around the world advertise high CRI values. However, the high CRI value criterion should be considered in HCL office lighting. Two recent studies [25; 49] showed that cool LED lighting had higher acceptance than warm LED lighting for work environments. However, this study found that acceptance was weaker with a CCT of 6500 K than with a CCT of 3000 K at a CRI < 80. This supports the finding that a high CCT does not necessarily result in high user acceptance. The color attributes for HCL in offices were obtained by investigating the correlation between brightness perception and user acceptance. For the reading task we set, LED lights with a CRI < 80 at 3000 K were rated as brighter than LED lights with a CRI \(\geq\) 80 at 3000 K, and LED lights with a CRI < 80 at 6500 K were rated brighter than LED lights with a CRI \(\geq\) 80 at 6500 K. However, LED lights with a CRI < 80 at 3000 K were rated as more acceptable than LED lights with a CRI \(\geq\) 80 at 3000 K, and LED lights with a CRI \(\geq\) 80 at 6500 K were rated as more acceptable than LED lights with a CRI \(<\) 80 at 6500 K. The results of the analysis indicate that the increase in brightness perception was accompanied by higher acceptance at 3000 K but lower acceptance at 6500 K. The level recommended for offices by the IESNA is 500 lux and that recommended by the KATS is 300 lux. It seems that offering the recommend illuminance level of 500 lux for HCL office spaces will not be accepted by users. The findings of this study confirmed that lighting for HCL in offices should be designed in consideration of the correlation between brightness perception and user acceptance. Figure 15: Interaction effect between the CRI and CCT for self-reported productivity (Q11) in experiment 2. In experiment 1, the lighting environment under LED lights with a CRI < 80 at 3000 K was more accepted than LED lights with a CRI \(\geq\) 80 at 3000 K. LED lights with a CRI \(<\) 80 at 3000 K had a higher value for the IES Rg than LED lights with a CRI \(\geq\) 80 at 3000 K. LED lights with a CRI \(<\) 80 at 3000 K had a negative D\({}_{uv}\) value, whereas LED lights with a CRI \(\geq\) 80 at 3000 K had a positive D\({}_{uv}\) value.