# Evaluating the Quality of Light Emitted by Smartphone Displays
## 4 Discussion  
First of all, the color gamut measurements revealed that 8 out of 9 tested smartphones meet the requirements of the sRGB standard. The oldest phone in the group, which features an LCD, has a narrower color gamut. LCDs achieved lower values compared to AMOLED displays. AMOLED displays produced similar values regardless of the phone's release date, with their color gamut area reaching 140-150% of the area defined by the standard. There is a noticeable difference between older and newer LCD models. It is rather connected with the type of LCD technology used. However, the older model (TFT LCD) does not meet the sRGB requirements, while the newer one (IPS LCD) has a color gamut that is more than 20% larger than required. Despite that, the newer LCD smartphone still did not match the AMOLED displays in color gamut. It indicates the advantage of AMOLED displays over LCDs. Almost all phones in this study exhibited excellent RGB linearity. The R\({}^{2}\) value for the linear trend fitting generally exceeded 0.97. It means that the luminance for each R, G, and B channel increases similarly. The linearity of response in the R, G, and B channels is crucial for displays, as it helps prevent color distortion and image discoloration. Thanks to the spectral distributions of radiation from the displays, fundamental differences can be observed in the radiation emitted by different types of displays. The SPD from smartphones with AMOLED displays is characterized by high radiant power density values in three typical wavelength ranges. These correspond to the blue, green, and red colors. It is due to the construction of these displays, which use LED sources emitting light in these three colors to create a color image. The spectral distribution of LCDs largely depends on the type of backlighting used. In the older model, the SPD resembles the spectrum of white LED light. In the newer model, there are also narrowband increases in radiant power density in the red light range. It is performed to expand the color gamut [50]. It must be stated that the blue light emitted by digital devices such as smartphones is often classified as hazardous. The radiation between approximately 400 nm and approximately 500 nm, characterized by the high energy, may harm the human eye and cause permanent effects of tissue degradation [13]. The peak emissions of blue light from smartphone displays obtained in this study are far lower than those humans encounter in the natural environment, resulting from solar radiation [51]. However, extensive exposure to visible radiation lasting very long may adversely affect the circadian rhythm [52; 53]. A precise assessment of how it is in the case of long-term use of these smartphones requires separate studies. Nevertheless, reducing blue light radiation from electronic devices, especially smartphones, is better because, as mentioned earlier, the daily usage time is increasing alarmingly. The growing awareness of the harmful effects of blue light radiation on health and well-being can be seen easily in the features available on smartphones. Older phones in this study did not have a built-in blue light filter. Nowadays, most models are equipped with such a filter, and many free apps claiming similar functionality are also available. In the SPDs of almost all selected displays, the highest radiant power density values occurred in the blue light wavelength range (from 380 nm to 480 nm). By comparing the SPD before and after applying the filter, it can be concluded that the blue light filters offered by manufacturers serve their purpose and reduce the highest blue light radiation values by about 20-30% without changing the rest of the visible spectrum. A more significant reduction in radiation across the entire visible spectrum was observed for the free app, especially for the blue light spectrum range. However, it must be considered that different apps can work differently, and further study of this phenomenon is required. High values characterize the CCT of the selected displays.