# Evaluating the Quality of Light Emitted by Smartphone Displays
## 4 Discussion
Each tested display without a blue light filter had a CCT above 6500 K. The high values of this parameter further indicate the dominance of blue light radiation emitted by the screen, which is probably linkedto the generally known higher efficiency of such blue LEDs compared to red and green ones. The highest CCT was exhibited by LCDs, probably due to the backlighting type used, which mainly uses white phosphor-converted white LEDs. The phosphor changes some blue light to green and red, but some remains [54]. In contrast, the high CCT for AMOLED screens is related to the fact that OLEDs responsible for blue pixels age faster than those responsible for green and red pixels [55]. The lowest values of CCT were usually obtained after activating a blue light filter app. It effectively lowered the CCTs. However, the yellowing of the display may not be well received by all smartphone users. The results of the refresh rate measurements of the phone screens were consistent with the data declared by the manufacturers. For phones where no information about this parameter was found from the manufacturer, the measured frequency was 60 Hz. It is the standard frequency for older models. However, high refresh rates are an energy-consuming solution, so phones with high refresh rates lower them in battery-saving mode, as confirmed by the results. The displays achieved quite low values of the Flicker Percent parameter, usually below 6%, regardless of the mode. Only one sample had values for this parameter exceeding 20%, which should be considered unfavorable from the perspective of using the screen. For many models in this study, an increase in the Flicker Percent parameter was observed after enabling battery-saving mode. The measurements of the spatial radiation distribution showed that the entire luminous flux from the smartphone display is sent into one hemisphere. LCDs have a narrower spatial radiation distribution than AMOLED displays, which is related to their construction. The second polarization filter used for backlighting the subpixels passes light polarized in a specific direction, significantly limiting the width of the spatial distribution. All smartphones in this study exhibited average luminance from approximately 300 cd/m\({}^{2}\) to 440 cd/m\({}^{2}\). Higher values were obtained for LCD than AMOLED displays. At the same time, uniformity values were reasonably good for all samples. For most models, the uniformity was around 0.80-0.85. Differences were observed in the luminance values for screens of different colors. LCDs were characterized by lower red and green luminance values than AMOLED displays. This feature is also influenced by the type of backlighting used in these phones and/or the type of color filter applied. The selected smartphone samples achieved relatively low static contrast values. The most noticeable differences occurred between the different types of displays. The contrast of LCDs was lower, close to 300:1, while the contrast of AMOLED displays was higher, close to 500:1. This is directly caused by how dark and bright pixels are generated in these types of devices. The backlighting in smartphones using LCD technology is always on, and black is achieved by not passing light through the polarization filter. In OLED displays, the image is created by organic diodes, which turn entirely off to display a black pixel. This is why smartphones with this type of display have much deeper blacks. By comparing the older and newer smartphone models, it is difficult to point out a clear improvement in radiation parameters in the visible range. Smartphone manufacturers often deliberately choose displays with worse light radiation parameters to increase energy efficiency or lower production costs. However, more attention is now paid to required standards, as evidenced by color space measurements, where newer models meet the sRGB standard, unlike the oldest model tested. Newer smartphones also generally exhibit higher screen luminance than older models with the same display type.