# Evaluating the Quality of Light Emitted by Smartphone Displays
## 3 Results
### Spectral Power Distribution and Blue Light Reduction  
Several similarities can be observed when analyzing the selected smartphone displays' SPD using the spectroradiometer, as shown in Figure 5. The most characteristic common feature is the high radiant power density values from 430 nm to 480 nm in the wavelength range. Except for sample 6, all samples reached the maximum radiant power density value within this wavelength range. The highest value, 3.21 mW/m\({}^{2}\)/nm, was obtained for sample 2 at a wavelength of 461 nm. The lowest value, 0.93 mW/m\({}^{2}\)/nm, was obtained for the older sample 8 at a wavelength of 456 nm. This result is more than three times smaller. The displays of both smartphones use AMOLED technology. However, those samples differ significantly in their release dates. Sample 2 was released in 2023, while sample 8 debuted in 2014. It indicates that despite recent reports about the harmful effects of blue light on human health [44; 45], this is not always considered when manufacturing electronic device screens. It is noticeable that built-in blue light filters are not present in older models released in 2014. A similar SPD was observed for all AMOLED displays. Three areas can be distinguished where the radiant power density increases noticeably. These wavelength ranges correspond to blue, green, and red colors. It directly results from how color images are generated using AMOLED technology, which uses organic light-emitting diodes emitting light in these three colors. More significant differences were observed in phones with LCDs in the SPD. Sample 9 is characterized by the SPD typical for white LED chips, which is most likely because such a light source was used to backlight the screen. For sample 6, the distribution also resembles the LED SPD, but there is an increase in radiation emission in the wavelength range responsible for the red color. This type of backlight is likely used to improve the available color gamut in the phone. Figure 5: The spectral power distributions for different variants: black continuous line—without internal filter, white line—with internal filter, and black dashed line—with dedicated app. As mentioned above, most smartphones currently have a built-in blue light filter due to the need to limit it. Many free apps also claim to offer similar functionality. By comparing the SPDs from smartphone screens before and after activating the manufacturer's filters, it can be concluded that they serve their purpose (white lines in Figure 5) but in minimalistic ways. For each phone model tested, a reduction in radiant power density was recorded in the desired wavelength range (from 380 nm to 480 nm). The most minor decrease of about 25% was noted for samples 1 and 5, which could be because these are budget versions. For some models, radiation in the red wavelength range was additionally increased. It can be seen for samples 3, 5, and 7. A decrease in the radiant power density for all wavelengths in the visible spectrum can be observed for the installed free app. This app more effectively reduces the emitted blue light values compared to the manufacturer's filter. For sample 9, the app reduces the maximum value in the blue light range by more than 80%. However, despite the significant reduction of emitted blue light, the restriction of other wavelengths makes the app less useful, as a similar effect can be achieved simply by dimming the screen. A high CCT characterizes all of the tested phones. For most of them, it is around 7000 K for the variant without any filters applied (Table 3). It is cold light, associated with daylight on a cloudy day. It may have a stimulating effect on humans [46]. The highest values in this case were obtained for smartphones 6 and 9, which are equipped with LCDs. Their CCTs were even closer to 8000 K. Some of the CCTs were also recorded for the two newest smartphone models, samples 1 and 2. It may also be due to using significantly cheaper and lower-quality displays since these are budget smartphones.