# Evaluating the Quality of Light Emitted by Smartphone Displays
### Display Technologies  
One of the oldest displays available on the first smartphones is the liquid crystal display (LCD) [18; 19]. This technology is constantly being improved to enhance image quality. An LCD is most commonly operated through twisted nematic (TN) technology. Figure 1a schematically shows the cross-section of such a display. The liquid crystals used in the display do not emit light but modify light coming from another source to present the image [20]. For this reason, special backlighting is usually used, which is placed directly behind the display or in an edge-lit configuration [21]. The backlight typically uses a cold cathode fluorescent lamp (CCFL) or light-emitting diodes (LED) [22]. The CCFL is inadequate for smartphones due to the size and power consumption, so LEDs are typically used. The backlight passes through the first polarizing filter, a set of electrodes, the surface of the alignment layer, and then reaches the liquid crystal layer. The liquid crystal layer is positioned between two alignment layers designed so that the liquid crystal molecules rotate 90 degrees, which is their resting position. Due to this twisting, the light changes its polarization direction. It allows the light to pass through the second polarizing filter, which results in the pixel's illumination on the screen [23]. When a voltage is applied to the electrodes, the liquid crystal molecules align according to the electric field, eliminating their twist. It prevents the light from changing its polarization direction, blocking it from passing through the second polarizing filter, resulting in a black pixel. Figure 1: Basic display technologies: (**a**) LCD and (**b**) OLED. Another type of LCD is the in-plane switching (IPS) [24]. This solution was proposed to widen the viewing angle and improve the performance of displays. It differs from the above-described TN technology primarily in the orientation of the electrodes (horizontal instead of vertical). Therefore, the liquid crystal molecules rotate differently. These molecules move in a single plane, remaining parallel to the surface of both filters [25]. Special filters are used to achieve colors on LCD. Each display pixel consists of smaller subpixels: red, green, and blue. A black matrix separates the subpixels to prevent light leakage [26]. The filter responsible for each subpixel only passes wavelengths corresponding to its specific color and absorbs all others. The human eye cannot distinguish between the individual subpixels due to their miniature size. Instead, it perceives the pixel as a single color, which is the result of the combination of the following three colors: red (R), green (G), and blue (B). OLED displays are gaining increasing popularity even for scientific research [27; 28; 29]. They use electroluminescent diodes made from organic compounds to produce images. Unlike LCD, the OLED displays do not require backlighting [30], and the light leakage is also eliminated by the black matrix [31]. Typically, they consist of two electrodes (anode and cathode), with organic semiconductor layers between them (Figure 1b). These layers can be divided into emissive and conductive layers, depending on the type of semiconductor used. The emissive layer comprises a \"p-type\" organic semiconductor, while the conductive layer is an \"n-type\" organic semiconductor. When voltage is applied and the junction is polarized in the conduction direction, the emissive layer becomes negatively charged, and the conductive layer becomes rich in positively charged holes. Due to electrostatic interaction, electrons and holes attract each other, leading to their recombination. This process involves electrons transitioning from the conduction band to one of the available states in the valence band. The electron then moves to a lower energy level, and the energy released in this process is emitted as a photon. Photon emission occurs closer to the emissive layer, as holes are more mobile carriers in organic semiconductors [32].