## 3 Measurement Method and Setup  
The aging process of all the tested devices was performed simultaneously using a dedicated environmental chamber. During the aging process, LEDs operated at the rated value of their forward current I\({}_{\text{F}}\) = 3 A at an ambient temperature of 80 \({}^{\circ}\)C. This ambient temperature was selected to make the aging process faster. According to the Arrhenius equation [42], an increase in temperature value equal to 55 \({}^{\circ}\)C causes over a 60-fold reduction in the lifetime of tested devices. This means that operation at a temperature of 80 \({}^{\circ}\)C for 1000 h corresponds to operation at a temperature of 25 \({}^{\circ}\)C for 60,000 h. During the test the humidity value was not registered. The aging process is carried out in the following mode:  
Step 1: The operation of the tested diodes in the above-described conditions for a predetermined time. Step 2: The measurement of the electrical, optical, and thermal parameters of the diodes under study. Steps 1 and 2 are performed alternately until the end of the test. The quasi-isothermal I-V characteristics, thermal resistance R\({}_{\text{th}}\), and luminous flux \(\Phi_{\text{V}}\) before the start of the heating cycle (t = 0) and after time intervals equal to 100 h, 250 h, 500 h, and 1000 h were measured. Figure 3: Solder paste stencil for SACX0307-REF (**a**), SACX0307-nanoTiO\({}_{2}\) (**b**), and SACX0307-TiO\({}_{2}\) (**c**). Figure 2: LED mounted on MCPCB (**a**) and assembled on heat sink (**b**). A measurement setup like that in the diagram shown in Figure 4 makes it possible to measure thermal resistance, the DC current-voltage characteristics, and illuminance of the LEDs. The measurements were performed for the samples mounted onto an aluminum heat sink (Figure 2b) and situated in a light-tight chamber. The papers by [PERSON], and [PERSON] [9, 43], provide a detailed description of the measurement method for thermal and optical parameters. In Figure 4, the blue color indicates the part of the power supply system of the tested LED during its aging in the environmental chamber. In turn, the black color indicates the part of the system used for measuring electrical, optical, and thermal parameters. During the aging process, a current of a fixed value regulated by a voltage source E\({}_{\text{A}}\) flows through the tested LED, and the environmental chamber provides the fixed value of the ambient temperature. The value of this current is limited by the resistor R\({}_{\text{A}}\) and measured with an ammeter. The limitation of the value of the diode forward current protects the tested devices against electrical damage, but it cannot protect these devices against thermal damage during the test. During the measurement of current-voltage characteristics, switch S is open, the voltage on the voltage source E\({}_{\text{M}}\) increases linearly, and the system consisting of instrumentation amplifiers, an A/D converter module, and a computer records the voltage values on the diode and the corresponding current values of this device. In turn, the thermal parameter (thermal resistance R\({}_{\text{th}}\)) is measured using the indirect electrical method described, among others, in [6]. In this method, it is necessary to switch the diode current from the heating supply (closed switch S) to the measuring supply (open switch S) and measure the diode forward voltage V\({}_{\text{H}}\) immediately after opening the switch S, which preceded obtaining the thermally steady state in the tested diode. Based on the previously measured thermometric characteristic V\({}_{\text{F}}\)(T), the measured voltage V\({}_{\text{H}}\), and the value of power dissipated in the diode before opening the switch S, the value of thermal resistance R\({}_{\text{th}}\) is determined. Figure 4: Measurement setup of the LED test system.