## 1 Introduction  
Ambient temperature has a significant impact on the luminous characteristics of LEDs. It also reduces the service life of LED light sources. Therefore, it is essential to identify ways to reduce the impact of ambient temperature on the light and electrical parameters of LEDs, thus increasing the efficiency of their use [1, 2]. Being part of lighting devices, LEDs are powered by constant voltage. This applies both to linear driver circuits with current stabilization and circuits with switching drivers. In the latter case, the output stage of the driver operates in pulse mode, while LEDs are still supplied with a constant voltage after a low-pass filter [3, 4]. Investigations were conducted on the temperature dependence of LEDs' electrical and luminous parameters in LED light sources (lamps, floodlights). They indicate that with an increase in ambient temperature, the luminous flux of LEDs decreases significantly, even when the LED current is constant [1, 2, 3, 4, 5]. Moreover, this decrease is most pronounced in light sources with linear drivers and is somewhat less in circuits with switching drivers [6]. In LED drivers with constant voltage stabilization, the luminous flux decreased significantly less with an increase in ambient temperature compared to circuits that stabilizedthe LED current [6; 7; 8]. A decrease in the LED luminous flux with temperature suggests that an increase in ambient temperature and consequently in the temperature of the LED p-n junction causes a decrease in the intensity of its radiation, which is due to the following factors [7; 8; 9; 10; 11; 12]:  
1. Radiation-free recombination due to deep impurity levels;
2. Recombination on the surface;
3. Loss of carriers in the barrier layers of the p-n junction [13; 14; 15; 16]. To determine the LED temperature, we measure its crystal temperature by causing a drop in the forward voltage at a constant current [17; 18]. The ambient temperature is kept within a specific range. The more significant the direct decrease in voltage with temperature, the higher the temperature of the LED crystal, and therefore the more significant the drop in its luminous flux. Thus, to estimate the temperature of the LED crystal, and hence its luminous flux, it is necessary to obtain the temperature dependence of the LED forward voltage at a constant current. This applies to LEDs powered by constant voltage sources and those powered by rectangular pulse sources at different ambient temperatures. A rectangular pulse voltage leads to a decrease in the crystal temperature and hence to a lower drop in the luminous flux of LEDs. As a result, the service life of LED light sources is extended [19; 20; 21; 22]. At the same time, to ensure the rated value of the current through the LED, its amplitude almost doubles. Such a doubling of the current leads to an increase in the LED's luminous flux at high excitation currents compared to that of LEDs powered with direct current [12]. Previous studies indicate that rectangular pulses are the most energy-efficient way to power LEDs [1; 2; 3; 6; 7; 10]. Their characteristics measured under such operating conditions indicate that the luminous flux and luminous efficiency significantly depend on both the pulse frequency and the pulse fill factor \(D\). Moreover, the influence of ambient temperature is not as significant as when a constant voltage powers LEDs. When a pulsed power supply to LEDs is used, pulse width modulation is used. In this case, the LED is supplied with a 0.3-100 kHz pulse-modulated voltage with a variable pulse filling factor \(D\), which is determined by the ratio of the pulse duration \(\tau_{0}\) to the oscillation period \(T\):  
\[D=\tau_{0}/T, \tag{1}\]  
Although the flicker of an LED light source is not perceived by human vision at frequencies above 100 Hz, a stroboscopic effect is still felt even at frequencies close to 300 Hz [12]. This accounts for the lower limit of the frequency range.