It is known that LEDs consist of a semiconductor chip which emits light energy near ultraviolet level and phosphors which converts the energy into radiation of a wide spectrum, which is visually perceived as a white color. The luminous flux is directly dependent on the radiation power of the crystal which is directly proportional to the operating current and inversely proportional to the transition temperature. It is obvious that the radiation increases with increasing current and decreases with increasing temperature.
From the viewpoint of saving the most important is the indicator of the light output: radiation power ratio to the power consumption. We have tested a series of LED XP-L-renowned manufacturer Cree and we found that for the most efficient LED light output is desirable to operate at low currents and low temperature of transition. Current 150 mA (at the transition temperature 25° C) gives efficiency almost 200 lm/W, and current of 3000 mA reduces the rate of up to 100 lm/ W.
Take for example a portable lamp. Lantern with bulb filament 3.6 with working current 1A gives luminous flux 30lm. The battery 10 Ah can work up to 10 hours, while the entire structure weighs 2 kg. For these characteristics the LED needs operating current 0.05A and battery capacity of 0.5 Ah, that will be much easier and cheaper, and the acquisition of more capacious battery will greatly increase the life of the LED. In addition, there is an opportunity to save money by purchasing a smaller radiator, or completely abandon the use of the radiator because often a luminaire’s housing performs this function itself, the design of which includes cooling fins.
We see that the reduction of the operating current for each of the LEDs makes their use as effective as possible, but at the same time, to preserve the value of the luminous flux we will have to increase the number of LEDs in a luminaire that will affect the cost of the light source.
To determine the number of LEDs required for best results in light output, take LED XP-L four LEDs and will during the experiment to increase the number of LEDs 4, adding an optical unit 2×2.
Originally this lamp operates at a maximum current of 3000 mA at the transition temperature of 85° C, giving a flow of 4134.4 lumens, power consumption 40.35 watts, light output is equal to 103.5 lm/ W, the thermal resistance of the heat sink will be about 0.862° C/W.
Add module of four LEDs. Luminous flux lm 4134.4 turns on current 1200 mA at the transition temperature of 84° C, the total power consumption would be 28.7 W (1.4 times smaller than in the 4-LED) and light output – 144.1 lm/W (1.4 times higher). The thermal resistance of the heat sink will rise to 1.744° C/W (twice, which will halve its size).
For twelve LEDs: total consumption – 25,878 watts, light output – 159.7 lm/W, the thermal resistance of the heat sink – 1.994 ° C/W. For sixteen LEDs: total consumption – 24,784 watts, light output – 167.35 lm/W, the thermal resistance of the heat sink – 2.144° C/W. We see that the most significant improvement occurred when we switched from 4 LEDs to 8 (light output increased by 40%, consumption decreased by 40%, and the thermal resistance of the heat sink has increased as much as two times).
Therefore, in our case the most advantageous purchase would be the lamp with two modules of 2×2 LEDs when the optional module will be marginally more expensive, and the reduced capacity of the radiator will allow even lower cost of the overall solution.
From experience we can conclude that the only correct choice of lighting solutions using LEDs makes the most economical and this choice can affect not only the operating cost of electricity, but also significantly reduce the initial purchase cost of lighting units.