LEDs are now an important part of semiconductor light sources for general lighting applications due to their various advantages. For some time now, LEDs have been significantly more efficient than traditional lighting technologies. But how do you define the efficiency of a solid-state lighting (SSL) system? In common, everyday language, when talking about the efficiency of light sources, we usually refer to their luminous efficacy, measured in lumens per watt (lm/W). Another measure is radiant power. The implications of effectiveness and efficiency need to be considered, discussing the decision-making process that product developers must follow, as this affects the efficiency of the overall system.
Calculating a functional metric involves determining the luminous flux of a light source (lm) versus the electrical input power (W). In contrast, the corresponding photometric radiance does not take into account the sensitivity of the human eye, but is purely a measure of efficiency. The electrical input power (W) is divided by the photometric output power (W) to give the percentage efficiency.The efficiency of an LED depends on many different factors. We will cover some of them in detail in this article.
Color Temperature Affects LED Efficiency
The choice of color temperature has a significant impact on lighting efficiency and can be used as a way to efficiently plan lighting solutions within the framework of existing lighting regulations. LEDs with high color temperatures (e.g. 5000K) are typically more efficient than
those with a lower color temperature (e.g. 3000K) are more efficient. The graphs in Figure 1 show the spectra (or spectral power distribution – SPD) of LEDs for different CCT values at color rendering index (CRI) Ra> 80. The SPD curves are built on the human sensitivity curve Vλ.
Figure 1 Spectral power distribution of 80-CRI LEDs at different CCT values
In order for LEDs to produce white light, LED chips that emit blue light are usually used. Some of the light from these lamps is converted to longer wavelengths (green, yellow, and red light) through a converter or phosphor, which adds all these colors together and then produces white light. But there are losses in the conversion process, and the more the wavelengths of the converted light increase, the more the losses increase, because the difference in energy between the higher energy level (blue) light and the lower energy level (red) light is converted to heat.
Minimizing losses requires accurate calculation of the absorbing and emitting wavelengths of the converter. However, a simplified situation is sufficient to explain the basic principles. For example, for a warm color temperature of 3000 K, a large amount of red light needs to be converted. However, this requirement results in greater losses and reduced luminous efficiency compared to 4000K. For a high color temperature of 5000K, blue light only needs to be converted to green light and less to red, which is why the luminous efficacy is increased compared to 4000K. The comparison of efficacy is shown in Figure 2.
Figure 2 Relative luminous efficacy of different CCTs at CRI Ra 80
Effect of color rendering on LED efficiency
As mentioned above, choosing the right converter, after conversion, the composition of the color spectrum has a decisive impact on the efficiency of the LED. Converter combinations have been developed specifically for different CRIs and are optimized for CRI as well as efficiency. The difference between CRIs 70, 80 and 90 can be seen very clearly when displaying red colors. In order to reproduce these color shades as realistically as possible, a high percentage of long-wave light is required; in other words, light from the red end of the spectrum.
Figure 3 shows the SPD of a 4000K LED at different CRI values. the high percentage of red in the CRI 90 version can be clearly seen. As mentioned above, producing such a high percentage involves high losses. In addition, most of the red energy produced clearly exceeds the sensitivity curve of the human eye for Vλ, which leads to a further reduction in luminous efficiency. The effect of different color temperatures on the luminous efficiency of LEDs is in the range of ±5%, and the effect of CRI at different values is usually in the range of ±15% (Figure 4).
Figure 3 SPD of 4000K-CCT LEDs at different CRI values
Figure 4 Relative luminous efficacy at different color rendering values at 4000K
LED Efficiency Tunable
LED optoelectronic semiconductors offer additional dimensions compared to conventional light sources, and luminaire manufacturers can adjust and set the efficiency or luminous efficacy, i.e. current density, on an application-specific basis.
LEDs are usually grouped according to brightness and color for a given operating current. For specific grouping conditions, it is therefore possible to adjust the efficiency appropriate to the application and desired level of luminous efficacy by varying the current density.
For example, if 130 lm/W luminous efficacy LEDs are used at a specified grouping current, the operating current can be reduced to 40%, which will ultimately increase luminous efficacy by 20% to 156 lm/W. If the operating current is increased to 140%, luminous efficacy will be reduced by 10% to 117 lm/W. Table 1 summarizes the variable current density.
Table 1 Variations in current density have corresponding efficacy effects
|Operating conditions 1||Grouping conditions||Operating conditions 3|
Table 2 Based on the theory of two different LEDs, product developers
Trade-offs in LED system design
|System A composed of LED1||System B composed of LED2||System C composed of LED2||System D composed of LED2|
The efficiency curve also depends on other parameters, such as the operating temperature or the maximum operating conditions to be met. At both points, the absolute luminous flux of the LED varies naturally. We will take a closer look at this effect in the next section.
Reducing system costs with more efficient LEDs
Over the course of development, LEDs have become brighter and therefore more efficient. However, some applications do not necessarily require higher efficiency. So why is there such a demand for brighter LEDs? Undoubtedly, one reason is that more efficient LEDs can significantly reduce costs at the solid-state lighting system level.
Take a look at an example. For the purposes of the following comparison, suppose a product is to be created with 100% efficiency and luminous flux. Here one of two LEDs can be selected: LED 1 with a luminance of 100% and the more efficient LED 2 with a luminance of 110%. Figures 5 and 6 graphically depict the two LEDs under system-level operating conditions.
System A with LED 1 is used as a reference. At a normal operating current of 100%, the system has a relative efficiency of 100% and a relative luminous flux value of 100%. The relative number of LEDs required for the system is also 100%.
If LED 2 is used, system B of FIG. 5 can be realized. in this system, the LEDs are operated at the same current and the same number of LEDs are used. as a result, the efficiency and brightness of system B is increased by 10%. Brighter can be a selling point for the luminaire or can provide options for the luminaire manufacturer to change the operating parameters.
Figure 5 This graph depicts the change in luminous efficacy as the operating current changes.
Figure 6 This graph shows the change in luminous flux relative to the operating current.
If there is no need to improve efficiency at the system level, perhaps because the system has reached the threshold for an energy efficiency rating, the efficiency can be reduced from 110% to 100% by increasing the current density. This means that in addition to the initial 10%, the new System C will be brighter, with a brightness level 42% higher than System A due to the same efficiency as System A.
But suppose the application may not require the greater luminous flux output. Table 2 summarizes this scenario and presents a System D option. Because System C is much brighter than System A, there is an option to reduce the number of LEDs. System D uses 70% of the number of LEDs in System A, thus significantly reducing the cost of the system.
This example can be very easily applied to systems with a large number of LEDs. It can also be used for single LED systems if the luminous flux package can be reduced and packaged LEDs with smaller chips can be used. The level of savings in each case depends on a variety of other parameters and may vary depending on the operating point.
These examples given here show that color temperature as well as CRI have a significant effect on the luminous efficiency of LEDs. Specifically, the higher the color temperature, the higher the luminous efficiency; the higher the CRI, the lower the luminous efficiency. In this regard, the key difference between LEDs and conventional light sources is that the efficiency of LEDs can be adjusted by the operating current. Because of the significant savings in LED costs, this should be fully considered when selecting an LED system.