1. Luminous brightness
Brightness is an important parameter of LED luminous performance. It means that the surface brightness of the luminous body in a certain direction is equal to the luminous flux radiated by a unit projected area on the luminous body’s surface in a unit solid angle. The unit of brightness is cd/m2. The brightness in the direction of the positive normal is
If the surface of the light source is an ideal diffuse reflection surface, the brightness L0 is a constant independent of direction. The luminous brightness of the surface of a clear blue sky and fluorescent lamps is about 7000cd/m2, and the luminous brightness of the sun’s surface from the ground is about 14×108cd/m2. The brightness of the LED is related to the applied current density J0. For a general LED, the increase in J0 and L0 also approximately increases. In addition, the brightness is also related to the ambient temperature. As the ambient temperature increases, ηC (composite efficiency) decreases, and L0 decreases. When the ambient temperature does not change, the current increase is enough to cause the temperature of the PN junction to rise, and the brightness becomes saturated after the temperature rises.
Between 1970 and 1995, LEDs have been gradually developing in order to continuously provide higher levels of brightness. However, since the mid-1990s, with the emergence of blue and white LEDs and the doubling of the average brightness of devices, the pace of LED innovation has been greatly accelerated. The improvement of LED brightness is mainly attributed to the progress of substrate materials. Starting from the original gallium arsenide phosphorous (GaAsP) products, in the late 1970s, it turned to nitrogen-doped GaAsP and gallium-phosphorus GaP to realize the earliest yellow and green LEDs. Then use single and double heterogeneous gallium aluminum arsenide (GaAlAs). In the early 1990s, a luminous flux of more than 0.1lm was achieved. Since the 1990s, various indium gallium combinations have become the base material for more novel and brighter color (including blue) LEDs.
Although LED has undergone the above development, there are still several problems, one of which is that the substrate tends to absorb most of the light generated by the LED. There are several solutions to this problem. Lumileds uses a patented transparent aluminum indium gallium phosphide (AlInGaP) substrate to solve this problem. Another method is to add a Bragg reflector grating layer to the substrate. This provides twice the brightness of the LED that a light-absorbing substrate can provide, but it will lose all the light emitted at a 90° angle. Vishay has improved on this method with an organic mirror attachment (OMA) technology, which attaches a mirror to a silicon substrate. All the light that reaches the mirror is reflected from the front of the device, so it can reach the same brightness level as using a transparent substrate, which is about four times that of a standard LED.
The lifespan of LEDs is generally very long. In the case of current density J0<1A/cm2, the lifespan can reach 1000000h, which can be continuously ignited for more than one hundred years. This is no light source can compete with it. The brightness of the LED declines with the lengthening of the working time, which is aging. The speed of aging is related to the current density J0 and the aging time constant r.
The phenomenon of LED luminous brightness attenuation of light intensity or light brightness with long-term work is called aging, and the aging degree of the device is related to the size of the external constant current source, which can be described as
Lt=L0e-t/r ( 1-2)
In the formula: Lt is the brightness after t time; L0 is the initial brightness.
Usually the time t for the brightness to drop to Lt=1/2L0 is called the lifespan of the LED. It takes a long time to determine t, and the life is usually calculated by extrapolation. The measurement method is to pass a certain constant current source to the LED and ignite it for 103~104h, and then measure L0 successively, calculate r by Lt=1000~10000, and substitute Lt=L0e-t/r; then let Lt=1/2L0 for generations, and the life t can be obtained.
For a long time, it has always been considered that the life of the LED is 106h, which means that a single LED is at IF=20mA. With the development and application of power LEDs, foreign scholars believe that the light attenuation percentage value of the LED should be used as the basis for its life. For example, the light attenuation of the LED is 35%, and the lifespan is greater than 6000h.