According to the law of refraction, when light is incident from the optically dense medium to the light-diffusing medium, when the incident angle reaches a certain value, that is, greater than or equal to the critical angle, full emission occurs. In the case of a GaN blue chip, the refractive index of the GaN material is 2.3, and when light is emitted from the inside of the crystal to the air, the critical angle θ0 = sin-1 (n2/n1) according to the law of refraction.
Where n2 is equal to 1, ie the refractive index of air, and n1 is the refractive index of GaN, from which the critical angle θ0 is calculated to be approximately 25.8 degrees. In this case, the light that can be emitted is only the light within the solid angle of the space where the incident angle is ≤ 25.8 degrees. It is reported that the external quantum efficiency of GaN chips is currently around 30%-40%. Therefore, due to the internal absorption of the chip crystal, the proportion of light that can be emitted outside the crystal is small. It is reported that the external quantum efficiency of GaN chips is currently around 30%-40%. Similarly, the light emitted by the chip is transmitted through the encapsulating material to the space, and the effect of the material on the light extraction efficiency is also considered.
Therefore, in order to improve the light extraction efficiency of the LED product package, it is necessary to increase the value of n2, that is, to increase the refractive index of the packaging material, so as to increase the critical angle of the product, thereby improving the package luminous efficiency of the product. At the same time, the encapsulation material absorbs light less. In order to increase the proportion of the emitted light, the shape of the package is preferably arched or hemispherical so that when the light is directed from the encapsulating material to the air, it is almost perpendicularly incident on the interface, so that no total reflection is produced.