Cold light sources: Cold light sources are light sources (fireflies, neon lights, LEDs, etc.) that are excited by chemical energy, electrical energy, and biological energy. Has very good optical, variable flash characteristics. When an object emits light, its temperature is not higher than the ambient temperature. This kind of light emission is called a cold light source. For example, an LED uses electron-hole pair recombination to emit light. Strictly speaking, LED light-emitting diodes are electroluminescent and also generate heat, but they are lower than light sources such as incandescent lamps. The electro-optical conversion efficiency of LED is about 30%, of which the internal quantum efficiency is about 70% (close to the theoretical limit), and the external quantum efficiency is about 50%. (This is only experimental data, not exact value)

Thermal light source: a light source excited by thermal energy, such as an incandescent lamp, which emits thermal radiation at a temperature of 3,000-4,000K. Incandescent lamps convert 80-90% of their energy into heat energy, and around 10% of their energy is converted into light energy. Therefore, the luminous efficiency is low. We are not defined as a cold light source or a hot light source according to the temperature of the lamp housing. The temperature around the lamp can only be judged whether the heat dissipation measures of the lamp are good or bad.

To distinguish between cold light sources and hot light sources, we cannot just distinguish them by definition. In fact, what we call a cold light source does not mean that no heat is generated during the process of emitting light, but that the way of emitting light is not converted from heat energy to light energy. Incandescent lamps are a typical process of converting electrical energy into heat energy, and then converting heat energy into light energy. The thermal energy loss is high and the luminous efficiency is low. According to the current development trend of LED white light, the era of cold light sources replacing hot light sources is not far away. Therefore, we can regard LED as a cold light source.