Generally speaking, a light source refers to an object that can emit light by itself. There are two main types of existing light sources, namely natural light sources and artificial light sources. Natural light refers to sunlight, and artificial light sources mainly include electric light sources, as well as oil lamps and candles.
Electric light sources are currently the absolute mainstream lighting and non-visual lighting sources. Electric light sources are divided into four types: thermal radiation light sources (such as incandescent lamps), gas discharge light sources, photoluminescence and electroluminescence light sources. LEDs are electroluminescence light sources.
LED is the fourth-generation lighting source after incandescent lamps, fluorescent lamps and high-pressure discharge lamps (HID). Compared with previous light sources, LEDs have incomparable advantages.
Compared with other light sources, the proportion of far red light in the spectrum of fluorescent lamps is very small, so the ratio of R/FR is generally high. Natural light R/FR is greater than incandescent lamps, but significantly smaller than Philips three-color, Panasonic three-color, and Panasonic standard fluorescent lamps (Liu Zailiang et al., 2004).
The ratio of red light to far red light is different between the sulfur lamp and the xenon lamp. The xenon lamp is 1, the sulfur lamp is 1.5, and the daytime sunlight is about 1.3. Traditional electric light sources, such as incandescent lamps, high-pressure sodium lamps, metal halide lamps, and fluorescent lamps, have a relatively complete spectrum. Only 15% of the radiant energy of incandescent lamps is photosynthetically active radiation, 75% is infrared radiation (850-2700nm), and 10% is dissipated by thermal energy consumption (> 2700nm).