The light emitted by the LED is not a single wavelength, and its wavelength is basically distributed as shown in Figure 1. It can be seen from Figure 1 that no matter what material is used for the LED, the spectral distribution curve of the LED has the strongest relative light intensity (the maximum light output), which corresponds to a wavelength, which is called the peak wavelength and is represented by λ0 . Only monochromatic light has a wavelength of λ0.
The luminous intensity of LED usually refers to the luminous intensity in the direction of the normal line (for cylindrical LEDs, its axis). If the radiant intensity in this direction is 1/683W/sr, the luminous intensity is 1 candela (symbol is cd). Since the luminous intensity of general LEDs is small, the luminous intensity is usually used as the unit of mcd. The luminous intensity or optical power output of the LED varies with the wavelength, and a distribution curve is drawn, that is, the spectral distribution curve. When this curve is determined, the relevant chromaticity parameters such as the dominant wavelength and purity of the device will also be determined accordingly.
The spectral distribution of the LED is related to the type, nature and PN junction structure (thickness of the epitaxial layer, doping impurities) of the compound semiconductor used in the preparation, and has nothing to do with the geometry of the device and the packaging method. Figure 2 depicts the spectral response curves of several LEDs made from different compound semiconductors and doping. In Figure 2:
Curve 1 is a blue InGaN/GaN-LED, with a spectral peak λp=460~465nm;
Curve 2 is a green GaPN-LED, with a spectral peak λp=550nm;
Curve 3 is a red GaPZn-O-LED, with a spectral peak λp=680~700nm;
Curve 4 is the LED using GaAs material, the emission spectrum peak λp=910nm;
Curve 5 is Si photodiode, usually used for photoreceiving.
For most semiconductor materials, due to the large refractive index, light tends to undergo multiple reflections before it escapes from the semiconductor. Since short-wave light is easier to absorb than long-wave light, the photon energy corresponding to the peak wavelength is smaller than the band gap Eg. For example, the photon energy corresponding to the peak wavelength of GaAs emission is 1.1 eV, which is 0.3 eV smaller than the band gap Eg at room temperature. Changing the value of x in GaAsl-xPx, the peak wavelength changes in the range of 620~680mm. The half width of the spectrum line is 200~300. It can be seen that the LED provides monochromatic light with a large half-width. Since the band gap of a semiconductor decreases with the increase of temperature, the peak wavelength emitted by it increases with the increase of temperature, and the temperature coefficient is about 2~3/℃.
At ±△λ on both sides of the peak of the LED spectrum, there are two points where the light intensity is equal to half of the peak (maximum light intensity). These two points respectively correspond to λp-△λ, and the width between λp + △λ is called spectral line width, also called half power width or half height width. The half-height width reflects the narrow spectral line width, that is, the parameter of the monochromaticity of the LED, and the half-width of the LED is less than 40nm. The half-width of the spectrum 2∆λ represents the spectral purity of the LED, which refers to the interval between the two wavelengths corresponding to the 1/2 peak light intensity in Figure 3.
16Figure 3 – The angular distribution of the luminous intensity of two different types of LEDs
Figure 3 shows the angular distribution of luminous intensity of two different types of LEDs. The coordinate of the vertical line (normal) AO is the relative luminous intensity (that is, the ratio of the luminous intensity to the maximum luminous intensity). Obviously, the relative luminous intensity in the normal direction is 1 cd, and the greater the angle from the normal direction, the smaller the relative luminous intensity. From Figure 3, the half-value angle or viewing angle value can be obtained. Half-value angle θ1/2 refers to the angle between the direction where the luminous intensity value is half of the axial intensity value and the luminous axis (normal direction). Two times the half-value angle is the viewing angle (or half-power angle).
For the LED’s spectral characteristics, it mainly depends on whether its monochromaticity is good, and it is necessary to pay attention to whether the main colors of red, yellow, blue, green, and white LEDs are pure. Because in many occasions, such as traffic signal lights, the color requirements are stricter, but it is observed that some of the LED signal lights in our country are green in blue and red in deep red. From this phenomenon, it is necessary and meaningful to conduct special research on the spectral characteristics of LEDs. Some LEDs emit light that is not monochromatic, that is, it has multiple peaks, not monochromatic light. To describe the chromaticity characteristics of LEDs, the dominant wavelength is introduced. The dominant wavelength is the wavelength of the dominant monochromatic light emitted by the LED that can be observed by the human eye. For example, GaP materials can emit multiple peak wavelengths, but there is only one dominant wavelength. As the LED works for a long time, the junction temperature rises and the dominant wavelength deviates toward long wavelengths.
Read more: Color rendering of light source