1. Luminous flux
Luminous flux Φ is the radiant energy that characterizes the total light output of the LED, and it marks the performance of the device. Φ is the sum of the energy that the LED emits in all directions, and it is directly related to the working current. As the current increases, the luminous flux of the LED increases. The unit of LED luminous flux is lumens (lm). The luminous flux is related to the LED chip material, the packaging technology level and the size of the external constant current source. At present, the maximum luminous flux of monochromatic LED is about 1lm, and the luminous flux of white LED is Φ≈1.5~1.8lm (small chip). For a power-level chip of 1mm×1mm, a white LED is made, and its luminous flux is Φ=18lm.
2. Luminous efficiency and visual sensitivity
(1) LED efficiency has internal efficiency (the efficiency of converting electrical energy into light energy near the PN junction) and external efficiency (the efficiency of radiation to the outside). Internal efficiency is only used to analyze and evaluate the characteristics of the pros and cons of the chip. The most important characteristic of LED is the ratio of radiant light energy (luminous quantity) to input electric energy, that is, luminous efficiency.
(2) Visual sensitivity. Human visual acuity has a maximum value (680lm/W) at λ=555nm. If the visual sensitivity is denoted as Kλ, the relationship between the luminous energy P and the visible luminous flux Φ is
(3) Luminous efficiency is the ratio of luminous flux to electric power. Luminous efficiency characterizes the energy-saving characteristics of the light source, which is an important indicator to measure the performance of modern light sources. The quantum efficiency of LED is
η=Number of photons emitted/Number of carriers in pn junction=
（1-2）
If the input energy is W=V×I, the luminous energy efficiency is
ηp=P/W (1-3)
If the photon energy hc=ev, then
η≈ ηp (1-4)
Then the total luminous flux is
Ф=(F/P)P=KηpW (1-5)
In the formula: K=F/P.
The efficiency of LEDs can be characterized by common terms for electric light sources, that is, the radiation efficiency ηe is used for infrared light, and the luminous efficiency ηl is used for visible light, but it is also characterized by internal quantum efficiency ηqi and external quantum efficiency ηqe. The internal quantum efficiency ηqi is
ηqi =NT/G (1-6)
In the formula: NT is the efficiency of radiation recombination to generate photons; G is the number of injected electrons and holes.
In this way, ηqi is equal to the number of photons that are injected into each electron-hole pair in the semiconductor, and the maximum can be close to 100%.
The external quantum efficiency ηqe is
ηqe=NT/G (1-7)
Where: NT is the efficiency of outputting photons from the LED.
In this way, ηqi is equal to the number of effective photons outside the output device produced by the injection of each electron-hole pair, generally only 0.01% to 13%. The ηqe of infrared emission can reach 15%, while the ηqe of green light drops below 1%.
The main reason for the significant drop in external quantum efficiency is the absorption of the semiconductor itself, which is caused by reflection loss and total reflection loss when light enters the air from the semiconductor. For example, the refractive index of GaAs is n=3.6, the reflection loss is 32%, and the total reflection loss of the LED with the structure shown in Figure 1 (a) is 96%, and the emitted light is only a few percent. The total reflection loss of the LED with the structure of Figure 1(b) is greatly reduced. The commonly used material for the spherical part in Figure 1(b) is transparent resin to reduce costs.
(5) Lumen efficiency is the main parameter to evaluate the characteristics of LEDs with external packaging. The high lumen efficiency of LEDs means that the energy of visible light radiating under the same external current is large, so it is also called visible light luminous efficiency.
A high-quality LED requires a large amount of light energy radiated outward, and as much light as possible, that is, high external efficiency. In fact, the outward light emission of the LED is only a part of the internal light emission, and the total luminous efficiency should be
A high-quality LED requires a large amount of light energy radiated outward, and as much light as possible, that is, high external efficiency. In fact, the outward light emission of the LED is only a part of the internal light emission, and the total luminous efficiency should be
η=ηiηcηe (1-8)
In the formula: ηi is the minority carrier injection efficiency in the PN junction region; ηc is the minority carrier and multi-carrier recombination efficiency in the barrier region; ηe is the external light extraction (light extraction efficiency) efficiency.
Because the refractive index of the LED material is very high ηi≈3.6, when the chip emits light perpendicular to the interface between the crystal material and the air (without epoxy packaging), it is reflected by the air, and its reflectivity is
(ni－1)2/(ni+1)2=0. 32
The reflected light accounts for 32%. Since the crystal itself absorbs a considerable part of the light, the external light extraction efficiency is greatly reduced. In order to further improve the external light emission efficiency ηe, the following measures can be taken:
Cover the surface of the chip with a transparent material with a higher refractive index (epoxy resin n=1.55 is not ideal).
Process the surface of the chip crystal into a hemispherical shape.
A semiconductor material with a large Eg is used as a substrate to reduce light absorption in the crystal. If n=2.4~2.6 low melting point, large thermoplastic glass is used as the cap, the efficiency of infrared GaAs, GaAsP, GaAlAs LED can be increased by 4 to 6 times.
Read more: Electrical characteristics of LED