Detailed knowledge of LED light-emitting diodes

First, LED basics

The LED is an abbreviation taken from the three words of Light Emitting Diode, which is translated into "light-emitting diode" in Chinese. As the name suggests, a light-emitting diode is a diode that can convert electrical energy into light energy. At present, different light-emitting diodes can emit light of different wavelengths from infrared to blue, and light-emitting diodes that emit purple or ultraviolet light have also been born. In addition to this, there are white LEDs that are coated with phosphors on blue LEDs to convert blue light into white light.

LED color and process:

The materials used to make the LEDs can produce photons with different energies, thereby controlling the wavelength of the light emitted by the LED, that is, the spectrum or color. The material used in the history of the first LED is arsenic (As) gallium (Ga), its forward PN junction voltage drop (VF, can be understood as lighting or operating voltage) is 1.424V, the emitted light is infrared spectrum . Another commonly used LED material is phosphorous (P) gallium (Ga), which has a forward PN junction voltage drop of 2.261V and emits green light.

Based on these two materials, the early LED industry used GaAs1-xPx material structure to theoretically produce LEDs from infrared light to any wavelength in the green range. The subscript X represents the percentage of phosphorus replaced by arsenic. The wavelength of the LED is typically determined by the PN junction voltage drop. Typical examples include red LEDs with GaAs 0.6P0.4, orange LEDs with GaAs 0.35P0.65, and yellow LEDs with GaAs 0.14P0.86. Since the manufacturing uses three elements of gallium, arsenic and phosphorus, these LEDs are commonly referred to as three-element luminous tubes. The GaN (gallium nitride) blue LED, the GaP green LED, and the GaAs infrared LED are called two-element LEDs. The latest technology is a four-element LED made of four elements of AlGaInN mixed with aluminum (Al), calcium (Ca), indium (In) and nitrogen (N). It can cover all visible light and some UV. The spectral range of light.

LED luminous intensity:

The unit of measurement of luminous intensity is illuminance unit (Lux Lux), luminous flux unit (Lumen Lumen), and luminous intensity unit (candle power).

1CD (candle light) refers to the luminous intensity of a completely radiated object at an area of ​​one-sixth of a square centimeter at the freezing point of platinum. (Formerly a whale oil candle with a diameter of 2.2 cm and a mass of 75.5 grams, burning 7.78 grams per hour, a flame height of 4.5 cm, luminous intensity along the horizontal direction)

1L (lumen) refers to the luminous flux of a 1 CD candle illuminated on a plane having a distance of 1 cm and an area of ​​1 cm 2 .

1Lux refers to the illuminance of 1L of luminous flux evenly distributed over an area of ​​1 square meter.

Generally, the active illuminator adopts a luminous intensity unit candle CD, such as an incandescent lamp, an LED, etc.; a reflective or penetrating object uses a luminous flux unit lumen L, such as an LCD projector, and the illuminance unit Lux Lux, which is generally used in photography and the like. . The three units of measure are numerically equivalent, but need to be understood from different angles. For example, if the brightness (light flux) of an LCD projector is 1600 lumens, and the size of the projection to the total reflection screen is 60 inches (1 square meter), the illumination is 1600 lux, assuming that the light exits from the light source 1 In centimeters, the area of ​​the light exit is 1 square centimeter, and the luminous intensity of the light exit is 1600CD. The true LCD projector, due to loss of light propagation, loss of reflection or light-transmissive film, and uneven distribution of light, will greatly reduce the brightness, and generally 50% efficiency is very good.

In actual use, light intensity calculations often use data units that are easier to map or use. For the LED display screen, the active illuminant generally adopts CD/m2 as the unit of luminous intensity, and the observation angle is used as an auxiliary parameter, which is equivalent to the illuminance unit of the screen surface; this value and the effective display area of ​​the screen Multiply, the illuminance of the entire screen at the optimal viewing angle is obtained. Assuming that the illuminance of each pixel in the screen is constant within the corresponding space, this value can be considered as the luminous flux of the entire screen. Generally, the outdoor LED display must have a brightness of more than 4000CD/m2 to have an ideal display effect under daylight. Ordinary indoor LED, the maximum brightness is about 700 ~ 2000 CD / square meter.