A brief discussion on the method of LED producing colored light

The color and efficiency of LED light are related to the materials and processes used to make it. Currently, red, green, and blue are widely used. Since LEDs have a low operating voltage (only 1.5-3V), they can emit light actively and have a certain brightness. The brightness can be adjusted by voltage (or current). They are also impact-resistant and vibration-resistant, and have a long lifespan (100,000 hours). Different materials used to make LEDs can produce photons with different energies, thereby controlling the wavelength of light emitted by the LED, that is, the spectrum or color.

The material used in the first LED in history was gallium arsenide (Ga), whose forward PN junction voltage drop (VF, which can be understood as the lighting or operating voltage) was 1.424V, and the light emitted was in the infrared spectrum. Another commonly used LED material is gallium phosphide (Ga), whose forward PN junction voltage drop is 2.261V, and the light emitted is green.

Based on these two materials, the early LED industry used GaAs1-xPx material structure, which can theoretically produce LEDs with any wavelength from infrared light to green light. The subscript X represents the percentage of phosphorus replacing arsenic. Generally, the wavelength color of the LED can be determined by the PN junction voltage drop. Typical examples include GaAs0.6P0.4 red LED, GaAs0.35P0.65 orange LED, GaAs0.14P0.86 yellow LED, etc. Since gallium, arsenic and phosphorus are used in manufacturing, these LEDs are commonly known as three-element light-emitting tubes.

GaN (gallium nitride) blue LED, GaP green LED and GaAs infrared LED are called two-element LEDs. The latest technology is to use AlGaInN, a four-element material that mixes aluminum (Al), calcium (Ca), indium (In) and nitrogen (N), to make four-element LEDs, which can cover the entire visible light spectrum and part of the ultraviolet light spectrum.

Luminous intensity: The units of measurement for luminous intensity include illuminance unit (Lux), luminous flux unit (Lumen), and luminous intensity unit (Candle power). 1CD (candle power) refers to the luminous intensity of a completely radiating object at the freezing point of platinum per one-sixtieth of a square centimeter. (Previously, it referred to a whale oil candle with a diameter of 2.2 cm and a mass of 75.5 grams, burning 7.78 grams per hour, with a flame height of 4.5 cm, and the luminous intensity in the horizontal direction.) 1L (lumen) refers to the luminous flux of 1 CD candlelight irradiated on a plane with a distance of 1 cm and an area of ​​1 square centimeter.

1Lux refers to the illumination when 1L of luminous flux is evenly distributed on an area of ​​1 square meter. Generally, active luminous bodies use the unit of luminous intensity, candlelight CD, such as incandescent lamps, LEDs, etc.; reflective or penetrating objects use the unit of luminous flux, lumen L, such as LCD projectors, etc.; and the unit of illumination, Lux, is generally used in photography and other fields. The three units of measurement are equivalent in value, but need to be understood from different perspectives. For example: if the brightness (luminous flux) of an LCD projector is 1600 lumens, and the size of its projection on the total reflection screen is 60 inches (1 square meter), then its illumination is 1600 lux. Assuming that its light outlet is 1 cm away from the light source and the area of ​​the light outlet is 1 square cm, the luminous intensity of the light outlet is 1600CD. However, due to the loss of light propagation, the loss of reflection or light-transmitting film, and the uneven distribution of light, the brightness of a real LCD projector will be greatly reduced, and generally 50% efficiency is good.

In actual use, light intensity calculations often use data units that are easier to measure or use them in a different direction. For active light sources such as LED screens, CD/square meter is generally used as the unit of luminous intensity, and the observation angle is used as an auxiliary parameter. It is equivalent to the unit of illumination lux on the surface of the screen. Multiply this value by the effective display area of ​​the screen to obtain the luminous intensity of the entire screen at the best viewing angle. Assuming that the luminous intensity of each graphic element in the screen is constant in the corresponding space, this value can be considered as the luminous flux of the entire screen. Generally, outdoor LED screens must reach a brightness of more than 4000 CD/square meter to have a relatively ideal display effect under sunlight. The maximum brightness of ordinary indoor LEDs is around 700 to 2000 CD/square meters.

The luminous intensity of a single LED is measured in CD and is also equipped with a viewing angle parameter. The luminous intensity has nothing to do with the color of the LED. The luminous intensity of a single tube ranges from a few mCD to 5000mCD. The luminous intensity given by the LED manufacturer refers to the point where the luminous intensity is the highest at the best viewing angle and the center position when the LED is lit at a current of 20mA. The shape of the top lens when encapsulating the LED and the position of the LED chip from the top lens determine the LED viewing angle and light intensity distribution. Generally speaking, the larger the viewing angle of the same LED, the smaller the maximum luminous intensity, but the accumulated luminous flux on the entire three-dimensional hemisphere remains unchanged. When multiple LEDs are arranged closely and regularly, their luminous spheres overlap each other, resulting in a more uniform distribution of luminous intensity on the entire luminous plane.

When calculating the screen luminous intensity, it is necessary to multiply the maximum point luminous intensity value provided by the manufacturer by 30% to 90% according to the LED viewing angle and LED emission density, as the average luminous intensity of a single tube. Generally, the luminous life of LEDs is very long, and manufacturers generally indicate that it is more than 100,000 hours. In fact, attention should also be paid to the brightness attenuation cycle of LEDs. For example, most UR red tubes used in car taillights will only be half as bright as before after being lit for more than ten to dozens of hours.

The brightness decay cycle is closely related to the material process of LED production. Generally, if economic conditions permit, four-element LEDs with slower brightness decay should be selected. Color matching and white balance: White is a mixture of red, green and blue according to the brightness ratio. When the brightness of green in the light is 69%, the brightness of red is 21%, and the brightness of blue is 10%, the human eye perceives pure white after the color mixing. However, the chromaticity coordinates of the red, green and blue colors of LED cannot achieve the effect of the full color spectrum due to the process and other reasons. Controlling the brightness of the primary colors, including the deviated primary colors, to obtain white light is called color matching. Before matching the colors for a full-color LED screen, in order to achieve the best brightness and the lowest cost, you should try to select LED devices with the luminous intensity of the three primary colors in a ratio of approximately 3:6:1 to form the pixels.

White balance requires that the three primary colors still produce pure white when synthesized at the same gray value. Primary colors and base colors: Primary colors refer to the basic colors that can synthesize various colors. The primary colors in color light are red, green, and blue. The following figure is a spectrum chart, and the three vertices in the chart are ideal primary color wavelengths. If the primary colors are biased, the area that can be synthesized will be reduced, and the triangle in the spectrum chart will be smaller. From a visual perspective, the colors will not only be biased, but also less rich.

The red, green and blue light emitted by LEDs are roughly divided into purple-red, pure red, orange-red, orange, orange-yellow, yellow, yellow-green, pure green, emerald green, blue-green, pure blue, blue-purple, etc. according to their different wavelength characteristics. Orange-red, yellow-green and blue-purple are much cheaper than pure red, pure green and pure blue. Green is the most important of the three primary colors because it accounts for 69% of the brightness of white and is in the center of the horizontal color arrangement table. Therefore, in the three-primary color composition method that balances the purity of color and price, in the three-primary color design application, it is usually achieved by adjusting the set LED current to achieve white balance and the maximum expected brightness value.

We generally use the simplest and most optimized color matching method as the color reproduction method for designing full-color display technology. White balance is one of the important signs to test color composition. The three primary colors of white light are generally mixed by the three primary colors of red, green and blue according to the brightness ratio. When the brightness of green in the light is 69%, the brightness of red is 21%, and the brightness of blue is 10%, the human eye perceives pure white after the color mixing. Early CRT TVs and current LCD displays are all composed in this way.