How LEDs Produce Colored Light

The luminous color and luminous efficiency of LED are related to the material and process of making LED. At present, red, green and blue are widely used. Due to the low working voltage of the LED (only 1.5-3V), it can actively emit light and have a certain brightness, and the brightness can be adjusted by voltage (or current), and it is shock-resistant, vibration-resistant, and has a long life (100,000 hours). Different materials for making LEDs can produce photons with different energies, so that the wavelength of light emitted by the LED can be controlled, that is, the spectrum or color.


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

Based on these two materials, the early LED industry used the 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 replaced by arsenic. Generally, the wavelength and color of the LED can be determined by the voltage drop of the PN junction.Typical ones are red LEDs of GaAs0.6P0.4, orange LEDs of GaAs0.35P0.65, yellow LEDs of GaAs0.14P0.86, etc. Because gallium, arsenic, and phosphorus are used in the manufacture, these LEDs are commonly known as three-element light-emitting tubes.


GaN (gallium nitride) blue LEDs, GaP green LEDs and GaAs infrared LEDs are called two-element light-emitting tubes. At present, the latest process is a four-element LED made of AlGaInN four-element material mixed with four elements of aluminum (Al), calcium (Ca), indium (In) and nitrogen (N), which can cover all visible light and some ultraviolet light. The spectral range of light.

Luminous intensity: The measurement units of luminous intensity are illuminance unit (lux), luminous flux unit (lumen Lumen), luminous intensity unit (candle power). 1CD (candle power) refers to a completely radiating object. The luminous intensity of an area of one-tenth of a square centimeter. (It used to refer to a whale oil candle with a diameter of 2.2 cm and a quality of 75.5 grams, burning 7.78 grams per hour, a flame height of 4.5 cm, and the luminous intensity along the horizontal direction) 1L (lumen) refers to 1 CD candlelight irradiated at a distance of 1 cm , the luminous flux on a plane with an area of 1 cm2.
1Lux (lux) refers to the illuminance at which 1L of luminous flux is evenly distributed on an area of 1 square meter. Generally active illuminants adopt the luminous intensity unit candle power CD, such as incandescent lamps, LEDs, etc.;Reflective or penetrating objects use lumen L as the unit of luminous flux, such as LCD projectors, etc.; Lux, the unit of illumination, is generally used in photography and other fields. The three units of measurement are numerically equivalent, 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 projected onto the total reflection screen is 60 inches (1 square meter), then its illuminance is 1600 lux, assuming that the distance between the light outlet and the light source is 1 cm, and the area of the light outlet is 1 square centimeter, then the luminous intensity of the light outlet is 1600CD. However, due to the loss of light transmission, loss of reflection or light-transmitting film, and uneven distribution of light, the brightness of a real LCD projector will be greatly reduced. Generally, an efficiency of 50% is good.

In actual use, light intensity calculations often use data units that are easier to survey and map or change directions. For active luminous bodies such as LED screens, CD/square meter is generally used as the unit of luminous intensity, and the viewing angle is used as an auxiliary parameter, which is equivalent to the illuminance unit lux on the surface of the screen; this value is effectively displayed on the screen. Multiply the areas to get 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 have a brightness of more than 4000CD/square meter to have an ideal display effect under sunlight. For ordinary indoor LEDs, the maximum brightness is about 700-2000 CD/square meter.

The luminous intensity of a single LED is in CD, and it is equipped with viewing angle parameters. The luminous intensity has nothing to do with the color of the LED. The luminous intensity of a single tube varies from a few mCD to 5000mCD. The luminous intensity given by the LED manufacturer refers to the point where the LED lights up at a current of 20mA, the best viewing angle and the center position with the highest luminous intensity. When packaging LEDs, the shape of the top lens 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 spherical surfaces overlap with each other, resulting in a relatively uniform distribution of luminous intensity across the entire luminous plane.


When calculating the luminous intensity of the screen, 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 the LED emission density, as the average luminous intensity of a single tube. Generally, the luminous life of LED is very long, and manufacturers generally mark it as more than 100,000 hours. In fact, attention should also be paid to the brightness decay cycle of LED. , the brightness is only half of the original.

The brightness attenuation period has a lot to do with the material process of LED production. Generally, four-element LEDs with slower brightness attenuation should be selected when economic conditions permit. 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%. Arrived in pure white. However, the chromaticity coordinates of the red, green and blue colors of the LED cannot achieve the effect of the full color spectrum due to the manufacturing process and other reasons. Controlling the brightness of the primary colors including the biased primary colors to obtain white light is called color matching. Before color matching for full-color LED screens, in order to achieve the best brightness and the lowest cost, try to choose LED devices with a ratio of 3:6:1 for the three primary colors to form pixels.White balance requires that the three primary colors synthesized under the same gray value are still pure white. Primary color, primary color: Primary color refers to the basic color that can synthesize various colors. The primary colors in color light are red, green, and blue. The figure below is a spectrum table, and the three vertices in the table are ideal primary color wavelengths. If the primary color deviates, the area where the color can be synthesized will decrease, and the triangles in the spectral table will shrink. From a visual point of view, the color will not only deviate, but also reduce the richness.


The red, green and blue light emitted by LEDs can be 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 and orange red. , Yellow-green, and blue-purple are much cheaper than pure red, pure green, and pure blue. Among the three primary colors, green is the most important, because green occupies 69% of the brightness of white, and is in the center of the color horizontal arrangement table. Therefore, in the three-primary color composition mode between the purity of the color and the price, in the three-primary color design application, the white balance and the maximum desired brightness value are usually achieved by adjusting the LED current.

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 the color composition. The three-primary-color white light is generally made by mixing 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%. It is pure white. The early CRT TVs to the current LCD liquid crystal displays are all composed in this way.