Talking about the method of LED producing colored light

The color and luminous efficiency of LEDs are related to the materials and processes for making LEDs. Currently, red, green and blue are widely used. Due to the low operating voltage of the LED (only 1.5-3V), it can actively emit light and have a certain brightness. The brightness can be adjusted by voltage (or current), and it is resistant to shock, vibration and long life (100,000 hours). 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), and its forward PN junction voltage drop (VF, which can be understood as lighting or operating voltage) is 1.424V, and 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 a red LED with GaAs 0.6P0.4, an orange LED with GaAs 0.35P0.65, and a yellow LED with GaAs 0.14P0.86. Since the manufacturing uses three elements of gallium, arsenic and phosphorus, these LEDs are commonly referred to as bismuth-emitting tubes.

The GaN (gallium nitride) blue LED, the GaP green LED, and the GaAs infrared LED are called two-element LEDs. The latest process 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.

Luminous intensity: The unit of measurement of luminous intensity is illuminance unit (lux), luminous flux unit (lumen Lumen), luminous intensity unit (candle power) 1CD (candle) refers to a completely radiated object, at the freezing point temperature of platinum, every six The luminous intensity of one tenth of a square centimeter area. (Formerly a whale oil candle with a diameter of 2.2 cm and a mass of 75.5 g, burning 7.78 g per hour, a flame height of 4.5 cm, and luminous intensity along the level) 1 L (lumen) means 1 CD candle illuminated at a distance of 1 cm The luminous flux on a plane of 1 square centimeter.

1Lux refers to the illuminance of 1L of luminous flux uniformly 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 cm, the light exit area is 1 cm ^ 2, and the luminous intensity of the light exit is 1600 CD. 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 illuminator, the active illuminant generally adopts CD/m2 as the unit of illuminance 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 illuminating intensity of each element 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 screen must have a brightness of 4000CD/m2 or more to have an ideal display effect under daylight. Ordinary indoor LEDs, the maximum brightness is about 700 ~ 2000 CD / square meter.

The luminous intensity of a single LED is in units of CD, and is equipped with a viewing angle parameter. The luminous intensity has no relationship with the color of the LED. The luminous intensity of a single tube varies from a few mCD to 5000 mCD. The luminous intensity given by the LED manufacturer refers to the point at which the LED illuminates at a current of 20 mA, with the highest luminous intensity at the best viewing angle and at the center position. The shape of the top lens and the position of the LED chip from the top lens when packaging the LED determine the LED viewing angle and intensity distribution. Generally speaking, the larger the LED viewing angle is, the smaller the maximum luminous intensity is, but the luminous flux accumulated on the entire three-dimensional hemisphere is unchanged. When a plurality of LEDs are relatively tightly discharged, the illuminating spheres are superimposed on each Other, resulting in a relatively uniform distribution of luminous intensity of the entire illuminating 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 emission density of the LED as the average luminous intensity of the single tube. Generally, the luminous life of LEDs is very long, and manufacturers generally indicate that they are more than 100,000 hours. Actually, attention should also be paid to the brightness decay period of LEDs. For example, most of the UR red tubes used for automobile taillights are illuminated for ten to several tens of hours. The brightness is only half of that.

The brightness decay period has a lot to do with the material manufacturing process of the LED. Generally, when the economic conditions permit, the four-element LED with slower luminance attenuation should be selected. Color matching, white balance: white is a mixture of red, green and blue colors according to the brightness ratio. When the brightness of green light is 69%, the brightness of red is 21%, and the brightness of blue is 10%, the human eye feels after mixing. It is pure white. However, the red, green, and blue enamel coordinates of the LED are not able to achieve the full chromatographic effect due to the process, etc., and the control color includes the brightness of the deviated ochre to obtain white light, which is called color matching. Before color matching for a full-color LED screen, in order to achieve the best brightness and the lowest cost, try to select a LED device with a three-color luminous intensity of approximately 3:6:1.

White balance requires that the 塬 color is still pure white synthesized at the same ash value. Twilight, base color: Twilight refers to the basic color that can be synthesized into various colors. The ochre in the shade is red, green, and blue. The graph below is the spectrum table. The three vertices in the table are ideal ochre wavelengths. If there is a deviation in the color, the area where the color can be synthesized will be reduced, and the corner shape in the spectrum table will be reduced. From a visual point of view, the color will not only be deviated, but the degree of richness will be reduced.

The red, green and blue light lines emitted by the LED are roughly classified 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. Yellow, green, blue and purple are much cheaper than pure red, pure green and pure blue. Green is the most important of the three colors, because green occupies 69% of the brightness in white and is at the center of the color horizontal row list. Therefore, in the 叁 primary color composition method between the purity and the price of the color, in the 叁 primary color design application, the white balance and the maximum desired brightness value are usually achieved by adjusting the set 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 check the color composition. The three primary colors of white light are generally red, green and blue. The three primary colors are mixed according to the brightness ratio. When the brightness of the green light is 69%, the brightness of the red is 21%, and the brightness of the blue is 10%, the human eye feels after the color mixing. It is pure white. The earlier CRT TVs have been composed of the current LCD liquid crystal display.

Editor: China Lighting Network Feng Yaoyuan

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