What is the color rendering index?

July 22, 2022

It is stronger because of sharing. Hello everyone, I am the old man. Two days ago, I argued with a group friend of a certain group about the RA of LED lights. The specific content of the debate and the process will not be described here. If you are interested, I can take a screenshot and show you a private message. Since many people are still not clear about some basic principles of LED, here, I combined my knowledge, consulted some materials, and consulted some industry leaders to give you a simple popular science. Please watch the video:


First define a premise, civilian-grade LED! The civilian-grade LED I define is what we ordinary people can see, touch, and buy. Those high-end, laboratory, and alien are not in the scope of discussion.


The earliest LEDs that we ordinary people come into contact with should be the small red lights on keyboards and game consoles when we were young. At that time, they were also called light-emitting diodes. Its English name abbreviation is what we now call LED, Light Emitting Diode.


The earliest LEDs we used started with red light, and later there were green light and yellow light diodes. Until 1993, the year I graduated from elementary school, a Japanese master named Shuji Nakamura invented high-brightness blue LEDs, and until 2014, master Nakamura, Hiroshi Amano and Isamu Akasaki jointly developed white LEDs. light, and won a Nobel Prize in Physics for it.


Of course, the method invented by the three masters at the time was still a bit complicated and the cost was a bit high. With the advancement of production technology, there are now multiple methods to synthesize white LEDs, but the invention of master Nakamura's blue diode is the foundation for all LEDs in the future. The cornerstone of technology.


Briefly explain the principle: The LED itself is basically the same as the fluorescent tube light-emitting principle we used in our early years. They all use light with a specific wavelength to excite phosphors to emit light. But the difference is that LED is semiconductor light, and fluorescent light is metal gas discharge light.


Since it comes to phosphors, let's list several that have been announced. Of course, there may be newer and more advanced formulas that have not been announced. If you use AlGaAs aluminum gallium arsenide or GaP gallium phosphide material, it can emit red light with a wavelength of 610~750nm, AlGaInP aluminum indium gallium phosphide material can make orange or yellow light with a wavelength of 570~610nm, and SiC carbide Silicon can make blue, and the wavelength is 450~500nm. Of course, there are also GaN gallium nitride and GaP gallium phosphide with a wavelength of 500~570 green light, and InGaN indium gallium nitride with a wavelength of 380~450 violet light.


When LED first entered the film and television industry a few years ago, there was only white light! White LEDs at the time had poor color rendering. Generally around 80, white light LEDs generally use blue LEDs to excite a variety of phosphor mixtures mentioned above. The blue light of the LED itself and other spectrums emitted by the phosphors are mixed to produce white light. In the early years, the technology of LED was immature and naturally there were not so many kinds of phosphors, and the CRI was not high. With the continuous upgrading of the formula, the precise proportioning LED technology is also becoming more and more mature.


There are many indicators to judge the pros and cons of LED lights. We generally refer to the color rendering index CRI and TLCI, etc. Its function is to define an indicator to reflect the ability of the light source to restore the true color of the object.


CRI is an evaluation standard for defining light sources in the last century. There are 15 reference colors in total. The average value of R1-R8 for the first 8 colors is what we often call Ra. If you add the color reproduction ability calculated by R9-R15 The average is called Re. As far as the current popular LED technology is concerned, R1~R15 are somewhat contradictory. If you just make the product Ra higher, it is easy to 99%, but you need to sacrifice R9~R15. Although the Ra looks good, the real color reproduction ability is not good. Because R9~R15 contains bright colors such as red, yellow, green, and blue. Therefore, the better color rendering ability should be that Ra and Re are very close, or Re can directly achieve 98+, but the current civilian-level technology has not reached that height.


Let's talk about the mixed color LED, which is the RGB full-color LED that has only emerged in the past two years. Generally, most of the LEDs we can buy are R, G, and B three-color LED lamp beads. In theory, the three primary colors of RGB are mixed into white in equal proportions. The color principle is actually not universal on RGB lamp beads, mainly because this kind of light only has the discrete spectrum of the three colors of RGB. And because the RGB three-color LED lamp beads have different wavelengths, the brightness under the same power is not the same.


For example, we use an iwata GL03, adjust the color under the same power, you will feel that the blue is obviously brighter than the red and green. Therefore, what needs to be adjusted for LED color mixing is the brightness ratio, which is usually about 69% green, about 21% red and about 10% blue. This is only a theoretical ratio, and the conversion rate of LED light and the brightness of each LED should also be considered. What is the color coordinate, what is the color coordinate of the light after the planned mixing, etc., and then accurately calculate the mixing ratio.


It can be said that the mixing ratio of each RGB LED product will not be the same, just like everyone's personality is different. After a series of complex adjustments, the mixed light is perceived as close to white by the human eye. Note that what is said here is the feeling, and the biggest difference between the mixed white and the white emitted by the white LED lamp bead is the color rendering RA! The color rendering of white and real white LED lamp beads mixed with only three color discrete spectra is a lot worse.


Well, so far we have only talked about three-color RGB. The iwata Master R light stick recommended to everyone recently is a five-color LED lamp bead color mixing. I also told you some color mixing processes of high and low color temperature during dismantling. One difficulty is to ensure that under the same color, such as HUE60° 180° 300°, the brightness changes from 1% to 100% to ensure that the color is not biased. This is also the focus of many domestic manufacturers in research and development. A watershed moment for judging RGB color LED lights.


Going back to the beginning, the group friend who has been arguing finally sent out two low color temperature pictures of Ra98, which does not really explain how good the so-called specially exported LED lights of the brand he emphasized. If you really want to judge, it is very simple to adjust to 5600K white, and mark both Ra and Re, and you will find out.


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