The Concept of Full Spectrum LEDs

The Concept of Full Spectrum LEDs

    Speaking of the most popular full spectrum LEDs in the current lighting industry, we must first talk about the concept of "full spectrum". The real "full spectrum" refers to the light emitted by the light source contains the spectrum curve of all wavelengths from ultraviolet light to visible light to infrared light, that is, the complete spectrum emitted by the sun (as shown in the spectrum curve in Figure 1), which is also the most comprehensive "full spectrum" in nature. The most talk about full spectrum LEDs refer to the narrower "full spectrum". The full spectrum LEDs refer to the light emitted by the LED light source in the visible light range close to the spectrum curve emitted by the sun in the visible light range (as shown in Figure 2). 

    The ultraviolet and infrared parts are removed. The main reason for removing these two parts is to make the full spectrum LEDs have the possibility of industrialization and make the full spectrum LEDs more "simple". If you want to add ultraviolet and infrared light to make a truly comprehensive full spectrum, there is basically no possibility of mass production and practical application, because the entire packaging system and subsequent applications will become very complicated and extremely difficult. Even if we remove the full spectrum of ultraviolet and infrared light, it is relatively "simple" to do but not that simple in reality. For example, to achieve a full spectrum CRI, the CRI must be very close to 100. However, many companies currently find it very difficult to increase the CRI from 96 to 98%, let alone to reach 99 or even above 99.

Figure 1     Full spectrum of sunlight (280nm-4000nm)

Figure 2     Spectral curve of sunlight in the visible range (380nm-780nm)

Ways of implement full-spectrum LEDs 

    Ways of implement full-spectrum LEDs From theoretical analysis, it can be concluded that it is nothing more than starting from two major directions: chips and phosphors. There are mainly two ways to use chips, one is to use chips to excite phosphors, while another is to use chips without adding phosphors. As for phosphors, they must be used in conjunction with chips, with different emission wavelengths and excitation wavelengths. In general, there are four main ways to implement full-spectrum LEDs: the 1st, use single-band blue chips to excite phosphors; the 2nd, use dual-band blue chips/tri-bands blue chips to excite phosphors; the 3rd, use purple light chips to excite phosphors; the 4th, use a combination of multiple chips. The following will explain the four methods one by one.

    1. The method of using a single-band blue chip to excite phosphors. This method is basically the same as the ordinary LED packaging method. The difference is that in order to make the spectrum curve emitted by the LED close to the full spectrum, a variety of phosphors will be added, such as green powder, yellow powder, red powder, and even orange powder, cyan powder, blue powder, etc. Although this method can also produce an effect close to the full spectrum, there will still be a relatively strong blue light peak. In addition, due to the low blue light excitation efficiency of cyan powder and blue powder and the reabsorption problem between phosphors, the emitted spectrum curve will still lack light in the range of 470-510nm.

    2. The method of dual-band blue light chip/triple-band blue light chip to excite phosphor. This method will greatly improve the effect of the single-band blue light chip method. By matching the high and low blue light wavelengths of the dual-band blue chip, and then using a variety of phosphors, the missing light in the 470-510nm range can be compensated. Dual-band blue chips usually choose two bands of 430-450nm and 460-480nm, and then use 490-510nm cyan powder, 510-550nm green powder, 550-580nm yellow powder, 580-600nm orange powder and 630-660nm crimson phosphor.Three-band blue chips're usually using a combination of chips with three bands of 430-440nm, 440-460nm and 460-480nm. The phosphor solution is similar to the dual-bands blue chip solution. This method can flexibly adjust the chip band and the phosphor type and ratio to achieve a closer solar spectrum (as shown in Figure 3), and the color index (CRI) can reach more than 98. However, this solution requires the addition of many types of phosphors, and the phosphor systems of different wavelengths may also be different. This will place higher requirements on phosphor proportioning personnel, and the ratio stability and batch consistency of the mass production process will also be more difficult to control. At present, some phosphor manufacturers will pre-mix two or more phosphors before giving them to the encapsultion for use. This method will wildly reduce the difficulty of powder mixing in the encapsultion, but it should be noted that the pre-mixed phosphors may have sedimentation and separation during transportation and storage, resulting in poor mixing effect. The main reason for the sedimentation and separation problem is that the phosphor manufacturer produces two different phosphors separately and then mixes them together. When there remains differences in the particle size and particle size distribution of the two phosphors, the phosphor with a larger particle size will be settled.

Figure 3     Dual-band blue light & triple-band blue light full spectrum curves (for reference)

    3. The way that the purple light chip excites the phosphor. This method has a relatively low light efficiency. The main reason is that the phosphors on the market are basically developed to match the blue light chip. The point where the mature phosphor has the highest excitation efficiency is usually in the blue light band. Although there is also an excitation peak in the purple light range, the excitation efficiency is much lower. In addition, the wavelength of the purple light chip is usually in the range of 385-405nm, and the efficiency of the chip itself is not high, resulting in a relatively low overall light efficiency, and the cost of the purple light chip is higher than that of the blue light chip. However, the spectrum produced by the purple light chip full spectrum solution can actually be as close to the solar spectrum as possible, and the spectrum saturation is high, while also avoiding the appearance of short-wave blue light (as shown in Figure 4). One thing that needs to be noted about the purple light full spectrum product is that during the long-term aging and use of the product, the phosphor will be more attenuated by the long-term radiation and excitation of the purple light, and it is more likely to have color drift and color temperature abnormalities in the later stage compared to the blue light chip solution. In addition, the purple light will cause greater damage to the organic materials used in the packaging, such as the packaging glue and the bracket plastic material, which will shorten the life of the LED. In addition, after long-term use, purple LEDs may leak purple light, which is something that requires special attention.

Figure 4:     Full spectrum of purple light spectrum curve (for reference)

    4. Multiple chips combinations. This method can use a combination of blue, cyan, green, yellow and red chips to achieve a full spectrum. In principle, this method is also suitable for achieving full-spectrum white light, but why do few people use this method to make a full spectrum? It may be affected by the following aspects. First, the light emitted by the chip usually has a narrow half-wave width, and it is difficult to achieve the effect of the spectrum emitted by a wide half-wave width material such as phosphor. Secondly, the electro-optical conversion efficiency of chips with different luminous colors varies greatly. The electro-optical conversion efficiency of blue light chips is usually higher, while the electro-optical conversion efficiency of other types of chips is relatively low. In this way, it is difficult to adjust to a state of light color balance in the same package. Thirdly, the attenuation of chips with different luminous colors during aging and use is very different. The blue light chip attenuates slowly, and the yellow and red light chip attenuates quickly. In this way, color drift and abnormal color temperature will occur during long-term aging and use, and it is difficult to achieve a good use effect. In addition, the multi-chip combination method can also add phosphors to achieve a full spectrum, which is closer to the way the chip excites phosphors. Usually, this method of adding multiple chips to phosphors is more difficult to mix, because the changes in the spectrum and color points will also be affected by other luminous color chips in the package.

In order to more clearly compare the above four LED full spectrum implementation methods, please refer to the following Table 1 Comparison of LED full spectrum implementation methods.

Table 1     Comparison of LED full spectrum implementation methods

Methods	Lighting Eff.	CRI	Cost	Encapsulation	Complex Performance	Realization Ways
Single brand blue excitation	Higher	Higher	Lower	Lower	Better	Chip+Phosphor
Dual-/tri-brands blue excitation	High	High	Low	Low	Good	Chip+Phosphor
Violet excitation	Low	High	High	Low	Normal	Chip+Phosphor
Multiple chips excitation	Lower	High	High	High	Normal	Chip

Application of full spectrum LEDs

    The above notes describe how to achieve full spectrum LED, but how to apply full spectrum LEDs? This is another very important question. Before explaining the application of full spectrum, we must first make it clear that there is another parameter that must be considered in the application of LED light source - color temperature. The sunlight is different at different times of the day or in different seasons. For example, the color temperature of the sun is about 2000K at sunrise in the morning, about 5000K at noon, and about 2300K at sunset. Therefore, full spectrum LEDs also needs to consider achieving the full spectrum closing to the sunlight effect at the corresponding color temperature based on different color temperatures. Of course, the full spectrum of different color temperatures can also be achieved through the above methods. The series of technical solutions're applied with full spectrum LEDs.

    Based on the above description, we can know that full-spectrum LED light sources can be used in almost any conventional lamps, such as our conventional household lamps, outdoor lamps, industrial lighting lamps, table lamps, plant lighting, etc. The specific application depends more on the price and people's acceptance. At present, the most widely used to grow lamps, table lamps etc. As your professionals: we need to consider: Is the current "full spectrum" good light source that people really need? If you want to communicate and discuss, please send us private messages.