Most of today’s lighting standard “white” led plant aquarium lights use InGaN semiconductors to emit light in the blue part of the spectrum, combined with yttrium aluminum garnet (YAG) phosphors (doped with cerium). The blue photons of most LEDs are absorbed by the phosphor and re-emitted in the yellow part of the spectrum. The mixture of the remaining blue photons and yellow illumination provides a good approximation of white light to the eye.
This combination of LED and phosphor is a proven technology with good performance but not perfect. An obvious disadvantage of this led is the lack of red-related color light components. The color temperature (CCT) of the light emitted by this led is relatively poor and the color rendering index (CRI) is relatively low. Combining YAG phosphors with other materials will increase red luminescence, leading to the availability of “warm” white led plant light, but at the cost of hard-won efficacy.
This article discusses how to make phosphor work for LED plant aquarium lighting to face the challenges, and then considers how the phosphor research impacts on the efficiency of solid-state lighting, CCT and CRI.
What is YAG phosphor
YAG is a professional chemical material with high phosphor and can increase the brightness of LED. The material contains rare earth element cerium (scientifically labeled Y3Al5O12: Ce3 +). It was synthesized for the first time in 1967. The synthesis method was obtained by improving the process of CRT phosphor material.
Working principle of YAG phosphor
The specific principle is that the blue photons emitted by the blue LED are absorbed by the outer electrons of the constituent molecules or atoms in the YAG phosphor. The electrons that absorb the photon energy contain higher energy and faster, and will transition to the higher energy region. This process is called electron transition. The energy will be released in the process of electron transition to a new stable state, Photons with longer wavelengths than absorbed photons (slightly red, some green and a large number of yellow), other parts of the energy will be radiated out as heat energy, and the electrons will change from an active state to a stable state.
White light generation principle of led plant aquarium light
The main yellow luminescence of YAG phosphor and the blue radiation directly from the led plant light are combined through the YAG:Ce coating “leakage”, which can be a good approximation of white light, which converts blue light into “white light” through the absorption and re-emission of the phosphor. The phenomenon is called the Stokes shift, named after the Irish physicist George G. Stokes, who described this effect in a paper in 1852. Figure above shows how the Stokes shift occurs through the absorption and emission curves of YAG phosphors.
Advantages of YAG phosphor
Figure above shows the relative spectral emission curve of modern white led plant light using YAG phosphors (in this case, OSRAM OSLON SSL 150 LED produces 136 lm (forward current 350 mA, forward voltage 3.1 V), with an efficiency of 125 lm /W. The dashed line is a function of the sensitivity of the eye, indicating how the eye responds to light of different wavelengths. Although the first peak (corresponding to the blue photon directly from the LED) is larger, it is less obvious because the eye is sensitive to light. Insensitive In contrast, the photons emitted by the phosphor are centered at 560 nm-the point where the eye best senses light.
Since it is a proven technology that produces acceptable results, the characteristics of YAG represent the benchmark for other LED phosphors. YAG phosphor can not only efficiently absorb blue photons, but also quickly release energy. After the atomic electrons of YAG phosphor absorb the energy of blue photons, they will quickly release yellow photons and energy, which effectively reduces the phenomenon of saturation quenching. This is a process in which the emitted photons are usually “submerged” and prevent the high photon flux in the phosphor matrix from escaping.
YAG phosphors have many advantages for other LED applications.First of all, YAG phosphor has a high photon conversion rate. Under the irradiation of blue light, YAG phosphor can convert 80% of the absorbed blue energy into longer-wavelength light. Secondly, YAG phosphor has good stability, even if it is excited by blue LED for a long time or is exposed to the air for a long time, it will rarely degrade. Finally, the synthesis of YAG phosphors is relatively simple and the cost is relatively low. Most of the materials used are materials used to synthesize traditional phosphor CRT, including high-purity chemical raw materials Y2O3, Al2O3, CeO2, etc.
Disadvantages of YAG phosphor
Despite the proven performance of LEDs, YAG phosphors are not perfect. Two practical problems are “temperature quenching” and relatively low chemical stability. The reason for temperature quenching is difficult to grasp, but in layman’s terms, increased temperature will cause electrons to usually absorb blue photons (and then emit yellow) “missing” (in other words, atoms are ionized). YAG phosphor will be affected by temperature quenching at about 200°C, and the LED will not experience abnormally high temperature during normal operation. In addition, the low chemical stability of the material limits the lifetime of the LED (defined as the point at which the luminosity of the device decreases), which reduces the yield by 70% when it is new. The manufacturer refutes this argument by pointing out that modern white led plant light can operate for 30,000 hours or more, but the researchers say this is measured under “ideal” operating conditions, and chemical instability may affect harsh environments The performance of the equipment used.
However, perhaps the biggest challenge facing YAG phosphors is the aesthetic challenge. Solid-state lighting manufacturers are keen to get consumers to accept this technology, but a common complaint is that the “dazzling” light produced by white led plant light has almost no “warmth” brought by traditional incandescent lighting. This feeling is due to the fact that the YAG phosphor produces “blue” white light with little red light content. The result is light with high CCT (5000 to 8300K). The lack of a significant red wavelength introduces another problem for YAG phosphor devices: poor CRI. CRI is a measure of the degree to which an illuminating light source reproduces the color of an object compared to sunlight (CRI is 100). Despite the obvious shortcomings, the CRI of an incandescent bulb is about 95. In contrast, cold white led plant light usually have a CRI of 70 to 80.
Generation of red light in LED plant aquarium light
LED manufacturers have solved the challenges of CCT and CRI to a certain extent, mixing YAG phosphor with another phosphor with red wavelength added to extend CCT to warmer areas and improve CRI. Replacing UV LEDs with blue LEDs can further improve CCT and CRI. LED manufacturers provide commercial UV products for this purpose. For example, Philips Lumileds offers an ultraviolet version of its Luxeon LED. The device emits a wavelength between 395 and 400 nm, and the minimum radiation intensity at 500 mA is 525 mW/sr. (Due to the inability of the eye to see ultraviolet light, the radiant intensity (watts/steradian) of UV LEDs is evaluated instead of the more familiar efficiency (lm/W) data cited by white LEDs.)
The main disadvantage of adding “red” phosphors is the reduced LED efficiency. To make matters worse, the temperature quenching threshold of the red phosphor is even lower than that of the YAG phosphor, further reducing the efficiency of the typical LED operating temperature.
MOKOLight white led plant aquarium light has a working voltage of 350 mA and 2.85 V, but the cool white (5000 K) version has a luminosity of 135 lm and an efficacy of 135 lm/W, while the brightness of the warm white (2700 K) is respectively It is 97 lm and 97 lm/W. Compared with cool white devices, warm white products of other manufacturers show similar low efficiency. Figure above shows the difference between the CCT and CRI of blue LED and YAG phosphor, and the mixture of UV LED and YAG phosphor and red phosphor.
Efforts on improving phosphor efficiency
In the past fifteen years, the pursuit of LED efficiency has been a key driving factor in the development of solid-state lighting components for major manufacturers. Significant progress has been made in the initial generation of photons and the extraction of photons from the mold. The last thing manufacturers want is the relatively low conversion efficiency of red phosphors, which affects the luminosity of their warm white LEDs.
Manufacturers invest more time and experience in the research of new phosphor materials, hoping to obtain phosphors with higher optical conversion rate, and phosphors can emit more color light and have better color rendering. The most promising candidate products are nitrides and oxynitride materials, which use another rare element metal europium (Eu) to replace the electroluminescence properties of cerium. Many of these new phosphors can be excited with purple or blue LEDs and match the room with the high-temperature quantum efficiency of YAG: Ce phosphors. In addition, the nitride phosphor will not degrade under high temperature/high humidity conditions, so it is suitable for LED lighting in harsh environments.
A series of high-efficiency nitrogen oxide phosphors are MSi2O2N2: Eu 2 + (where M = Ca 2 +, Sr 2 +, Ba 2 +) composition, with an emission range of 575 to 675 nm, at temperatures exceeding 200°C , The quantum efficiency is greater than 85% (Figure above). The yellow, orange and red EU 2 + color light transformed by these nitride phosphorus is combined with the green light generated by YAG. The synthesized white light has better color temperature and higher color rendering than the previously synthesized white light. At present, the warm white light on the market is basically made of the phosphorescent mixture of caalsin3: EU 2 + and YAG: CE combined with blue LED.
One disadvantage is that although these new materials have great potential, synthesizing them is much more difficult than traditional phosphors. However, this has not prevented some enterprising manufacturers from commercializing nitride phosphors. Intematix added red nitride materials to its phosphor product line in 2011. The company’s phosphors are used in high-efficiency warm white LEDs for general lighting, providing customers with additional incentives to protect them from certain patent licensing issues.
The company claims that the new phosphor will enable the lighting market to create warm white applications with higher efficiency and higher CRI (up to 98) than traditional YAG phosphor solutions. It is said that the use of this new type of phosphor allows customers to (legally) circumvent certain patents related to the application of YAG phosphors, otherwise it will incur licensing fees.
Development prospect of fluorescent led plant aquarium light
High-efficiency warm white led plant light are about to be launched. As manufacturers try to convince consumers that solid-state lighting is a practical alternative to traditional lighting, the efficacy of LEDs has been greatly improved. However, further growth in photon production and light extraction is difficult to find, so LED manufacturers are increasingly turning their attention to other aspects of chip characteristics for better performance.
One aspect is phosphors, especially those used to add red wavelengths to the output of white led plant light to make the device appear warmer. Traditional solutions have expanded the CCT and CRI of the product to meet consumers’ needs for various temperature options and faithful color reproduction, but negate some of the hard-won effects obtained from the introduction of other technologies. Phosphors based on europium-doped nitrides and oxynitrides are expected to produce new materials with good CCT and CRI, but with a quantum efficiency comparable (or better) to pure YAG phosphors used in modern high-efficiency cold white led plant light. Some materials are now being introduced to the market, and engineers should expect a new generation of more efficient warm white led plant light to be launched soon.
