RGB lamp percentage component of radiation by color. An inside look: LED light bulbs. Features of LEDs of different designs

But growing flowers in our winter conditions is not easy. I’ll tell you about what helps in growing plants - special light, phytolamps.

Happy spring holidays, dear ladies! What is a spring holiday without flowers?

About homemade lamps I have already written several articles for plants



Now I’ll tell you about special LEDs for plants with a “full spectrum”
The process is highly dependent on the light spectrum.


Therefore, it is more effective to use light as close as possible to 445nm and 660nm. It is also recommended to add an infrared LED. Quite a few copies have been written about all this on the relevant forums. I won’t theorize, I’ll move on to practice. This time, in the vastness of ALI, I purchased 3-watt “full spectrum” LEDs for plants.

Product characteristics

• Power: 3W (there is 1W in the same lot)
• Working current: 700mA
• Operating voltage: 3.2-3.4V
• Chip manufacturer: Epistar Chip
• Chip size: 45mil
• Spectrum: 400nm-840nm
• Certificates: CE, RoHS,
• Lifespan: 100,000 hours
• Purpose: lamps for plants

The price of LEDs is quite attractive.
The packaging is very simple.




In appearance, the LED is similar to its cold and warm white brothers.




The packaging was left over from previously used LEDs.

LED testing

To begin with, check the power and take the current-voltage characteristic
Computer power supply, used by me as a laboratory one and the good old PEVR-25, personifying a great era)))


Measuring current/voltage with a simple device, since special accuracy is not required here. Well, and a heatsink, so as not to overheat the LED while I’m mocking it. Additionally, I measured the illumination in each mode at a distance of approximately 15-20 cm to assess the effectiveness of the glow at different currents.

I increased the LED power to 7.5W, I thought he would die, but no, he survived!


Let's see what the graph of voltage and illumination versus current gives.


The voltage changes fairly linearly. There are no signs of crystal degradation at a current of 1.5A. Everything becomes more interesting with lighting. After approximately 500mA, the dependence of illumination on current decreases. I conclude that 500-600mA is the most effective mode of operation with this LED, although it will work quite well at its rated 700mA.

Spectral analysis

I used a spectroscope for spectral analysis

We shine light into one tube with the source being studied, and into the other, we illuminate the scale. We look at the finished spectrum through the eyepiece


Unfortunately, this spectroscope does not have a special attachment for photography. The picture was visually very beautiful and did not want to be produced on a computer. I tried different cameras, phones and tablets. As a result, I settled on , with the help of which I somehow managed to take pictures of the spectrum. I completed the scale numbers in the editor, since the camera did not want to focus normally.


This is what I ended up with
Solar spectrum

Fluorescent table lamp
The spectral lines of mercury are clearly visible

As a radiator I use a U-shaped 30mm aluminum profile. There are 10 LEDs on 1m of profile (about 20W). During continuous operation, such a lamp heats up to no more than 45C.

I make driver housings from electrical cable channel.

To glue the LEDs to the profile I use Kazan sealant, although hot-melt adhesive would also work.

Then I connect everything with wires, I insulate the contacts with heat shrink

Now the driver and phytolamp are ready

A couple of hours of running shows that the thermal calculation was done correctly and there will be no overheating even during long-term operation

The light from the lamp is softer than that of separate 440nm and 660nm LEDs. It is less blinding to the eyes.

It's time to take stock

LEDs with “full spectrum” fully justify their purpose and are suitable for making phytolamps.

The declared power and spectrum correspond to the declared characteristics, although the infrared component could not be verified.

The required spectrum in such LEDs is achieved using a special phosphor, so the design of the diodes themselves can be anything. You can take powerful matrices of 20W and higher for use in greenhouses. For illuminating seedlings and indoor plants These LEDs are quite enough.

Exit inspection passed!

The intensity of photosynthesis under red light is maximum, but under red light alone, plants die or their development is disrupted. For example, Korean researchers have shown that when illuminated with pure red, the mass of grown lettuce is greater than when illuminated with a combination of red and blue, but the leaves contain significantly less chlorophyll, polyphenols and antioxidants. And the Faculty of Biology of Moscow State University has established that in the leaves of Chinese cabbage under narrow-band red and blue light (compared to illumination with a sodium lamp), the synthesis of sugars is reduced, growth is inhibited and flowering does not occur.

Rice. 1 Leanna Garfield Tech Insider - Aerofarms

What kind of lighting is needed to get a fully developed, large, fragrant and tasty plant with moderate energy consumption?

How to evaluate the energy efficiency of a lamp?

Basic metrics for assessing the energy efficiency of phytolight:

• Photosynthetic Photon Flux (PPF), in micromoles per joule, i.e., in the number of light quanta in the range of 400–700 nm emitted by a lamp that consumed 1 J of electricity.
• Yield Photon Flux (YPF), in effective micromoles per joule, i.e., in the number of quanta per 1 J of electricity, taking into account the multiplier - the curve McCree.

PPF always turns out a little higher than YPF(curve McCree normalized to one and in most of the range less than one), so the first metric is beneficial for lamp sellers. The second metric is more profitable to use for buyers, since it more adequately assesses energy efficiency.

Efficiency of DNAT

Large agricultural enterprises with extensive experience and counting money still use sodium lamps. Yes, they willingly agree to hang the LED lights provided to them over the experimental beds, but they do not agree to pay for them.

From Fig. 2 it is clear that the efficiency sodium lamp highly dependent on power and reaches a maximum at 600 W. Characteristic optimistic value YPF for a sodium lamp 600–1000 W is 1.5 eff. µmol/J. Sodium lamps 70–150 W are one and a half times less efficient.

Rice. 2. Typical spectrum of a sodium lamp for plants (left). Efficiency in lumens per watt and in effective micromoles of commercial sodium greenhouse light brands Cavita, E-Papillon, "Galad" and "Reflex" (on right)

Any led light silt having an efficiency of 1.5 eff. µmol/W and reasonable price, can be considered a worthy replacement for a sodium lamp.

The questionable effectiveness of red-blue phytolights

In this article we do not present the absorption spectra of chlorophyll because it is incorrect to refer to them in a discussion of the use of light flux by a living plant. Chlorophyll invitro, isolated and purified, only really absorbs red and blue light. In a living cell, pigments absorb light in the entire range of 400–700 nm and transfer its energy to chlorophyll. The energy efficiency of light in a sheet is determined by the curve " McCree 1972"(Fig. 3).

Rice. 3. V(λ) - visibility curve for humans; RQE- relative quantum efficiency for the plant ( McCree 1972); σ r And σ fr- absorption curves of red and far-red light by phytochrome; B(λ) - phototropic efficiency of blue light

Note: the maximum efficiency in the red range is one and a half times higher than the minimum efficiency in the green range. And if you average the efficiency over a somewhat wide band, the difference becomes even less noticeable. In practice, the redistribution of part of the energy from the red range to the green range sometimes, on the contrary, enhances the energy function of light. Green light passes through the thickness of the leaves to the lower tiers, the effective leaf area of ​​the plant increases sharply, and the yield of, for example, lettuce increases.

Plant lighting with white LEDs

The energy feasibility of lighting plants with common LED white light lamps was studied in the work.

The characteristic shape of the spectrum of a white LED is determined by:

• the balance of short and long waves, correlating with color temperature (Fig. 4, left);
• the degree of spectral occupancy, which correlates with color rendering (Fig. 4, right).

Rice. 4. Spectra of white LED light with the same color rendering, but different color temperature CCT (left) and with the same color temperature and different color rendering R a (on right)

The differences in the spectrum of white diodes with the same color rendering and the same color temperature are subtle. Consequently, we can evaluate spectrum-dependent parameters only by color temperature, color rendering and luminous efficiency - parameters that are written on the label of a conventional white light lamp.

The results of the analysis of the spectra of serial white LEDs are as follows:

1. In the spectrum of all white LEDs, even with a low color temperature and maximum color rendering, like sodium lamps, there is very little far-red (Fig. 5).

Rice. 5. White LED spectrum ( LED 4000K R a= 90) and sodium light ( HPS) in comparison with the spectral functions of plant sensitivity to blue ( B), red ( A_r) and far red light ( A_fr)

Under natural conditions, a plant shaded by a canopy of alien foliage receives more distant red than near red, which in light-loving plants triggers the “shade avoidance syndrome” - the plant stretches upward. Tomatoes, for example, at the growth stage (not seedlings!) need far red to stretch, increase growth and the total occupied area, and therefore the harvest in the future.

Accordingly, under white LEDs and under sodium light the plant feels like it is under the open sun and does not stretch upward.

2. Blue light is needed for the “sun tracking” reaction (Fig. 6).

Rice. 6. Phototropism - turning leaves and flowers, stretching stems towards the blue component of white light (illustration from Wikipedia)

One watt of white LED light contains twice as much phytoactive blue component as one watt of sodium light. Moreover, the proportion of phytoactive blue in white light increases in proportion to the color temperature. If, for example, you need to turn decorative flowers towards people, they should be illuminated from this side with intense cold light, and the plants will turn around.

3. The energy value of light is determined by color temperature and color rendering and can be determined with an accuracy of 5% using the formula:

where is the luminous efficiency in lm/W, is the general color rendering index, is the correlated color temperature in Kelvin.

Examples of using this formula:

A. Let us estimate for the basic values ​​of the parameters of white light what the illumination should be in order to provide, for example, 300 eff. for a given color rendering and color temperature. µmol/s/m2:

It can be seen that the use of warm white light with high color rendering allows the use of slightly lower illumination levels. But if we take into account that the luminous efficiency of warm light LEDs with high color rendering is somewhat lower, it becomes clear that by choosing color temperature and color rendering there is no energetically significant win or loss. You can only adjust the proportion of phytoactive blue or red light.

B. Let's evaluate the applicability of a typical general purpose LED grow light for growing microgreens.

Let a lamp measuring 0.6 × 0.6 m consume 35 W and have a color temperature of 4000 TO, color rendition Ra= 80 and luminous efficiency 120 lm/W. Then its efficiency will be YPF= (120/100)⋅(1.15 + (35⋅80 − 2360)/4000) eff. µmol/J = 1.5 eff. µmol/J. Which, when multiplied by the consumed 35 W, will be 52.5 eff. µmol/s.

If such a lamp is lowered low enough above a bed of microgreens with an area of ​​0.6 × 0.6 m = 0.36 m 2 and thereby avoiding light loss to the sides, the lighting density will be 52.5 eff. µmol/s / 0.36m 2 = 145 eff. µmol/s/m2. This is approximately half the usually recommended values. Therefore, the power of the lamp must also be doubled.

Direct comparison of phytoparameters of different types of lamps

Let's compare the phytoparameters of a conventional office ceiling LED lamp produced in 2016 with specialized phytoluminaires (Fig. 7).

Rice. 7. Comparative parameters of a typical 600W sodium lamp for greenhouses, a specialized LED phytolight and a lamp for general lighting premises

It can be seen that an ordinary general lighting lamp with the diffuser removed when lighting plants is not inferior in energy efficiency to a specialized sodium lamp. It is also clear that the red-blue light phyto-lamp (the manufacturer is deliberately not named) is made at a lower technological level, since its total efficiency (the ratio of the power of the luminous flux in watts to the power consumed from the network) is inferior to the efficiency of an office lamp. But if the efficiency of red-blue and white lamps were the same, then the phytoparameters would also be approximately the same!

It is also clear from the spectra that the red-blue phyto-luminaire is not narrow-band, its red hump is wide and contains much more far-red red than that of the white LED and sodium lamp. In cases where far-red is required, using such a luminaire alone or in combination with other options may be advisable.

Assessment of the energy efficiency of the lighting system as a whole:

Rice. 8. Audit of phytolighting system

Next model UPRtek- spectrometer PG100N According to the manufacturer, it measures micromoles per square meter, and, more importantly, the luminous flux in watts per square meter.

Measuring luminous flux in watts is an excellent feature! If you multiply the illuminated area by the luminous flux density in watts and compare it with the consumption of the lamp, the energy efficiency of the lighting system becomes clear. And this is the only indisputable efficiency criterion today, which in practice differs by an order of magnitude for different lighting systems (and not by several times, or even more so by percentages, as the energy effect changes when the shape of the spectrum changes).

Examples of using white light

Examples of lighting hydroponic farms with both red-blue and white light are described (Fig. 9).

Rice. 9. From left to right and top to bottom farms: Fujitsu, Sharp, Toshiba, growing farm medicinal plants in Southern California

The farm system is quite well known Aerofarms(Fig. 1, 10), the largest of which was built near New York. Under white LED lamps in Aerofarms They grow more than 250 types of greens, harvesting over twenty harvests a year.

Rice. 10. Farm Aerofarms in New Jersey ("Garden State") on the border with New York

Direct experiments comparing white and red-blue LED lighting
There are very few published results of direct experiments comparing plants grown under white and red-blue LEDs. For example, this result was briefly shown by the Moscow Agricultural Academy named after. Timiryazev (Fig. 11).

Rice. eleven. In each pair, the plant on the left is grown under white LEDs, on the right - under red-blue LEDs (from presentations I. G. Tarakanova, Department of Plant Physiology, Moscow Agricultural Academy named after. Timiryazev)

In 2014, Beijing Aviation and Space University published the results of a large study of wheat grown under different types of LEDs. Chinese researchers have concluded that it is advisable to use a mixture of white and red light. But if you look at the digital data from the article (Fig. 12), you notice that the difference in parameters when different types lighting is by no means radical.

Figure 12. Values ​​of the studied factors in two phases of wheat growth under red, red-blue, red-white and white LEDs

However, the main focus of research today is to correct the shortcomings of narrowband red-blue lighting by adding white light. For example, Japanese researchers found an increase in the weight and nutritional value of lettuce and tomatoes when white light was added to red light. In practice, this means that if the aesthetic appeal of a plant during growth is not important, it is not necessary to abandon the already purchased narrow-band red-blue lamps; white light lamps can be used additionally.

The influence of light quality on the result

The fundamental law of ecology “Liebig barrel” (Fig. 13) states: development is limited by the factor that most deviates from the norm than others. For example, if water is fully provided, minerals And CO 2, but the light intensity is 30% of the optimal value - the plant will produce no more than 30% of the maximum possible yield.

Rice. 13. Illustration of the limiting factor principle from training video on YouTube

The plant's response to light: the intensity of gas exchange, consumption of nutrients from solution and synthesis processes is determined in the laboratory. The responses characterize not only photosynthesis, but also the processes of growth, flowering, and the synthesis of substances necessary for taste and aroma.

In Fig. Figure 14 shows the plant's response to changes in the wavelength of light. The intensity of sodium and phosphorus intake from the nutrient solution was measured by mint, strawberries and lettuce. Peaks on such graphs are signs of stimulation of a specific chemical reaction. The graphs show that excluding some ranges from the full spectrum for the sake of saving is the same as removing part of the piano keys and playing a melody on the remaining ones.

Rice. 14. The stimulating role of light for the consumption of nitrogen and phosphorus by mint, strawberries and lettuce (data provided by Fitex company)

The principle of the limiting factor can be extended to individual spectral components - for a full result, in any case, you need a full range of. Removing some ranges from the full spectrum does not lead to a significant increase in energy efficiency, but the “Liebig barrel” may work - and the result will be negative.
Examples demonstrate that ordinary white LED light and specialized “red-blue phytolight” have approximately the same energy efficiency when lighting plants. But broadband white comprehensively satisfies the needs of the plant, which are expressed not only in stimulating photosynthesis.

Removing green from the continuous spectrum so that the light turns from white to violet is a marketing ploy for buyers who want a “special solution” but are not qualified customers.

White light adjustment

The most common general purpose white LEDs have poor color rendering Ra= 80, which is due primarily to the lack of red color (Fig. 4).

The lack of red in the spectrum can be compensated by adding red LEDs to the lamp. Such a solution is promoted by, for example, CREE. The logic of the “Liebig barrel” suggests that such an additive will not harm if it is truly an additive and not a redistribution of energy from other ranges in favor of red.

Interesting and important work was done in 2013–2016 by the Institute of Biomedical Problems of the Russian Academy of Sciences: they studied how the addition of 4000 white LEDs to the light affects the development of Chinese cabbage TO / Ra= 70 light narrowband red LEDs 660 nm.

And we found out the following:

• Under LED light, cabbage grows about the same as under sodium light, but it has more chlorophyll (the leaves are greener).
• The dry weight of the crop is almost proportional to the total amount of light in moles received by the plant. More light - more cabbage.
• The concentration of vitamin C in cabbage increases slightly with increasing illumination, but increases significantly with the addition of red light to white light.
• A significant increase in the proportion of the red component in the spectrum significantly increased the concentration of nitrates in the biomass. It was necessary to optimize the nutrient solution and introduce part of the nitrogen in ammonium form so as not to exceed the maximum permissible concentration for nitrates. But in pure white light it was possible to work only with the nitrate form.
• At the same time, an increase in the proportion of red in the total light flux has almost no effect on the weight of the crop. That is, replenishment of the missing spectral components affects not the quantity of the crop, but its quality.
• The higher moles per watt efficiency of a red LED means that adding red to white is also energetically efficient.

Thus, adding red to white is advisable in the particular case of Chinese cabbage and quite possible in the general case. Of course, with biochemical control and correct selection fertilizers for a specific crop.

Options for enriching the spectrum with red light

The plant does not know where the quantum from the white light spectrum came from, and where the “red” quantum came from. There is no need to make a special spectrum in one LED. And there is no need to shine red and white light from one special phyto-lamp. It is enough to use general-purpose white light and additionally illuminate the plant with a separate red light lamp. And when a person is near the plant, the red light can be turned off using a motion sensor to make the plant look green and pretty.

But the opposite solution is also justified - by selecting the composition of the phosphor, expand the spectrum of the white LED towards long waves, balancing it so that the light remains white. And you get white light with extra-high color rendering, suitable for both plants and humans.

Open questions

It is possible to identify the role of the ratio of far and near red light and the advisability of using the “shade avoidance syndrome” for different crops. One can argue into which areas during analysis it is advisable to divide the wavelength scale.

It can be discussed whether the plant needs wavelengths shorter than 400 nm or longer than 700 nm for stimulation or regulatory function. For example, there is a private report that ultraviolet radiation significantly affects the consumer qualities of plants. Among other things, red-leaved varieties of lettuce are grown without ultraviolet radiation, and they grow green, but before sale they are irradiated with ultraviolet light, they turn red and are sent to the counter. And is the new metric correct? PBAR (plant biologically active radiation), described in the standard ANSI/ASABE S640, Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms, prescribes taking into account the range of 280–800 nm.

Conclusion

Chain stores choose more shelf-stable varieties, and then the buyer votes with rubles for brighter fruits. And almost no one chooses the taste and aroma. But as soon as we become richer and begin to demand more, science will instantly provide the necessary varieties and recipes for the nutrient solution.

And in order for the plant to synthesize everything that is needed for taste and aroma, it will require lighting with a spectrum containing all the wavelengths to which the plant will react, i.e., in the general case, a continuous spectrum. Perhaps the basic solution will be white light with high color rendering.

Acknowledgments
The author expresses sincere gratitude for the assistance in preparing the article to the employee of the State Scientific Center of the Russian Federation-IMBP RAS Ph.D. n. Irina Konovalova; the head of the Fiteks project, Tatyana Trishina; company specialist CREE Mikhail Chervinsky

Literature

Literature
1. Son K-H, Oh M-M. Leaf shape, growth, and antioxidant phenolic compounds of two lettuce cultivars grown under various combinations of blue and red light-emitting diodes // Hortscience. – 2013. – Vol. 48. – P. 988-95.
2. Ptushenko V.V., Avercheva O.V., Bassarskaya E.M., Berkovich Yu A., Erokhin A.N., Smolyanina S.O., Zhigalova T.V., 2015. Possible reasons for a decline in the growth of Chinese cabbage under combined narrowband red and blue light in comparison with illumination by high- pressure sodium lamp. Scientia Horticulturae https://doi.org/10.1016/j.scienta.2015.08.021
3. Sharakshane A., 2017, Whole high-quality light environment for humans and plants. https://doi.org/10.1016/j.lssr.2017.07.001
4. C. Dong, Y. Fu, G. Liu & H. Liu, 2014, Growth, Photosynthetic Characteristics, Antioxidant Capacity and Biomass Yield and Quality of Wheat (Triticum aestivum L.) Exposed to LED Light Sources with Different Spectra Combinations
5. Lin K.H., Huang M.Y., Huang W.D. et al. The effects of red, blue, and white light-emitting diodes on the growth, development, and edible quality of hydroponically grown lettuce (Lactuca sativa L. var. capitata) // Scientia Horticulturae. – 2013. – V. 150. – P. 86–91.
6. Lu, N., Maruo T., Johkan M., et al. Effects of supplemental lighting with light-emitting diodes (LEDs) on tomato yield and quality of single-truss tomato plants grown at high planting density // Environ. Control. Biol. – 2012. Vol. 50. – P. 63–74.
7. Konovalova I.O., Berkovich Yu.A., Erokhin A.N., Smolyanina S.O., O.S. Yakovleva, A.I. Znamensky, I.G. Tarakanov, S.G. Radchenko, S.N. Lapach. Justification of optimal plant lighting regimes for the Vitacycle-T space greenhouse. Aerospace and environmental medicine. 2016. T. 50. No. 4.
8. Konovalova I.O., Berkovich Yu.A., Erokhin A.N., Smolyanina S.O., Yakovleva O.S., Znamensky A.I., Tarakanov I.G., Radchenko S.G., Lapach S.N., Trofimov Yu.V., Tsvirko V.I. Optimization of the LED lighting system for a vitamin space greenhouse. Aerospace and environmental medicine. 2016. T. 50. No. 3.
9. Konovalova I.O., Berkovich Yu.A., Smolyanina S.O., Pomelova M.A., Erokhin A.N., Yakovleva O.S., Tarakanov I.G. Influence of parameters light mode on the accumulation of nitrates in the aboveground biomass of Chinese cabbage (Brassica chinensis L.) when grown with LED irradiators. Agrochemistry. 2015. No. 11.

White LED

Unlike traditional incandescent and fluorescent lamps, which produce white light, LEDs generate light in a very narrow range of the spectrum, i.e. give an almost monochrome glow. That is why LEDs have long been used in control panels and garlands, and today they are especially effectively used in lighting installations that emit a specific primary color, for example, in traffic lights, signs, and signal lights.

Principle of a white LED

The design principle of a white LED is not very complicated; the implementation technology is complex.

In order for an LED to emit white light, it is necessary to resort to additional technical elements and technical solutions. The main ways to obtain white light in LEDs are:

applying a layer of phosphor to blue crystals;

applying several layers of phosphor to crystals that emit light close in color to ultraviolet; RGB systems, in which a glow is achieved by mixing the light of many monochrome red, green and blue diodes.

white

The second method is a recently developed technology for producing solid-state white light sources based on a combination of a diode emitting a glow similar in color to ultraviolet and several layers of phosphor made of phosphorus of various compositions.

In the latter case, white light is produced in the classical way by mixing three basic colors (red, green and blue). The quality of white light is improved by complementing the RGB configuration with yellow LEDs to cover the yellow part of the spectrum.

Advantages and disadvantages of old LEDs

Each of these methods has its positive and negative sides. Thus, white phosphor LEDs, manufactured on the principle of combining blue crystals with a phosphor phosphor, are characterized by a fairly low color rendering index, a tendency to generate white light in cold tones, heterogeneity in the hue of the glow with a fairly high luminous flux and a relatively low cost.

White phosphor LEDs, obtained on the basis of a combination of diodes with a glow close to ultraviolet color and multi-colored phosphors, have an excellent color rendering index, can generate white light of warmer shades and are characterized by greater uniformity of glow shades from diode to diode.

However, they consume more electricity and are not as bright as the first ones.

In turn, RGB LEDs make it possible to create dynamic lighting effects in lighting installations with a change in the color of the glow and different tones of white light and can potentially provide a very high color rendering index. At the same time, LEDs of individual colors react differently to the operating current, ambient temperature and brightness control, and therefore RGB LEDs require rather complex and expensive control systems to achieve stable operation.

So that lamps based on white LEDs provide better quality light, i.e. a more complete spectrum is used in the design of lamps. As mentioned above, the main materials for the manufacture of efficient LEDs are binary semiconductor compounds A III B V and their solid solutions. In Fig. Figure 4.4 shows the emission spectra at room temperature of some typical commercially produced LEDs in relative units.

LEDs based on gallium arsenide are the most efficient GaAs with band gap ∆ E= 1.45 eV. Consequently, the maximum of the spectral characteristics of the radiation itself GaAs observed at wavelength λ max=1.24/1.4 = 0.9 µm, which corresponds to the infrared region. When doping GaAs various impurities (tellurium, selenium, lithium, etc.) having different depths in the band gap, LEDs can emit in the range λ max= 0.9…0.96 µm. LEDs on GaAs have the highest quantum efficiency ( η external=10...30% depending on the design). It is important that the emission spectrum GaAs-LEDs correspond very well to the photosensitivity spectrum of the most common Si-photodiodes.

LEDs for longer wavelengths are manufactured based on direct-gap solid solutions Ga X 1p 1's As And Ga X 1p 1's As 1st R at . For them, quasi-interband radiative recombination is predominant.

It is important that the maximum emission spectrum of such LEDs is determined by the composition of the solid solution. Changing X And at, it is possible to produce an LED for a given region of the spectrum, for example, coinciding with the minimum of losses in an optical fiber or with the maximum of the absorption spectrum of any substance whose concentration is to be controlled. LEDs for the spectrum region λ >5 microns can be made on the basis of lead chalcogenides: Rb X SP 1- x Those and mercury: Cd X Hg 1- x Those.

Gallium phosphide ( GaP) has a band gap ∆ E = 2.25 eV, which determines the wavelength of the radiation λ max=0.56 µm. This corresponds to the green color of the glow. When doped with impurities ( N, O 2 , Zn) such LEDs can emit red, yellow, green light. Thus, GaP LEDs are designed to operate in the visible part of the spectrum. For GaP - η external = 7…0,7 %.

Light-emitting diodes for the short-wave region of the visible spectrum, operating in the blue, indigo and violet ranges, can be created on the basis of gallium nitride GaN and heterojunctions using solid solutions Ga X In 1- x N And Ga 1- x Al x N. LED based GaN give off radiation λ max=0.44 µm, but with very low efficiency η external 0,5 %.

Silicon carbide is used for the same purpose. SiC. Although diodes based SiC have small η external 0.01%, but have high time and temperature stability. Based on them, standard radiation sources are created.

Fig.4.4. Emission spectra of LEDs.

For emitting diodes of both infrared and visible radiation, ternary compounds made on the basis of a gallium-aluminum-arsenic solid solution are widely used GaAlAs. Solid solutions based on gallium-arsenic-phosphorus are also used GaAsP and indium gallium phosphorus I nGaP. R According to the general indicator ( izl GaAlAs, performance) Ga most fully satisfies the requirements of optoelectronics. In this material some of the atoms GaAs in crystal Al replaced by atoms ∆ E. As the fraction of substituted atoms increases, the band gap varies from GaAs=1.45 eV ( ∆ E) before =2.16 eV (pure AlAs  max). Thus, such LEDs can emit at a wavelength η external =1,2…12 %.

=0.6...0.9 µm, i.e. generate radiation in both the visible and infrared regions of the spectrum. The external quantum yield for this material is Brightness GaP LED illumination or radiation power depends almost linearly on the current through the diode over a wide range of current changes. The exception is red GaP- LEDs, in which brightness saturates as the current increases. With a constant current through the LED, its brightness decreases with increasing temperature. For the Reds

- for LEDs, an increase in temperature compared to room temperature by 20 o C reduces their brightness by about 10%, and for green ones - by 6%. As temperatures rise, the lifespan of LEDs decreases. The lifespan of an LED also decreases as its current increases.

White LED

Powerful white LED

• There are two types of white LEDs:
• Multi-chip LEDs, more often three-component (RGB LEDs), containing three semiconductor emitters of red, green and blue light, combined in one housing.

Phosphor LEDs, created on the basis of an ultraviolet or blue LED, containing a layer of a special phosphor that, as a result of photoluminescence, converts part of the LED radiation into light in a relatively wide spectral band with a maximum in the yellow region (the most common design). The emission of the LED and phosphor, when mixed, produce white light of various shades.

The first red semiconductor emitters for industrial use were obtained by N. Holonyak in 1962. In the early 70s, yellow and green LEDs appeared. The light output of early low-efficiency devices reached the single lumen level by 1990. In 1993, Suji Nakamura, an engineer at Nichia (Japan), created the first high-brightness blue LED. Almost immediately, LED RGB devices appeared, since blue, red and green colors made it possible to obtain any color, including white. White phosphor LEDs first appeared in 1996. Subsequently, the technology developed rapidly and by 2005, the luminous output of LEDs reached 100 lm/W or more. LEDs appeared with different shades of glow, the quality of light made it possible to compete with incandescent lamps and with already traditional fluorescent lamps. The use of LED lighting devices in everyday life, indoors and outdoors has begun. street lighting.

RGB LEDs

White light can be created by mixing different colored LEDs. The most common trichromatic design is made up of red (R), green (G) and blue (B) sources, although bichromatic, tetrachromatic and more are also found. multicolor options. A multicolor LED, unlike other RGB semiconductor emitters (luminaires, lamps, clusters), has one complete housing, most often similar to a single-color LED. The LED chips are located next to each other and share a common lens and reflector. Since semiconductor chips have a finite size and their own radiation patterns, such LEDs most often have uneven angular color characteristics. In addition, to obtain the correct color ratio, it is often not enough to set the design current, since the light output of each chip is unknown in advance and is subject to changes during operation. To set the desired shades, RGB lamps are sometimes equipped with special control devices.

The spectrum of an RGB LED is determined by the spectrum of its constituent semiconductor emitters and has a pronounced line shape. This spectrum is very different from the spectrum of the sun, therefore the color rendering index of the RGB LED is low. RGB LEDs make it possible to easily and widely control the color of the glow by changing the current of each LED included in the triad, adjusting the color tone of the white light they emit directly during operation - up to obtaining individual independent colors.

Multicolor LEDs have a dependence of light output and color on temperature due to the different characteristics of the emitting chips that make up the device, which results in a slight change in the color of the glow during operation. The service life of a multicolor LED is determined by the durability of the semiconductor chips, depends on the design and most often exceeds the service life of phosphor LEDs.

Multicolor LEDs are used primarily for decorative and architectural lighting, in electronic signage and video screens.

Phosphor LEDs

Spectrum of one of the phosphor LED options

Combining a blue (more often) or ultraviolet (less often) semiconductor emitter and a phosphor converter allows you to produce an inexpensive light source with good characteristics. The most common design of such an LED contains a blue gallium nitride semiconductor chip modified with indium (InGaN) and a phosphor with a maximum re-emission in the region yellow color- yttrium-aluminum garnet doped with trivalent cerium (YAG). Part of the power of the initial radiation of the chip leaves the LED body, dissipating in the phosphor layer, the other part is absorbed by the phosphor and re-emitted in the region of lower energy values. The re-emission spectrum covers a wide region from red to green, but the resulting spectrum of such an LED has a pronounced dip in the green-blue-green region.

Depending on the composition of the phosphor, LEDs are produced with different color temperatures (“warm” and “cold”). By combining different types of phosphors, a significant increase in the color rendering index (CRI or R a) is achieved, which suggests the possibility of using LED lighting in conditions critical to the quality of color rendering.

One way to increase the brightness of phosphor LEDs while maintaining or even reducing their cost is to increase the current through the semiconductor chip without increasing its size - increasing the current density. This method is associated with a simultaneous increase in requirements for the quality of the chip itself and the quality of the heat sink. As the current density increases, the electric fields in the bulk of the active region reduce the light output. When the limiting currents are reached, since areas of the LED chip with different impurity concentrations and different band gaps conduct current differently, local overheating of the chip areas occurs, which affects the light output and the durability of the LED as a whole. In order to increase the output power while maintaining the quality of spectral characteristics and thermal conditions, LEDs are produced containing clusters of LED chips in one housing.

One of the most discussed topics in the field of polychrome LED technology is its reliability and durability. Unlike many other light sources, an LED changes its light output (efficiency), radiation pattern, and color tint over time, but rarely fails completely. Therefore, to estimate the period beneficial use take, for example for lighting, a level of reduction in luminous efficiency up to 70% of the original value (L70). That is, an LED whose brightness decreases by 30% during operation is considered to be out of order. For LEDs used in decorative lighting, a brightness reduction level of 50% (L50) is used as an assessment of the lifespan.

The service life of a phosphor LED depends on many parameters. In addition to the manufacturing quality of the LED assembly itself (the method of attaching the chip to the crystal holder, the method of attaching the current-carrying conductors, the quality and protective properties of the sealing materials), the lifetime mainly depends on the characteristics of the emitting chip itself and on changes in the properties of the phosphor over the course of operation (degradation). Moreover, as numerous studies show, the main factor influencing the service life of an LED is temperature.

Effect of temperature on LED service life

During operation, a semiconductor chip emits part of the electrical energy in the form of radiation and part in the form of heat. Moreover, depending on the efficiency of such conversion, the amount of heat is about half for the most efficient emitters or more. The semiconductor material itself has low thermal conductivity; in addition, the materials and design of the housing have a certain non-ideal thermal conductivity, which leads to the heating of the chip to high temperatures (for a semiconductor structure). Modern LEDs operate at chip temperatures in the region of 70-80 degrees. And a further increase in this temperature when using gallium nitride is unacceptable. High temperature leads to an increase in the number of defects in the active layer, leads to increased diffusion, and a change in the optical properties of the substrate. All this leads to an increase in the percentage of non-radiative recombination and absorption of photons by the chip material. An increase in power and durability is achieved by improving both the semiconductor structure itself (reducing local overheating), and by developing the design of the LED assembly, and improving the quality of cooling of the active area of ​​the chip. Research is also being conducted with other semiconductor materials or substrates.

The phosphor is also susceptible to high temperatures. With prolonged exposure to temperature, re-emitting centers are inhibited and the conversion coefficient, as well as the spectral characteristics of the phosphor, deteriorate. In the first and some modern designs With polychrome LEDs, the phosphor is applied directly to the semiconductor material and the thermal effect is maximized. In addition to measures to reduce the temperature of the emitting chip, manufacturers use various ways reducing the influence of chip temperature on the phosphor. Isolated phosphor technologies and designs LED lamps, in which the phosphor is physically separated from the emitter, can increase the service life of the light source.

LED housing made of optically transparent silicone plastic or epoxy resin, is subject to aging under the influence of temperature and over time begins to fade and turn yellow, absorbing part of the energy emitted by the LED. Reflective surfaces also deteriorate when heated - they interact with other elements of the body and are susceptible to corrosion. All these factors together lead to the fact that the brightness and quality of the emitted light gradually decreases. However, this process can be successfully slowed down by ensuring efficient heat removal.

Phosphor LED design

Diagram of one of the white LED designs. MPCB ​​- printed circuit board with high thermal conductivity.

A modern phosphor LED is a complex device that combines many original and unique technical solutions. The LED has several main elements, each of which performs an important, often more than one function:

All LED design elements are tested thermal loads and must be selected taking into account the degree of their thermal expansion. And an important condition for a good design is manufacturability and low cost assembling the LED device and installing it in the lamp.

Brightness and quality of light

The most important parameter is not even the brightness of the LED, but its luminous efficiency, that is, the light output from each Watt of electrical energy consumed by the LED. The luminous efficiency of modern LEDs reaches 150-170 lm/W. The theoretical limit of the technology is estimated at 260-300 lm/W. When assessing, it is necessary to take into account that the efficiency of a lamp based on LEDs is significantly lower due to the efficiency of the power source, the optical properties of the diffuser, reflector and other design elements. In addition, manufacturers often indicate the initial efficiency of the emitter at normal temperature. While the temperature of the chip during operation is much higher. This leads to the fact that the actual efficiency of the emitter is 5 - 7% lower, and that of the lamp is often twice as low.

The second equally important parameter is the quality of the light produced by the LED. There are three parameters to assess the quality of color rendering:

Phosphor LED based on an ultraviolet emitter

In addition to the already widespread combination of a blue LED and YAG, a design based on an ultraviolet LED is also being developed. A semiconductor material capable of emitting in the near ultraviolet region is coated with several layers of a phosphor based on europium and zinc sulfide activated by copper and aluminum. This mixture of phosphors gives re-emission maxima in the green, blue and red regions of the spectrum. The resulting white light has a very good characteristics quality, but the effectiveness of such transformation is still low.

Advantages and disadvantages of phosphor LEDs

Considering the high cost LED sources lighting compared to traditional lamps, there are compelling reasons to use such devices:

• The main advantage of white LEDs is high efficiency. Low specific energy consumption allows them to be used in long-running sources of autonomous and emergency lighting.
• High reliability and long service life suggest possible savings on lamp replacement. In addition, the use of LED light sources in hard-to-reach areas and outdoor conditions reduces maintenance costs. Combined with high efficiency, there are significant cost savings when using LED lighting in some applications.
• Light weight and size of devices. LEDs are small in size and suitable for use in hard-to-reach places and small portable devices.
• Lack of ultraviolet and infrared radiation in the spectrum allows the use of LED lighting without harm to humans and for special purposes (for example, for lighting rare books or other objects exposed to light).
• Great job at negative temperatures without reduction, and often with improvement of parameters. Most types of LEDs exhibit greater efficiency and longer life as temperatures drop, but power, control, and design components may have the opposite effect.
• LEDs are inertia-free light sources; they do not require time to warm up or turn off, such as fluorescent lamps and the number of on and off cycles has no effect negative influence on their reliability.
• Good mechanical strength allows LEDs to be used in harsh operating conditions.
• Ease of power regulation by both duty cycle and supply current regulation without compromising efficiency and reliability parameters.
• Safe to use, no risk of electric shock due to low supply voltage.
• Low fire hazard, possibility of use in conditions of explosion and fire hazard due to the absence of incandescent elements.
• Moisture resistance, resistance to aggressive environments.
• Chemical neutrality, no harmful emissions and no special requirements for disposal procedures.

But there are also disadvantages:

Lighting LEDs also have features inherent in all semiconductor emitters, taking into account which the most successful application can be found, for example, the direction of radiation. The LED shines only in one direction without the use of additional reflectors and diffusers. LED lights best suited for local and directional lighting.

Prospects for the development of white LED technology

Technologies for manufacturing white LEDs suitable for lighting purposes are under active development. Research in this area is stimulated by increased public interest. The prospect of significant energy savings is attracting investment in process research, technology development and the search for new materials. Judging by the publications of LED manufacturers and related materials, specialists in the field of semiconductors and lighting engineering, we can outline the paths of development in this area:

see also

Notes

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Literature

• Schubert F.E. LEDs. - M.: Fizmatlit, 2008. - 496 p. - ISBN 978-5-9221-0851-5
• Weinert D. LED Lighting: A Handbook. - Philips, 2010. - 156 p. - ISBN 978-0-615-36061-4

Links

• US Department of Energy website about LED lighting
• Led Professional. Scientific and technical journal about LEDs and LED lighting, Austria
• LEDs Magazine. Scientific and technical magazine about LEDs and LED lighting. USA
• Semiconductor lighting technology. Russian magazine about LEDs and LED lighting

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