LEDs with a narrow emitting spectrum. Harm of LED lamps to vision and human health

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.

History of invention

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, in indoor and outdoor lighting, has begun.

RGB LEDs

White light can be created by mixing different colored LEDs. The most common trichromatic design is made from red (R), green (G) and blue (B) sources, although bichromatic, tetrachromatic and more multi-chromatic variants are found. 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 maximum re-emission in the yellow region - 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 useful life, for example for lighting, a level of reduction in luminous efficiency of up to 70% of the original value (L70) is taken. 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 early and some modern polychrome LED designs, 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 methods to reduce the influence of chip temperature on the phosphor. Isolated phosphor technologies and LED lamp designs, in which the phosphor is physically separated from the emitter, can increase the service life of the light source.

The LED housing, made of optically transparent silicone plastic or epoxy resin, is subject to aging under the influence of temperature and begins to dim and yellow over time, 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 ​​is a high thermal conductivity printed circuit board.

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 experience thermal stress 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 of assembling an LED device and installing it in a 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 very good quality characteristics, but the efficiency of such conversion is still low.

Advantages and disadvantages of phosphor LEDs

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

• The main advantage of white LEDs is their 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.
• The absence 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 illuminating rare books or other objects exposed to light).
• Excellent performance at sub-zero temperatures without reducing, and often even improving, 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 does not negatively affect 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 luminaires are 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 manufacturers of LEDs and related materials, specialists in the field of semiconductors and lighting engineering, it is possible to outline development paths in this area:

see also

Notes

1. , p. 19-20
2. Cree MC-E LEDs containing red, green, blue and white emitters. LED Professional. Archived
3. Vishay VLMx51 LEDs containing red, orange, yellow and white emitters. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
4. Cree XB-D and XM-L Multicolor LEDs. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
5. Cree XP-C LEDs containing six monochromatic emitters. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
6. Nikiforov S.“S-class” of semiconductor lighting technology // Components and technologies: magazine. - 2009. - No. 6. - P. 88-91.
7. Truson P. Halvardson E. Advantages of RGB LEDs for lighting devices // Components and technologies: magazine. - 2007. - No. 2.
8. , p. 404
9. Nikiforov S. Temperature in the life and operation of LEDs // Components and technologies: magazine. - 2005. - No. 9.
10. LEDs for interior and architectural lighting (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
11. Siang Ling Oon LED solutions for architectural lighting systems // : magazine. - 2010. - No. 5. - P. 18-20.
12. RGB LEDs for use in electronic displays (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
13. Turkin A. Gallium nitride as one of the promising materials in modern optoelectronics // Components and technologies: magazine. - 2011. - No. 5.
14. LEDs with high CRI values. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
15. Cree's EasyWhite technology. LEDs Magazine. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
16. Nikiforov S., Arkhipov A. Features of determining the quantum yield of LEDs based on AlGaInN and AlGaInP at different current densities through the emitting crystal // Components and technologies: magazine. - 2008. - No. 1.
17. Nikiforov S. Now electrons can be seen: LEDs make electric current very visible // Components and technologies: magazine. - 2006. - No. 3.
18. LEDs with a matrix arrangement of a large number of semiconductor chips (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
19. White LED Lifetime Archived from the original on November 23, 2012. Retrieved November 10, 2012.
20. Types of LED defects and methods of analysis (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
21. , p. 61, 77-79
22. LEDs from SemiLEDs (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
23. GaN-on-Si Silicon LED Research Program. LED Professional. Retrieved November 10, 2012.
24. Cree Isolated Phosphor Technology. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
25. Turkin A. Semiconductor LEDs: history, facts, prospects // Semiconductor lighting technology: magazine. - 2011. - No. 5. - P. 28-33.
26. Ivanov A.V., Fedorov A.V., Semenov S.M. Energy-saving lamps based on high-brightness LEDs // Energy supply and energy saving – regional aspect: XII All-Russian meeting: materials of reports. - Tomsk: St. Petersburg Graphics, 2011. - pp. 74-77.
27. , p. 424
28. White LEDs with high light output for lighting needs. Phys.Org™. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
29. LED Lighting Basics. U.S. Department of Energy. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
30. Sharakshane A. Scales for assessing the quality of the spectral composition of light - CRI and CQS // Semiconductor lighting technology: magazine. - 2011. - No. 4.
31. Ultraviolet LEDs SemiLED with a wavelength of 390-420 nm. (English) . LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
32. , p. 4-5
33. Active cooling systems from the Nuventix campaign. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
34. N.P.Soschin Modern photoluminescent materials for efficient solid-state lighting devices. Conference materials. (Russian) (February 1, 2010). Archived
35. O.E.Dudukalo, V.A.Vorobiev(Russian) (May 31, 2011). Archived from the original on October 27, 2012.
36. Tests of accelerated temperature degradation of phosphors (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
37. Research and Markets Releases New 2012 Report on LED Phosphor Materials (English) . LED Professional. Archived from the original on December 10, 2012. Retrieved November 30, 2012.
38. Intematix presented a set of phosphors for high-quality color rendering (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
39. Lumi-tech proposed SSE phosphor for white LEDs. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
40. Red phosphorus from Intematix (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
41. Quantum dot LEDs (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
42. Osram's 609 nm red all-diode prototype with 61% efficiency. LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
43. Transition to GaN-on-Si structure (English). LED Professional. Archived from the original on November 23, 2012. Retrieved November 10, 2012.
44. Tim Whitaker Joint venture to make ZnSe white LEDs (English) (December 6, 2002). Archived from the original on October 27, 2012. Retrieved November 10, 2012.
45. , p. 426

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

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?

I have already written several articles about homemade lamps 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 h
• Purpose: lamps for plants

LED testing

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

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 housings for drivers from electrical cable ducts.

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

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. These LEDs are sufficient for illuminating seedlings and indoor plants.

Exit inspection passed!

Ecology of consumption. Ideally, a spectrophotometer is needed to assess the quality of the lamp's emission spectrum. As a last resort, you can use spectrophotometers for profiling/calibrating monitors (for example, ColorMunki) - if you have such a device.

Ideally, a spectrophotometer is needed to assess the quality of the lamp's emission spectrum. As a last resort, you can use spectrophotometers for profiling/calibrating monitors (for example, ColorMunki) - if you have such a device. There is no point in buying spectrophotometers at home to evaluate lamps; they cost from hundreds to tens of thousands of dollars.

Nevertheless, for the needs of geologists and jewelers, simple spectroscopes based on a diffraction grating are produced. Their cost is from 1200 to 2500 rubles. And it's a fun and useful thing.

The spectroscope looks like this:

You need to look into the eyepiece (on the left, where the cone is), while the lens (on the right) should be directed towards the radiation source.

A diffraction grating splits light into a spectrum (like a rainbow or an optical prism).

Before delving into the spectra of real lamps, let me remind you of some general information. (This is discussed in some detail in the book in the chapter “Quality of Light”).

Here I will show two SDL spectra with an exceptionally high color rendering index of 97:

Cold light:

You can see that the color temperature is 5401 K, index 97. The main thing is that you can see what colors visible to the eye the spectrum consists of.

Warm light:

Temperature 3046 K, index also 97.

A spectrophotometer - unlike a spectroscope - shows not just which colors form the spectrum, but also gives their intensity. It is clearly visible that in the spectra of both lamps there are all the colors that make up white (“every hunter wants to know where the pheasant sits,” i.e. red, orange, yellow, green, blue, indigo, violet). The difference in color temperature is achieved through the relative contributions of cool (blue-cyan) and warm (yellow-red) components.

I am forced to mention that this spectroscope is intended for mobile use using the eyes. It is extremely inconvenient to fix the image, since the eyepiece is small and there are no devices for fixation on the camera. Therefore, you need to hold the camera with one hand, the spectroscope with the other, and control the shooting with your voice. At the same time, you still need to keep the direction towards the light source; small deviations from the normal lead to distortion of the colors of the spectrum. Of the almost dozen different cameras that I have at home, the Samsung tablet turned out to be the best. The camera is only 5 megapixels, but the software is good, and the size and position of the lens on the device body allows you to more or less conveniently attach the spectroscope. White balance was fixed as “daylight”, ISO 400. The pictures were not processed, only straightened and cropped. The numbers on the right indicate the color rendering index of the source (100 - daylight in cloudy weather, 99 - incandescent lamp). I'm not very happy with the quality of the photos - but I couldn't take it better.

So, let's start from top to bottom and use specific examples to try to understand what you need to pay attention to in such spectra.

Daylight and incandescent: an ideal spectrum that includes all of the above colors.

SDLs with color rendering indices of 87 and 84 also demonstrate almost the full spectrum. The problem is usually the red part - while yellow and orange are usually sufficient, deep red shades are most often absent. They are not visible here either. It can also be assumed (for example, by the amount of blue in the spectra) that manufacturers use different 5736SMD LEDs. Those. We are not dealing with the same lamp purchased from different sellers - but with different manufacturers.

SDL with index 78 (its analysis is given in the chapter “Example of assessment testing” in the book) along with the trimmed red part also demonstrates a small amount of blue. (It may seem that in comparison with the spectrum of a lamp with index 84, this is not the case. But here you need to remember that 84 is a warm lamp, T = 2900. And 78 is cold, T = 5750 K, there is, by definition, much more blue) . This is precisely the main disadvantage of simple budget SDLs, which produce supposedly white light due to the blue or purple radiation of the LED and the yellow-orange light of the phosphor. To the right of blue lies blue - but from the described combination it “does not work.” Therefore, there is usually a dip there in the SDL spectrum. Due to this (plus a deficiency of deep red) the color rendering index drops.

The lowest spectrum is a high-quality compact fluorescent lamp (CFL, T=2700 K, resource 12,000 hours, declared color rendering index of at least 80). And here you can clearly see how this formally rather high value is achieved. The manufacturer itself calls this the “Tricolor system”. Those. it uses a phosphor of 3 components, each of which emits light in a narrow band. (Of course, it is not at all easy to make such a lamp, since a careful selection of the combination of phosphors is required.) It is the presence of such vertical stripes (for example, violet, green, yellow) that is a sign of low-quality light sources. The second consequence of the line spectrum of the source is the physical absence of some colors in principle (in the figure, for example, there is practically no yellow and very little blue). It is obvious that the light of such lamps is of little use to the eyes, despite the formally quite high performance. Such lamps should be used in lamps with high-quality diffusers (although, of course, this will not change the spectrum of the lamp).

Conclusion: in the spectra of light sources with a high color rendering index, all colors of the spectrum should be present and there should be no intense narrow bands.

Separately, I would like to warn against haste in analyzing the spectra. In my line of work, I talked a lot with spectroscopists and noticed an ironclad pattern: the more qualified and professional the specialist, the more cautious and evasive he is in his conclusions. From the best of them, the professor, head of the spectroscopy laboratory, it was generally impossible to achieve a clear conclusion (which at first, when I was young, irritated me wildly). The eye is undoubtedly the best optical instrument in existence. But the analysis and interpretation of spectra is an endlessly complex topic. There are a huge number of different factors at work. Therefore, I strongly recommend only the simplest qualitative assessment of spectra with the eyes, without attempts at cunning reasoning and far-reaching conclusions. It is best to alternately look at the spectrum of the lamp being evaluated and at the ideal spectrum of daylight or FL. Those. clear comparison with each other. published

The emission spectrum of an LED is determined by the bandgap of the semiconductor material used, the type of dopant, the doping level, and the radiative recombination mechanism. 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. According to the general indicator ( R izl, performance) GaAlAs most fully satisfies the requirements of optoelectronics. In this material, some of the atoms Ga in crystal GaAs replaced by atoms Al. As the fraction of substituted atoms increases, the band gap varies from ∆ E=1.45 eV ( GaAs) before ∆ E=2.16 eV (pure AlAs). Thus, such LEDs can emit at a wavelength  max=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 η external =1,2…12 %.

Brightness 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 GaP- 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.

I have already written several articles about homemade lamps for plants.
Using regular blue and red LEDs
Using special spectrum LEDs 440nm and 660nm

Today I’ll tell you about special “full spectrum” LEDs for plants. For these LEDs, the required emission spectrum is achieved by a special phosphor, which provides secondary radiation.

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 h
• Purpose: lamps for plants

Appearance

Packaging from the store

For convenience, I transfer it to packaging from white LEDs

Enough admiration, let's move on to testing

Testing at different currents

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.

Started with a very tiny current of 30mA

I gradually increased the current to 1.5A and the power to 7.5W, I thought he would die, but no, he survived!

The graph of voltage and illumination versus current looks like this

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, and the decrease in brightness is due to simple overheating.

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 an endoscope, 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.

For analysis I used the free Cell Phone Spectrophotometer program
Having struggled with errors, as written in the article, associated with different decimal point formats in different Windows, I got the following spectrograms

sunlight

Fluorescent table lamp. The spectral lines of mercury are clearly visible

“Full spectrum” LEDs from this review

It is not possible to check the presence of the 840 nm infrared component on this device, but in the visual range the spectrum of LEDs is quite suitable for their intended purpose. The maximum luminescence occurs at 440nm and 660nm. The spectral band in this range is wider and smoother than that of separate monochrome LEDs.

The design of the lamp is extremely simple. For production I took:

• LEDs 3W “full spectrum” – 10 pcs.
• LED driver 10×3W 600mA (Completely suitable)
• U-shaped aluminum profile 30mm - 1m
• Wires, Kazan sealant, piece of electrical cable channel 25×20

I cut and mark the profile

I make housings for drivers from electrical cable ducts.

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 and even with prolonged operation the temperature will not rise above 45C

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. These LEDs are sufficient for illuminating seedlings and indoor plants.

Exit inspection passed!

For those who are too lazy to assemble such lamps themselves,