Cold white LED emission spectrum. LEDs for growing plants. White LED light is harmful to your eyesight

Tile 19.10.2023

Tile

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

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 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

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!

With the development of LED technology, more and more areas of application are constantly being found for it; it is gradually replacing fluorescent and conventional incandescent lamps. LEDs are much more practical during operation, consume 10 times less electricity, are more durable, and are resistant to mechanical stress. Due to the properties of LEDs to provide radiation in certain spectra of the light range, they began to be actively used for growing plants.

Light spectrum intervals that promote plant growth

It is known that all plants develop through the process of photosynthesis; deeper studies have shown that it occurs more actively in blue and red light. Statistics from various experiments show how some plants differ in the composition of chlorophyll, the intensity of photosynthesis depends on this. Depending on the stage of growth, different plant crops absorb a certain part of the light spectrum.

Greens such as onions, parsley, dill grow more actively in the blue spectrum (wavelength 445 nm). At an early stage of development, this range is also preferred by vegetable seedlings. When the period of flowering, ovary and fruit ripening begins, light of the red spectrum in the range of 660 nm is actively absorbed. Some vegetable crops benefit from broad-spectrum white light for favorable growth.

Having studied these properties, you can understand that LEDs are the easiest to adapt to the technology of growing plants in greenhouse conditions under artificial lighting.

Artificial lighting sources

Previously, white LEDs, fluorescent or gas-discharge lamps with a wide spectrum of radiation were actively used for plants in greenhouses. Such lighting is not entirely effective in stimulating plant growth. Much energy is wasted on lighting in the yellow-green range, which is useless for the growth of seedlings.

At the first stage, simple red and blue LEDs and LED strip were used. But these diodes had a fairly wide scattering range beyond the red and blue spectrum, high cost and low illumination intensity. In the process of successive improvements, the LED crystals began to be covered with a layer of phosphor, which has the properties of transmitting only blue and red rays. New phytolamps emit purple light. Technologies using phosphor made it possible to achieve maximum effect in all respects:

low production costs;
maximum concentration of radiation energy in the blue and red ranges;
maximum radiation intensity;
economical mode of electricity consumption.

Such LEDs ensure the active process of photosynthesis, stimulating plant growth. Work to improve the parameters of the emitted spectrum is constantly ongoing; manufacturers are trying to make phytophotodiodes, bringing it as close as possible to the spectrum of sunlight. One of the modern examples is full-spectrum phyto-LEDs Bridgelux 35 mm and Epistar, the first has a more convex diffuser lens.

Appearance of Bridgelux 35 mm

Technical characteristics of Bridgelux 35 mm:

rated power – 1 W;
voltage from 3.0 to 3.4 V;
current – 350 mA;
full color spectrum for plants 400–840 nm;
service life – 50,000 hours;
beam dispersion direction – 120 degrees;
Dimensions – Ø chip with housing 9 mm, Ø lens 5.6 mm, height of the entire chip structure 6 mm.

The peculiarity of these phyto-LEDs is that they do not require several chips with different emission spectra - blue or red. In this case, everything is mounted in one chip with a wide spectrum of illumination, where blue and red colors predominate.

Comparative analysis of the spectra of a red LED and a phytophotodiode

The intervals of yellow, green and other spectra are significantly reduced. This allows you to concentrate energy on emitting useful colors.

The main advantages of phytoLEDs

The emission spectrum covers the entire range from 400 to 840 nm.
The distribution of radiation intensity of parts of the spectrum is as close as possible to sunlight.
The problem of using several types of LEDs with different spectra is solved when red and blue LEDs are inserted into the lamp.
PhytoLED effectively stimulates plant growth throughout the entire development period: before flowering, during flowering, fruit set and ripening. No need to change light sources at different stages. The phytophotodiode is assembled on the basis of a single crystal.

Lamps with phyto-LED elements, which have a full spectrum of sunlight, work 1.9 times more efficiently than simple phytolamps with peaks in the red and blue range. And 1.2 times better than assemblies using individual diodes of different spectrums.

An example of a design for illuminating seedlings with phyto-LEDs

It has been noticed that under phytolamps of the red and blue spectrum the sprouts grow higher, but there are fewer ovaries on the flowers. Full spectrum phytophotodiodes have less intense blue light than red light. The contrasts of the spectrum are balanced so that LEDs for plants do not provide significant growth in height, but the maximum number of fruits.

The superiority of full-spectrum phytophotodiodes over other models is obvious. In order for them to be used even more widely, it remains to improve the details to increase the intensity of the light flux.

A band with a maximum in the yellow area (the most common design). The emission of the LED and phosphor, when mixed, produce white light of various shades.

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Subtitles

History of invention

The first red semiconductor emitters for industrial use were obtained by N. Kholonyak in 1962. In the early 70s, yellow and green LEDs appeared. The light output of these, at that time still inefficient, devices reached one lumen by 1990. In 1993, Shuji 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 efficiency 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 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 emissions from LEDs of different colors. 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 allow you 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 luminous efficiency 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

Combining a blue (more often), violet or ultraviolet (not used in mass production) semiconductor emitter and 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 with different color temperatures (“warm” and “cold”) are produced. By combining different types of phosphors, a significant increase in color rendering index (CRI or R a) is achieved. As of 2017, there are already LED panels for photography and filming, where color rendering is critical, but such equipment is expensive, and manufacturers are few and far between.

One of the ways 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 volume of the active region reduce the light output. When limiting currents are reached, since sections of the LED chip with different impurity concentrations and different band gap widths conduct current differently, local overheating of the chip sections 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 assess 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 has decreased by 30% during operation is considered to be out of order. For LEDs used in decorative lighting, a dimming level of 50% (L50) is used as a life estimate.

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 case 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

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 190 lm/W. The theoretical limit of the technology is estimated at more than 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. There are three reasons for this [ ]: the first is due to the fact that the difference between the energy of the incident and emitted quanta is lost during fluorescence (turns into heat), and in the case of ultraviolet excitation it is much greater. The second reason is that part of the UV radiation not absorbed by the phosphor does not participate in the creation of the luminous flux, unlike LEDs based on a blue emitter, and an increase in the thickness of the phosphor coating leads to an increase in the absorption of luminescent light in it. And finally, the efficiency of ultraviolet LEDs is significantly lower than that of blue ones.

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:

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 producing 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:

Notes

, p. 19-20.
MC-E LEDs from Cree, containing red, green, blue and white emitters Archived November 22, 2012.
LEDs VLMx51 from Vishay, containing red, orange, yellow and white emitters(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
Multicolor LEDs XB-D and XM-L from Cree(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
LEDs XP-C from Cree, containing six monochromatic emitters(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
Nikiforov S.“S-class” of semiconductor lighting technology // Components and Technologies: magazine. - 2009. - No. 6. - pp. 88-91.
Truson P. Halvardson E. Advantages of RGB LEDs for lighting devices // Components and Technologies: magazine. - 2007. - No. 2.
, p. 404.
Nikiforov S. Temperature in the life and operation of LEDs // Components and Technologies: magazine. - 2005. - No. 9.
LEDs for interior and architectural lighting(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
Xiang Ling Oon. LED solutions for architectural lighting systems // Semiconductor lighting technology: magazine. - 2010. - No. 5. - pp. 18-20.
RGB LEDs for use in electronic scoreboards(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
High CRI LED Lighting | Yuji LED (undefined) . yujiintl.com. Retrieved December 3, 2016.
Turkin A. Gallium nitride as one of the promising materials in modern optoelectronics // Components and Technologies: Journal. - 2011. - No. 5.
LEDs with high CRI values(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
Cree EasyWhite technology(English) . LEDs Magazine. Retrieved November 10, 2012. Archived November 22, 2012.
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: Journal. - 2008. - No. 1.
Nikiforov S. Now electrons can be seen: LEDs make electric current very visible // Components and Technologies: magazine. - 2006. - No. 3.
LEDs with a matrix arrangement of a large number of semiconductor chips(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
Service life of white LEDs(English) . U.S. Department of Energy. Retrieved November 10, 2012. Archived November 22, 2012.
Types of LED defects and analysis methods(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
, p. 61, 77-79.
LEDs from SemiLEDs(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
GaN-on-Si Silicon LED Research Program(English) . LED Professional. Retrieved November 10, 2012.
Cree's isolated phosphor technology(English) . LED Professional. Retrieved November 10, 2012. Archived November 22, 2012.
Turkin A. Semiconductor LEDs: history, facts, prospects // Semiconductor Lighting Engineering: magazine. - 2011. - No. 5. - pp. 28-33.
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.
, p. 424.
Reflectors for LEDs based on photonic crystals(English) . Led Professional. Retrieved February 16, 2013. Archived March 13, 2013.
XLamp XP-G3(English) . www.cree.com. Retrieved May 31, 2017.
White LEDs with high light output for lighting needs(English) . Phys.Org™. Retrieved November 10, 2012. Archived November 22, 2012.

There are two common ways to obtain white light of sufficient intensity using LEDs. The first is the combination of chips of three primary colors - red, green and blue - in one LED housing. By mixing these colors, white is obtained; in addition, by changing the intensity of the primary colors, any color shade is obtained, which is used in manufacturing. The second way is to use a phosphor to convert the radiation of a blue or ultraviolet LED into white. A similar principle is used in fluorescent lamps. Currently, the second method prevails due to the low cost and greater light output of phosphor LEDs.

Phosphors

Phosphors (the term comes from the Latin lumen - light and the Greek phoros - carrier) are substances that can glow under the influence of various types of excitations. Based on the method of excitation, there are photoluminophores, x-ray phosphors, radioluminophores, cathodoluminophores, and electroluminophores. Some phosphors come in mixed excitation types, for example, photo-, cathode-, and electroluminophore ZnS·Cu. Based on their chemical structure, they distinguish between organic phosphors - organoluminophores, and inorganic ones - phosphors. Phosphores that have a crystalline structure are called crystallophosphors. The ratio of emitted energy to absorbed energy is called quantum efficiency.

The glow of a phosphor is determined both by the properties of the main substance and by the presence of an activator (impurity). The activator creates luminescence centers in the main substance (base). The name of activated phosphors consists of the name of the base and the activator, for example: ZnS·Cu,Co means ZnS phosphor activated with copper and cobalt. If the base is mixed, then the names of the bases are listed first, and then the activators, for example, ZnS, CdS Cu, Co.

The appearance of luminescent properties in inorganic substances is associated with the formation of a phosphor base in the crystal lattice during the synthesis of structural and impurity defects. The energy that excites the phosphor can be absorbed both by luminescent centers (activator or impurity absorption) and by the phosphor base (fundamental absorption). In the first case, absorption is accompanied either by the transition of electrons inside the electron shell to higher energy levels, or by the complete removal of an electron from the activator (a “hole” is formed). In the second case, when energy is absorbed by the base, holes and electrons are formed in the main substance. Holes can migrate throughout the crystal and become localized at luminescence centers. Emission occurs as a result of the return of electrons to lower energy levels or the recombination of an electron with a hole.

Phosphors in which luminescence is associated with the formation and recombination of opposite charges (electrons and holes) are called recombination phosphors. They are based on semiconductor-type connections. In these phosphors, the crystal lattice of the base is the medium in which the luminescence process develops. This makes it possible, by changing the composition of the base, to widely vary the properties of the phosphors. Changing the band gap when using the same activator smoothly changes the spectral composition of the radiation over a wide range. Depending on the application, there are different requirements for the parameters of the phosphor: type of excitation, excitation spectrum, emission spectrum, emission output, time characteristics (glow rise time and afterglow duration). The greatest variety of parameters can be obtained with crystal phosphors by changing the activators and the composition of the base.

The excitation spectrum of various photoluminophores is wide, from short-wave ultraviolet to infrared. The emission spectrum is also in the visible, infrared or ultraviolet regions. The emission spectrum can be wide or narrow and strongly depends on the concentration of the phosphor and activator, as well as on temperature. According to the Stokes-Lommel rule, the maximum of the emission spectrum is shifted from the maximum of the absorption spectrum towards long waves. In addition, the emission spectrum usually has a significant width. This is explained by the fact that part of the energy absorbed by the phosphor is dissipated in its lattice, turning into heat. A special place is occupied by “anti-Stokes” phosphors, which emit energy in a higher region of the spectrum.

The energy output of the phosphor radiation depends on the type of excitation, its spectrum and the conversion mechanism. It decreases with increasing concentration of the phosphor and activator (concentration quenching) and temperature (temperature quenching). The brightness of the glow increases from the beginning of excitation for varying periods of time. The duration of the afterglow is determined by the nature of the transformation and the lifetime of the excited state. Organoluminophores have the shortest afterglow time, crystal phosphors have the longest.

A significant part of crystal phosphors are semiconductor materials with a band gap of 1-10 eV, the luminescence of which is caused by an activator impurity or crystal lattice defects. Fluorescent lamps use mixtures of crystal phosphors, for example, mixtures of MgWO4 and (ZnBe)2 SiO4·Mn] or single-component phosphors, for example calcium halophosphate activated by Sb and Mn. Phosphors for lighting purposes are selected so that their glow has a spectral composition close to the spectrum of daylight.

Organic phosphors can have high yield and fast response. The color of the phosphor can be selected for any visible part of the spectrum. They are used for luminescent analysis, production of luminescent paints, signs, optical brightening of fabrics, etc. Organic phosphors were produced in the USSR under the brand name luminors.

During operation, the phosphor is subject to changes in parameters over time. This process is called phosphor aging (degradation). Aging is mainly caused by physical and chemical processes both in the phosphor layer and on its surface, the emergence of non-radiative centers, and the absorption of radiation in the changed phosphor layer.

Phosphor in LED

White LEDs are most often made using a blue InGaN crystal and a yellow phosphor. The yellow phosphors used by most manufacturers are modified yttrium aluminum garnet doped with trivalent cerium (YAG). The luminescence spectrum of this phosphor is characterized by a maximum wavelength of 530..560 nm. The long-wave part of the spectrum is longer than the short-wave part. Modification of the phosphor with gadolinium and gallium additives allows you to shift the maximum of the spectrum to the cold region (gallium) or to the warm region (gadolinium).

The spectral data of the phosphor used in Cree is interesting. Judging by the spectrum, in addition to YAG, a phosphor with an emission maximum shifted to the red region has been added to the phosphor composition of the white LED.

Unlike fluorescent lamps, the phosphor used in LEDs has a longer service life, and the aging of the phosphor is determined mainly by temperature. The phosphor is most often applied directly to the LED crystal, which becomes very hot. Other factors affecting the phosphor are of much less importance for service life. Aging of the phosphor leads not only to a decrease in the brightness of the LED, but also to a change in the shade of its glow. With severe degradation of the phosphor, a blue tint of the glow is clearly visible. This is due to a change in the properties of the phosphor, and to the fact that the LED chip’s own radiation begins to dominate in the spectrum. With the introduction of technology (remote phosphor), the influence of temperature on the rate of phosphor degradation is reduced.

Light-emitting semiconductor devices are widely used to operate lighting systems and as indicators of electrical current. They refer to electronic devices that operate under the influence of applied voltage.

Since its magnitude is insignificant, such sources belong to low-voltage devices and have an increased degree of safety in terms of the effects of electric current on the human body. The risk of injury increases when high voltage sources are used to illuminate them, for example, a household home network, which require the inclusion of special power supplies in the circuit.

A distinctive feature of the LED design is the higher mechanical strength of the housing than that of Ilyich and fluorescent lamps. When used correctly, they work long and reliably. Their lifespan is 100 times greater than that of incandescent filaments, reaching one hundred thousand hours.

However, this indicator is typical for indicator structures. Powerful lighting sources use increased currents, and their service life is reduced by 2–5 times.

A conventional indicator LED is made in an epoxy housing with a diameter of 5 mm and two contact leads for connection to electric current circuits: . Visually they differ in length. The new device without cut contacts has a shorter cathode.

A simple rule helps to remember this position: both words begin with the letter “K”:

When the legs of the LED are cut off, the anode can be determined by applying a voltage of 1.5 volts to the contacts from a simple AA battery: light appears when the polarities match.

The light-emitting active semiconductor single crystal has the shape of a rectangular parallelepiped. It is placed near a parabolic-shaped reflector made of aluminum alloy and mounted on a substrate with non-conducting properties.

At the end of the light transparent body made of polymer materials there is a lens that focuses the light rays. Together with the reflector, it forms an optical system that shapes the angle of radiation flux. It is characterized by the directional pattern of the LED.

It characterizes the deviation of light from the geometric axis of the overall structure to the sides, which leads to increased scattering. This phenomenon occurs due to the appearance of minor technology violations during production, as well as the aging of optical materials during operation and some other factors.

At the bottom of the case there can be an aluminum or brass belt that serves as a radiator to remove the heat generated by the passage of electric current.

This design principle is widely accepted. On its basis, other semiconductor light sources are created, using other forms of structural elements.

Principles of light emission

The p-n type semiconductor junction is connected to a constant voltage source in accordance with the polarity of the terminals.

Inside the contact layer of p- and n-type substances, under its influence, the movement of free negatively charged electrons and holes, which have a positive charge sign, begins. These particles are directed towards the poles that attract them.

In the transition layer, charges recombine. Electrons pass from the conduction band to the valence band, overcoming the Fermi level.

Due to this, part of their energy is released with the release of light waves of different spectrum and brightness. Wave frequency and color rendition depend on the type of mixed materials from which it is made.

To emit light inside the active zone of a semiconductor, two conditions must be met:

1. The width of the bandgap in the active region should be close to the energy of emitted quanta within the frequency range visible to the human eye;

2. The purity of the semiconductor crystal materials must be ensured to be high, and the number of defects affecting the recombination process must be as low as possible.

This complex technical problem can be solved in several ways. One of them is the creation of several layers of p-n junctions, when a complex heterostructure is formed.

Effect of temperature

As the source voltage level increases, the current through the semiconductor layer increases and the glow increases: an increased number of charges enter the recombination zone per unit time. At the same time, heating of the current-carrying elements occurs. Its value is critical for the material of internal current guides and the substance of the p-n junction. Excessive temperature can damage and destroy them.

Inside LEDs, the energy of electric current is converted into light directly, without unnecessary processes: unlike lamps with incandescent filaments. In this case, minimal losses of useful power are formed due to low heating of the conductive elements.

Due to this, high efficiency of these sources is created. But, they can only be used where the structure itself is protected and blocked from external heat.

Features of lighting effects

When holes and electrons recombine in different compositions of p-n junction substances, unequal light emission is created. It is usually characterized by the quantum yield parameter—the number of isolated light quanta for a single recombined pair of charges.

It is formed and occurs at two levels of the LED:

1. inside the semiconductor junction itself - internal;

2. in the design of the entire LED as a whole - external.

At the first level, the quantum yield of correctly made single crystals can reach a value close to 100%. But, to ensure this indicator, it is necessary to create large currents and powerful heat removal.

Inside the source itself at the second level, part of the light is scattered and absorbed by structural elements, which reduces the overall radiation efficiency. The maximum value of the quantum yield is much lower here. For LEDs emitting a red spectrum, it reaches no more than 55%, and for blue ones it decreases even more - to 35%.

Types of color transmission of light

Modern LEDs emit:

White light.

Yellow-green, yellow and red spectrum

The p-n junction is based on gallium phosphides and arsenides. This technology was implemented in the late 60s for indicators of electronic devices and control panels of transport equipment, and billboards.

In terms of light output, such devices immediately surpassed the main light sources of that time - incandescent lamps - and surpassed them in reliability, service life and safety.

Blue spectrum

Emitters of the blue, blue-green and especially white spectra have long resisted practical implementation due to the difficulties of comprehensively solving two technical problems:

1. limited size of the band gap in which recombination occurs;

2. high requirements for the content of impurities.

For each step in increasing the brightness of the blue spectrum, an increase in the energy of the quanta was required by expanding the width of the band gap.

The issue was resolved by including silicon carbides SiC or nitrides into the semiconductor substance. But the developments of the first group turned out to have too low efficiency and low quantum emission yield for one recombined pair of charges.

The inclusion of solid solutions based on zinc selenide in the semiconductor transition helped to increase the quantum yield. But such LEDs had increased electrical resistance at the junction. Due to this, they overheated and quickly burned out, and complex heat removal structures for them did not work effectively.

For the first time, a blue-emitting diode was created using thin films of gallium nitride deposited on a sapphire substrate.

White spectrum

To obtain it, one of three developed technologies is used:

1. mixing colors using the RGB method;

2. applying three layers of red, green and blue phosphor to an ultraviolet LED;

3. covering a blue LED with layers of yellow-green and green-red phosphor.

In the first method, three single crystals are placed on a single matrix, each of which emits its own RGB spectrum. Due to the design of the lens-based optical system, these colors are mixed to produce a total white tint.

In an alternative method, color mixing occurs due to sequential irradiation of three constituent layers of phosphor with ultraviolet radiation.

Features of white spectrum technologies

RGB technique

It allows:

use various combinations of monocrystals in the lighting control algorithm, connecting them one by one manually or by an automated program;

cause different color shades that change over time;

create spectacular lighting systems for advertising.

A simple example of such an implementation is . Similar algorithms are also widely used by designers.

The disadvantages of RGB LED design are:

uneven color of the light spot in the center and edges;

uneven heating and heat removal from the surface of the matrix, leading to different aging rates of p-n junctions, affecting color balancing, changing the overall quality of the white spectrum.

These disadvantages are caused by different arrangements of single crystals on the base surface. They are difficult to eliminate and configure. Due to this RGB technology, models are among the most complex and expensive designs.

LEDs with phosphor

They are simpler in design, cheaper to manufacture, and more economical when calculated per unit of luminous flux.

They are characterized by disadvantages:

in the phosphor layer, losses of light energy occur, which reduce light output;

the complexity of the technology for applying a uniform layer of phosphor affects the quality of color temperature;

The phosphor has a shorter lifespan than the LED itself and ages faster during operation.

Features of LEDs of different designs

Models with phosphor and RGB products are created for various industrial and domestic applications.

Eating methods

The indicator LED of the first mass production consumed about 15 mA when powered from slightly less than two volts of direct voltage. Modern products have increased characteristics: up to four volts and 50 mA.

LEDs for lighting are powered by the same voltage, but consume several hundred milliamps. Manufacturers are now actively developing and designing devices up to 1 A.

In order to increase the efficiency of light output, LED modules are being created that can use a sequential voltage supply to each element. In this case, its value increases to 12 or 24 volts.

When applying voltage to the LED, polarity must be taken into account. When it is broken, the current does not pass and there will be no glow. If an alternating sinusoidal signal is used, then the glow occurs only when a positive half-wave passes through. Moreover, its strength also changes proportionally according to the law of the appearance of the corresponding current value with a polar direction.

It should be taken into account that with reverse voltage, breakdown of the semiconductor junction is possible. It occurs when exceeding 5 volts on a single crystal.

Control methods

To adjust the brightness of the emitted light, one of two control methods is used:

1. the magnitude of the connected voltage;

The first method is simple but ineffective. When the voltage level drops below a certain threshold, the LED may simply go out.

The PWM method eliminates this phenomenon, but it is much more complicated in technical implementation. The current passed through the semiconductor junction of a single crystal is supplied not in a constant form, but in a pulsed high frequency with a value from several hundred to a thousand hertz.

By changing the width of the pulses and pauses between them (the process is called modulation), the brightness of the glow is adjusted over a wide range. The formation of these currents through single crystals is carried out by special programmable control units with complex algorithms.

Emission spectrum

The frequency of the radiation coming out of the LED lies in a very narrow region. It is called monochromatic. It is radically different from the spectrum of waves emanating from the Sun or the incandescent filaments of conventional lighting lamps.

There is much debate about the effect of such lighting on the human eye. However, the results of serious scientific analyzes of this issue are unknown to us.

Production

In the production of LEDs, only an automatic line is used, in which robotic machines operate using pre-designed technology.