The LEDS have been used for 40 years, mainly for signage applications (Calculator, light appliance, etc.). The LEDs technology continued to improve since. New LED lighting applications grow every day, thus improving performance in terms of power, energy and life of the new lighting equipment.
General technical features
The market for LEDs
Bright future for LED lighting?
Luminous flux (Lumens) O
This is the amount of energy emitted by a source in the form of visible radiation in all directions, per unit of time.
Emission angle (degrees)
It is the angle of half light intensity of the LED, which is the total angle (in both directions with respect to the optical axis) within which the luminous intensity (in Candelas (cd)) is more than half of the maximum light intensity.
Luminous efficacy (Lumens/Watt)
It defines the ability of light to produce an important from a Watt (W) power absorbed flow. There are three luminous efficiencies, depending on whether you consider:
the only light source.
the light source and its power supply (transformer, Rectifier, ballast…).
light source, its power supply and the optical losses of installation (broadcaster, reflector…).
Luminous efficiencies of the LEDs tested in laboratory are still significantly higher than those marketed LEDs.
The lumen unit depends on the sensitivity of the eye to light. Therefore, it is easier to have high luminous efficiencies in orange, wavelength to which humans are very sensitive.
Color temperature (Kelvin)
It is the apparent color emitted by a white light source. It is expressed in degrees Kelvin (0 ° K = – 273 ° C). Pull on the yellow-red and warm hues lights have a color temperature 3000 ° K (2700 to 2900 ° K for “incandescent” lights). Shoot the blue and have a temperature of between 5,000 and 10,000 color lights cold shades ° K (6,500 ° K for colour luminaires “light of day”). According to the rule of Kruithof, the apparent color is hot (the temperature of color is low), the level of illumination (in lux) may be low without harming the feeling of well being.
(IRC) color rendering index
This index between 0 and 100 defines the ability of a light source to render the different colors of the objects it illuminates, compared to a source of reference. Solar light has a 100 IRC, while sodium vapor lamps (used in road tunnels for example) have a 20 IRC. In stores, schools or offices, IRC should always be greater than 80.
Here are assessments that can be drawn to an IRC:
IRC 0-50: very bad
IRC between 50 and 70: bad
IRC between 70 and 80: fair
IRC between 80 and 90: good
IRC between 90 and 100: very good
General technical features
There are two main types of LEDs: inorganic (LED) and organic (OLED). Just note that the organic LEDs (OLEDs) are developing, and that the first applications starting to hit the market. While the LEDs provide point sources similar to the lamps incandescent, OLEDs could replace more extensive sources such as fluorescent tubes. At the moment the poor luminous efficiency of OLEDs, as well as their short lives are that they are not used for lighting. On the other hand, manufacturers plan to use them term for display and signage applications. We will focus in the following only the (inorganic) led.
The LEDs inorganic classics are overdoses (diodes) P – N junctions in order to emit a visible or ultraviolet radiation when a current passes through in the passing sense. The radiation emitted by a conventional LED (red, green) is almost monochromatic (spectral line). The influence of white LEDs is dichromatic or polychromatic according to the technique used (see below).
The different formats
The forms of the LEDs can be very different depending on the model. LEDs can be: round 5mm (classic), 3mm round, square 7.6 mm miniatures (CMS), or even mounted on a special circuit (see the LED Luxeon of Lumileds)
In all types of LEDs light semi¬conductrice (or ‘in’) chip is completely encapsulated in a transparent plastic material, usually an epoxy resin, who plays the role of lens and determines the angle of light emission. This format makes the LEDs very resistant to shocks, in contrast with a glass bulb.
The power supply
The LEDs still work in current continuous low voltage (from 0.5 to 3 Volts per LED according to color). They are often mounted in series to increase the level of tension. They are generally powered by DC 9V, 12V or 24V, from batteries, batteries or solar cells. Continuous, the supply circuit is very simple: just add correctly sized between the voltage source and the LED resistance.
Example: you can plug a classic red LED (1.2 volts) in series with a 180 Ohm resistor, all powered by 5V. The current then reached 21 my [(5-1,2) / 180]. Notice that three quarters of the energy is lost in the resistance (80 mW compared to 25 in the LED).
In alternative, (alternative/continuous) converters can supply lights to LEDs from the 230 V. However these converters can have very bad performance (below 50%), which reduces the luminous efficiency of the whole. Therefore, it is important to properly design these power supplies to reduce consumption. Conventional LEDs absorbing a current of 20 mA to 0.025 W power in 1, 2V. But the power of some recent LEDs now reached 5W
The duration of life
Life of LEDs is defined as the period before which the LED will no longer emit only 50% of the initial luminous flux. According to the manufacturers of LEDs, these estimated lives are greater than 100,000 hours (more than 11 years continuous), sometimes even 150,000 hours. It is 100 times higher than for a conventional incandescent lamp. It is currently the major interest of the LEDs. Conventional LEDs lost 30% of their luminous efficiency after 3,000 hours of use because of the yellowing of epoxy encapsulation. This problem has been resolved on the latest generation of LEDs.
The luminous efficiency of LEDs depends on the technology used. It varies a lot with the color emitted by the LED, as well as with the manufacturer. The table below gives some orders of magnitude, for the best LEDs (standard models produce 100 to 1,000 times less).
How to make white light from LEDs?
Conventional LEDs emit only around a wavelength given (spectral line), whereas white light solar or incandescent emits on the range of the visible spectrum in a continuous and homogeneous way. Different methods are used to create the white light or pseudo-blanche from LEDs. Note that these methods are relatively recent because they rely on the blue LEDs show or UV, which were particularly complex to realize, and have been commercially available until 1990.
Method 1 – Mix of colors LEDs
The recent development by blue LEDs from Nichia, to produce white light from a mixture of red, green, and blue LEDs in the same light, spot, or ‘table’ to LEDs. The resulting color temperature will depend on the relative numbers of each type of LEDs. This method to two advantages: the overall luminous efficiency is good (of the order of 25 lumens per Watt and more), and it allows to vary the intensities of each group of LEDs in order to obtain the desired color temperature. It is the most effective method at the moment. A KPI of 80 can be reached using 3 types of carefully selected LEDs, but the best performance is achieved with 4 or 5 groups of LEDS of different colors. This method will probably be the most used long term. But the homogeneous mixture of colors is very difficult and expensive.
Method 2 – Blue LED and phosphorus (s)
This method uses the principle of fluorescence. The white LED is made from a diode emitting in blue (GaN). A luminescent layer at base of phosphorus is deposited above to convert a portion of the radiation emitted by a yellow radiation. The resulting visible light is a mixture of blue and yellow, seen as a pseudo-blanc. These LEDs have a luminous efficiency of 15 to 25 lumens per Watt and more, according to the manufacturers (Nichia, CREE). These are the most popular on the market today. Their color temperature is approximately 4,000 ° K and their IRC is 75, or weaker (IE pretty bad). A “halo” embarrassing is also present for these LEDs: blue light is directive, yellow light emitted is multidirectional.
Method 3 – Schubert method
This method developed by the Boston University professor Fred Schubert, to create a pseudo-blanc without phosphorus. This LED is called PRS – LED, for Photon Recycling Semi-conductor LED. The principle is this: at the level of the semiconductor, a primary source is excited by injected electric current. This active region then emits a visible glow blue (InGaN) which part will be absorbed by a secondary active region (AlInGaP) “recycling” these emitted photons. This recycling allows the re-broadcasting of light of a higher wavelength (yellow or red), which, combined with residual and complementary light emitted by the primary source, give a source-dichromatic pseudo-blanche. This method is not yet used industrially.
Method 4 – UV and three LEDS phosphores or more
This method uses a LED UV radiation that is absorbed by different Phosphors, which emit so simultaneously complementary colours. The IRC is so similar to that of fluorescent lamps (good). But the fact that UV radiation is not visible directly (unlike the blue light in method 2), requires that the UV emission is very effective, to compete with method 2.
What are the strengths and weaknesses of white LEDs for lighting?
Are the LEDs more efficient than other white light sources?
From a purely technical point of view the answer is “no, not yet… but it will be soon…”, as shown in the table below.
Incandescent Halogen CFL Fluorescent tube Sodium high pressure LED white
Note: including the losses of the ballast tanks, the LBC have an average luminous efficiency of around 45-60 Lm/W and 36 W about 80 Lm/W T8 tubes.
LEDs allow currently energy savings?
Yes, in some applications, due to the optical characteristics (colors, directivity, type of power supply…) LEDs, LEDs can advantageously replace “traditional” lighting with less important, a luminous flux (in lumens) and a demand for an equivalent service. They are particularly interesting for applications that require a very directional lighting.
Are the LEDs profitable from an economic point of view?
The reliability and the service life of the LEDs allow in the places where the maintenance of the luminaires is problem to quickly absorb the extra cost of purchase.
Benefits of LED technology
Instant start (unlike the lamps or fluorescent tubes).
Life much longer than a conventional incandescent or even fluorescent lamp (50 000 to 100 000 hours from 6 000 to 15 000 hours for the fluorescent and maximally a thousand hours for incandescent lamps).
Reliability: high resistance to shock, vibration and crash.
Insensitivity to the repeated ignitions and low temperatures
Directivity: show the current LEDs angle can vary from 15 ° to 120 °. So we can get to the choice of the lighting very directive without adding reflectors or lenses (reading lights, torches), or of much more diffuse lighting.
Possibility of control of light intensity very easy by simple changes in the supply voltage.
Possibility of temperature control of colour (coloured LEDs mix case, method 1).
Can be used in low-power and low-voltage (direct use on batteries).
Wide range of possible colours (without use of filters).
Small size and aesthetics (compared to the LBC).
Ease of mounting on a circuit board, traditional or CMS
Low to very low power consumption (a few tens of milliwatts) thanks to a very good performance.
Much smaller than conventional lamps. By assembling several LEDs, can be achieved with innovative shapes lighting.
Operating on very low voltage (TBT), pledge of security and ease of transport. There is for campers to led flashlights powered by a simple hand slow motion (“crank lamp’) dynamo.
Asset security, compared to conventional light systems, their light inertia is almost zero. They ignite and extinguish in a very short time, which allows the use in transmission of signals at short distances (optocouplers) or long (fiber optics). the LED immediately reach their nominal light intensity.
Because of their power, the conventional LEDs 5 mm almost not heat and do not burn your fingers. For the fixtures of power greater than 1 W, there will be a heat dissipation without which the LED will be severely damaged or destroyed as a result of warming up. Indeed, a LED converts about 20% of electrical energy into light, the rest being released as heat.
The market for LEDs
Manufacturers of LEDs
The three leading manufacturers of LEDs in the world are companies NICHIA (Japan), Philips LUMILEDS (Europe/USA) and created (United States). Another large manufacturer is AGILENT.
Signalling by led
Led luminaires market is not very developed in France. But LEDs can now be used with profit for the following applications of signaling:
The LEDS in this application are in some cases 10 times more effective than the lamps incandescent usually used, especially their directivity and their color. Maintenance is also very strongly reduced. Incandescent lamps used for these fires have a low yield.
Example of a light pedestrian:
2 x 40 W incandescent,
6 to 10 W with LEDs. Large fires are still very expensive, but the progress of this technology have been rapid in recent years.
The city of Grenoble embarked on the adventure and has invested 150,000 Euros to change all the lights in LED lights. The investment was repaid in 3 years and the city saves 55,000 Euros per year. The replacement of incandescent by LED represents a triple interest. These saving important consumption, a gain of maintenance thanks to their vastly superior longevity, and also offer greater security. In fact, the colorful LED light avoids laying caches of color on the boxes, thus significantly improving visibility.
Urban markup, road safety, and automotive.
Many distributors propose systems markup and signalling led. A system of marking up a motorway in Sweden has been implemented from LEDs. The tags are powered by a photovoltaic system. Manufacturers are also starting to use LEDs for the brake lights (third) indicators and dashboards.
BAEH and BAES
The use of LEDs for night light and emergency lighting BAES (block independent of light relief) or BAEH allows to considerably reduce the maintenance on these devices. LEDs are now routinely used for the night light function, but the use of LEDs for lighting is still rare.
The led lighting
Flashlights and frontal
There are many torches, headlamps and LED lights white or blue on Petsinclude. The LEDs are particularly popular for these applications, because of their resistance to shocks and their long life. They are usually powered with batteries 1.5 V and contain only a few LEDs (from 3 to 10 for torches, but up to 60 for a dive light). We can find from 15 Euros. The use of LEDs also lets you design torches magnetic without battery, which is not too cumbersome: shaking the lamp for 30 seconds, recharge with a magnet sliding inside a coil, a capacitor capable of powering a torch for nearly an hour. This system is built to last for years.
Luminaires in height
In factories, supermarkets, amphitheatres,… outside light sources are often placed more than three meters in height which makes maintenance more difficult and costly. Very important life of LEDs can then incredibly reduce maintenance operations. The extra cost of LEDs can be amortized quickly. There are unfortunately not yet a lot of fixtures for this type of application in France.
Lighting in “developing” countries
Two billion people have no access to electric light. The LEDs could eventually replace the ineffective and dangerous kerosene lamps in the villages who want it. Professor Irvine-Halliday has developed a lamp LEDs running on rechargeable batteries from renewable energy (pico-turbines, small wind turbines and solar cells). A village of 60 homes equipped with these lamps absorbs a power of 100 Watts. Yet the lighting is of better quality than traditional lighting (kerosene, candle, oil…), and is sufficient to illuminate the main jobs of the housing.
The “design” lighting
The first fixtures to use LED technology are made by great designers. This technology allows to develop new forms. The quality of lighting is also appreciated. Artemide, Lucepla, Ingo Maurer developed desktop or ceiling lights from LED lamps.
LEDs are often used in interior decoration: there are spots colored LEDs for lighting of exhibits, pebbles, Garden torches, “strips” of LEDs that can be used as nose of markets or to mark up theaters, replacement of the colorful neon signs, dichroic lamps… These applications don’t require high power, and their cost is justified by their decorative aspect.
Bright future for LED lighting?
Thomas Edison incandescent lamps could well disappear if the LEDs keep all their promises.
Over the past years, geometric adaptations of electronic chips have made progress in terms of the emission of light. We provide more light for a same electricity consumption. You can get more clarity by enlarging the chip. There is no reason known light efficiency of white LEDs to achieve a 200 lumens or more per watt.
According to the OIDA (Optoelectronics Industry Development Association) in Washington, DC, white LEDs could cut by half the amount of electricity used for lighting in the United States by 2020.