LEDs consist of semiconducting material mixed with impurities to create a p-n function similar to a diode. Atoms in the n-type material have free electrons, and n-type materials have electron holes. A positive voltage on the p-side and a negative voltage on the n-side generate forward bias circuit. When voltage is applied, the atoms get pushed towards the junction and become close. The n-type atoms donate extra electrons to the p-type atoms. The recombination of the extra electrons with the holes in p-type material leads to energy being released in the form of photons. The LED material is selected so that the wavelength of the photons falls within the visible part of the spectrum.
LEDs can be categorized into two major types: Low Power LEDs and High Power LEDs.
Low Power LEDs
In low power LEDs, the maximum driving current is limited to 20 mA. This is also limited by heat dissipation through epoxy encapsulation. These LEDs are available in 3, 5, and 8 mm sizes, out of which 5 mm is the most common. The typical forward voltage is 3.2V in such LEDs. The LED power is limited to 0.1 W range and light output rarely exceeds 2-3 lumen.
High Power LEDs (High Brightness LEDs)
In high power LEDs, the maximum driving current falls in the 350 - 1000 mA range. They are typically available in 1-5 watt packages, but can go up to 40 W for multi-chip lamps. Commercially available 1 W packages can produce in excess of 100 lumens/watt. Heatsinks are essential with high power LEDs, as they perform effective heat dissipation. A device can be destroyed in seconds if the heat from the high power LED is not removed. High power LEDs are frequently used to replace incandescent bulbs in torches, or can be set in an array to form a powerful LED lamp.
Advantages of High Brightness LED
The lumen (lm) is a measure of total amount of visible light (luminous flux) produced by a light source.
It is calculated by multiplying the intensity (in candela) by the angular span over which the light is emitted. With the symbol Φv for lumen, Iv for candela and Ω for the angular span in steradian, the relation is:
Φv = Ivx Ω
Candela (cd), or candlepower, is the base measurement of luminous intensity. It describes the brightness of the light source in a particular direction. Laser pointers or spotlights have the highest candela rating since the majority of their light is focused in a single direction. If a light bulb generates 1 cd and blocks part of the light, every direction not covered still produces 1 cd. This happens as the same intensity of light is seen from any non-obscured direction at same distance.
Illuminance (lux) is a measure of the amount of light falling onto and spreading over a given surface area. Illuminance is measured using a chroma meter, a lux meter, or an illuminance spectrophotometer. The SI unit of illuminance is lux (lx). It is correlated with the eye’s visual perception for brightness of an illuminated area.
One lux measures one lumen per square meter. A lamp which indicates its brightness in lux usually includes the distance from the bulb because any change in distance or bulb type varies the lux level. As an example, a 2000-lumen lamp shines on one square meter of the surface which is lit at 2000 lx. If the lamp backed away to shine on four square meters, the surface will now be lit with only 500 lx.
Photometric Unit’s Conversion
To make the conversion between Candela (cd), lumen (lm), the following formula can be used.
lm/m2 = lux (lx)
lm/cm2 = phot (ph)
lm/ft2 = footcandle (fc)
lm/sr = cd
Conversion between fc and lux is made according to the following equation:
1 fc =10.764 lux
Light emitting diodes are crafted using semiconductor compounds mixed at different ratios to produce different wavelengths of colour. The forward voltage is also different for each different compound. Different compounds emit light in specific regions of the visible light spectrum and thus produce different intensity levels. The following table shows the color of each different compound along with their forward voltage.
An LED emits one color depending on the specific composition of its materials. When red, green and blue LEDs are combined in a single device, millions of colors can be created by controlling the relative intensity of each color.
Correlated Color Temperature (CCT)
Correlated color temperature (CCT) is a scale of the color of a light source, the latter being defined by chromaticity coordinates when heated to a particular temperature. It is measured in degrees Kelvin (K). The CCT rating for an LED measures the warmth or coolness of its appearance. The LEDs with a CCT rating less than 3200 K are considered warm sources and LEDs with a CCT rating above 4000 K are generally considered cool in appearance. In figure 1, Point A represents an LED with chromaticity coordinates of (0.24, 0.59). As this point lies on the 3000 K is temperature line, the LED has a CCT of 3000 K. Light appearance depends on the color temperature and is usually categorized in three different ways:
- Warm White: Color temperature from 2000K to 3500K displays a warm white appearance, imparting a calm and cozy feeling. These kinds of LEDs are generally used in living rooms, bedrooms and for decorative lighting.
- Natural White: Color temperature from 3500K to 5500K displays a natural white appearance.
- Cool White: Color temperature above 5000K displays a cool white appearance imparting a bright and vibrant atmosphere. These types of LEDs are normally used in work environments, task lighting and offices.
Figure 1: chromaticity coordinates
Color Rendering Index (CRI)
The color rendering index (CRI) is a measure of a light source’s ability to expose the colors of various objects in comparison with an ideal, or natural, light source. It describes how good the variations in color shades are revealed and how they appear to the human eye. It is specified from 0 to 100 showing the accuracy of rendering color for a given LED when compared to a reference light source. The higher the CRI is, the better the color rendering ability is. LEDs with a CRI of 85 to 90 are considered good at color rendering, and a CRI of more than 90 is considered excellent. The following table shows achievable CRI of different light sources and their rating as per the index:
Like any other mass manufacturing process, LED chips are manufactured in millions and thus available with slight differences in color appearance and light output. Binning is the method of sorting LED chips into bins so that all LEDs in a specific bin have the same characteristics. Binning systems manage different variations in LED performance during mass production. They also ensure specific lighting standards of LEDs. Most manufacturers sort their production into three different binning types:
- Luminous Flux (Lumen)
Luminous intensity varies, for example from 210 mcd to 320 mcd at 350mA as per the datasheet of an LED. If we randomly select these parts and connect in series, the brightness may vary wildly, even using the same current for all LEDs. To avoid this issue, manufacturers follow the binning process and categorize LEDs with approximately the same lumen intensity in a given tolerance at constant current, as shown in the following table:
- Forward Voltage (Volts)
Forward voltage drop in LEDs varies from device to device and causes some issues. For example, the LED datasheet shows the forward voltage varies between 1.75 V and 3.75 V at 350 mA, - which is a significant variation. For minimum value (1.75 V), LEDs can work with a low-value series resistance at 3.3 V. However for the maximum value (3.75 V) sample, the LEDs will not even turn on. This issue is avoided by binning the LEDs according to forwarding voltage, and keeping the current constant. The following table shows the bin code for different forward voltages at 350 mA forward current:
- Color Temperature (Kelvin)
For the binning process according to color, it is necessary to introduce and understand the chromaticity diagrams, as shown in figure 2a. This is a CIE 1931 Chromaticity Diagram, which describes the color as seen by the human eye in full daylight. The diagram labels color along x- and y-axes with individual coordinates related to a unique color. The two-dimensional coordinates of this diagram are related to specific colors. For example, coordinate 0.3, 0.3 is white light, whereas 0.0082, 0.5384 is greenish cyan, and 0.7066, 0.2934 is HeNe laser red. The figure 2B is the CIE 1931 Chromaticity Diagram for a white LED, where coordinate 0.31, 0.35 shows cool white color with bin code A, whereas the coordinate 0.45, 0.42 comes in warm white with bin code H.
LED Selection Criteria
The following parameters are essential when it comes to LED. They should be taken into consideration.
- Luminous Flux and Intensity
Luminous flux is the photometrically weighted radiant flux or power of light and is measured in lumen (lm). It differs from the measure of the total power of light emitted. The luminous flux takes into account varying sensitivity of the human eye to different wavelengths of light. One lumen is the luminous flux of light produced by an LED that emits 1 cd of luminous intensity over a 1 sr solid angle. In the datasheet, the luminous flux is represented in the table below. It indicates that at constant 350 mA forward current, a red LED of bin code M has a luminous flux in the range of 45.7 lm to 87.4 lm:
Luminous intensity is the quantity of light emitted by an LED in a particular direction per unit solid angle. The quantity of light produced from a source in one second is called lumen and evaluated based on visual sensation. One lumen per steradian is the unit of luminous intensity and also known as a candela.
- Radiant Flux
Radiant flux, or radiant power, is the radiometric parameter of an LED and is defined as the radiant energy emitted, reflected or transmitted per unit of time. It is measured in watts. In the datasheet, it is represented in the following table and indicates that, for a particular LED of bin code P at a constant forward current of, say, 700mA, the radiant power is 1.6W to 2.0W and, at 1000mA forward current, it is typically 2.3 W:
- Peak Wavelength
Peak wavelength is the single wavelength where the radiometric emission spectrum of the LED reaches its maximum. It does not represent any apparent emission of the light source, as perceived by the human eye, but, rather, by photo-detectors. In the datasheet, it is represented in the following table, where an LED of bin code U5 has the peak wavelength 390 nm to 395 nm at a constant forward current of 700 mA:
- Dominant Wavelength
Dominant wavelength is the single wavelength observed by the human eye. Normally, one light source involves multiple wavelength spectrums rather than a single wavelength. When we look at the light, our brains observe those multiple spectrums as a single color of light with a single specific wavelength. If the LED is used to illuminate or indicate something for human operators, we should consider the dominant wavelength for an LED selection.
- Viewing Angle (FWHM) Viewing angle is the maximum angle at which we can view the display with an acceptable visual performance. The LED industry defines a viewing angle as the full angle at which brightness is half of the brightness from the dead center. If ø is the angle from off center (0°) where the LED’s brightness is half, then 2ø is defined as the full viewing angle. In the datasheet, it is denoted by Full-Width Half Maximum (FWHM).
- Forward Voltage
The forward voltage rating is the minimum voltage difference between the anode and cathode to allow current flow. LEDs are current-dependent devices with their forward voltage drop depending on the semiconductor compound (color) and on the forward biased LED current. Some common LEDs have a forward operating voltage from 1.2 to 3.6 V, with a forward current of 350 mA. The exact voltage drop is decided by the manufacturer due to the different construction materials and wavelengths used. It is an important parameter in the binning process.
- DC Forward Current
The forward current (If) of an LED is the appropriate current flowing across the LED's leads, from the anode to cathode, to power on the LED. Different kinds of LEDs require a different forward current. However, for the majority of visible-light LEDs, it usually requires approximately 20 mA forward current for low power and 350 mA for 1 W high power LEDs. We should be careful and must not apply excess current to an LED to avoid the risk of destroying the LED. Two specifications are listed on an LED's datasheet for the maximum current an LED can receive. These are the peak forward current and the continuous forward current. We must not apply more current to an LED than specified.
- Thermal Resistance, Junction to Solder Point (Thermal Management)
A simple resistor network can evaluate the LED system’s thermal resistance as in an electrical circuit. The resistors denote thermal resistances, the electrical current denotes the heat flow and the equivalent temperatures within the system correspond to electrical voltages. It is measured in °C /w. The details of thermal management are described in another document: Thermal Consideration in LED Lighting.
- ESD Withstand Voltage
An ESD (electrostatic voltage discharge) protected LED can withstand up to the specified voltage of the ESD event. It is the release of static electricity when two objects come into contact. It is a generally known hazard during the manufacturing, shipping and handling of LEDs. For example, Cree XLamp LEDs contain ESD protection devices and are classified as class 2 in the MIL-STD‑883 Human Body Model, meaning they survive ESD events up to 2 kV. One troublesome aspect of ESD events is that they sometimes do not cause an immediate catastrophic failure. Instead, these latent failures may become catastrophic hundreds or thousands of hours after the ESD event.
- LED Types and Package
An LED die, or chip, is the fundamental source of light incorporated into a suitable package for electrical connection. LEDs are available in various types of package, depending on various application parameters like low power, high power, single color, multicolor, through-hole type, surface mount type and chip-on-board (COB) type LEDs. These are used to make complete LED lighting modules for various types of applications, like floodlights for a playground, stage lights, LED strips to replace fluorescent tubes, LED lamps and panel lights.
- Low power LEDs: These work on low current below 75 mA and are available in single and multicolor in different sizes and packages. They are useful where a low amount of light is required and the application is battery operated.
- High power LEDs: These mostly come in packages of greater than 1 watt. They are driven at relatively high currents of typically 350, 700, or 1000 mA and produce several hundred lumens per 1 W. High power LEDs come in different packages such as 2-PLCC, 4-PLCC, 4-SMD, 3528, 5050 and 2835 among others.
- Chip-on-Board (cob) LEDs:
These are LED chip arrays joined directly to a substrate to form a single module. The individual LEDs used in a COB are chips and are not packaged. The chips are mounted in less space to achieve the highest potential of the LED chips. They produce more light in a lighting panel compared to multiple individual lights, as in the case of when multiple SMD LEDs are mounted closely together.
- LED Array:
This is an assembly of LED packages on a printed circuit board, or substrate, with optical elements and thermal, mechanical and electrical interfaces.
- LED Module:
This part of the LED light source includes one or more LEDs connected in series or parallel to the load side of the power source. The electrical, electronic and optical component can also be a part of an LED module.
- LED Luminaire:
This is a complete LED lighting unit consisting of a light source and driver together with parts to distribute light, to position and protect the light source, and to connect the light source to a branch circuit. The light source itself may be an LED array, LED module or LED lamp.
Discrete LEDs are generally reflow soldered to a metal core printed circuit board (MCPCB), which is subsequently attached to a heat sink. They use discrete wires to deliver power to the LED. A thermal interface material (TIM) must be applied between the MCPCB and the heat sink to maintain thermal performance. The following table shows some typical tolerances for a single-layer MCPCB. Tolerances vary for different PCB manufacturers and designers are advised to consult the PCB manufacturer for its exact specifications.
LED Standard and Certifications
Standardized methodologies are used to characterize LED sources and the finished solid state light products so that product performance and reliability can be compared and the product duly certified. A few are described in the following text.
Photo-biological Safety Standard of LED Lighting
Photo-biological Safety Standard was published by the International Electro-technical Commission (IEC) for the safety of LEDs and Lamps system. The IEC 62471 deals with the guidelines for conducting evaluations to assess the risk of damage to the skin and eyes caused by light. The photo-biological risk may be divided into different risk groups based on the degree of photo-biological damage. The following are a few examples of photo-biological risks:
- Eye damage caused by near ultraviolet radiation
- Retinal damage caused by blue light
- Retinal damage caused by blue light from miniature light sources
- Retinal heat injuries
- Retinal heat injuries caused by low visibility light
- Eye damage caused by infra-red radiation
- Heat injuries to the skin
North American Product Safety Standards for Solid-State Lighting
Product safety standards in North America are issued by organizations like the Underwriters Laboratories (UL), American National Standards Institute (ANSI), the National Fire Protection Association (NFPA) and Canadian Standards Association (CSA), respectively.
UL 8750 is a newly published standard covering LED equipment that is an integral part of a luminaire, including LED drivers, controllers, arrays, modules and packages.
In Europe, LED lighting products must conform to applicable European Directives and CE Marking guidelines. Then the manufacturer issues a Declaration of Conformity, stating that their products fulfil all requirements. For solid-state lighting products, the two principal directives utilized in CE Evaluations are the Low Voltage Directive (73/23/EEC) and the EMC Directive (89/336/EEC).
The LM-80 deals with the method of measuring the lumen reduction of LED packages, modules and arrays. It is not a measurement of LED system performance or reliability, but a useful tool to analyze LED products. It only describes how to measure the performance of part of an LED luminaire (LED light source) over a period of time under certain conditions. LM-80 is not a measure of the lifetime of a component or the LED lamps and luminaires which use that component. It doesn’t provide a guide to the long-term performance of LED components but only an important part of the equation. It is important to know how quickly the light output of an LED will depreciate under various temperature and current conditions, and how the color point has shifted at the same conditions. Those measurements will enable them to assess how the LED component is expected to perform under similar circumstances. It helps manufacturers to earn a desired ENERGY STAR rating for their products.
It is an approved method for the Electrical and Photometric Measurements of Solid-State Lighting Products published by Illuminating Engineering Society of North America (IES). LM-79 testing captures the performance characteristics of LEDs. It applies to luminaires and replacement lamps rather than LED packages, modules or arrays. The documents based on LM-79 standard contain key types of measurements, including electrical characteristics, lumen output, the spatial distribution of light and color attributes.
A variety of electrical measurements can be performed as part of LM-79 testing, and a few of them are:
LED Lighting Luminaire
A luminaire is an electrical device containing an LED lamp, which provides illumination. All LED luminaires have a fixture body and one or more LED lamps. The following are a few examples of different applications for a luminaire:
LED flood lights produce bright white light with a broad angle. They are useful in theaters, warehouses, perimeters of houses, playgrounds and stadiums. They are brighter than CFL or halogen lights.
LED Stage Lights
LED stage lights are mainly three types: PAR cans, strip lights and moving head. These LED lights accomplish the needs of various stage light requirements for all kinds of celebrations.
LED Light Bar
LED Light Bars are used in display cases and showcases. They are made from miniature LEDs connected on a single aluminum plate. LED Light Bars have a lifespan of nearly 30,000 hours, depending on number of LEDs.