Get Educated

Mag-Led has created this page to be a fluid center for technical definitions, terminology, and LED jargon for lighting professionals young and old. If you are new to the industry, we recommend reading through the below, as it will help a great deal to have a basic understanding of these terms. Please feel free to email us if would like to contribute to this knowledge base. 

LED – A light-emitting diode (LED) is a two-lead semiconductor light source that resembles a basic pn-junction diode, except that an LED also emits light. When a LEDs anode lead has a voltage that is more positive than its cathode lead by at least the LEDs forward voltage drop, current flows. Electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs are often small in area (less than 1mm) and integrated optical components may be used to shape its radiation pattern.

BLUE LED – The first blue LED with sufficient luminance for practical application was the Gallium Nitride (GaN) LED that Nichia introduced in December 1993. It emitted pure blue light with a luminance of over 1cd, which made it possible for the first time to create large-size full-color displays. The introduction of GaN, which replaced the then-predominant Zinc Selenide (ZnSe), was a turn in a new direction and GaN is still widely used today. The structure of a blue LED is typically an Aluminum Nitride (AIN) or GaN type semiconductor layer on a sapphire or Silicon Carbide (SiC) substrate. Light is emitted from the semiconductor’s so-called active layer, which consists of a stack of alternating layers of p-type GaN and n-type materials that provide pn-junctions. In fact, all high luminance LEDs make use of pn-junctions.

ULTRAVIOLET LEDs – LEDs with an emission wavelength of less than 400nm are usually called “ultraviolet LEDs” (UV LEDs), but within that group, those with relatively long wavelengths of around 380nm are sometimes called “near-ultraviolet LEDs” (near-UV LEDs) and those with relatively short wavelengths of less than 300nm are called “deep ultraviolet LEDs” (deep UV LEDs). Since light of short wavelengths is highly effective for sterilization, UV LEDs are used in refrigerators and other consumer appliances as disinfection and deodorizing and in combination with phosphorous  substances they are also used for LEDs that emit visible light.

INFRARED LEDs – The term “infrared LED” is typically used for LEDs whose emission wavelength is 700nm or longer – mostly in the invisible range of the spectrum. Infrared LEDs are commonly used as light sources in remote controls, distance sensors, photo couplers, and printer heads, as well as for infrared communication.

GREEN LEDs – Green LEDs emit light in the green range of the visible spectrum and typically have an emission wavelength peak around 560nm. They are used for lighting applications (light fixtures, illumination), indicators (signaling), LED display light sources and backlights for LCD panels.

When configuring LED displays and LCD backlight light sources by combining red, green and blue LEDs to obtain white light with high luminance and with proper white balance, the proportion of the respective luminous intensities (RGB) needs to be approximately 3:6:1 or 3:7:1, to match the luminous sensitivity of the human eye. However, the efficacy of green LEDs is lower than of that of red and blue LEDs, and to compensate for that, more than one green LED may be used in a cluster, for example:  RGGB instead of RGB.

The main cause of the reduced efficacy of GaN type semiconductor material in green LEDs (that is, the lower efficacy when emitting green LEDs as opposed to blue light), is an effect called “piezo polarization”, which results from choosing the polarity plane of the GaN crystal, the c-plane (0001), as the crystal growth pane and the c-plane’s normal vector orientation (c-axis) as the crystal growth axis. Research is under way now into changing the growth axis to minimize piezo polarization and improve the efficacy of green LEDs.

RED LEDs – Red LEDs emit light in the red range of the visible spectrum and typically have an emission wavelength that peaks around 620-630nm. They find wide application, for example, in lighting applications (light fixtures, illumination), indicators (signaling), car tail lamps and in backlights for LCDs.

The semiconductor material for red LEDs is Aluminum Gallium Indium Phosphide (AlInGaP), a material that is also called “quaternary material”, since it is composed of four different chemical elements. When talking about LEDs for visible light, the expression “quaternary material” usually refers to AlInGaP.

AlInGaP can be used to produce light in the range red to yellow, and since its introduction in the 1990s, it has been further developed to dramatically increase its brightness. One technology that has helped to substantially improve crystal quality is vapor-phase epitaxy (VPE) technology, represented by metalorganic chemical vapor deposition (MOCVD). Before AlInGaP became available, the commonly used material for red LEDs had been GaAs and at that time liquid phase epitaxial growth (LPE) technology was used to grow crystals.

WHITE LEDs – Broadly speaking, there are three different ways to produce white light. First, a certain part of the light emitted from a blue LED chip hits phosphorous material to produce light of another color. Second, light from a near-UV chip hits several phosphorous materials to produce light of several colors. Third, three separate LEDs (R, G, B) produce light of distinct colors. In all three cases mixing the colors results in a kind of white light.

The most commonly used method is the first one, using a blue LED with a fluorescent material. This material can be a yellow phosphorous substance alone or a combination of red with yellow or green substances.

If one uses a yellow phosphorous substance alone, it will convert part of the blue light into yellow light and the resulting mix is a so called pseudo-white light, light with a very low red component and a bluish hue, i.e. a high color temperature. To mitigate this, a red phosphorous substance is added to obtain a more balanced white light. If the red portion of the light is made even stronger the result is a light bulb color LED, a chip whose light resembles that of an incandescent light bulb, i.e. it has a low color temperature.

LUMINOUS EFFICACY – The luminous efficacy indicates how well a certain light source transforms electric energy into light energy, and its unit is lm/W.

High hopes have been placed on the white LED as the light source of the next generation, to supersede the incandescent light bulb and fluorescent lamp, and there had been concerns about whether or not the combined efficacy of fluorescent tube lighting at 100 lm/W could be achieved, but with recently developed white LEDs this milestone has been reached. It needs to be noted however that this luminous efficacy represents the efficacy of the light source only and is not the same as the lamp and auxiliary efficacy (the combined efficacy of a LED installed in a fixture).

The external quantum efficiency is the number of photons emitted from a LED package, divided by the number of electrons flowing into it, and in the case of a white LED based on a blue LED chip with phosphorous substances it is arrived at by multiplying the following 4 factors: the internal quantum efficiency (the number of photons exiting the emission layer of the LED chip divided by the number of electrons flowing into it), the light extraction efficacy of the chip (the number of photons leaving the chip divided by the number of photons being generated), the conversion efficiency of the phosphorous substances and the light extraction efficiency of the package.

Since even in a LED chip itself, i.e. without a package or any phosphorous substances being involved a certain percentage of the photons being generated in the emission layer is absorbed or reflected and therefore does not leave the chip, the external quantum efficiency is in all cases lower than the internal quantum efficiency. The ratio of the two is the light extraction efficacy of the chip. In the case of a white LED with a luminous efficacy of 100lm/W, only 32% of the electric input energy reaches the outside in the form of light; the remaining 68% turns into thermal energy.

LED DROOP – LED droop is an effect whereby the luminous efficacy of a chip decreases under high input power conditions. LED manufacturers are keen on finding a solution, since that would allow larger input power and an increase of the luminous flux from a given device. This would make it possible to use a smaller number of chips for a given flux, which translates directly into a reduction of the cost per unit of luminous flux.

COLOR TEMPERATURE – The wavelength of light (color) emitted by a blackbody radiator (a hypothetical object that completely absorbs outside light) varies with its temperature and each color of light has therefore a corresponding color temperature; expressed in Kelvin (K). Using color temperature values, one can compare the color of different fluorescent lamps and white LEDs as well as the hues/tints of the white levels of displays. In general, light at a low color temperature has a reddish tinge and at a high color temperature it appears bluish.

A typical white LED, with a color rendering index (Ra) in the range of 70, is the combination of a blue LED chip with a yellow phosphorous substance and its color temperature is over 6000K of standard daylight. Incandescent light bulb color LEDs have a color temperature of around 3000K, which is achieved by adding an additional red phosphorous substance. Using blue LED chips with several phosphorous substances combined, one can also obtain colors in the range of 4000K to 5000K.

In practical applications, the color temperature of a lamp can be chosen depending on where it will be used. For example, many people prefer office light that is close to bright daylight, and thus the lamps used in offices will have a higher color temperature, while light bulbs used for households and restaurants will often have a lower color temperature in order to resemble incandescent light bulbs.

COLOR RENDERING – The color rendering (or sometime called color rendition) indicates how closely a light source reproduces the colors that would be achieved with natural illumination. It is usually expressed using the General Color Rendering Index (Ra), with a value between 0 and 100. The closer the Ra is to 100, the more natural colors appear when illuminated by that light. In common indoor lighting applications such as homes and offices an Ra of at least 80 is desirable, with an Ra of 70 or higher for corridors. In rooms where color rendering is particularly important, such as in art galleries, museums, stores, etc., a value of over 90 is considered suitable.

The current widely available white LEDs used for indoor lighting can be roughly divided into low-Ra and high-Ra products. There is a trade-off between luminous efficacy and Ra, so that in a high-Ra design the luminous efficacy can drop by 20 to 30 percent.

A typical white LED produces pseudo-white light using a blue LED chip combined with yellow phosphorous material, but only gets an Ra of around 70. By adding a red phosphorous substance, Ra can be increased to over 80. The development of white LEDs with an Ra of over 90, based on blue LED chips combined with green and red fluorescent substances to yield less pronounced spectral peaks and valleys, is being considered. In addition, white LEDs with Ra of over 90 can also be produced by combining near-UV LED chips with multiple phosphorous substances.

LUMINOUS FLUX/INTENSITY, LUMINANCE, ILLUMINANCE – Luminous flux refers to the overall brightness of a light source, expressed in lumens (lm). It is typically used to quantitatively describe the brightness of a light source used for illumination. Luminous flux takes the spectral sensitivity of the human eye to radiated light into account. 1 lm (lumen) is defined as the amount of light from an isotropic light source of 1cd (candela) intensity that is available within a solid angle of 1sr (steradian; the unit of the solid angle at the center of a sphere that subtends/circumscribes an area on the surface of that sphere).

Luminous intensity refers to the brightness of a light per solid angle in a certain direction, measured in candelas (cd). 1cd is defined as the magnitude of the monochromatic emission of a light source at a frequency of 540×1012Hz (555nm), in a specified direction and with a power level of 1/683W/sr.

Luminance is the luminous intensity per unit area of light that is emitted from, or passes through, a given surface within a given solid angle and reaches the eye of an observer, measured in cd/m2. Luminance, too, is dependent on the spectral sensitivity of the human eye and it is typically used to characterize the surface brightness of liquid crystal displays (LCD) and plasma display panels (PDPs).

Illuminance is the total luminous flux incident on a surface per unit area, measured in lux (lx) or lm/m2. Illuminance is typically used to quantitatively express the brightness of illuminated flat surfaces, such as in the comparison of the effective illumination performance of various lamps for a given surface.

The relationship between these four physical quantities can be summed up as follows: if we multiply the luminous intensity by the unit solid angle we get luminous flux; luminance is the luminous intensity per exposed unit area and Illuminance is the luminous flux per exposed unit area.

JUNCTION TEMPERATURE – In a LED, the junction temperature is the temperature of the emission layer inside the chip; this temperature increases when the LED is turned on. This heat needs to be removed, because a higher junction temperature means lower luminous efficacy and since lower efficacy means more heat, there is the potential for a vicious circle or runaway effect that can destroy the LED.

To what degree the junction temperature increases depends on the LEDs mode of operation and the input current. That is to say, although higher input current will in any case lead to a higher junction temperature, when the LED operates in pulse mode, the junction temperature will not go up as much as when the LED is continuously lit.

EMISSION SPECTRUM – The term “emission spectrum” refers to the distribution of light intensity versus wavelength. In the blue LED (a mono-chromatic LED), the peak lies around 470nm, again with the light intensity in the neighboring UV and green ranges falling below the detection threshold. In contrast with that, an incandescent light bulb has a wide visible emission spectrum where the intensity gradually increases from blue in the 400nm range to near infrared in the 700nm, but there is no sharp drop off and emission can be detected not only in the infrared range but also in the UV range. With fluorescent lamps, portions of the emission wave length of the combined fluorescent substances becomes visible as peaks of the emission spectrum.

Comparing to the typical red, green and blue LEDs that have one peak in the emission spectrum, the emission spectrum of white LEDs is very different. For example, there may be two peaks, one in the blue range and one in the yellow range, or there may be three peaks as well. The reason is that the white light of a white LED is a mix of light of different wavelengths each with peaks and valleys in their spectrum.

GaN (GALLIUM NITRIDE) – Gallium nitride (GaN) is a compound semiconductor composed of gallium (Ga) and nitrogen (N). It has a bandgap of 3.45 eV (equivalent to about 365 nm in terms of light wavelength) which is three times wider than that of silicon (Si). Taking advantage of this characteristic, GaN is mainly used in optical devices. By mixing indium (In) or aluminum (Al) to control the bandgap, light-emitting devices such as LEDs and blue-violet semiconductor lasers have already been put to practical use.

As GaN has a wide bandgap, it can generate blue and green light beams, which have comparatively short wavelengths. Indium gallium nitride (InGaN), which is obtained by adding In to GaN, is used in blue LEDs and blue-violet semiconductor lasers. In addition to the wide bandgap, GaN has excellent properties as a semiconductor material, such as a high breakdown field, high field saturation velocity and high thermal conductivity. Moreover, a GaN-based device with a high-electron-mobility transistor (HEMT) structure has a high carrier mobility and as a result is suitable for high-frequency devices. This is because a region called a “two-dimensional electron gas layer” is generated, where electrons flow at a high speed. Further, because a GaN HEMT device has a higher breakdown field compared with Si and gallium arsenide (GaAs), it has good voltage resistance, meaning that a higher voltage can be applied. Thus, a GaN HEMT device can improve power-added efficiency and reduce power consumption when it is used in high-frequency power amplifier circuits in mobile phone base stations, etc.

SUBSTRATES – A semiconductor layer, which serves as the light-emitting area of LEDs and semiconductor lasers, is formed by crystal growth on a substrate. The type of substrate used is selected depending on the LED emission wavelength. LED chips utilizing GaN-based semiconductor materials, e.g., blue and white LEDs use sapphire, SiC and Si as the substrate, while chips using semiconductor materials based on aluminum indium gallium phosphide (AlInGaP), such as red LEDs, employ GaAs substrates.

The reason why the substrates are selected depending on the LED emission wavelength is because it is necessary to choose an inexpensive substrate material with a lattice constant as close as possible to that of the semiconductor crystal, which serves as the LED emission area. Doing so can minimize the difference between the lattice constants (lattice mismatch), prevent crystal defects in the semiconductor layer and also reduce the unit price of LEDs. Devices such as blue-violet semiconductor lasers, which have high current and light output densities, use expensive GaN substrates. GaN substrates are also used in some blue LEDs.

EPITAXIAL GROWTH – Epitaxial growth is a technology for growing crystal layers with aligned axes on a crystalline substrate. It is used to produce crystal layers free from impurities and defects. There are two types of epitaxial growth processes : vapor phase epitaxy (VPE) to deposit crystal layers by a gas reaction above the substrate and liquid phase epitaxy (LPE) to grow crystal layers by bringing the substrate into contact with a solution.

GaN-based light emitting devices such as blue and white LEDs and blue-violet semiconductor lasers are generally produced using the metal organic chemical vapor deposition (MOCVD), which is one type of VPE. As the materials, MOCVD uses organic metal gases, etc. With the use of MOCVD equipment, GaN-based semiconductor layers are epitaxially grown on a sapphire or SiC substrate when producing blue LEDs or on a GaN substrate when making blue-violet lasers.