LED - Light Emitting Diode Glossary

Are you new to the world of LEDS? Here is some basic information about LEDS and their properties.

Ambient Light: The surrounding light within an environment.

Anode: The "positive" terminal of a diode.

Ambient Temperature: The surrounding temperature within an environment.

Candela: Unit of luminous intensity. One candela equals one lumen per steradian.

Candlepower: Luminous intensity expressed in candelas.

Cathode: The "negative" terminal of a diode.

Chip: A very small square of semi-conducting material. Also known as a "die," it is the "active" light-emitting component of an LED.

Color: (Dominant Wavelength) LEDs are designed to give off a specific color emission. The dominant wavelength is a quantitative measure of an LED color as perceived by the human eye and is usually measured in nanometers (a billionth of a meter). In order to specify an LED, you must specify the color or dominant wavelength range required for your application. Some applications may have color constraints in order to meet specific government specifications or regulatory guidelines.

Color Bin: LEDs are sorted according to their wavelength or CIE coordinates into different groupings or "bins."

Die: See Chip

Diffused: A diffuser scatters the light emitted and widens the viewing angle, resulting in a more even distribution of light. Diffusing the encapsulant may also reduce the intensity of the device. Diffused LEDs are ideal for applications where the LED acts as an indicator, rather than an illuminator. Diffused LEDs are also usually tinted.

Dominant Wavelength: A quantitative measure of the color of an LED as perceived by the human eye. It is usually measured in nanometers (a billionth of a meter).

Encapsulant Finish: The outer case which can be water clear, diffused or tinted.

Flange: A flange is a rim that's added to the base of some LED lamps. The flange increases the diameter at the base, adding stability to the device, especially when mounted flush to a surface. The flange may also aid in fitting the LED into various secondary optic assemblies.

Forward Current: Current through a diode in the direction of its greatest conduction.

Forward Voltage (VF): The voltage across a diode for a given forward current.

Illuminance: Density of the luminous flux incident upon a surface. It is the quotient of the luminous flux by the area of the surface when the latter is uniformly illuminated.

Intensity Bin: LEDs are sorted according to their intensity values into different groupings or "bins."

Leadframe: A metallic frame used for mounting and connecting LED chips. The leadframe functions as the electrical leads of the device.

Lumen: Unit of measure representing the total amount of light flux (energy) in any particular situation, e.g., from a lamp, through an aperture, etc.

Luminance: Density of luminous flux leaving a surface in a particular direction. It is the quotient of the intensity of the source in the direction of measurement by the projected area of the source in that direction.

Luminous Efficiency: The ratio of light output to power input. Luminous Intensity A measure of the visibility of a light source generally expressed in candelas. It is defined as luminous flux per unit solid angle (steradian) in a given direction.

Package: LEDs are available in either leaded through hole or surface mount packages. Through-hole LEDs are ideal for wave solder circuit board applications. Most through hole LED's are 2-leaded devices. Common through-hole package sizes include 3mm (T-1) and 5mm (T-1 3/4) diameter parts. Surface-mount packages are best used with reflow assembly. SMD devices are also useful when package size constraints are an issue.

Peak Wavelength: The maximum wavelength of an LED.

Physical Attributes: LEDs can also be created with specific physical variations to make it unique, like a flange or standoff.

Reverse Current: Current flowing through a diode in the direction opposite to the direction of maximum conduction.

Reverse Voltage (VR): Voltage across the diode for a given reverse current.

Stand-offs: Stand-offs allow the LED to be positioned without coming in direct contact with the printed circuit board (PCB). Stand-offs are ideal for applications where multiple LEDs are required to be at identical heights above a common plane, such as a circuit board.

T-1: An LED lamp package. Type T1 = 0.125 inch lamp diameter or a 3.18mm (Referred to as a 3mm) lamp diameter.

T-1 3/4: An LED lamp package. Type T1 ¾ = 0.2188 inch lamp diameter or a 5.56mm lamp diameter.

Tint: Tinting an LED can be accomplished by adding a dye to the encapsulant similar in color to the light emitted. This allows identification of the LED color in the OFF state, as well as offering color filtering of direct sunlight. Tinting may also reduce the intensity of the device.

Water Clear: Encapsulated finish that is colorless in appearance.

Viewing Angle: The off-axis angle where the display may be satisfactorily viewed (defined by lenses, obstructions, or intensity decrease). The viewing angle referred to as the 2-half theta angle (2Q * ) indicates how focused the light is when emitted from the LED. This angle is determined by measuring the angle from direct on-axis to the angle where intensity falls to * of the on-axis intensity, and then multiplying this difference by 2. The shape of the encapsulant dome lens controls the viewing angle of the LED. Flat LEDs without domes generally have a very wide viewing angle. The choice of viewing angle also affects the LED intensity.

As the viewing angle is increased, more area is covered by light, however the on-axis intensity is reduced. For applications where the LED will project light onto a small area, or on an object far from the emitter, a narrow viewing angle is ideal. For applications where the LED will project light onto a large area, or on an object very close to the emitter, a wide viewing angle is ideal. For instance, backlighting an indicator or switch in a cavity of less than 18mm would typically call for a surface mount LED with a 120ƒ viewing angle. Illuminating an object of several square inches in size, that just two feet away would require a 5mm LED with a 20ƒ or smaller viewing angle. Oval-viewing angle LEDs are available for applications requiring illumination of areas that are not square or circular. These LEDs have a wider viewing angle in either the horizontal or vertical direction.