Absorption
The loss of light as it passes through a material, generally due to its conversion to other energy forms (typically heat).

Absorption spectrum
Also called spectral window of absorption. The spectrum formed by radiation that has been filtered through a material medium, in contrast to emission spectrum.

Ambient temperature
Ta, The temperature of air or liquid surrounding a device.

Anode
The part of an electrical circuit in which the electrons leave (a cathode-ray tube) or enter (an electrolytic cell) a unit in the circuit. It is the positive or side of a terminal.

Aperture
An opening or hole through which radiation or matter may pass.

Average Current
May be broadly defined as the root mean square of the current waveform applied.

Averaged LED Intensity
A term coined by the CIE to describe quantities measured in “near field” conditions. These values are obtained using two different geometries labeled “Condition A with a measurement distance of – 316mm” and “Condition B with a measurement distance of – 100mm”. Both conditions involve the use of a detector with a circular entrance aperture of 100mm2. (Refer to CIE 127 for additional details).

Beam
A bundle of light rays that may be parallel, converging or diverging.

Bin
The systematic dividing of various LED performance parameters such as (Brightness, Wavelength, Forward Voltage and CCT) into smaller groups in order to meet the electrical and/or aesthetic requirements of an assembly.

Blackbody
An ideal body that completely absorbs all radiant energy striking it and, therefore, appears perfectly black at all wavelengths. The radiation emitted by such a body when heated is referred to as blackbody radiation. A perfect blackbody has an emissive of unity.

Brightness resolution
The degree to which a pixel in a digital image represents the analog brightness of the corresponding point in the original image. It is dependent largely on the number of bits devoted to representing the image processing system’s gray scale.

Burn-in
The operation of a LED or other component prior to its use in its intended application, as a means of testing and stabilizing it.

Cathode
The negative “-“ electrode of a device in an electrical circuit.

Chromaticity
The qualities of color associated with hue and saturation, but not brightness or lightness.

Chromaticity coordinates
Proportions of standard primaries (tristimulus values) required for a color match; ratios of each tristimulus value of a color to their sum. In the CIE colorimetric system, designated X, Y and Z.

Chromaticity diagram
The plane diagram produced by plotting one of the three chromaticity coordinates (X, Y, Z) against another. The most common diagram is the CIE (X, Y) diagram, which is plotted in rectangular coordinates.

CIE
Commission Internationale de l’Eclairage, the international commission on illumination.

CIE illuminant C
6500 K color temperature source that produces light which simulates the daylight produced by an overcast sky.

CIE system
Methodology for specifying color based on the CIE sources, observers, and coordinate system.

CMOS (complementary metal oxide semiconductor)
An integrated circuit that uses both PMOS and NMOS devices on the same substrate, resulting in extremely low power dissipation.

Color
The attribute of visual experience that can be described as having quantitatively specifiable dimensions of hue, saturation, and brightness or lightness. The visual experience, not including aspects of extent (e.g., size, shape, texture, etc.) and duration (e.g., movement, flicker, etc.).

Color temperature
A colorimetric concept related to the apparent visual color of a source (not its temperature). For a blackbody, the color temperature is equal to the temperature in kelvin.

CRI (Color Rendering Index)
The calculated rendering of an object. CRI is based upon a 0 – 100 scale. The higher the CRI, the more natural the colors appear.

DC forward current
Continuous direct current applied which is constant over time.

Die
The heart of an LED, sometimes called – chip.

Diffuse density
The logarithm of the reciprocal of diffuse transmittance. Diffuse density results when a sample is diffusely illuminated.

Diffuse transmission
Transmission accompanied by diffusion or scatters to the extent that there is no regular or direct transmission.

Diffuser
A device used to scatter or disperse light emitted from a source, usually by the process of diffuse transmission.

Diode
A two-electrode device with an anode and a cathode that passes current in only one direction. It may be designed as an electron tube or as a semiconductor device.

Discrete
An individual circuit component, complete in itself, such as a resistor, diode, capacitor or transistor. It is used as an individual and separable circuit element.

Display
The observable illustration of an image, scene or data on a screen such as a console or cathode-ray tube, seen as a graph, report or drawing.

Dominant wavelength
For an LED of a given color, dominant wavelength is the single wavelength representation of the LEDs perceived color. Put another way, if the color of a single wavelength DW is indistinguishable from the color of a given LED, then that LED has a dominant wavelength of DW.

Doping
The addition of impurities to another substance, usually solid, in a controlled manner that produces desired properties. Silicon doping with small amounts of other semi metallic elements increases the number of electrical carriers.

Dot matrix display
A display format consisting of small light-emitting elements arranged as a two-dimensional array. Various elements are energized to depict a character. The typical matrix is 5 x 7 dots.

Dual Display
Refers to a TWO Digit seven segment displays in a unit.

Dual inline package (DIP)
A package for electronic components that is suited for automated assembly into printed circuit boards. The DIP is characterized by two rows of external connecting terminals or pins, which are inserted into the holes of the printed circuit board.

Duty Factor
Pulse width divided by the period.

Efficacy
The output of a light source divided by the total electrical power input to that source, expressed in lumens per watt (lm/W).

EPI Wafer
The growth of crystalline materials layers on top of a wafer substrate.

Epoxy
Common name for a variety of adhesives used for lens bonding, fiber optic splicing and other photonics applications. The term is actually a prefix denoting the presence of an epoxide group in a molecule.

Fall time
Measurement of the interval during which a photo detector’s signal and output current drops from 90 to 10 percent.

Far-field diffraction pattern
The diffraction pattern of a source such as a light-emitting diode, injection laser diode or the output end of an optical waveguide observed at an infinite distance from the source.

Far-infrared (FIR)
That part of the infrared spectrum from about 15um to 1000um. Flux: Time rate of flow of energy; the radiant or luminous power in a beam.

Frequency
Reciprocal of period, expressed in Hz (1/s).

Goniometer
A spectrometer or autocollimator used to measure prism angles.

Gray scale
In image processing, the range of available gray levels. In an 8-bit system, the gray scale contains values from 0 to 255.

HBT
Heterojunction Bipolar transistors

HEMT
High Electron Mobility Transistors

Hue
The perceptual term for that aspect of color described by words such as red, yellow or blue. Achromatic colors, such as white, gray and black, do not exhibit hue.

Illumination
The general term for the application of light to a subject.

Incandescence
The emission of light by thermal radiation of a temperature high enough to render the source of radiation visible.

Incandescent lamp
A lamp that emits light when an electric current passes through a resistant metallic wire situated in a vacuum tube.

Incidence
Flux incident per unit area of a surface.

Infrared (IR)
The invisible portion of the electromagnetic spectrum that lies between about 0.75um and 1000um.

InP- (Indium Phosphide)
A semiconductor material made of indium and phoshide. It has superior electron velocity over silicon and GaAs and is also has direct band gap making it useful for optoelectronic devices.

Integrating sphere
A hollow sphere coated internally with a white diffusing material and provided with openings for incident beam, specimen and detector used for measuring the diffuse reflectance or transmittance of objects.

Intensity
Flux per unit solid angle.

Junction diode
A semiconductor device with the property of conducting current more easily in one direction than the other. It has two terminals containing a single crystal of semi conducting material that ranges from P-type at one terminal to N-type at the other.

Junction temperature
(Tj), the temperature at the PN junction within a semiconductor device. LED (light-emitting diode): A PN junction device that gives off light radiation when biased in the forward direction.

LED
A Light Emitting Diode (LED) is a solid-state semiconductor device that converts electrical energy directly into light.

Lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.

Light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 380nm to 750 nm.

Liquid crystal display (LCD)
An alphanumeric display formed by a layer of liquid crystal material sandwiched between two sheets of glass; a transparent conductive coating on the glass is etched to form the character segments. An applied voltage causes the appropriate segments to darken as the molecules in the liquid crystal change their arrangement.

Lumen (lm)
The SI unit of luminous flux, equal to the luminous flux emitted per unit solid angle by a standard point source having a luminous intensity of 1 candela.

Luminance (LV)
Luminous flux per unit solid angle per unit area of emitting surface at an angle with respect to surface normal, in candela per square meter or nits.

Luminous flux
Defined as the total luminous energy per unit time emitted by the light source into a sphere (360) surrounding the light source, where the luminous flux is the radiant flux multiplied by the human eyes sensitivity. The measurement unit for luminous flux is the lumen (lm).

Luminous Intensity (IV)
The amount of luminous flux emitted into a very small solid angle at a defined angular orientation from the light source. The unit of luminous intensity is the lumen/steradian (lm/sr), or candela (cd).

Mechanical center
The physical center of the lens; it is on the axis of the lens, halfway between the front and rear vertex.

Multiplex
Carrying out several functions simultaneously in an independent but related fashion. (In a 7-segment display, combining individual segments and pins for multiplex operation)

MWIR
Mid wavelength infrared light in the range of 3.0-8.0 micron.

Nit
Unit of measurement of brightness (luminance) equal to one candela per square meter.

Optical center
The point on the axis of a lens that is the image of the nodal points. Rays through the optical center emerge parallel to entering rays.

PCB
“Printed Circuit Board” typically made from FR-$ ceramic or metal core material.

Peak Current
Maximum instantaneous current that is applied in pulsed operation.

Peak wavelength
The wavelength at which the radiant intensity of a source is maximum (As seen by a photo detector).

Period
Time interval from one point to its next/consecutive occurrence in a repeating waveform (s).

Phosphor
A chemical substance that exhibits fluorescence when excited by ultraviolet radiation, x-rays or an electron beam. The amount of visible light is proportional to the amount of excitation energy. If the

Photo detector
A device that senses light or other electromagnetic energy.

Photometry
Photometry deals with light energy of wavelengths that can cause visual sensation. The human visual range is typically from 380nm to 780nm. Wavelengths outside this range do very little in stimulating our eye.

Pixel
Contraction of “picture element.” A small element of a scene, often the smallest resolvable area, in which an average brightness value is determined and used to represent that portion of the scene. Pixels are arranged in a rectangular array to form a complete image.

PLCC-2 (Plastic Leaded Chip Carrier)
PLCC-2 is a SMT LED packaged in the industry standard PLCC package. In general, its exterior dimensions are similar to the other surface mount LED components found in the market. PLCC-2 has wide viewing angle, available in full selection of color. Its application mainly toward Interior Automotive, Electronic Signs & Signals, Office automation, Home appliances & Industrial Equipment

PN junction
The transition boundary between P-type and N-type materials in a semiconductor.

Point light source
1. With respect to angular subtense, a source of light, such as a star, that is very small. In a lab, a point source may be simulated by imaging a large source onto and through a pinhole, or by focusing a parallel bundle of light, such as that from a laser, with a precise lens to a point image. 2. In lens design, a fictitious infinitely small source of radiation.

Point Source Emitter
is a semiconductor diode similar in structure to a standard LED, however the light is emitted through a well defined circular area- typically 23 um-200um in diameter. The light produced appears as a “spot”.

Power Dissipation
Work per unit time consumed by a device, generally the product of current and voltage.

Pulse Width
The interval of device ON time in a period.

Radiometry
Light is electromagnetic energy. Radiometry deals purely with that energy without consideration on how it stimulates our visual system i.e. the eye.

Ripples
The approximately concentric waves that form on a surface that has been polished without an oscillation of the polishing lap.

Rise time
Measurement of the time elapsed during the current output change from 10 to 90 percent in a photoconductor.

Saturation
Saturation tells us the purity of the given color. The spectral width of an LED is several tens of nanometers which mean that an LED only contains wavelengths that are at the very most several tens of nanometers apart. Most everyday objects reflect light that spread over a wide range wavelength, much more than an LED. Generally, the tighter the range of wavelengths, the more saturated it is. LEDs are very saturated (>95% for most die types) but not as saturated as lasers which have spectral widths of an order of magnitude narrower.

Spectrometer
A kind of spectrograph in which some form of detector, other than a photographic film, is used to measure the distribution of radiation in a particular wavelength region.

Steradian
Solid angle subtending an area on the surface of a sphere equal to the square of the radius. There are 4 pi steradians in a sphere.

Storage temperature
The temperature range in which a device may be stored. Ultraviolet (UV): That invisible region of the spectrum just beyond the violet end of the visible region. Wavelengths range from 1 to 400 nm.

Substrate
A solid substrate or medium to which another substance is applied. The substance (such as silicon or InP-Indium Phosphide) serves as a foundation for the growth of semiconductor materials such as photo diodes or transistors.

SWIR
Short wave Infrared light in the range of 1.4 -3.0 microns.

Thermal Resistance
A measure of the heat transfer capacity of a LED. A lower thermal resistance is usually preferred.

Viewing Angle
Typically defined as the included angle which encompasses 50% of maximum intensity.

Wafer
A thin slice of semiconductor material used in the fabrication of electronic devices. A substrate with an epitaxial layer on it. The individual light emitting elements cut from the substrate are typically referred to as chips or die.