Beam Angle: Generally specified as the off-axis angle where the output power drops to 50% of the peak value. Can be specified from 50% to 50% point, or peak to 50%. Generally speaking if the value is referred to as Half Intensity Beam Angle or FWHM, the value is from 50% to 50% points.
Breakdown Voltage: Breakdown Voltage (VBR) is the maximum reverse bias that can be applied, beyond which the diode readily conducts current. Exceeding the breakdown voltage with no current-limiting protection can cause severe device degradation.
Candela: A measurement of luminous intensity. Visible LEDs are usually specified in candela (cd) or millicandela (mcd). Angle of measurement is critical when comparing lensed (narrow beam) products from different vendors. Value is pegged to the human eye response, making the peak wavelength a critical factor in the final value. IRLEDs have a value of nearly zero, because they do not emit appreciable levels of visible light.
Centroid Wavelength: The wavelength value where half of the light energy is at shorter and half the energy is at longer wavelengths. Value is stated in nanometres (nm) or microns (µm). This value is of interest to people in the test and measurement industries. Not commonly specified for standard LED products of any wavelength.
Dominant Wavelength: The colour, or perceived wavelength of a light source by the human eye. Also called the hue wavelength. Most visible LEDs are specified by the dominant wavelength.
Full-Width-Half-Max: Usually abbreviated as FWHM. Used most commonly when discussing beam angle or spectral bandwidth. In both cases it refers to the distance from 50% to 50% point, or –3db to –3db point. Beam angle value is specified in degrees and spectral bandwidth values are specified in nanometres.
Light Emitting Diodes: Commonly abbreviated as LED, LEDs, or IRLEDs in the case of infrared devices. Refers to a diode which when forward biased converts electrical current to light in a non-coherent waveform.
Lumens: Unit of Luminous Intensity. A measurement of total visible energy emitted from a point source. Output is measured in an integrating sphere with a detector whose spectral sensitivity approximates the human eye. This value is not commonly specified for NIR or UV LEDs because the human eye is insensitive to these wavelengths.
Peak Wavelength: The wavelength value with the highest amount of energy radiating from the source. Most commonly specified for non visible (infrared) LEDs. (See Dominant Wavelength)
Power Output: Value is expressed in Watts or milliWatts. A radiometric measurement of the total light energy radiating from an emitter regardless of wavelength. Measurement is made with an integrating sphere. This figure of merit is most commonly used with IRLEDs. It is the optical output measurement that is most easily correlated from one measurement facility to another.
Radiant Intensity: Radiant measurement of on-axis intensity. This value must be known to calculate optical power incident on a detector that is greater than 15mm from the LED. The angle of measurement is a critical component when comparing data sheets from one vendor to the next.
