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.

Candela
A measurement of luminous intensity. Visible LED’s 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. Infrared LED’s 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 nanometers (nm) or microns (µ). This value is of interest to people in the test and measurement industries. Not commonly specified for standard LED products of any wavelength.

Dark Current
Usually abbreviated as I_D. The current flowing through a reverse biased photodiode when light is not incident upon a photodiode. Higher reverse bias voltages result in higher dark currents. Dark current is not present in zero photodiode bias circuits (see shunt resistance).

Dominant Wavelength
The color, or perceived wavelength of a light source by the human eye. Also called the hue wavelength. Most visible LED’s 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 nanometers.

Junction Capacitance
P-N junctions have an inherent capacitance similar to a parallel plate capacitor. The junction capacitance is proportional to the active area of the semiconductor. In the case of photodiodes, the junction capacitance can be reduced by reverse biasing it. The junction capacitance in conjunction with the inherent series resistance of the diode is not the limiting factor for response time of the device (see response time).

Light Emitting Diodes
Commonly abbreviated as LED, LED’s, or IRLED’s in the case of infrared light emitting diodes. Refers to any diode which when forward biased converts electrons (electrical current) to photons (light) in a non-coherent waveform.

Lumens
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 LED’s.

Peak Wavelength
The wavelength value with the highest amount of energy radiating from the source. Most commonly specified for non visible (infrared) LED’s.

Photoconductive Detector
Common name given to a photodiode operated in a reverse bias mode. This mode decreases junction capacitance, increases speed and linearity. It also increases noise current by introducing dark current to the circuit.

Photodiode
Generic name given to any diode used as a light detector. Device has no internal gain like a photodiode or photodarlington. Directly converts photons (light) into electrons (current). It is linear over at least 6 decades of light input. Average saturation point is 10mW/cm². Used extensively where light must be accurately measured or higher speeds (greater than 30KHz) is required. Response is measured in Amps/Watt (A/W).

Photovoltaic Detector
Common name given to a photodiode operated in a zero bias mode. This mode is commonly used in lower speed applications where rise time and junction capacitance are less important than minimizing dark current and thus reducing noise current.

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 IRLED’s. 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 6 inches from the LED. The angle of measurement is a critical component when comparing data sheets from one vendor to the next.

Response Time
Also known as rise or fall time. The period of time it takes an emitter or detector to go from the 10%-90% point, emitting and detecting respectively, or the 90%-10% point. RC time constant of the device is almost never the limiting factor. The speed of the device is almost always due to the transit time of the semiconductor material and the distance from the depletion region to edge of device.

Short Circuit Current
Operation of photodiode in a condition where a voltage bias is not allowed to generate across the diode. Usually accomplished by using a transimpedence amplifier circuit.

Shunt Resistance
The zero bias resistance of a photodiode. In practical measurements, most manufacturers put a 10mV reverse bias on the photodiode and measure current. The ratio between the bias voltage and current determine the shunt resistance value. This value must be known to determine noise current generated by the photodiode in a photovoltaic, short circuit current mode circuit.

Spectral Response
The conventional method for determining the sensitivity of photodiodes. The term is expressed in Amps/Watt (A/W). The monochromatic wavelength the measurement is done at must be specified as well.

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Supplying Power to the LED
LED’s, like all diodes are current driven devices. The light output is directly proportional to the input current provided. It is not enough to provide a constant voltage to the LED. From one LED to the next, the voltage at a specific current can vary by as much as 0.4V. This can result in a 2:1 difference in the current supplied and will affect the power output from one LED to the next by the same ratio.

There are a few different methods for supplying current to the LED. An example of an accurate and stable circuit is shown in Figure 1.

Figure 1.

This circuit is commonly referred to as a constant current source. Note that the supply current is determined by the supply voltage (V_cc) minus Vin divided by R₁, (V_cc-V_in)/R₁.

In applications where the operating temperature range is narrow (less than 30°C) or the output of the LED is not critical, a simple circuit utilizing a current limiting resistor may be used as shown in Figure 2.

Figure 2.

The current value is found by applying the equation I=(V_cc-V_f)/R_L. To be absolutely certain of the current flow in the circuit, each LED V_F would have to be measured and the appropriate load resistor specified. In practical commercial applications Vcc is designed to be much larger than V_F and thus the small changes in V_F do not affect the overall current by a large amount. The negative aspect of this circuit is a large power loss through R_L.

In a few instances, primarily in the surveillance industries, current limited power supplies or batteries are used as the power source. If the supply has a maximum current rating less than the LED maximum rating it can be connected directly without any associated electronics. Please note this is not a method we recommend but has been used with varying success by end users.

For pulsing applications it is important that the LED be driven by a low impedance driver. Opto Diode recommends a low impedance FET output for the final driver stage. If this is not possible, then a load resistor must be placed between the LED and output driver to minimize any inductive spikes that may occur. Instantaneous damage to the LED chip may occur if the circuit is not properly connected. For breadboard prototyping or small production runs we recommend an off the shelf Laser Diode Driver be used to pulse the LED.

Attributes of IRLED’s over Temperature
Peak Wavelength, Forward Voltage and Power Output are all affected by changes in the junction temperature. The junction temperature in a given circuit is affected by the input current, ambient temperature and level of heat sinking of the package.

Peak Wavelength
For applications where an optically unfiltered photodiode is the receiver the shift in wavelength is not critical. However, for wavelength specific applications such as CCD or Night Vision goggle illumination high junction temperatures can play a critical role. The wavelength shift is typically 0.35nm/C shift in the junction temperature. For instance, if the junction temperature rises from 40°C to 100°C, the wavelength may shift from 885nm to 906nm.

Forward Voltage
The forward voltage gets smaller as the junction temperature increases. The typical shift is approximately –0.1%/°C. This can be important if the current source does not have enough headroom at low temperatures to support the increased forward voltage of the LED. If figure 2 circuit is employed at high temperatures the current will increase and may potentially damage the device from excess current and junction temperature.

Power Output
The power output efficiency of Near IRLED’s decreases by approximately –0.5%/°C. In applications where there are large temperature swings, the variation in LED output will be far greater than the drop in LED power output caused by long term degradation.

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The United States applies different emission limits than other countries so care should be taken to apply the proper Standard and emission limit.
In the United States, ANSI Z136.1-2000, American National Standard for Safe Use of Lasers, is the standard used to define emission limits for Lasers. Appendix H of this Standard refers manufacturers to ANSI RP27.1 and RP27.3 to apply Safety Standards for LED’s.
Western Europe, and other Countries, apply IEC 60825-1, Safety of Laser Products as the standard for LED’s and Lasers. This document does not make a distinction or apply different emission limits between LED’s and lasers.
Reflector cups, multiple LED’s, external lenses and/or other optical components applied by the equipment manufacturer may significantly alter the emission level. It is the responsibility of the equipment manufacturer to test and verify the actual emission in the system. Opto Diode will be available to provide values at the component level as requested.

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