LED Specifications & Parameters

- summary of different LED specifications and LED parameters found in LED datasheets.

When choosing LEDs for particular applications it is necessary to comprehend the different LED specifications or LED parameters.

There is a variety of different LED specifications, each of which will have an effect on the choice of the particular LED used. With such a variety of different LEDs available, the LED specifications cane be matched to the requirements for the particular application rather than having to make do.

Some of the major LED specifications are outlined in the paragraphs below.

LED colour

Obviously the colour is a major LED specification or LED parameter. LEDS tend to provide a single colour. The light emission extends over a relatively narrow light spectrum.

The colour emitted by an LED is specified in terms of its peak wavelength (lpk) - i.e. the wavelength which has the peak light output. This is measured in nanometers (nm).

The colour of the LED, i.e. the peak wavelength of the emission from the LED is a function mainly of the chip material and its fabrication. Typically process variations give peak wavelength variations of up to ±10nm.

When choosing colours within the LED specification, it is worth remembering that the human eye is most sensitive to hue or colour variations is around the yellow / orange area of the spectrum - around 560 to 600 nm. This may affect the choice of colour, or position of LEDs if this could be a problem.

LED light intensity value, Iv

The LED specification for light intensity is important. The light intensity is governed by a variety of factors including the LED chip itself (including the design, individual wafer, the materials, etc.) , the current level, encapsulation and other factors.

The LED light intensity specification is not of crucial importance for most indicator applications, but with LEDs being used for lighting, this parameter is needed to be able to specify exactly what is needed in many situations.

The light output from an LED is quantified in terms a single point, on-axis luminous intensity value (Iv). This is specified as millicandella, mcd.

The lv measurement for LEDs cannot easily be compared with the values of mean spherical candle power, MSCP used for incandescent lamps.

The luminous intensity value for an LED must be quoted for a given current. Many LEDs will operate at currents of around 20mA, but the light output of an LED increases with increasing current.

LED current / voltage specification

LEDs are current driven devices and the level of light is a function of the current - increasing the current increases the light output. It is necessary to ensure that the maximum current rating is not exceeded. This could give rise to excessive heat dissipation within the LED chip itself which could result in reduced light output and reduced operating lifetime.

In operation, LEDs will have a given voltage drop across them which is dependent upon the material used. The voltage will also be slightly dependent upon the level of current, so the current will be stated for this.

Most LEDs require an external series current limiting resistor. Some LEDs may include a series resistor and will state the overall operating voltage.

LED reverse voltage

LEDs are not tolerant to large reverse voltages. They should never be run above their stated maximum reverse voltage, which is normally quite small. If they are then permanent destruction of the device will almost certainly result.

If there is any chance of a reverse voltage appearing across the LED, then it is always best to build in protection into the circuitry to prevent this. Normally simple diode circuits can be introduced and these will adequately protect any LED.

LED angle of view specification

In view of the way in which LEDs operate, the light is only emitted over a certain angle. While this LED specification may not be important for some applications, it is of great importance for others.

The angle of view is normally defined in degrees - °. For early devices, the angle of view was normally relatively small. More recent devices may have a much wider angle of view.

LED specification for operational life

The light intensity of a LED does diminish gradually with time. This means that a LED has an operational life.

This LED specification is of particular importance when a LED or LEDs are to be used for lighting applications. It is not normally as crucial when the LED is used as an indicator - here a catastrophic failure is of greater importance.

The LED specification for its operational life is generally defined in the following terms:

L70% = Time to 70% of illumination (lumen maintenance)

L50% = Time to 50% of illumination (lumen maintenance)

The standards state that during these times, the LED should not exhibit any major shifts in chromaticity.

The rationale behind these figures is that 70% lumen maintenance equates to a 30% reduction in light output. This is around the figure for the threshold for detecting gradual reductions in light output.

Where light output is not critical, the 50% lumen maintenance figure may be more applicable. However for applications where lights may be placed side by side the 80% lumen maintenance figure should be considered.

Figures for LED operational life may be of the order of 50 000 hours or more dependent upon the lumen maintenance figure used.

By Ian Poole

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