Basics of LED

LED stands for “light-emitting diode”. It is a basic pn-junction diode, which emits light when activated. When a proper voltage is applied to the two leads of an LED, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

LEDs are produced in a variety of shapes and sizes. One of the most common package is 5mm through hole LED. The following picture shows a blue, a pure green, and a red LED in 5 mm diffused cases:

3 LEDs

The detailed structure of a 5mm LED is show below:

5mm LED structure

 

When comparing different LEDs, we often refer to some major specifications as listed below:

Color

One important parameter of LED is its emission color. In general, LEDs tend to provide a single color. The light emission extends over a relatively narrow light spectrum. Depending on its chip material, pn-junction structure, and the fabrication process, different LED emits light of different colors. Typically process variations give peak wavelength variations of up to ±10nm.

The color emitted by an LED is usually specified in terms of its peak wavelength – i.e. the wavelength which has the peak light output. This is usually measured in nanometers (nm). For example, the first high-brightness blue LED demonstrated by Shuji Nakamura of Nichia Corporation in 1994 has a peak wavelength of 450nm.

Besides peak wavelength, the color of an LED is also often specified in terms of its dominant wavelength. Since color is the perception of light by human eyes, people like to use the color perceived by the human eyes to describe the LED color. The eye essentially gets a weighted average of all wavelengths emitted by the LED and perceives a dominant “color” based on this averaging. Because this color is an average of all emitted colors, it will not necessarily correspond to the wavelength of peak intensity. Therefore, you will often notice that peak wavelength of an LED is not the same as its dominant wavelength.

 

Light intensity value

The amount of light emitted from an LED is quantified by a single point, on-axis luminous intensity value (Iv). LED intensity is specified in terms of millicandela (mcd). Luminous intensity does not represent the total light output from an LED. Depending on the output light distribution, the light intensity value may be dramatically different even the total light output from LEDs are similar. For example, if the viewing angle of an LED is smaller, the measured light intensity value of it will be higher if the total light output is the same. Therefore, when comparing different LEDs, both the luminous intensity and the spatial radiation pattern (viewing angle) must be taken into account.

Luminous intensity is roughly proportional to the amount of current (If) supplied to the LED. The greater the current, the higher the intensity. Of course, there are design limits. To compare different LEDs, the luminous intensity value for an LED must be quoted for a given current. Many low power LEDs will operate at currents of around 20mA.

 

Current / Voltage

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.

The connection and calculation of the current limiting resistor is shown below:

led_circuit

So the resistor value should be R = (V1-V2)/I.

 

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.

 

Angle of view (or viewing angle)

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.

 

Operational life time

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

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 70% 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.

 

 

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