What is a LED diode?
Light-emitting diode (LED) is a type of semiconductor, which produces light when electric current flows through it. They usually emit different colors of visible light, but it can also be infrared or ultraviolet light. LEDs are available in many different forms – from commonly used, cylindrical-shaped, through-hole mounted elements, through microscopic surface-mounted components, all the way to large chip-on-board (COB) arrays. What features distinguish them from other types of semiconductors? You will find the answer in this article.
What are the applications of light emitting diodes?
LED technology accompanies us in everyday life – as indicator lights in various types of electronic devices, as well as backlight of LCD displays and monitors. It is also a standard in most modern lighting products. It is very likely that in the near future, due to rapidly advancing technology LEDs may completely replace traditional lighting solutions. Therefore, it is worth learning about the properties of this universal source of light.
What are the most important parameters of LEDs?
Below we present a summary of the most important parameters and characteristics of LED diodes:
Case and mounting method
LED diodes come in many forms and types – there are numerous shapes and sizes, mounting methods, shapes and colors of the lens and even the position of the emitter in the casing. There are also diodes containing many separate emitters in one casing. This results in a very wide range of possible applications – LEDs can be found both in street lamps and compact, portable devices. Mounting method is also an important aspect to consider when selecting the diode. For example, SMD (surface mount) components allow for machine assembly and take up less space, but they cannot be mounted directly into the enclosure or front panel of the device.
Maximum forward current
This parameter varies depending on the type and purpose of the diode – it is usually determined by the manufacturer. For example, a 5mm red diffusion LED used e.g. in backlit warning signs has 50mA maximum current. Exceeding this value may result in the LED burning out, while lower current allows for extending its lifetime. The current flowing through the diode depends on the voltage – the higher the voltage, the higher the current. The intensity of emitted light also depends on the current.
Forward voltage is related to the color of the emitted light, which in turn is related to the photon energy. For a given color of diode, the range of forward voltages is generally constant, with acceptable limits of about 1.2V – 5V. For example, for a typical red diode the voltage ranges from 1.6V to 2.4V, for a blue diode – from 2.8V to 3.6V.
Color of light (wavelength)
LEDs usually emit only one specific color of light, in a very narrow wavelength range. The color depends mainly on the type of semiconductor material used, and more specifically – on the size of the band gap of a given semiconductor. The wider the gap, the shorter the wavelength emitted. White LEDs used in lighting contain a phosphor layer or coating to achieve a broader light spectrum. There are also various types of multicolor LEDs, used in many specific applications. An example of such a diode is a 5mm RGB diffused LED, offered by our company.
Luminous flux (brightness)
It is a physical quantity used in visual photometry, measuring the total power of light emitted from a given source. The unit of luminous flux is lumen (lm). The luminous flux takes into account the sensitivity of the human eye to different wavelengths, which is determined by the luminosity function. For example, a green LED achieves much higher luminous power than a blue LED of the same power, because the human eye perceives green light much better than blue.
This value determines the luminous flux that a given light source is capable of producing per unit of delivered power. It is expressed in lumens per watt (lm/W). In case of LEDs, the luminous efficacy is usually within the range of 70-120 lm/W, while high-performance emitters are capable of exceeding 200 lm/W. For comparison, traditional incandescent bulbs have a luminous efficiency of about 12 lm/W and halogen bulbs – about 20 lm/W. This significant difference in efficiency is particularly important in lighting applications, allowing for much lower energy consumption.
Color rendering index (CRI or Ra)
This parameter is relevant mainly to white LEDs used in lighting products. It determines how faithfully the colors of objects illuminated by a given light source are reproduced. The value is expressed by a number in the range from 0 to 100 – the higher its value, the more naturally the colors are rendered. Typically, LED lighting lacks in color reproduction when compared to incandescent light sources – the CRI value of white LEDs is usually in the range of 80 – 90, while incandescent and halogen lamps reach a CRI value close to 100. However, there are some high-end lighting diodes capable of CRI value exceeding 95.