Characteristics of Lasers versus LEDs
The modern world seems to be awash in light-emitting devices. Consumers are presented with LED light bulbs, LED panels, and LED screens; laser data storage systems, laser pointers, and laser facelifts; and lights so small they can be embedded in jewelry. Most people are not quite sure what these things are or how they work. Clearly, they involve light. But are LEDs and lasers the same thing? What does LED stand for? How do they make light bulbs that small? We answer these pressing questions and others in this comprehensive guide.
Light is electromagnetic radiation and it travels around in waves. Different types of light are distinguished by their wavelength, which is the distance from the peak of one wave to the next wave. Human eyes evolved to detect light of certain wavelengths emitted by the Sun (visible light) and our eyes are capable of distinguishing among wavelengths of that light. We experience different wavelengths of light as different colors. Light with a longer wavelength is perceived as red, while a shorter wavelength looks blue.
A diode is a type of semiconductor. A semiconductor is a material that can conduct electricity, but only sort of and sometimes. Metal wires are conductors: they easily conduct electricity. We wrap metal wires in plastic, an insulator, which does not conduct electricity, to keep the electricity inside the wire. Semiconductors are called "semi" because they are in between conductors and insulators in their capacity to conduct electricity. This ability to sort of and sometimes conduct electricity makes semiconductors very useful in electronics because it allows us to control when and how well they conduct electric currents.
A diode used to make light is more technically called a PIN diode. It is called a PIN diode because it has a P-N junction made out of two layers (the P and N layers) separated by a space. The P stands for positive: this part of the material has electrons. The N stands for negative: it has electron holes, an absence of electrons. The I stands for intrinsic and it is the empty layer between the P and N layers.
When an electric current is fed into the P-N junction of a PIN diode, it transfers energy to the electrons in the P layer. These excited electrons then move into the intrinsic layer and combine with the electron holes from the N layer. However, the electrons cannot remain excited for very long and they quickly release their energy in the form of light. They then return to their normal state until they receive some more electricity.
Engineers can precisely modulate what wavelength of light a PIN diode emits by controlling the structure and ions of the semiconducting material. A PIN diode can be designed to emit light in any desired wavelength. However, since the color it emits is controlled by its structural characteristics, each PIN diode can only emit light in one wavelength.
Devices that produce light from a PIN diode are small and compact, use very little energy, and waste very little of that energy in the form of heat. Since the electrons are capable of undergoing an infinite number of excitation/light emission cycles, the diode never uses up its raw materials and it can continue emitting light indefinitely as long as the electricity keeps flowing.
Normal light travels around in a disorganized fashion (technically, it is called incoherent). When you point a flashlight in one direction, the light spreads out the further it moves away from the source because of its random, disorganized movement.
Unlike normal light, lasers emit light that is coherent and unidirectional: the wavelengths are all lined up and traveling together in a tight, directed beam instead of wandering off in all directions. When you point a laser in one direction, the light remains tightly confined in a beam no matter how far it moves away from the source.
The acronym LED stands for "light-emitting diode," although some people have jokingly changed it to mean "light for every day" to distinguish it from laser light. LEDs emit normal light, namely, incoherent light. It is exactly like light emitted from the Sun, a candle, or a flashlight, except it consists of only one wavelength of light. If you point an LED in one direction, the light will spread out as it moves away from the source.
In an LED PIN diode, the intrinsic region is wide with a large, open surface area surrounding the gap between the P and N layers. As the excited electrons release their energy inside the intrinsic region, it easily escapes out through the wide gap and into the surrounding area, creating light. LED devices use reflectors outside of the diode to bounce the emitted light in the desired direction.
In PINs used to create lasers, the intrinsic region is very narrow and the edges of the gap, rather than being open, are coated with a reflective material. As the excited electrons emit their energy, it bounces off the reflective material back into the intrinsic region to re-excite the electrons, over and over. This intense stimulation causes population inversion to occur, turning the light into a coherent laser beam that can only escape from the intrinsic region through one tiny point that is not coated with a reflective material.
This key difference in the intrinsic region between laser diodes and LEDs profoundly affects the nature of the light emitted, changing it from normal light into laser light.
LEDs can be much more powerful than LEDs and produce a brighter beam of light. However, they are not necessarily more powerful; for example, the laser in a laser pointer is not very powerful, producing less than 5 milliwatts of energy. In comparison, a laser scalpel can emit up to 80,000 watts focused at a single point.
An average LED light consuming 100 watts of input power will emit around 40 watts of energy in the form of light and 60% in the form of heat. A key point to keep in mind is that since the energy output of an LED is not focused in a laser beam, it spreads out and diffuses as it moves away from the diode. Thus, even a strong LED light's power output rapidly diminishes as it moves away from the source, unlike a laser.
Each PIN diode produces light with only a specific wavelength. LED devices that can change color or produce different colors use multiple PIN diodes that get turned off or on in different proportions to produce the desired color effect. All colors visible to the human eye can be produced by mixing red, green, and blue together (the RGB color system). Devices that use LEDs to produce realistic images use just red, green, and blue LEDs to produce their colors, including white light. Lasers can be used similarly to generate different colors, although their higher cost to construct and other characteristics generally preclude their use as just color- and light-emitting devices.
LEDs are much more durable and have a longer lifespan than laser diodes. The tiny, mirrored PIN in a laser diode is not as robust as the PIN in an LED.
LEDs are rapidly taking over all lighting tasks in the modern world and are expanding into new uses that were impossible to achieve using previous forms of lighting (light-up wallpaper, for example). Tiny, energy-efficient lights are being incorporated into all kinds of devices.
Laser diodes are more of a niche product; if an LED can do the task, they are preferred due to their lower costs and increased durability. Lasers have, however, firmly established themselves in many fields as essential tools.
The wave of LED technology innovations has just gotten started. Energy-efficient LED technology has transformed lighting applications, entertainment, and a wide range of consumer products. Creative new uses of LEDs seem to pop up almost daily. In the field of laser diodes, the recent development of extremely high-powered lasers that can be produced from small, compact devices may lead to as yet unimagined tools, weapons, and toys.
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