LEDs-Wavelengths and Applications

Light-emitting diodes (LEDs) are semiconductors that convert electrical energy into light energy. The color of the emitted light depends on the semiconductor material and composition, with LEDs generally classified into three wavelengths: ultraviolet, visible, and infrared.

The wavelength range of commercially available LEDs with single-element output power of at least 5 mW is 275 to 950 nm. Each wavelength range is made from a specific semiconductor material family, regardless of the manufacturer. This article will provide an overview of the LED operation and a brief look at the industry. Various types of LEDs, the corresponding wavelengths, materials used in their composition, and some applications for the specific lamps will also be discussed.

LEDs are semiconductor diodes that emit light when an electrical current is applied in the forward direction of the device — an electrical voltage that is large enough for the electrons to move across the depletion region and combine with a hole on the other side to create an electron-hole pair must be applied. As this occurs, the electron releases its energy in the form of light, and the result is an emitted photon.

The bandgap of the semiconductor determines the wavelength of emitted light. Shorter wavelengths equal greater energy, and therefore higher-bandgap materials emit shorter wavelengths. Higher-bandgap materials also require higher voltages for conduction. Short-wavelength UV-blue LEDs have a forward voltage of 3.5 V, while near-IR LEDs have a forward voltage of 1.5 to 2.0 V.

The overriding factor as to whether or not a specific wavelength is commercially available has to do with market potential, demand and industry-standard wavelengths. This is particularly pronounced in the 420- to 460-nm, 480- to 520-nm and the 680- to 800-nm regions. Because there are no high-volume applications for these wavelength ranges, there are no high-volume manufacturers providing LED products for these ranges. However, it is possible to find small- or medium-size suppliers offering products to fill these particular wavelengths on a custom basis.

Light Emitting Diodes (LEDs) can emit light across a wide range of wavelengths, from ultraviolet (UV) through visible light to infrared (IR). Each wavelength range offers unique properties that are harnessed in various applications. Below is a detailed overview of LED wavelengths and their applications:

Ultraviolet (UV) LEDs (200–400 nm)

• UV-C LEDs (200–280 nm):
  • Sterilization and Disinfection: UV-C LEDs are widely used in water purification systems, air sterilizers, and surface disinfectants. By disrupting the DNA and RNA structures of bacteria and viruses, they render them unable to reproduce, effectively achieving sterilization.
  • Medical Applications: They are used in applications such as air disinfection in operating rooms and sterilization of medical equipment.
• UV-B LEDs (280–315 nm):
  • Phototherapy: UV-B LEDs are used in medical treatments for skin conditions like psoriasis and vitiligo. UV-B radiation stimulates vitamin D production in the skin, aiding in therapeutic processes.
  • Dermatology: They help treat skin conditions such as eczema and acne vulgaris.

• UV-A LEDs (315–400 nm):
  • Photolithography: UV-A LEDs are crucial in semiconductor manufacturing for creating intricate patterns on silicon wafers.
  • Curing Applications: They are used in the rapid curing of inks, coatings, and adhesives, enhancing production efficiency.
  • Fluorescence Analysis: They play a role in non-destructive testing and forensic analysis. For example, substances that fluoresce under UV-A light, such as bodily fluids at crime scenes, can be detected.
    

Visible Light LEDs (400–700 nm)

• Blue LEDs (430–470 nm):
  • Dental Curing Instruments: Blue LEDs are used in dental curing devices to harden dental composites during procedures like fillings and sealants.
  • Phototherapy: Blue LED phototherapy is used to treat neonatal jaundice by breaking down excess bilirubin in infants.
  • Horticultural Lighting: Blue light promotes vegetative growth in plants by enhancing chlorophyll absorption, making it beneficial for plant cultivation.
    

• Green LEDs (520–530 nm):
  • Traffic Signal Lights: Green LEDs are used in green traffic signals due to their high visibility.
  • Indicators and Displays: They are commonly used in electronic devices as status indicators and display screens.

• Amber LEDs (580–590 nm):
  • Traffic Signal Lights: Amber LEDs are used in amber or yellow traffic signals.
  • Warning Lights: They serve as caution signals and alerts in industrial settings.
• Red LEDs (630–640 nm):
  • Traffic Signal Lights: Red LEDs are standard for red stop signals in traffic systems.
  • Indicators and Displays: They are used in various signaling devices and electronic displays.
  • Medical Applications: Red light promotes cell metabolism and accelerates wound healing. For instance, 630–640 nm red light can reduce pain and inflammation in muscles and joints.
• Deep Red LEDs (660–680 nm):
  • Blood Oximetry and Analysis: Deep red LEDs are used to measure blood oxygen saturation and assist in medical diagnostics and laboratory equipment.
  • Photodynamic Therapy: They are applied in light-activated drug treatments for cancer cells.
  • Horticultural Lighting: Deep red light stimulates flowering and fruiting in plants, enhancing agricultural productivity.
  • Medical Aesthetic Treatments: 660 nm red light is used in hair growth devices to treat conditions like androgenetic alopecia. Studies have shown that continuous exposure to 660 nm red light can increase hair density.
    

Infrared (IR) LEDs (700 nm–1 mm)

• • Near-Infrared LEDs (700–850 nm):
    • Night-Vision Illumination: They provide invisible illumination that can be detected by night-vision equipment and CCD cameras, used in night-vision devices.
    • Sensing and Detection: They are integral to proximity sensors, optical encoders, and safety light curtains in automation systems.
  • Near-Infrared LEDs (850–940 nm):
    • Photoelectric Controls: They are used in remote sensing and switching devices for industrial automation.
    • Covert Illumination: They enable surveillance systems that remain undetectable to subjects being monitored.
  • Mid-Infrared LEDs (1,400–3,000 nm):
    • Thermal Imaging and Monitoring: Mid-infrared LEDs are used in thermal imaging cameras for predictive maintenance and process monitoring by detecting heat signatures.
    • Gas Detection Equipment: They help identify gas emissions in industrial processes, ensuring environmental compliance.
  • Deep Infrared LEDs (3–25 μm):
    • Spectroscopy and Material Analysis: They are used to determine chemical compositions and molecular structures.
    • Quality Control: They ensure process verification in manufacturing.
      

• White LEDs: White LEDs are primarily used for general lighting. They are also applied in LCD backlighting, automotive headlights, and street lighting.
• RGB LEDs: RGB LEDs are widely used in advertising displays, decorative lighting, and stage lighting. By mixing different intensities of red, green, and blue light, they produce a wide range of colors.

LEDs are more reliable than lasers, generally cost less and can be driven with lower-cost circuitry. The European Union has now joined with the U.S. in classifying LEDs as a separate entity. Fortunately, LEDs do not carry the same eye safety concerns or warnings that lasers and laser diodes do. On the other hand, LEDs cannot be made into extremely small, highly collimated and optically dense spots. In applications where extremely high power density within a small area is required, a laser is almost always required.

LEDs are now used in a large number of diverse markets and applications. Their high reliability, high efficiency, and lower overall system cost compared with lasers and lamps make these devices very affordable and attractive to both consumer and industrial segments. Each individual LED technology and/or color has been developed to address specific uses and requirements.