Understanding Lasers, Light Sources, and Other Energy-Based Technology

Characteristics of Light

Light is a form of energy that is made up of photons. Photons are particles that travel in space as electromagnetic waves. Photons will eventually come into contact with atoms, which exist in nature in their lowest state of energy, the “ground state.” When atoms absorb photons, they are propelled into higher energy states because their orbiting electrons are displaced further from the nucleus. When photons stop hitting atoms, the atoms instantaneously fall back toward their ground states, and in so doing, release energy in the form of photons. The process of energy transfer from energized atoms back to their resting states via the emission of photons is called electromagnetic radiation. It was originally described by Albert Einstein in “The Quantum Theory of Radiation.”

Electromagnetic Spectrum

The photons that are released as electromagnetic radiation move at a specific wavelength and frequency based on properties of the emitting atom. The electromagnetic spectrum is the range of frequencies at which photons travel in nature ( Fig. 1.1 ). It encompasses low-frequency radio waves, visible light, ultraviolet radiation, and high-frequency gamma rays.

Light, composed of a stream of photons, travels at a constant velocity “the speed of light” in a vacuum. The speed of light is a product of wavelength and frequency (c = λ xƒ). Since the speed of light is constant, the energy of the photon is proportional to its frequency and inversely proportional to its wavelength. Therefore, high energy photons have higher frequencies and lower wavelengths. Lasers take advantage of the different properties of photon frequencies and wavelengths to treat a wide variety of dermatologic diseases.

Lasers

Laser is an acronym that stands for Light Amplification by Stimulated Emission of Radiation . A laser is comprised of an energy source and an optical resonator. The energy source, which may be an electrical current, flashlamp, or a second laser, functions to excite atoms within the optical resonator into high energy states. The optical resonator contains a medium (gas, liquid, solid, or crystal) that provides the source of atoms ( Fig. 1.2 ). The medium determines the wavelength of the photons that are emitted as electromagnetic radiation. Lasers often take the names of the specific medium they contain: Neodymium-doped: Yttrium Aluminium Garnet (Nd:YAG), Ruby, carbon dioxide (CO ₂ ), etc.

The medium is enclosed in a tube between two parallel mirrors, one that is completely reflective and one that is semi-transparent. The energy source fires photons at the atoms in the medium which displaces their electrons further from the nuclei and brings them to higher energy levels. Atoms then emit photons of a specific wavelength when the atoms fall back into their ground states. If an emitted photon collides with an atom already in the excited state, another photon of the same wavelength will be emitted. This chain reaction leads to photons of the same wavelength bouncing off the walls and reflective mirror in the optical resonator until they are in parallel with the semi-transparent mirror. The semi-transparent mirror acts as a filter to emit only photons of a specific wavelength that are traveling in the same phase and direction. The exiting stream of photons is known as the laser beam.

Properties of Laser Light

Laser light is unique because it is monochromatic, which means that the stream of photons is composed of a single wavelength of light. This differs from other light sources, such as sunlight, light bulbs, and intense pulsed light, which emit photons with many different wavelengths. Monochromaticity carries great therapeutic importance because it allows for laser light to target specific compounds in the skin.

Laser light is also coherent and collimated ( Fig. 1.3 ). Coherence is the state in which photons travel together in both time and space, while collimation is the state in which photons travel in parallel to one another. Coherent and collimated light is able to travel longer distances without significant divergence or loss of intensity. In this manner, lasers are able to treat spot sizes that are near-equivalent to the wavelength of light.

Intense Pulsed Light (IPL)

Lasers are unique in that they are monochromatic. Intense pulsed light (IPL) devices are not lasers— they are light sources that produce electromagnetic radiation that spans a range of wavelengths, approximately 500–1200 nanometers (nm). Different filters are inserted into the IPL device to allow for a more specific range of wavelengths. A specific filter permits light of that wavelength and longer wavelengths to pass through. For example, a 560 nm filter permits light with wavelengths 560 nm and above to exit the device.