BASIC LIGHT THEORY It is difficult to describe light since its properties consist of both waves and particles (photons) at the same time. Because of the difficulty in describing light correctly, in this book light is simply described as light, wavelengths, or as the light of a specific wavelength. All light is however electromagnetic radiation and the distance between two crests of a photon is measured in nanometres (nm), which is one billionth of a meter. It is the distance between the crests of a photon that is called wavelength. WAVELENGTH
Depending on the distance between the crests of the photon (wavelength) our eyes perceives light as different colours. For example we perceive wavelengths around 530 nm as green light and wavelengths around 600 nm as red. With our naked eyes we are able to see light with wavelengths between 400 and 800 nm. Outside this wavelength spectrum light becomes invisible to our eyes. Wavelengths below 400 nm are called ultraviolet light and those above 800 nm infrared light. Light from the common light sources around us consists of a mixture of wavelengths. The light from an incandescent or fluorescent lamp contains wavelengths from just below 400 nm and above. The sun, however, generates wavelengths in a much wider spectrum, from 200-300 nm and rising. A major difference between laser light and other kinds of light sources is that the la-
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Laser Therapy – a practical guide
ser light consists of one single wavelength, for example 633 nm. Therefore, laser light is known as a very “clean” light. 1 WAVELENGTH AND ABSORPTION
The lights ability to penetrate tissues varies with the wavelength. The light in the red and infrared spectrum has the best penetration through human tissue and wavelengths between 800-960 nm and 990- 1200 nm reach down to a depth of about 3-5 cm. (1) In practice, you can see this for yourself by shining with a flashlight through a hand. We perceive the light from the flashlight as a yellow / white light. However, putting the flashlight to one side of the hand we see red light on the other side. This is because only light with wavelengths in the red / infrared spectrum have the ability to penetrate through the hand. The rest is absorbed in the skin and other tissue. (See picture). How deep and fast the light is able to penetrate human tissue depends on several parameters: for example the tissue itself, the wavelength used, if the laser is simultaneously pressed against the tissue and the output power of the laser (how strong the light is). Laser light has been shown in studies to be able to reach anywhere from a few millimetres to about nine centimetres deep below the skin surface (measured on animals). (2, 3)
We perceive the light from the flashlight as a yellow / white light. However, putting the flashlight to one side of the hand we see red light on the other side. This is because only light with wavelengths in the red / infrared spectrum have the ability to penetrate through the hand. The rest is absorbed in the skin and other tissue.
Laser Therapy – a practical guide
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Laser Therapy – a practical guide
“Since laser therapy has a superior effect on healing, pain and inflammation, I would not withhold any patient the method, alone or as a complement to other treatment. For me, laser therapy is also an obvious complement to physical exercise therapy because the rehab period is shortened considerably” ANDERS BERGSTRÖM, LICENSED PHYSIOTHERAPIST, OMI DEGREE (ORTHOPEDIC MEDICINE INTERNATIONAL)
A super pulsed laser (a pulsed rather than continuous light beam, super pulsed = short pulses of high intensity) with wavelength 904 nm was found in one study to have the best penetration over time, two to three times better than 810 nm. A super pulsed laser with a wavelength of 904 nm may therefore be the best type of laser for deep targets. (4,5) For superficial targets at the depth of 0-1 cm, both red and infrared light can be used. A target of 2-5 cm depth should be treated using a laser with a near infrared or infrared light. The concept of depth in laser theory relates to the depth where one can see local therapeutic effects of laser light, so-called therapeutic depth. Generally, this is considered to be at 4-5 cm depth for near-infrared light and infrared light. For red light it is 0-2 cm down from the skin surface. Note that the depth may increase if the therapist simultaneously puts pressure on the tissue trough the laser during treatment. 1 PHOTON ENERGY
Light with wavelengths below 400 nm is named ultraviolet radiation a, b and c (uva, uvb, uvc). When the distance between wave crests is below 400 nm it means that the light particle / photon itself has high energy - in other words that light bends in very high speed. Light with high photon energy can be dangerous to humans. The energy can be strong enough to break the dna bindings in our cells. Exposure to such radiation over a sufficiently long time without any protection of the skin, can cause cancer. This is why one should use sunscreen with protection against uva and uvb rays if you do not protect your skin with clothing or stay in the shadow. uvc is absorbed mainly by the ozone layer. Wavelengths used in the treatment of pain in the musculoskeletal system usually have a wavelength between 780 and 910 nm. In these wavelengths the photon energy is too weak to damage our cells. 1
Laser Therapy – a practical guide
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