Once the laser beam is absorbed by its target, the light energy is converted into heat, which in turn, causes lysis (destruction) of the target cells- selective photothermolysis. The idea is to destroy the target cells without damaging the surrounding tissue, and that is achieved by controlling the amount of energy delivered to the target ( pulse duration of the beam) and by cooling the surrounding skin. This cooling is achieved by devices internal to the laser eg cryogen sprays, contact cooling: or by external means eg Zimmer machines, ice packs.

Other types of tissue interactions with lasers include

1. photoacoustic effects- ultra-short (nano- or pico- second), very high intensity pulses literally blast pools of dye in tattoos, creating particles small enough to be absorbed by the white blood cells.
2. photochemical effects- see photodynamic therapy–  chemicals, called photosensitizers, are rubbed on the skin, taken orally, or injected into the blood , and taken up by the specific cells that we wish to target by the laser or light device. This approach is used for the treatment of specific skin conditions such as  psoriasis,or actinic keratoses; and has been adapted for the treatment of some internal cancers or precancerous conditions, such as Barrett’s oesophagus.
3. The heat effect of the laser alters the configuration of collagen fibres, causing shrinkage or tightening (e.g. CO2 lasers) or, by causing the localized release of a protein called heat shock protein- stimulates the production of collagen, resulting in tissue remodelling and smoothing (scars, stretch marks, wrinkles).

Low-level lasers (so called “cold”lasers), in contrast to the high level lasers described above – have a stimulating effect on cellular metabolism– and are used for laser acupuncture ( eg smoking cessation), wound healing , and for the relief of pain or inflammation. They can also be used to stimulate hair growth on the scalp; and research is being done on the role of LLLT in limiting brain damage in the immediate post-stroke period. This mode of action is referred to as Biomodulation.

Light Devices

In contrast to lasers, light devices emit a broad spectrum of wavelengths , usually in the visible spectrum, and includes Intense Pulsed Light -IPL  and Light- Emitting Diodes -LED. Because they are broad spectrum, their targeting is less specific than a true laser.

These devices are used for a wide variety of purposes-  for the treatment of photopigmentation (sun damage), rosacea, broken capillaries or fine spider veins;  for hair removal; or to  activate different chemicals – as in photodynamic therapy.

Photodynamic Therapy– a chemical – called a photosensitizer– is applied to a target tissue, and then activated by a light source: For instance, in the treatment of skin conditions, including acne, psoriasis, and also skin cancers, the photosensitizer can be applied topically and locally excited by a light source. In the local treatment of internal tissues and cancers, after photosensitizers have been administered intravenously, light can be delivered to the target area using endoscopes and fiber optic catheters.

Common use for Photodynamic therapy in Cosmetic medicine is for the treatment of  resistant acne and for the treatment of multiple actinic keratoses (see Skin- Brown Spots).

A photosensitizer- Levulan® Kerastick® is applied and left on for an hour , then activated by exposure to blue light  using the BLU-U®  device. (see image, right)

An alternate is to use a different photosensitizer topical methyl 5-aminolevulinate (MAL) and activate with red light wavelengths from an IPL device. The photosensitizer is taken up by rapidly dividing cells such as bacteria on the skin’s surface, and sebum producing cells, thus reducing the bacterial count and excess sebum production which are the causative factors behind acne.

IPL devices are high energy , tissue destructive devices, capable of causing burns, scarring and permanent pigment changes in the wrong hands; therefore should only be used in medical clinics by properly trained personnel.

IPL devices differ from lasers in that a broad bandwidth of light is generated by an IPL machine , as opposed to a single wavelength of light generated by a laser, This means that IPL targeting is less specific than laser, i.e. an IPL device may target hemoglobin as well as melanin, while a laser would target only one specific chromophore. This is because IPL devices incorporate a filter which screens out all light below a certain wavelength, but allow all wavelengths above it. Additionally, the beam generated by a laser is very narrow and intense , whereas an IPL device sends out a diffuse flash of light which targets skin all around the intended target.

On the other hand, LED devices are low-energy, and for the most part, can be safely used by the general public. Various hand-held LED devices emitting light in the blue or red visible spectrum are available for the treatment of acne & brown spots (eg TANDA). LED devices emitting BLUE LIGHT are also used to activate hydrogen peroxide gels in Light-activated Tooth Whitening. (See below)

Full Spectrum “Bright Light” Therapy for Seasonal affective disorder

A very interesting effect of light is its effect on circadian hormonal production by the brain. Melatonin, Melanocyte stimulating Hormone (MSH), GH (Growth Hormone), and ACTH ( adrenal- corticotrophin  hormone), are among those hormones whose secretion is directly affected by the brain’s perception of light, and in turn, these hormones have an effect on mood, immune function , and tissue regeneration.

Many people who live in Northern climates, with long periods of deprivation of daylight  notice a deterioration of their mood – dysphoria or depression, in the winter; this has been termed SEASONAL AFFECTIVE DISORDER. They will respond to exposure to full spectrum bright light in the form of dawn simulators (a type of alarm clock that doesn’t use sound, but gradually builds up the intensity of ambient light to a peak just before the person is due to wake up) or light “boxes” see below; this is yet another example of photo biomodulation