When treating a pigmented lesion, the light absorption characteristics of melanin are most important. Melanin is the target pigment or chromophore and is heated by absorption of the laser light to the point where the lesion is destroyed. The surgeon chooses a wavelength that is well absorbed by melanin. Generally a wavelength in the Blue/Green range is needed. Blue has higher absorption (but more scattering) than the green. Because of the increase of likelihood of scarring with the blue wavelengths, the typical choice for a pigmented lesion would be the green.
-- Principles of Pediatric Dermatology, Dr. Mahmoud Hijazy
How can freckles be treated?
To treat freckles that are already present, several safe and effective methods are available:
1. Bleaching creams -- Products containing hydroquinone and kojic acid can be purchased without a prescription. Higher concentrations of hydroquinone (over 2%) require a prescription. These products can lighten freckles if they are applied consistently over a period of months.
2. Tretinoin -- Sometimes used in conjunction with other bleaching creams, tretinoin (vitamin A acid, Retin-A) also helps to make freckles lighter when applied over a period of time.
3. Cryosurgery -- A light freeze with liquid nitrogen can be used to treat freckles.
4. Laser treatment -- Several lasers, especially ones that produce a green light, can lighten and eliminate freckles safely and effectively. Like cryosurgery, this is a simple and safe procedure with a high success rate and a low risk of scarring.
[re Pigmented Lesions]
Firstly, a specific wavelength, which is optimally absorbed by the target tissue, is needed, and this depends on the chromophore absorption curve of that target. Chromophores are substances used to absorb the laser light. In skin, they include haemoglobin in vascular lesions, melanin in pigmented lesions, and water in most tissues. While considering the selectivity of absorption, one must remember that in general, the longer the wavelength, the deeper the penetration of the light; and the longer the wavelength, the smaller the absorption coefficient. These properties would guide the operator to choose the suitable wavelength to treat specific skin lesions. In treating vascular lesion, yellow light is usually used; while in pigmented lesions, green light is usually used in epidermal lesions and red to infrared light (with longer wavelength) for dermal lesions.
-- Laser therapy in dermatology: what have we
learnt in the last decade? L Y Chong The Hong Kong Practitioner VOLUME 23 August 2001 pages 323-330.
Green light is effective and less painful than red light in photodynamic therapy of facial solar keratoses.
Photodynamic therapy (PDT) with topically applied delta-aminolevulinic acid (ALA) is increasingly employed to treat patients with multiple solar keratoses and superficial skin tumors. For these indications, ALA-PDT has been shown to be highly efficient. Treatment of multiple or extended lesions, however, is substantially hampered by the fact that ALA-PDT is associated with burning pain during the irradiation procedure. The standard irradiation devices commonly used for ALA-PDT emit red light around 630 nm. In the present half-side comparison study we have observed that ALA-PDT employing a green light irradiation device (543-548 nm) is equally effective, as compared with standard red light ALA-PDT. In contrast to red light ALA-PDT, however, green light ALA-PDT caused only little tingling and burning but no pain. These observations indicate that green light ALA-PDT is superior to standard ALA-PDT, because it is associated with less unwanted side effects.
-- Fritsch C, Stege H, Saalmann G, Goerz G, Ruzicka T, Krutmann J. Department of Dermatology, Heinrich Heine University Duesseldorf, Germany. Photodermatology, photoimmunology & photomedicine1997 Oct-Dec;13(5-6):181-5.