Abstract—The process of laser absorption in the skin during laser irradiation was a critical point in medical application treatments. Delivery the correct amount of laser light is a critical element in photodynamic therapy (PDT). More amounts of laser light able to affect tissues in the skin and small amount not able to enhance PDT procedure in skin. The knowledge of the skin tone laser dependent distribution of 635 nm radiation and its penetration depth in skin is a very important precondition for the investigation of advantage laser induced effect in (PDT) in epidermis diseases (psoriasis). The aim of this work was to estimate an optimum effect of diode laser (635 nm) on the treatment of epidermis diseases in different color skin. Furthermore, it is to improve safety of laser in PDT in epidermis diseases treatment. Advanced system analytical program (ASAP) which is a new approach in investigating the PDT, dependent on optical properties of different skin color was used in present work. A two layered Realistic Skin Model (RSM); stratum corneum and epidermal with red laser (635 nm, 10 mW) were used for irradiative transfer to study fluence and absorbance in different penetration for various human skin colors. Several skin tones very fair, fair, light, medium and dark are used to irradiative transfer. This investigation involved the principles of laser tissue interaction when the skin optically injected by a red laser diode. The results demonstrated that the power characteristic of a laser diode (635 nm) can affect the treatment of epidermal disease in various color skins. Power absorption of the various human skins were recorded and analyzed in order to find the influence of the melanin in PDT treatment in epidermal disease. A two layered RSM show that the change in penetration depth in epidermal layer of the color skin has a larger effect on the distribution of absorbed laser in the skin; this is due to the variation of the melanin concentration for each color. Keywords—Photodynamic therapy, Realistic skin model, Laser, Light penetration, simulation, Optical properties of skin, Melanin. F. H. Mustafa is with the Universiti Sains Malaysia, Penang,11800, Malaysia (corresponding author to provide phone: 0060-174179468; e-mail: science_farhad@yahoo.com). M. S.Jaafar is with the Universiti Sains Malaysia, Penang, 11800 ,Malaysia (corresponding author to provide phone: 0060-194785418; e-mail: msj@usm.my ). A.H. Ismail is with the Universiti Sains Malaysia, Penang,11800 ,Malaysia (corresponding author to provide phone: 0060-196890766; e-mail: asadhawlery@hotmail.com). A. F. Omar is with the University Sains Malaysia, Penang,11800, Malaysia (corresponding author to provide phone: 0060-194494449; e-mail: thinker_academy@yahoo.com). Z. A. Timimi is with the Universiti Sains Malaysia, Penang,11800, Malaysia (corresponding author to provide phone: 0060-164306900; e-mail: Zahra_Malayzsia2007@yahoo.com). H. A. Houssein is with the University Sains Malaysia, Penang,11800, Malaysia (corresponding author to provide phone: 0060-174992602; e-mail: h_saltani@yahoo.co.uk,msj@usm.my). I. INTRODUCTION DT is a form of cancer treatment. It is a photochemical process that shows a complex interaction between fluence, photosensitizer concentration and oxygen concentration. Low fluence rates cause more damage than high fluence rates for the same total fluence. [1-2]. PDT is based on the ability of light to cause chemical changes in human tissue. The choice of continuous wave diode laser for PDT is determined by two main factors: the absorption spectrum of the photosensitizers and the wavelength and penetration depth of the light. The first and most widely used class of photosensitizers in PDT is the porphyries, which absorb light maximally in the Soret band ranging from 500 and 635 nm [3]. The light distribution in the tissue depends on the tissue optical properties of various tissue layers at the laser wavelength and on the thickness of those layers. The total problem of light propagation in tissue is referred to as tissue optics [4]. Development of optical methods in the modern medicine for the area of diagnostics, therapy and surgery has stimulated for the investigation of an optical property of various biological tissues. Since the efficacy of laser treatment depends on the photon propagation and fluence rate distribution within irradiated tissues. The skin is the most important tissues for photodynamic therapy of cancer and other diseases [5]. Laser's radiation, is commonly used in dermatology, which accounts for the attention paid to practical aspects of the interaction between radiation and human skin and to the problem of selective absorption and depth of radiation penetration into the skin. As a wavelength increases into the visible and near-infrared optical region of the spectrum, radiation penetrates more deeply into the skin [6]. The knowledge about skin physiology of specific sub-populations leads to an increase in specific therapeutic options, for example, for sensitive skin and ethnic groups. The understanding and quantification of racial differences in skin functions are important for the treatment and prevention of skin diseases and skin care [7]. Human skin color varies significantly between individuals dependent on race, sun exposure and age. Normal skin color originates in the presence of specific chromospheres such as melanin, hemoglobin, bilirubin and carotene. However, scattering due to the inhomogeneous distribution of lipids, water and proteins within each cell, as well as the random distribution of cell also has a very important impact on the visual appearance of skin [8-9]. Red Diode Laser in the Treatment of Epidermal Diseases in PDT Farhad H. Mustafa, Mohamad S. Jaafar , Asaad H. Ismail, Ahamad F. Omar, Zahra A. Timimi and Hend A. A. Houssein P World Academy of Science, Engineering and Technology International Journal of Biomedical and Biological Engineering Vol:4, No:10, 2010 532 International Scholarly and Scientific Research & Innovation 4(10) 2010 scholar.waset.org/1307-6892/2373 International Science Index, Biomedical and Biological Engineering Vol:4, No:10, 2010 waset.org/Publication/2373