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Melasma is a common hypermelanosis that occurs in areas exposed to sunlight, especially the face. It occurs rarely before puberty and is commonly observed in women, especially women of childbearing age. It is also observed in 10% of men. New melasma often arises and existing melasma may worsen during pregnancy. Symptoms appear as brown macules with irregular borders and occur symmetrically in the photodistribution of sunlight on the face. They are often fused in a reticular pattern. They are darkened by sunlight, which makes them worse during summer and improves during winter, i.e., there is a seasonal change. Melasma is common among Asian and Hispanic women who live in areas with strong sunlight. It occurs frequently in patients with Fitzpatrick skin types IV, V, and VI [1].

Although there are many side effects that can occur after laser treatment, the most common and problematic side effect in darker skin including Koreans is postinflammatory hyperpigmentation (PIH). Therefore, a full understanding and management of PIH are necessary. In this chapter, we go through PIH in detail.

Many papers and dermatology laser texts usually describe lasers about white skin. In separate articles or chapters laser treatment of “darker skin” or ethnic, nonwhite, pigmented skin is described [1–4]. Here, the term “darker skin” refers to all races except Caucasians, including Chinese, Japanese, Indian, Pakistani, Hispanic, Latino, African, African-Caribbean, and African American. Fitzpatrick skin types are III, IV, V, and VI [5].

Skin rejuvenation is a treatment that targets age-related pigmentation disorders and telangiectasia or improves wrinkles and skin texture [1]. It can be classified by treatment methods; ablative rejuvenation (AR) by CO2 laser and erbium-doped yttrium aluminium garnet (Er:YAG) laser or nonablative rejuvenation (NAR), which induces a selective thermal reaction to the dermis and subcutaneous fat while sparing the epidermis. It can also be classified by treatment targets; type I rejuvenation targets pigmentation and vascular disorders while type II rejuvenation focuses on improving wrinkles and skin texture. For example, AR and NAR are both treatment modalities for type II rejuvenation (Table 9.1). Generally, NAR refers to rejuvenation treatment that uses nonablative methods to improve wrinkles and skin texture (type II rejuvenation).

In “Chap. 1,” we reviewed that the most important theory regarding lasers is the theory of selective photothermolysis. In practice, however, there are cases in which the theory of selective photothermolysis does not apply and sometimes it even deviates from the theory of selective photothermolysis [1]. Next, we will review three advanced theories developed from the selective photothermolysis theory (Table 3.1).

Hair is a cylindrical fiber consisting of tightly adhered, cornified epithelial cells that only mammals have [1]. It is present anywhere on the skin except on palms and soles, distal ends of fingers and toes, boundaries of mucous membranes, and the glans penis.

There are many types of scars. They vary according to the shape of the scar surface, shape of the scar itself, color, and chronology. Scars may be classified by scar surface (flat, elevated, depressed, or irregular) by morphology (atrophic, normorophic, hypertrophic, or keloid) by color (depigmented, hypopigmented, hyperpigmented, or discolored) or by chronology (acute, chronic, immature, or mature) [1].

Absorption curves for water are very low in the ultraviolet and visible range and the absorption curve rises in the near-infrared range and is the highest in the far-infrared. The absorption curve of water in far-infrared peaks at 2940 nm and then decreases, then gradually increases (Fig. 10.1) [2].

During Apple’s iPad2 launch presentation, Steve Jobs said, “The reason that Apple is able to create products like the iPad is because we’ve always tried to be at the intersection of technology and the liberal arts… And it’s the combination of these two things that I think has let us make the kind of creative products like the iPad.” Similarly, we are at the intersection of physics and dermatology. Doctors who treat skin are too ignorant of physics, while physicists and laser-manufacturing companies do not know what is important for skin treatment because they have no experience in dermatology.

Hemoglobin (Hb) includes oxygen-bound oxyhemoglobin (HbO2), oxygen-free deoxyhemoglobin (deoxyHb), and methemoglobin (metHb), which underwent protein denaturation by heat. The absorption curve of HbO2 reaches a peak at 418 nm, reaches a small peak at 542 and 577 nm, then decreases, and finally forms a smaller peak at 940 nm. DeoxyHb reaches peaks at 430 and 555 nm and reaches a peak once again between 800 and 1200 nm (Fig. 6.1) [1, 2].

The world consists of light and matter (Fig. 1.1). When light and matter meet, they interact with each other and make various physical and chemical changes. For example, if you stand under the clear autumn sky, you can feel your body warming up even in the cool weather. This phenomenon occurs when light is converted into heat in the skin. Conversely, light can be created by applying energy to a material. This phenomenon is used when laser is made.