# 2.1 Basic Laser Concepts (Wavelength, Energy, Pulse Duration) --- ## 1. Wavelength (파장) ### A. Definition - 레이저가 방출하는 **빛의 파장 (nm)** = 광자가 가진 에너지 크기와 조직 내 흡수 특성을 결정. ### B. Key Principles - **Selective photothermolysis (선택적 광열분해)**: 특정 파장이 특정 **chromophore (색소, 물, 혈관 등)** 에 선택적으로 흡수되어 치료 효과 발생. - **Common chromophores**: - Melanin (멜라닌, 400–1100 nm 흡수) - Hemoglobin (헤모글로빈, 400–600 nm peak) - Water (물, >1000 nm에서 강력 흡수) ### C. Examples in Dermatology - **532 nm Nd:YAG**: superficial epidermal pigment (lentigo, freckles). - **755 nm Alexandrite / 694 nm Ruby**: melanin-targeting (tattoos, nevi). - **1064 nm Nd:YAG**: deeper penetration → dermal pigment, vascular lesions. - **10,600 nm CO₂**: absorbed by water → tissue vaporization/ablation. --- ## 2. Energy (에너지) ### A. Definition - **Energy (Joules, J)** = Power × Time. - Amount of energy delivered to tissue determines treatment efficacy vs. risk of damage. ### B. Fluence (에너지 밀도, J/cm²) - More clinically relevant than absolute energy. - **Fluence = Energy / Spot size area**. - Determines how much energy per unit area is absorbed by chromophore. ### C. Clinical Relevance - Too low → insufficient effect (pigment not destroyed). - Too high → collateral damage, blistering, scarring, PIH. - Example: laser toning for melasma uses **low fluence 1064 nm** repeatedly to avoid PIH. --- ## 3. Pulse Duration (펄스 지속시간) ### A. Definition - The length of time energy is delivered in one pulse (milliseconds, microseconds, nanoseconds, picoseconds). ### B. Principle of **Thermal Relaxation Time (TRT, 열이완시간)** - Each target (melanosome, hair follicle, vessel) has a TRT = time it takes to cool down by 50% after heating. - To selectively destroy a target: **Pulse duration ≤ TRT of target**. - Example: - **Melanosome TRT ~ 1 microsecond** → Q-switched lasers (ns) or picosecond lasers effective. - **Hair follicle TRT ~ 10–100 ms** → long-pulse lasers effective. ### C. Clinical Relevance - **Q-switched lasers (ns)**: tattoo ink, melanosomes (high peak power). - **Picosecond lasers (ps)**: smaller particles, faster photomechanical effect, fewer side effects. - **Long-pulse lasers (ms)**: hair removal, vascular lesions (match vessel TRT). --- ## 4. Integration in Clinical Practice |Parameter|Definition|Clinical Impact|Example| |---|---|---|---| |**Wavelength (파장)**|Distance between peaks of laser light|Determines chromophore target & depth|532 nm (melanin, superficial) / 1064 nm (deep pigment, vessels) / 10,600 nm (water, ablation)| |**Energy / Fluence (에너지/에너지 밀도)**|Total energy per unit area (J/cm²)|Determines intensity of tissue effect|Low-fluence Nd:YAG for melasma vs. high-fluence for tattoo removal| |**Pulse Duration (펄스 시간)**|Time energy is delivered|Must match or be less than target’s TRT|Q-switched (ns) for pigment, long-pulse (ms) for hair/vascular| --- ## 5. Clinical Pearls 1. **Wavelength = “What target?”** (chromophore selectivity). 2. **Fluence = “How strong?”** (efficacy vs. side effects). 3. **Pulse duration = “How long?”** (matching TRT prevents collateral damage). 4. Wrong combination = complications (e.g., high fluence long-pulse 532 nm on Asian skin → severe PIH risk). 5. Mastery of these 3 variables = safe & effective laser practice. --- ✅ **Summary for practice:** In laser dermatology, **wavelength** determines _which chromophore is targeted_, **fluence (energy/spot size)** determines _treatment intensity_, and **pulse duration** must be matched to the _thermal relaxation time of the target_ to achieve selective destruction with minimal collateral damage. ---