# Laser and Light Therapy: Principles and Applications

## Introduction

Laser and light-based devices are fundamental tools in dermatologic practice, with applications spanning vascular lesions, pigmented lesions, tattoo removal, hair removal, skin resurfacing, and inflammatory skin disease. A thorough understanding of laser physics, tissue interactions, and device selection is essential for safe and effective treatment.

## Laser Physics Fundamentals

### Basic Principles

**LASER** stands for Light Amplification by Stimulated Emission of Radiation. Laser light is **monochromatic** (single wavelength), **coherent** (waves in phase), and **collimated** (parallel beam). The **gain medium** determines the wavelength: solid media produce Nd:YAG lasers, liquid media produce pulsed dye lasers, gas media produce CO2 lasers, and semiconductor media produce diode lasers. Wavelength determines both the **depth of penetration** and **chromophore selectivity**.

### Selective Photothermolysis (Anderson and Parrish, 1983)

Selective photothermolysis is **the foundational principle** of modern cutaneous laser therapy. It requires three elements: first, a wavelength preferentially absorbed by the **target chromophore**; second, a pulse duration equal to or shorter than the **thermal relaxation time (TRT)** of the target; and third, sufficient **fluence** (energy per area, measured in J/cm2) to damage the target. The **thermal relaxation time** is the time for the target to lose 50% of its peak temperature and is proportional to the square of the target size. TRT examples include approximately 1 microsecond for melanosomes, 1 to 10 milliseconds for dermal blood vessels, and 10 to 100 milliseconds for hair follicles.

### Extended Theory of Selective Photothermolysis

For targets larger than the desired damage zone, such as hair follicles, **longer pulses** can still achieve selective damage if the pulse duration is shorter than the TRT of the target but longer than the TRT of the surrounding epidermis. This allows effective treatment while minimizing collateral thermal damage.

<image>Diagram illustrating selective photothermolysis: a laser pulse targeting an intravascular chromophore (oxyhemoglobin) with wavelength-specific absorption, pulse duration matched to the thermal relaxation time of the vessel, and resulting selective vessel damage with epidermal preservation</image>

## Chromophores and Wavelength Selection

### Oxyhemoglobin

Oxyhemoglobin has **absorption peaks** at 418 nm (Soret band), 542 nm, and 577 nm. The lasers used to target it include the **pulsed dye laser (PDL) at 585 to 595 nm** (the gold standard), the KTP at 532 nm, and the Nd:YAG at 1064 nm for deeper vessels.

### Melanin

Melanin has **broad absorption** that decreases from UV through near-infrared wavelengths. Shorter wavelengths (532 nm, 694 nm) target superficial pigment, while longer wavelengths (755 nm, 1064 nm) penetrate deeper. Melanin absorption creates a **competing risk** of epidermal injury in darker skin types, which is managed by using longer wavelengths and longer pulse durations.

### Water

Water has **major absorption** at 1927 nm (thulium), 2940 nm (Er:YAG), and 10,600 nm (CO2). Water is the chromophore for ablative resurfacing lasers. Er:YAG has 10 times greater water absorption than CO2, providing more precise ablation with less residual thermal damage.

### Tattoo Pigment

Tattoo pigment is an exogenous chromophore, and different colors require different wavelengths for removal. Black ink responds to 1064 nm (Nd:YAG), 694 nm (ruby), and 755 nm (alexandrite). Red ink requires 532 nm (KTP/Nd:YAG frequency-doubled). Green ink responds to 694 nm (ruby) and 755 nm (alexandrite). Blue ink is targeted by 694 nm (ruby) and Q-switched 1064 nm.

| Chromophore | Absorption Peaks/Range | Primary Laser(s) | Clinical Application |
|-------------|----------------------|-----------------|---------------------|
| Oxyhemoglobin | 418, 542, 577 nm | PDL 585–595 nm, KTP 532 nm, Nd:YAG 1064 nm | Vascular lesions, port wine stains |
| Melanin | Broad (decreasing UV→IR) | Alexandrite 755 nm, Ruby 694 nm, Nd:YAG 1064 nm | Pigmented lesions, hair removal |
| Water | 1927, 2940, 10600 nm | Er:YAG 2940 nm, CO2 10600 nm, Thulium 1927 nm | Ablative resurfacing |
| Tattoo (black) | Broad | Nd:YAG 1064 nm, Ruby 694 nm, Alexandrite 755 nm | Tattoo removal |
| Tattoo (red) | ~532 nm | KTP 532 nm | Tattoo removal |
| Tattoo (green) | ~694–755 nm | Ruby 694 nm, Alexandrite 755 nm | Tattoo removal |

## Major Laser Categories and Applications

### Vascular Lasers

The **pulsed dye laser (PDL) at 585 to 595 nm** is the gold standard for port wine stains, hemangiomas, telangiectasias, rosacea, hypertrophic scars, warts, and psoriasis. The **KTP at 532 nm** treats superficial telangiectasias and facial erythema, though high melanin absorption limits use in darker skin. The **long-pulsed Nd:YAG at 1064 nm** treats deep blue veins, leg veins, and deeper vascular lesions and is safer in darker skin types. **Intense pulsed light (IPL)** is a broadband (500 to 1200 nm) filtered light source that is versatile for vascular and pigmented lesions but is not technically a laser.

### Pigment Lasers

**Q-switched** (nanosecond) and **picosecond** lasers generate ultra-short pulses for photomechanical destruction of pigment. The **Q-switched Nd:YAG (1064 nm/532 nm)** is versatile, safe in darker skin at 1064 nm, and is the workhorse for tattoo removal. The **Q-switched ruby (694 nm)** is excellent for blue-black tattoos and dermal pigment. The **Q-switched alexandrite (755 nm)** treats blue, black, and green tattoos. **Picosecond lasers** (755 nm, 1064 nm, 532 nm) use shorter pulse durations to create more efficient photomechanical disruption, improving clearance of tattoos and pigmented lesions with fewer treatments.

### Hair Removal Lasers

The target for laser hair removal is **melanin in the hair follicle**, specifically the follicular stem cells and bulge. The **alexandrite at 755 nm** is effective for skin types I to III and is the gold standard for lighter skin. The **diode at 810 nm** is versatile and effective for skin types I to IV. The **long-pulsed Nd:YAG at 1064 nm** is the safest for skin types IV to VI, with deeper penetration but lower melanin absorption. Pulse duration should approximate the TRT of the hair follicle (10 to 100 ms), and **anagen phase** hairs are most responsive because they are melanin-rich and attached to the papilla.

| Laser | Wavelength | Skin Types | Key Features |
|-------|-----------|------------|--------------|
| Alexandrite | 755 nm | I–III | Gold standard for lighter skin; high melanin absorption |
| Diode | 810 nm | I–IV | Versatile; good penetration depth |
| Long-pulsed Nd:YAG | 1064 nm | IV–VI | Safest for dark skin; deepest penetration; lowest melanin absorption |

### Ablative Resurfacing Lasers

The **CO2 laser (10,600 nm)** vaporizes tissue layer by layer with a significant residual thermal damage zone (50 to 150 micrometers) that stimulates collagen remodeling. The **Er:YAG laser (2940 nm)** has 10 times greater water absorption, providing precise ablation with minimal thermal damage (10 to 40 micrometers) but less hemostasis. **Fractional ablative** technology creates microscopic treatment zones (MTZs) surrounded by intact tissue, dramatically accelerating healing and reducing complications. Indications include photoaging, acne scarring, rhinophyma, and epidermal nevi.

### Non-Ablative Fractional Lasers

The **1550 nm erbium-doped fiber (Fraxel)** is a fractional non-ablative laser that treats acne scars, photoaging, melasma, and surgical scars. The **1927 nm thulium** is more superficial and excellent for dyschromia and superficial photodamage. These devices create microscopic columns of thermal coagulation without epidermal disruption at the surface. Multiple sessions are required, but downtime is less than with ablative fractional treatments.

<image>Wavelength absorption spectrum chart showing the relative absorption coefficients of oxyhemoglobin, melanin, and water across wavelengths from 400 to 11000 nm, with common dermatologic laser wavelengths marked (KTP 532, PDL 595, ruby 694, alexandrite 755, diode 810, Nd:YAG 1064, erbium 1550, CO2 10600)</image>

## Complications and Safety

### Common Adverse Effects

**Post-inflammatory hyperpigmentation** is most common in skin types III to VI and is managed by using longer wavelengths, longer pulse durations, lower fluences, and skin preparation with hydroquinone. **Burns and blistering** result from excessive fluence or inappropriate settings. **Scarring** is rare with proper technique but carries the highest risk with ablative resurfacing. **Herpes reactivation** necessitates antiviral prophylaxis for perioral ablative procedures. **Infection** (bacterial, viral, or candidal) is a particular concern after ablative resurfacing.

### Laser Safety

**Protective eyewear** that is wavelength-specific must be worn by the operator and patient, with corneal shields used for periorbital treatment. **Plume evacuation** is essential because laser plume contains viable viral particles and chemical carcinogens. **Fire hazards** require avoidance of flammable prep solutions, supplemental oxygen near the operative field, and dry drapes. **Reflective hazards** are mitigated by using non-reflective (anodized or ebonized) instruments.

<image>Comparison of fractional versus fully ablative laser treatment patterns: fractional treatment showing microscopic columns of ablation (microthermal zones) surrounded by islands of intact tissue enabling rapid reepithelialization, versus full-field ablation with confluent tissue removal</image>

## Key Clinical Pearls

Selective photothermolysis requires matching wavelength to chromophore, pulse duration to TRT, and sufficient fluence -- these three elements must all be present for selective tissue destruction. For darker skin types, longer wavelengths (1064 nm Nd:YAG) and longer pulse durations should be used to minimize epidermal melanin competition. PDL at 595 nm is the gold standard vascular laser, and its purpuric threshold can be avoided with longer pulse durations and lower fluences (subpurpuric settings). Fractional technology, both ablative and non-ablative, revolutionized resurfacing by dramatically reducing downtime and complication rates. Picosecond lasers improve tattoo clearance rates compared to Q-switched nanosecond lasers, particularly for resistant colors.

## References

1. Anderson RR, Parrish JA. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. *Science*. 1983;220(4596):524-527.
2. Bernstein EF, Schomacker KT, Basilavecchio LD, et al. A consensus guide for laser and energy-based device treatment. *Dermatol Surg*. 2021;47(7):925-933.
3. Hruza GJ, Avram MM, eds. *Lasers and Lights*. Procedures in Cosmetic Dermatology. 4th ed. Elsevier; 2018.
4. Sakamoto FH, Lopes JD, Anderson RR. Photodynamic therapy for acne vulgaris: a critical review from basics to clinical practice. *J Am Acad Dermatol*. 2010;63(2):183-193.