Proper parameter selection is fundamental to achieving both efficacy and safety in laser therapy. For Q‑switched and picosecond lasers, fluence settings are primarily influenced by the following four factors:

1. Laser Wavelength

Commonly used wavelengths include 1064nm, 755nm, 694nm, and 532nm.

Within the 400‑1200nm spectral range, the longer the wavelength, the lower the absorption coefficient of target chromophores such as melanin. Consequently, longer wavelengths require higher fluence to achieve equivalent therapeutic effects. Conversely, shorter wavelengths are more susceptible to scattering and absorption during penetration, resulting in more pronounced energy attenuation.

This wavelength‑dependent behavior guides clinical decision‑making: deeper pigmented lesions generally necessitate longer wavelengths paired with higher fluence to effectively reach and act upon the target.

2. Pulse Duration

Q‑switched lasers typically deliver pulse widths in the nanosecond range (e.g., 5ns), while picosecond lasers offer 900ps, 700ps, or even 450ps.

Longer pulse durations favor photothermal effects with relatively weaker mechanical disruption; shorter pulse durations amplify optoacoustic mechanical effects. For picosecond lasers with shorter pulse widths, peak power increases significantly when fluence is held constant. Therefore, within a certain range, lower fluence may suffice for effective pigment fragmentation—provided that the energy level remains above the threshold required for adequate disruption.

3. Lesion Color (Pigment Density and Thickness)

The color depth of a pigmented lesion reflects the density and accumulation thickness of pigment granules.

Darker lesions exhibit stronger absorption and attenuation of laser energy, allowing efficient energy deposition and disruption at the target site. Consequently, darker lesions should be treated with relatively lower fluence to avoid overtreatment, whereas lighter lesions may require higher fluence to achieve sufficient fragmentation.

4. Clinical Endpoints

Clinical endpoints—such as erythema, purpura, and pinpoint bleeding—serve as immediate visual indicators of treatment dose adequacy.

The more pronounced the desired endpoint response, the higher the fluence required. Clinicians should continuously monitor skin reactions during treatment and adjust parameters in real time to avoid adverse effects resulting from overly aggressive dosing.

A thorough understanding and balanced integration of these four determinants are essential for achieving precise, safe, and effective outcomes in Q‑switched and picosecond laser therapy.