Understanding Laser-Tissue Interaction: The Anderson Principles Explained
By John Hoopman, CLMSO
Laser Safety Officer | Clinical Educator
Introduction
Mastering light-based treatments in aesthetic medicine begins with one fundamental principle: understanding how laser energy interacts with tissue. One of the most important frameworks in this field is based on the work of Dr. R. Rox Anderson, whose pioneering research laid the groundwork for what we now know as selective photothermolysis. This article will guide you through the Anderson principles in a clear, clinically useful way—so you can apply them in practice with confidence and safety.
The Origins of Selective Photothermolysis
Coined by Anderson and Parrish in the 1980s, “selective photothermolysis” refers to the precise targeting of specific chromophores (light-absorbing molecules) in the skin using controlled laser parameters. The goal: destroy or alter the target without damaging surrounding tissues.
This is the scientific foundation for procedures like hair removal, vascular lesion treatment, and pigmentation correction. The effectiveness of any laser treatment depends on your ability to follow this principle in real-time clinical settings.
The Three Key Parameters
Anderson’s framework emphasizes the importance of matching three key parameters to your chosen chromophore:
- Wavelength: Must match the absorption peak of the chromophore.
- Pulse Duration: Should be equal to or shorter than the target’s thermal relaxation time (TRT).
- Fluence (Energy Density): Must be sufficient to cause the desired effect without overheating adjacent structures.
Each of these parameters must be adjusted based on the patient’s skin type, the depth of the target, and the competing chromophores in the treatment area.
Chromophores: The Targets
The primary chromophores in aesthetic laser medicine are:
- Melanin: Found in hair follicles, pigmented lesions, and epidermis.
- Hemoglobin/Oxyhemoglobin: Present in blood vessels and vascular lesions.
- Water: A universal chromophore that absorbs light in ablative resurfacing treatments.
Clinical Application Example
Imagine treating a superficial vascular lesion (telangiectasia) on Fitzpatrick II skin. Using Anderson’s principles, you would:
- Select a wavelength around 532 nm to match hemoglobin absorption.
- Choose a pulse duration under the TRT of the vessel (typically ~1–2 ms).
- Adjust fluence to ensure blanching without purpura or epidermal injury.
This strategic approach transforms your work from guesswork to science-backed treatment.
Anderson’s Contribution to Modern Laser Protocols
Most modern manufacturer protocols are rooted in Anderson’s work. However, many of these presets are built for broad safety rather than individualized optimization. Understanding the why behind the settings empowers you to go beyond presets and achieve superior outcomes.
When You Don’t Follow the Principle
Ignoring Anderson’s guidelines leads to complications such as:
- Post-inflammatory hyperpigmentation (PIH)
- Scarring
- Insufficient treatment effect
This happens when energy diffuses into unintended tissue structures. Staying within the window of selective photothermolysis is how you avoid these issues.
Thermal Relaxation Time (TRT): The Most Overlooked Factor
TRT is the time it takes for tissue to cool down to half its peak temperature. If the pulse duration exceeds TRT, heat escapes and damages surrounding tissue. Always remember:
Pulse Duration <= TRT = Precision
This principle helps guide your pulse settings for everything from fractional lasers to IPL and beyond.
Energy Stacking and Dwell Time
Another clinical consideration is the stacking of pulses or repeated passes. If you fire multiple pulses in succession without allowing enough cooling time, you exceed the safe thermal window. Energy “stacks” in the tissue and raises the risk of burns or hyperpigmentation.
Why This Still Matters Today
Even with advanced devices, smart presets, and built-in cooling, the science of Anderson remains as important as ever. Devices do not treat patients—clinicians do. And the best clinicians understand and apply the underlying physics.
Recommended for All Practitioners
These principles apply whether you’re a laser safety officer, dermatologist, aesthetic nurse, or medical spa provider. Whether you’re treating with a 532, 755, 1064, CO2, Er:YAG, or IPL system, Anderson’s work should shape your decisions every step of the way.
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John Hoopman is a Certified Laser Safety Officer and educator who turns scientific principles into practical, safe, and effective clinical outcomes. If you’re ready to go beyond presets and become a laser expert:
Become the expert your patients trust—guided by physics, not presets.