The balance between safety and efficacy relies on the principle of thermal relaxation. To achieve this balance, pulse width and spot size must be precisely calibrated to the specific cooling properties of the target tissue, such as a hair follicle. This ensures that destructive heat remains confined to the target while dissipating harmlessly from the surrounding healthy skin.
Core Insight: The governing law of laser safety is Thermal Relaxation Time (TRT). By adjusting the pulse width to match the time it takes for the target to release 50% of its heat, the system maximizes damage to the lesion or follicle while preventing heat accumulation in adjacent soft tissue or underlying structures like teeth.
The Role of Pulse Width in Thermal Control
Matching Thermal Relaxation Time
The pulse width determines the duration that laser energy interacts with the tissue.
To destroy a target effectively, the pulse width is typically set to match or slightly exceed the target's thermal relaxation time.
For example, a pulse width of 30 milliseconds is often ideal for hair follicles. This duration allows heat to build up sufficiently to destroy the follicle structure.
Targeting the "Bulge" (Stem Cells)
In professional hair removal, the goal is permanent destruction, not just temporary vaporization of the hair shaft.
Pulse widths in the 10ms to 100ms range allow heat to slowly diffuse from the hair shaft into the surrounding follicle stem cells (the bulge).
This specific timing ensures the regenerative capacity of the hair is destroyed without allowing time for the heat to spread outward into the dermis.
High-Peak Power for Vaporization
For targets other than hair, such as pigment or lesions, much shorter pulse widths are required.
Nanosecond or microsecond pulses deliver high peak power to achieve rapid vaporization.
Because the energy is delivered so quickly, heat does not have time to conduct to surrounding areas, protecting healthy skin from thermal damage.
The Function of Spot Size in Energy Delivery
Controlling Energy Density
Spot size works in tandem with pulse width to define the intensity of the treatment.
Adjusting the spot size allows the operator to control the volume of tissue being exposed to the laser energy.
Proper calibration ensures that the energy density (fluence) is sufficient to damage the target without overwhelming the skin's ability to cool down.
Preventing Deep Conduction
In sensitive anatomical areas, such as the upper lip, uncontrolled heat can penetrate too deeply.
A specific concern in these areas is the conduction of heat to underlying teeth, which can cause pain or damage.
By optimizing the spot size alongside a short pulse width (e.g., 30ms), the system prevents excessive heat conduction to these deeper, non-target structures.
Understanding the Trade-offs
The Risk of Excessive Heat Accumulation
If the pulse width is too long relative to the target's size, heat will "leak" into surrounding tissues.
This creates a risk of acute erythema (redness) or long-term pigmentary changes, particularly in darker skin types (e.g., Type IV).
Operators must balance the need for energy accumulation against the risk of epidermal burns.
Balancing Ablation vs. Coagulation
In scar treatment or skin resurfacing, the goal changes from destruction to stimulation.
A pulse width of 500 microseconds, for example, creates a balance between tissue ablation (removal) and thermal coagulation (heating).
This precise thermal control stimulates deep collagen regeneration while sparing the delicate mucosal tissue from secondary burns.
Making the Right Choice for Your Goal
When configuring laser parameters, your objective dictates the specific settings required for safety and success.
- If your primary focus is Hair Removal: Select a pulse width (10–100ms) that allows heat to diffuse from the shaft to the stem cells but not into the dermis.
- If your primary focus is Sensitive Areas (e.g., Lips): Utilize a precise spot size and controlled pulse width (approx. 30ms) to prevent heat conduction to underlying teeth.
- If your primary focus is Scar Treatment: Use shorter pulses (e.g., 500 microseconds) to balance ablation with coagulation for collagen remodeling.
- If your primary focus is Pigment/Lesion Removal: Opt for nanosecond-range pulses to vaporize the target before heat can spread to the surrounding skin.
Mastering these parameters allows you to exploit the physics of heat transfer, ensuring the target is eliminated while the surrounding tissue remains untouched.
Summary Table:
| Parameter | Target Application | Typical Range | Mechanism of Action |
|---|---|---|---|
| Pulse Width (Long) | Hair Removal (Follicle) | 10ms - 100ms | Thermal diffusion to stem cells (the bulge) |
| Pulse Width (Short) | Pigment & Lesions | Nanoseconds | Rapid vaporization with zero thermal spread |
| Pulse Width (Mid) | Scars & Resurfacing | ~500 Microseconds | Balances tissue ablation with collagen stimulation |
| Spot Size | All Treatments | Varies by Area | Controls energy density and prevents deep conduction |
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References
- Server Mutluay Ünal, Mehmet BİÇER. Evaluation of Heat on the Teeth During Upper Lip Laser Epilation: Clinical Experiment. DOI: 10.5336/dentalsci.2022-90918
This article is also based on technical information from Belislaser Knowledge Base .
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