Laser and Light Therapies

Learning Objectives

• Explain Terms: polychromatic vs monochromatic, collimated, coherent, wavelength, spectral range, nanometer, pulse width.

• Identify Radiation Types: Electromagnetic radiation for cosmetic light therapies; ionising vs non-ionising forms.

• IPL vs LASER: How they differ and their clinical uses.

• Identify Chromophores: Main ones in the skin.

• Light-Skin Interaction: How light interacts with skin/tissue.

• Analyze Absorption Graph: Identify aspects specific to cosmetic light therapies.

• Chromophore & Spectral Ranges: For treatments like hair reduction, vascular/pigmented lesions, skin resurfacing.

Timeline and Historical Context

• 3000–30 BC: Heliotherapy (sun therapy) in Ancient Greece, Egypt, Rome; herb/light treatments in India; sun worshipped for health.

• 1903: Dr. Niels Finsen develops first artificial light source for lupus, wins Nobel Prize.

• 1916: Einstein proposes LASER theory.

• 1940s: Light energy used for WWII soldier wounds.

• 1960: Theodore Maiman publishes first laser for surgery; low-power therapeutic benefits emerge.

• 1967: Endre Mester discovers low-dose laser increases tissue healing.

• 1990s: Low-level laser therapy widely used in medical and sports institutions.

Light Energy: Fundamental Concepts

• Light: A form of electromagnetic radiation.

• Two Behaviors:

  • Wave-like: (diffraction, transmission)

  • Particle-like: (photons)

• Photons: Chargeless energy bundles; carried by oscillating electric/magnetic fields; travel at speed of light.

• Wave-particle duality: "We have two contradictory pictures of reality…separately neither of them fully explains the phenomena of light, but together they do." — Albert Einstein.

• Key Relationships:

  • Wavelength ($ext{distance between peaks}$) in meters.

  • Frequency ($ext{number of peaks per second}$) in Hz.

  • Relation: $f= rac{c}{<br>ho}$

  • Photon Energy: $E=hf= rac{hc}{<br>ho}$

• Propagation: Light as waves in media; can refract, reflect, scatter (e.g., sunlight in water droplets).

Waveform of Light

• Wavelength (
  ho): Distance between successive peaks (meters).

• Frequency (f): Number of peaks per second (Hz).

• Dispersion: Light separates into colors in media due to varying refractive indices for different wavelengths.

Electromagnetic Radiation: Ionising vs Non-Ionising

• Distinction: Based on wavelength and energy (frequency).

• Non-ionising Radiation: Energy insufficient to ionize atoms.

  • Examples: Visible light, Microwaves, Infrared, Radio.

• Ionising Radiation: Energy sufficient to remove electrons, creating ions.

  • Main Forms:

    • Alpha particles (protons, neutrons)

    • Beta particles (electrons)

    • Gamma rays and X-rays (high-energy photons)

Light Emission: Atomic Model and Photons

• Bohr’s Atomic Model:

  • Nucleus: Protons (+), Neutrons (neutral).

  • Electrons: Orbit nucleus in stable ground state.

• Quantum States:

  • Electrons: Occupy discrete energy levels.

  • Absorption: Specific energy raises electrons to higher, excited states.

  • Emission: Excited states are short-lived; returning to ground state emits photons with specific wavelengths.

• Spontaneous Emission:

  • Excitation and de-excitation occur randomly.

  • Most atoms remain in resting state.

  • Examples: Sunlight, many light sources, IPL.

• Stimulated Emission:

  • Excited electron absorbs another photon of equal energy.

  • Releases two identical photons (same energy, wavelength, direction) when returning to ground state.

  • Enables amplification and population inversion.

• Result: Produces laser light with high brightness and coherence.

• Laser Light Characteristics:

  • Monochromatic: Single, distinct wavelength (single color through prism).

• Contrast IPL/White Light:

  • Polychromatic: Multiple wavelengths.

• Chromaticity:

  • Laser: Collimated & coherent.

  • IPL: Non-collimated & non-coherent.

Laser Emission and Light Sources

• Laser Emission:

  • Light Amplification by Stimulated Emission of Radiation.

  • Medium: Determines wavelength (gas, liquid, solid).

  • Emission Modes: Continuous, pulsed, or Q-switched.

  • Pulse Width: How long energy is emitted on skin.

  • Q-switched lasers: Produce pulses in nanosecond range ($ext{pulse width} heta 10^{-9} ext{s}$).

• IPL Emission:

  • Light Source: Flash lamps (e.g., xenon bulbs).

  • Filters: Exclude shorter, potentially damaging wavelengths.

  • Cutoff Filters: Tailor wavelength ranges to target structures.

    • Vascular Lesions: $(500-670 ext{ nm}, 870-1400 ext{ nm})$

    • Pigmented Lesions: $(525-1200 ext{ nm})$

  • Pulse Durations: Millisecond range.

  • Applications: Photoepilation (permanent hair removal).

• Key Contrasts Summary:

  • Laser: Monochromatic, coherent, collimated.

  • IPL: Broadband, non-coherent, non-collimated.

Intense Pulsed Light (IPL) and Skin Optics

• Importance: Critical for device selection, safety, effectiveness in light-tissue interactions.

• Four Major Tissue Interactions:

  1. Reflection: Light bounces off.

     • Main Cause: Stratum corneum (skin surface).

     • Minimizing: Perpendicular incidence.

     • Amount: ~4{-}6 ext{%} at 90° incidence.

  2. Scatter: Photons redirected within tissue.

     • Main Cause: Collagen in dermis.

     • Impact: Inversely related to wavelength and spot size; longer wavelengths scatter less, penetrate deeper.

  3. Transmission: Light passes through tissue.

     • Effect: No effect if not absorbed or scattered.

  4. Absorption: Energy taken by chromophores.

     • Requirement: Necessary for biological effects (photothermal, photochemical, photomechanical).

Absorption, Reflection, Scattering, Transmission: Skin Optics Details

• Grothus-Draper Law of Photobiology: Absorbed photon energy converts to thermal energy, heating target tissue.

• Absorption: Depends on wavelength and chromophore absorption spectra.

• Reflection: Minimized by perpendicular exposure; ~4–6% at 90° incidence.

• Scattering: Primarily by collagen; inversely proportional to wavelength and spot size; longer wavelengths penetrate deeper.

• Transmission: Light passing without interaction has no clinical effect.

• Penetration Depth vs. Wavelength:

  • Shorter Wavelengths ($ext{ heta }300{-}400 ext{ nm}$): Superficial penetration, high scattering.

  • Longer Wavelengths ($ext{ heta }600{-}1200 ext{ nm}$): Deeper penetration, reduced scattering.

  • Very Long Wavelengths: Increased water absorption, reduced penetration.

• Visible to Near-Infrared ($ext{ heta }400{-}1200 ext{ nm}$): Longer wavelengths generally penetrate deeper.

• Mid-IR and Far-IR Lasers (e.g., Er:YAG at 2940 nm; CO2 at 10600 nm): Different patterns due to high water absorption; do not follow general depth rule.

Absorption Coefficient and Chromophores

• Graph Axis: X-axis = Wavelength (nm); Y-axis = Absorption Coefficient ($ext{cm}^{-1}+$).

• Key Chromophores and Ranges:

  • Melanin: Strong UV absorption, decreases through visible, lower beyond ~750 nm (but still present).

  • Oxyhemoglobin (HbO2) & Deoxyhemoglobin (Hb): Multiple peaks in visible range ($ext{ heta }400{-}600 ext{ nm}$).

  • Water: Peaks at infrared wavelengths (e.g., ~2940 nm for Er:YAG, ~10600 nm for CO2).

• Absorption Spectrum Overview:

  • Melanin: Dominant in UV, decreasing into visible, limited into near-IR.

  • Hemoglobin: Peaks within 400–600 nm.

  • Water: Minimal in visible, sharply increases in IR.

• Transparency Curve: Shows penetration depth for each wavelength.

• Extended Theory: Clinical target vs. absorber chromophore concepts.

Absorption Spectrum: Chromophores and Treatment Ranges

• Chromophore Absorption Examples:

  • Melanin: UV and visible (pigment-targeting).

  • Oxyhemoglobin/Deoxyhemoglobin: Visible to near-IR (vascular targeting).

  • Water: Mid to far-IR (ablative lasers).

  • Tattoo Inks: Specific visible wavelengths.

  • DNA/RNA/Proteins: UV range.

• Specific Practical Ranges:

  • Acne Treatment: 400–450 nm (targets porphyrins).

  • Photo-rejuvenation (pigment + vessels): 500–650 nm (targets melanin & oxyhemoglobin).

  • Permanent Hair Reduction: 600–1200 nm (targets hair melanin for deeper penetration).

• Selective Photothermolysis (Extended Theory): Distinguishes absorber (generates heat) from distant target (damaged by heat diffusion).

• Absorption Spectrum: Lists chromophores for various tissue targets (tattoos, sebaceous glands, fat).

Selective Photothermolysis

• Core Concept: Targeted, controlled tissue injury using laser light, minimizing collateral damage.

• Key Components (Anderson & Parrish, 1983):

  1. Wavelength: Preferentially absorbed by target chromophore.

  2. Pulse Duration: Equal to or shorter than Thermal Relaxation Time (TRT) of target to confine heat.

  3. Exposure Dose (Fluence): Sufficient to damage target within pulse duration.

• Commonly Targeted Chromophores: Melanin (hair, epidermis), water (epidermis, dermis), haemoglobin (blood vessels).

• Extended Theory:

  • Absorber vs. Target: Heat generated by absorber diffuses to the target.

  • Thermal Damage Time (TDT): Duration for entire target to cool by a prescribed fraction (e.g., 63%).

  • Hair Removal Example: Melanin in hair shaft/matrix is absorber; stem cells are target. TDT may need to be longer than TRT for heat to diffuse to non-pigmented stem cells for permanent removal.

• Practical Implications:

  • Hair Removal: Pulse duration ~TRT of hair follicle for safety.

  • Large Targets/Heat Diffusion Need: TDT considerations may require longer pulses than TRT.

Absorption Spectrum and Chromophore Mapping (More Details)

• Chromophore–Range Mapping:

  • DNA, RNA, Proteins: UV, blue to green visible light.

  • Oxyhemoglobin: Red absorption peaks (vascular coloration).

  • Deoxyhemoglobin: Near-IR and visible peaks.

  • Melanin: Broad visible absorption, decreasing in near-IR.

  • Water: Strong IR absorption (prominent at ~2940 nm (Er:YAG), ~10600 nm (CO2)).

• Tattoo Inks: Absorption spectrum indicates regions for removal.

• Transparency Curves: Predict light penetration depth.

• Clinical Target vs. Chromophore: Sometimes direct match (tattoo pigment), sometimes different (hair removal, where pulse width > TRT for heat diffusion).

Intense Pulsed Light Therapy (IPL) vs Laser Therapy: Indications and Applications

• IPL Therapy: Photothermal effects via broad-spectrum, filtered light.

• IPL Indications:

  • Vascular lesions

  • Pigmented lesions

  • Photoaged skin

  • Excess hair growth

  • Acne vulgaris

• IPL Spectral Coverage & Targets:

  • Broadband output (400–1200 nm, device-dependent) with cutoff filters.

  • Vascular: 500–670 nm and 870–1400 nm.

  • Pigmented: 525–1200 nm.

• Laser Therapy: Monochromatic, coherent light, high irradiance.

  • Versatile: Cutting, coagulating, ablation, imaging.

• Eye Safety: Highly susceptible to injury; mandatory protection needed.

• Medical Applications: Beyond dermatology (GIT, ophthalmology, urology, etc.).

Skin-Tissue Interactions in Clinical Context

• Clinical Effect: Requires energy absorption and conversion to heat.

• Energy Thresholds & Heat Effects:

  • >50°C: Tissue injury, inflammation, repair begins.

  • >60°C: Protein/DNA denaturation, tissue coagulation.

  • >100°C: Intracellular water boils, vaporization (ablation).

• Photothermal Response: Underpins most laser/IPL therapies.

• Photomechanical (Photoacoustic) Response:

  • Rapid, high-energy pulses; rapid thermal expansion, shock waves.

  • Uses: Breaking calculi (bladder/ureter), nanoparticle/photoacoustic effects.

  • Requires: Very high energies, short pulse durations (nanoseconds).

• Photochemical (Photodynamic) Responses:

  • Involves photosensitizers (endogenous/exogenous).

  • Examples: Acne vulgaris (porphyrins); topical ALA for precancerous lesions (activates ROS, immune response).

  • PDT Uses: Actinic keratosis, skin cancer, acne.

  • Demonstrations: Topical photosensitizer + irradiation (e.g., red LEDs) for Bowen’s disease.

Clinical Targeting Principles: Photothermolysis in Practice

• Selective Photothermolysis (Reiterated):

  • Wavelength: Matches chromophore absorption spectrum.

  • Pulse Duration: Matches TRT to confine heat.

  • Fluence: Sufficient for target damage.

• Extended Theory Nuances:

  • Absorber vs. Target: Heat diffusion is key.

  • Hair Removal: Melanin (absorber), stem cells (target). TDT may exceed TRT to allow heat diffusion to stem cells.

Summary of Key Spectral Ranges and Their Clinical Relevance

• Acne Treatment (porphyrins): ~$400{-}450 ext{ nm}$ ($ext{blue light}$).

• Photo-rejuvenation (pigmentary + vascular): ~$500{-}650 ext{ nm}$.

• Hair Reduction (melanin in hair): ~$600{-}1200 ext{ nm}$ (deeper penetration).

• Tattoo Removal: Nd:YAG Q-switched (1064 nm or 532 nm) for pigment fragmentation.

• Water Absorption: Limits deep penetration for mid/far-IR lasers (Er:YAG at 2940 nm, CO2 at 10600 nm).

Practical Takeaways for Exam Preparation

• Distinguish Laser vs. IPL:

  • Laser: Monochromatic, coherent, collimated; precise short pulses.

  • IPL: Broadband, non-coherent, divergent; uses filters; targets multiple structures.

• Understand Four Tissue Interactions: Reflection, Scatter, Transmission, Absorption; how chromophores lead to effects.

• Master Selective Photothermolysis: Match pulse duration to TRT, ensure adequate fluence.

• Explain Mechanism Roles: Photothermal, photomechanical, photochemical clinical applications.

• Recognize Absorption Spectra: Informs wavelength choice for treatments (hair, vascular/pigmented lesions, resurfacing); water absorption in high-wavelength lasers.

• Acknowledge Safety: Ocular protection, minimizing collateral damage.

Notes on Formulas Used

• Frequency-Wavelength-Speed: $f= rac{c}{<br>ho}$

• Photon Energy: $E=hf= rac{hc}{<br>ho}$

• Fluence (Energy density): (F) in $\text{J/cm}^2$

• Power (Rate of energy delivery): (P) in watts; $P = ext{Energy}/ ext{Time}$

• Irradiance (Power density): $\frac{P}{A}= ext{W/cm}^2$

• Tissue Responses: Temperature thresholds: >50°C, >60°C, >100°C (thermal effects).

• Pulse Width, TRT, TDT: Times used for optimizing target damage while sparing surrounding tissue.

References (Selected from transcript)

• Ash, C., Dubec, M., Donne, K. et al. Effect of wavelength and beam width on penetration in light-tissue interaction using computational methods. Lasers Med Sci 32, 1909–1918 (2017).

• Cios A, Ciepielak M, Szymański Ł, et al. Effect of Different Wavelengths of Laser Irradiation on the Skin Cells. Int J Mol Sci. 2021;22(5):2437.

• Gupta, P., & Khare, R. Laser Physics and Technology. Springer India. 2015.

• Heidari Beigvand, H., et al. Assessment of Laser Effects on Skin Rejuvenation. J Lasers Med Sci. 2020;11(2):212–219.

• Landthaler M, Ulrich H, Hohenleutner S, et al. Role of Laser Therapy in Dermatology – Clinical Aspects. Dermatology 2004;208:129-134.

• Meschede, D. Optics, Light and Lasers. 2017.

• Talor, R., et al. Lasers in tattoo and pigmentation control: PicoSure system. Med Devices 2016;9:63–67.