Physics of Light

Laser Properties

• Velocity of light = frequency x wavelength
• Energy (Watts) = 6.63 x 10^34 x frequency
• F = velocity of light in air in a vacuum / L (where L is the wavelength)

Laser Characteristics

• Monochromatic (one color)
• Thin beam (collimated)
• Coherent (in phase waves)
• Can focus into the same point
• Much more intense than regular bulb light

Laser Applications in Ophthalmic Treatment

• Color is important (monochromatic)
• Collimated is necessary to focus the light
• Has a specific focus point

Light Spectrum

• UV (100 nm - 400 nm): UV-C (100 nm - 280 nm), UV-B (280 nm - 315 nm), UV-A (315 nm - 400 nm)
• Visible light (400 nm - 760 nm): blue (400 nm - 500 nm), green (500 nm - 550 nm), yellow (550 nm - 590 nm), orange (590 nm - 620 nm), red (620 nm - 760 nm)
• IR (760 nm - 1 million nm): IR-A (760 nm - 1400 nm), IR-B (1400 nm - 3000 nm), IR-C (3000 nm - 1 million nm)

Scattering

• Scattering is the result of eye color
• Raleigh scattering: occurs when light encounters small molecules (e.g., blue eyes)
• Mie scattering: occurs when light encounters larger particles (e.g., collagen fibers)

Eye Color Determinants

• Blue eyes: lack of pigment in the iris stroma, Raleigh scattering
• Green eyes: some pigment in the iris stroma, scattering by collagen fibers
• Brown eyes: dense pigment in the iris stroma, absorption of light
• Grey eyes: large collagen molecules in the stroma, Mie scattering

Light Absorption

• Tissues with nucleic acids and amino acids absorb UV-C light
• Skin and eye pigment (melanin) absorb UV light well
• Macular pigment absorbs blue light
• Hemoglobin absorbs visible light in the order of blue, green, yellow, and red
• Cornea absorbs UV-C, IR-C, UV-B, IR-B, and very little IR-A and UV-A
• Lens absorbs UV-B and UV-A
• Retina absorbs IR-A light

Damage Mechanism

• Photothermal: heat involved, temperature increase causes tissue damage
• Photochemical: light triggers chemical reactions
• Photomechanical: light causes mechanical damage (e.g., ionization, photo destruction)

Photoscoagulation

• Increasing temperature causes denaturation of proteins, coagulation, and cell shrinkage
• Diabetic retinopathy treatment involves coagulation of the retina
• Shortening of collagen fibers occurs between 50-70°C
• Temperature increase above 100°C causes water content in tissue to explode (photovaporization)
• Extreme temperatures (150°C) cause carbonization of tissue (photocarbonization)
• Melting occurs at 300°C

Thermal Damage

• Heat conduction, exposure time, thermal relaxation time, and spot size affect thermal damage
• Selective photothermolysis reduces collateral damage by controlling these factors