UV Radiation & Light Interactions

• Light has properties used in medicine.

Light and Refraction

• Light's speed changes when transitioning between materials.
• The index of refraction (n) is the ratio of light's speed in a vacuum (c) to its speed in a material (v): n = c/v
• c represents the speed of light in a vacuum.
• v represents the speed of light in a material.

Light as Wave and Particle

• Light acts as both a wave (interference and diffraction) and a particle (single molecule absorption).

Light Absorption

• Absorbed light energy typically becomes heat, which is the basis for using IR light to heat tissues and using laser beams to "weld" retinas or coagulate blood vessels.

Fluorescence

• Fluorescence occurs when a lower energy light photon is emitted after a light photon is absorbed.

Light Reflection

• Light reflects from surfaces, with two types of reflection:
  • Specular reflection: occurs from smooth surfaces like mirrors.
  • Diffuse reflection: occurs from rough surfaces, scattering light in many directions.

Light Measurement and Units

• Light is categorized by wavelength as UV, Visible, and IR.
• Wavelength was measured in:
  • Microns (μ): 1 μ = 10⁻⁶ m
  • Angstroms (A°): 1 A° = 10⁻¹⁰ m
  • Nanometers (nm): 1 nm = 10⁻⁹ m
• Ultraviolet light has wavelengths from 100 to 400nm
• Visible light wavelengths range from 400 to 700 nm.
• IR light has wavelengths from 700 to 10⁴ nm.
• UV is further subdivided:
  • UV-A: 320 – 400 nm
  • UV-B: 290 - 320 nm
  • UV-C: 100 – 290 nm
• Visible light is measured in photometric units:
  • Illuminance: the quantity of light striking a surface
  • Luminance: the intensity of a light source.
• UV and IR radiation use radiometric units:
  • Irradiance: the quantity of light striking a surface.
  • Radiance: the intensity of a light source.

Applications of Visible Light in Medicine

• Endoscopy: Uses light to view body openings without obstruction, sometimes using instruments like:
  • Ophthalmoscopes: for examining the eyes
  • Otoscopes: for examining the ears
• Endoscopes are used for viewing internal body cavities.
  • Cystoscopes: examine the bladder
  • Proctoscopes: examine the rectum
  • Enteroscopes: examine the small intestine
  • Colonoscopes: examine the large intestine
  • Hysteroscope: examine the uterus
  • Bronchoscopes: examine the air passages into the lungs.
• Some endoscopes are:
  • Rigid tubes with a light source and optical attachments to magnify tissues.
  • Flexible endoscopes made of fiber optics, providing access to areas unreachable by rigid endoscopes, these can also take tissue biopsies.

Transillumination

• Transillumination is the transmission of light through body tissues.
  • Detection of hydrocephalus: Light scatters in infants' skulls due to excess cerebrospinal fluid (CSF).
  • Detection of pneumothorax: Light indicates the degree of lung collapse in infants.
  • Recovering from Jaundice: Visible light (phototherapy) helps premature infants recover from jaundice by excreting bilirubin.

Applications of UV and IR Light in Medicine

• UV photons have higher energy used to sterilize medical instruments using UV at below 290 nm.
• UV light effects on the skin:
  • Beneficial: converts skin products into vitamin D
  • Harmful: causes tanning, and a major cause of skin cancer because it is easily absorbed by DNA
• UV light isn't visible in cells because it's absorbed before reaching the retina.
• IR rays focused by the eye can be hazardous when visible light is filtered, which can cause retinal burns.
• About half of sunlight's energy is in the IR region, and it produces warmth.
• IR light is better at heating deep tissues.
• Heat lamps with 1000-2000nm wavelengths are used for physical therapy.

IR Photography

• Two types of IR photography exist in medicine:
  • Reflective: Uses 700-900nm wavelengths to show veins.
  • Emissive: Uses long IR heat waves (14000-900nm) to measure body temperature (thermography).

Laser in Medicine

• Laser production involves population inversion.
• Laser operation relies on stimulated emission.
• Properties of stimulated emission:
  • Amplification: One photon in, two photons out.
  • Direction: Emitted photon travels in the same direction.
  • Coherence: Emitted photons are in phase.
• A laser emits a narrow, coherent beam of single-wavelength light and is an acronym for Light Amplification by the Stimulated Emission of Radiation.
• Laser energy is stored in the laser material.
• A laser beam can be focused to a small spot, reaching high power densities.
• Lasers in medicine can cause rapid temperature increases, destroying tissue.
• Tissue can withstand 70°C for 1s, but temperatures above 100°C cause destruction.

Types of Lasers

• Pulse Laser:
  • Ruby laser (λ = 694 nm).
  • Semiconductor (λ varies with applied current).
  • Glass laser.
• Continuous Wave Laser:
  • Neon-helium laser (λ = 632.8 nm)
  • Argon laser (λ = 488-514 nm).

Applications of Lasers

• Lasers are used as bloodless surgical knives.
• Lasers in eye surgery:
  • Used for photocoagulation of the retina to repair detachments by heating blood vessels.
  • A 1ms laser flash is efficient for welding the retina, pain-free, and requires no anesthesia.
  • Laser energy for photocoagulation depends on the spot size used.
  • The ophthalmologist determines the correct dose visually during treatment.
  • The minimum reactive dose (MRD) is the minimum laser energy that will damage the retina.
  • The MRD for a 50μm spot is 2.4mJ in 0.25s.
  • Photocoagulation needs 10-50x the MRD (24-120mJ for a 50μm spot in 0.25s).
• Lasers in dermatology for abnormal skin:
  • Surgical CO2 laser (10600 nm): for different skin or mucosal diseases
  • Erbium: YAG laser (2940 nm): for superficial cutaneous lesions and refreshing the skin.
  • Vascular: DYE laser (595 nm) and Nd:YAG (1064 nm or 532 nm) because their safe profiles and wide areas of use.
  • Aesthetic: removal of benign pigmented lesions, hair, tattoos, and patient resurfacing

Explores UV radiation for sterilization and jaundice treatment in newborns. Discusses transillumination effectiveness, wavelength relationships, and light source brightness assessment. Highlights safety aspects of infrared rays and skin cancer risks from prolonged sun exposure.