Glossary of Terms


Common terms used in reference to laser technology and specific conditions and treatments.

Ablation
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Tissue removed by laser vapourisation (phase transition from liquid to gas by boiling).
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Absorption
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Photoabsorption of energy by tissue. Linear for visible absorption spectrums. Energy is transferred to tissue.
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Absorption coefficient
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Attenuation coefficient. Characterises how easily a material can be penetrated by light.
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Absorption peak
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Maximal absorption of chromosphere on absorption curve.
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Active medium
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Medium by which the laser is known (eg CO2 laser).
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ALA
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Aminolevulinic acid – photoactive porphyrin (5-ALA or methyl-ALA).
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Angioma
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Capillary haemangioma – mass of tiny blood vessels – flat or raised.
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Attenuation
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Light in tissue dissipates progressively. Attenuation coefficient (absorption coefficient).
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Beam delivery
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Laser beam by way of articulated arm by mirrors or via fibreoptic cable.
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Beers’ law
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The Beer-Lambert law states that there is a logarithmic dependence between transmissivity of light through a substance and the product of the absorption coefficient and the path length.
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Biologic effects
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Tissue effects of light which can be photothermal, photomechanical or photochemical.
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Biostimulation
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Low intensity lasers in the red/IR range may produce beneficial microcirculatory effects to stimulate cellular processes.
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Caviation
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Formation of a cavity eg by ‘Q’ switched or other very short pulsed wide lasers.
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Chromophore
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A part of the molecule responsible for its colour. The colour is seen when that molecule absorbs certain wavelengths and transmitted or reflects others.
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Class
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Referring to lasers (medical) – Class I – IV.
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CO2 laser
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Carbon dioxide lasing medium giving a far infrared (10600nm) output.
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Coagulative necrosis
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Cell death with denatured proteins (structural and lysosomal) caused in this context by light induced tissue heating >62°C.
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Coherent
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Waves in phase (laser).
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Collimated
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Waves parallel – no convergence or divergence.
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Continuous wave (CW)
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Laser beam present continuously while in operation. Sometimes pulsed CW.
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Corneal shields
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Eye protection placed directly onto cornea under topical anaesthesia.
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DCD
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Dynamic cooling device (cryogen spray pre-treatment of skin).
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Defocussed
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In clinical laser practice a beam can be ‘defocussed’ by a ‘pullback’ technique.
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Delivery System
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Transmission of EMR to treatment site - optical fibres for shorter wavelengths up to 1500nm, articulated aims generally beyond that.
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Density
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Referring to the density (percentage coverage) of microthermal zones in fractional photothermolysis.
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Dermis
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Deeper layer of skin giving structural strength and containing blood vessels and adnexae.
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Divergence
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Laser beams have low divergence - ideally no divergence. Beams are parallel.
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Ectatic
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Dilatation of hollow structure (vessel) – from the Greek.
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Electromagnetic Spectrum
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Range of wavelengths from ultraviolet to radio waves.
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EMR
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Electromagnetic radiation (includes UV, visible light and IR wavelengths) as well as X-rays and radio waves.
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Energy
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Refers to radiant energy of electromagnetic waves. The unit of energy is the joule (J). The unit of energy per unit time is a watt (W) which is power.
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Epidermis
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The outer layer of skin. The thickness of the epidermis varies depending on anatomical site.
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Er: glass
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Erbium doped glass (silicate/phosphate) laser emitting at 1550nm.
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Er:YAG
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Erbium doped YAG (Yttrium aluminium garnet) laser emitting at 2940nm.
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Erythema
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Redness (of the skin).
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Excimer laser
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‘Excited dimmer’ laser utilising noble gas halides to deliver a number of different UV wavelengths. XeCe @ 308nm.
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Excitation
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Gas lasers are excited electrically and solid-state and liquid lasers are optically pumped.
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Flash lamp
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Flash tube electric are lamp producing intense, incoherent full spectrum white light for very short durations through ionisation of xenon gas.
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Fluence
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Radiant exposure (energy per unit area – normally joules cm2).
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Focal length
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Distance between lens and focal point.
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Focussed
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Collimated laser light can be focussed through optical lenses.
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Frequency
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Temporal frequency is the number of occurrences per unit time and is the reciprocal of the period (as in wavelength and frequency).
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Gating
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Mechanical structure to produce a pulsed beam.
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Gaussian
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Bell curve shape characteristic of most laser beam output profile. Not suitable for ‘Q’ switched energy delivery.
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Hazard Classification
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Direct and indirect viewing hazardous – applies to all class 4 lasers.
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Holmium
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A rare earth element used to ‘dope’ lasing media – often together with other rare earths such as Erbium and Thulium.
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Hyperpigmentation
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Excessive colour often after inflammation (PIH) post inflammatory hyperpigmentation. May be a racial tendency.
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Hypopigmentation
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Loss of skin pigmentation. May be post injury, postinflammatory,immunological, genetic or idiopathic.
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Intensity
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Power density or irradiance.
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Irradiance
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Fluence per unit time. Power delivered per unit area (power density).
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Keloid
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Nodular mass of scar tissue which continues to develop over time and spreads beyond the initial site.
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Kirchhoff’s laws
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A hot object produces a continuous spectrum - a hot rarefied gas produces an emission line spectrum. A cool gas in front of a continuous light source produces an absorption line spectrum.
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KTP
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Potassium titanyl phosphatase (frequency doubling crystal) used to halve Nd:YAG 1064nm wavelength to 532nm.
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KTP laser
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532nm (wavelength halved) Nd:YAG laser at 1064nm.
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Laser
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Light amplification by stimulated emission of radiation (an acronym).
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Laser medium
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Solid state (eg Nd:YAG), gas (eg CO2) or liquid (eg dye). Produces light of a particular wavelength.
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Lentigines
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Pigmented epidermal macules – sun exposed surfaces – benign.
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Melanin
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Pigment from active melanocytes (skin type dependent) eumelanin and pheomelanin.
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Melanocytic naevi
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>0.5mm diameter (usually acquired), otherwise lentigines.
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Melanosomes
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Melanin containing organelles (giant in café au lait macule - CALM).
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Microsecond
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10-6 seconds (0.000001 second).
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Microthermal zone
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MTZ is the submillimetre pathway through tissue in fractional photothemolysis.
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Millisecond
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10-4 seconds (0.0001 second).
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Monochromatic
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The coherent light emitted by lasers (distinct from polychromatic) is of an individual (pure) wavelength.
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Nanosecond
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10-9 second (0.0000000001 seconds).
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Nd:YAG
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Neodymium: yttrium aluminium garnet. Solid state lasing medium which emits radiation at 1064nm.
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Optical coupling
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With a light skin can be enhanced with water, alcohol, gel or oil as well as used with dermoscopy.
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Optical density
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(OD). A measure of the extent to which a substance transmits light.
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Optical interface
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At which light passes from one medium to another.
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Photoablation
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Light induced ablation/vapourisation of tissue which may occur with no thermal effect (Excimer laser).
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Photoacoustic
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Light that generates an acoustic shock wave in tissue (akin to thunder following lightening).
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Photochemical
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Light induced chemical change in tissue following absorption of visible, UV or IR radiation.
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Photochemotherapy
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PUVA. Oxsorolen is ingested and acts as a photosensitiser with UVA.
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Photodynamic therapy
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(PDT). Aminolevaline acid is activated by various visible light sources.
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Photomechanical
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Light induced mechanical (fracture) change in tissue utilising extremely high power densities delivered in extremely short pulses.
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Photon
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An elementary particle, the quantum of light and other forms of electromagnetic radiation. Characterises light – tissue reactions at a molecular level.
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Photothermal
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Light induced heat change (various types) in tissue.
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Plank’s Law
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Energy is proportional to frequency and inversely proportional to wavelength. High temperature objects emit high energy photons (shorter wavelengths) E=hc/λ.
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Poikiolderma of Civatte
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Pigmentation and telangiectasia (sides of neck) sun. Induced/genetic.
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Port wine stain
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Vascular malformation – complex heterogeneous congenital vascular lesion.
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Power
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Rate of energy delivery watts (W) = J/s. Overall ‘strength’ of laser beam.
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Power Density
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This is critical to effect on tissue with increased power through lens focusing.

Power / Beam Area = W/cm2
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Pulse repetition
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Rate of pulse generation (per second – Hertz).
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Pulse width
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Pulse duration measured by photodiodes. Determines the rate of delivery of laser power.
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Pulsed dye laser
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(PDL). A rhodamine dye solution is used as a lasing medium. Various wavelengths can be emitted, depending on additives. The Vbeam Perfecta™ emits at 595nm.
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Pulsed mode
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In contrast to continuous mode. Modern medical lasers are pulsed. Energy is released over very short time spans.
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Quality switched
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‘QS’. Rapid gating of laser output enables high peak power over very short periods leading to photomechanical effects.
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Reflection
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Photons reflected from the skin’s surface.
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Refraction
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Photons knocked ‘sideways’ within the skin.
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Remittance
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Photons sent back out through the skin and ‘lost’.
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Repetition rate
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Hertz rate of laser pulsing.
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Resonator
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Laser cavity/tube. Where the laser energy is produced.
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Rhodamine
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Lasing medium for pulsed dye laser.
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Scatter
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Deflection from a straight path through tissue - changes in direction.
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Selective photothemolysis
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Chromophore absorbs strongly with sparing of adjacent structures.
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Spatial coherence
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Describes the correlation between signals at different points in space.
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Spectral Lines
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Each element produces its own unique set of spectral lines when it emits light. Spectral lines occur when an electron jumps from one energy level to another and is the result of interaction between a quantum system and a photon.
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Spectroscopy
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Analysing components of light received from objects. Can also determine velocity (redshift).
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Spot size
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1mm+ up to 12mm for most current cutaneous lasers.
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Stimulated emission
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Is the process by which an atomic electron interacts with an electromagnetic wave drops to a lower energy level thereby transferring its energy to that field.
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Telangiectasia
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Fixed, dilated vessels (x Greek).
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Thermal
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The thermal mechanism involves conversion of laser energy into heat.
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Thermal relaxation time
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’T’. Time taken for a chromophore to cool to 50% of its base state.
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Thermoelectric cooling
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(TEC) System for contact cooling, which utilises the heat flux between two different metals. Also known as a Peltier device.
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Top Hat
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Flat topped beam profile giving the optimal shape for power output in many laser applications. May be made up of micro-pulses or pulse trains as in the pulsed dye laser.
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Transmission
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Where no energy is lost during passage through tissue.
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Transmissivity
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Measure of the ability of a material to transmit EMR.
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Wavelength
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The special period of a sinusoidal wave.
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Wien’s Law
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The dominant wavelength of radiation emitted by a blackbody is inversely propositional to its temperature. λmax = 2.9 x 103/T.
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Xenon arc
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Bright white light produced by passing electricity through ionised xenon gas. Mimics sunlight and has multiple absorption spectra.
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