Optical Parameters
N(r,ŝ): At a given point r, is defined as the number of photons per unit volume
moving in the direction of unit vector ŝ.
Radiance: Used to describe the propagation of photon power. Is related to power
flowing through infinitesimal area , located at r, in the direction of unit vector ŝ. Unit
Fluence rate: At a given point in space, the radiant power incident on a small
sphere, divided by the cross sectional area of that sphere.
Radiant Energy: Delivery of energy described in terms of radiant power, power
emitted or transferred or received as radiation.
Radiant Emittance: Energy radiant flux leaving an element of the surface
divided by the area of that element.
Irradiance: At a point of a surface , the radiant energy flux incident on a element
of the surface divided by the area of the surface.
Source term: The amount of photon energy per second absorbed locally in
tissue. Unit [W/m3]
Laser tissue Interaction
Different types of lasers react differently with tissue.
Interaction depends on:
•The wavelength of the laser
•Power density and exposure time
•Optical and thermal property of the tissue being irradiated
•Laser beam size on the tissue
•If exposure is CW or pulsed wave radiation
Some common interactions….
Causes dielectric breakdown in tissue caused by shock wave plasma
expansion resulting in localized mechanical rupture.
Causes photo dissociation or breaking of the molecular bonds in tissue.
Converts light energy into heat energy. This causes the tissue to heat up
and vaporize.
Very low-power irradiation inactivates cell function by means of induced
toxic chemical processes
Causes target cells to start light-induced chemical reactions.
Laser-tissue interaction – absorption:
Absorption spectra of main absorbers: melanin in skin and
hemoglobin (Hb/HbO2) in blood
•The broad absorption band of water in the IR can also be used to
transfer optical energy into kinetic energy (heat) in tissue
•In the visible range, light absorption by water is low, which can be
used for in vivo interrogation of tissue
•In UV, light scattering in tissue increases significantly - less
penetration depth in tissue
Laser-tissue interaction – thermal
Thermal effects of laser radiation (raised temperature in tissue)
Note that protein and collagen denature at around 60°C
Heating tissue at 60°C can lead to cell necrosis - can be used to
kill cancer cells
Thermal effects…..
Can also be used to join or bond soft tissues such as blood
vessels, skin, nerve fibers, various tubes/tracts etc.
In Helium laser Lithotripsy, the principal interaction is thermal
Laser-tissue interaction – Laser ablation
•With enough photon energy, laser can be used to ionize molecules in
biological tissue.
•Even better, tissue can be removed very precisely without any appearance
of thermal damage using short pulses of laser
•The absorption of laser energy in tissue follows Beer-Lambert’s law
•There exists a threshold at which the absorbed energy is high
enough to cause decomposition of tissue
•Ultra short laser pulses can be used to cut biological tissue into thin slices
without any previous preparation of the tissue.
•Focusing ultra short laser pulses on biological tissue, optical breakdown
Occurs within the focus of the laser pulse
•Laser eye surgery - Ablation of corneal tissue is achieved
by using a pulsed ArF laser (λ=193nm)
•Wrinkle removal (skin resurfacing) and pigment removal from skin
Lasers in Ophthalmology
(laser-assisted in situ
LASIK is a painless,patchless 15
minutes procedure which aims at
permanently altering the focusing
power of the eye by reshaping the
corneal surface with the laser light.
1. Anesthetic drops are used
and a lid speculum is used
to hold your eyelids wide
2. No injections are used and
you will not feel any pain,
only some discomfort may
be felt.
3. A ring is placed on your
eye and a slight pressure
will be applied to create
4. Your doctor will then
make a corneal flap.
5. The suction ring is
6. The doctor will then lift the
flap and hold it back on its
hinge and dry the exposed
7. The laser beam will then
be focused over your eye
and you will be asked to
stare at a green blinking
light. This light is to help
you to fixate your eyes on
one spot, once the laser is
8. The laser light will reshape
the cornea.
9. A computer controls the
amount of laser energy
delivered to your eye.
10. The flap is put back into
11. A black goggle will be
placed over your eye at
the end of the procedure
to protect your eyes
12. There are no stitches.
Therapeutic and
Diagnostic applications
of Lasers in
Transmission and Absorptive properties
of Ocular tissues
- Light entering the eye can be reflected,
transmitted, scattered or absorbed.
-In the visible and the near infrared
spectrum the absorption characteristics of
ocular tissues is determined by group of
chromophores within the tissue.
-Ocular chromophores include melanin
located in the retinal pigment epithelium,
iris pigment epithelium, hemoglobin
located in red blood cells within blood
- Due to its transmission characteristics cornea provides effective window for
vision, photocoagulation, photodisruption, imaging and examination of
intraocular structures.
Photothermal Laser applications
A] Mechanisms of photothermal laser tissue interaction:
1. Occur when laser energy is absorbed by target tissue and converted into heat.
2. Low temperature elevations cause cell damage without causing structural
alterations to the tissue (photoheating).
3. Greater temperature elevations cause thermal coagulation of tissue with cellular
death and structural damage to the tissue (photocoagulation).
4. To minimise unwanted thermal damage, a wavelength should be selected that is
preferentially absorbed by the target tissue and the laser exposure duration
should be shorter than the thermal relaxation time of the tissue which is given by
Clinical and research applications for photothermal lasers in ophthalmology
Photodisruptive laser tissue interaction
Photochemical Laser applications:
Photoablation and photodynamic therapy
Photodynamic therapy
-Utilizes a selective laser wavelength to activate a photosensitizing agent that in
turn causes damage or destruction to malignant or abnormal tissue.
-A sensitizer is a molecule or compound that produces singlet oxygen or free
radicals when irradiated by a particular wavelength.
-Phototoxicity results from the biochemical interaction of tissues with the singlet
oxygen and radicals and is a function of photosensitizers quantum efficiency and
absorption spectra, its concentration in tissue and wavelength and fluence of the
-Hematoporphyrin derivative
-A complex mixture of monomeric and aggregated porphyrins .
-It is a tetrapyrrole aromatic macrocycle related to heme
-Protoporphyrin IX absorbs light, exciting it to an excited singlet state.
-Resulting in excited triplet protoporphyrin IX
-Energy is transferred from triplet protoporphyrin IX to triplet oxygen, resulting in
excited singlet oxygen.
-Singlet oxygen reacts with biomolecules, fatally damaging some cells in the
treatment area.
1. Visual Acuity Measurement:
-Spatial coherence of lasers is used.
-The retinal functional health is evaluated when it is obscured by cataract.
-Helpful in deciding whether or not cataract surgery will be benefit to the patient.
2. Laser Doppler Velocimeter:
-Light scattered by moving blood cells is shifted in frequency and is used to
measure the rate of blood flow in the veins and arteries of the retina.
-Temporal coherence of lasers is used.
- Useful in viewing retina and its supporting structures including blood vessels,
nerve bundles and underlying layers.
4. Spectroscopic diagnosis of Ocular diseases:
-Uses fluorescence spectroscopy and Raman spectroscopy to detect various
abnormal states of ocular tissues and ocular diseases.
- Understanding of biochemical changes associated with cataract formation.
Lasers in Dermatology
The choice of laser depends on the colour, depth and chemical nature of the
tattoo ink.
Two to ten treatments are often necessary.
Black: QS ruby, alexandrite or Nd:YAG
Blue and green*: QS ruby, alexandrite
Yellow*, orange*, red: QS Nd:YAG or PDL
Tatoo Removal
QS Ruby laser
QS Alexandrite laser
QS Nd -YAG laser
-A port-wine stain is a vascular birthmark consisting of superficial and deep
dilated capillaries in the skin which produce a reddish to purplish discolouration of
the skin.
-The flash lamp pumped dye laser, a yellow light laser, has been the most
successful at destroying stains in infants and young children. The neodymium YAG
laser is used to treat the nodules that may develop in some adult port-wine stains.

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