The Light
Properties of Light :
1. Its electromagnetic radiation .
2. Behaves both as a wave and as a particle .
3. Transverse wave .
4. The speed of light in vacuum  3 x 108 m/sec-1.
5. Refractive index ( n ) = C / V
C= speed of light in vacuum .
V= speed of light in medium .
6. Light divided into three categories :
a. UV – light ( wave length () < 400 m )
b. V- light ( = 400 – 700 m ).
c. If light ( ≥ 700 nm).
UV = Ultraviolet light .
V= visible light .
IR = Infrared light .
The light in Medicine :
Absorption of the light :
1. Absorption of the light produce chemical and
electrical energy .
Example :- ( light photon )  absobed in eye ( Cones
and reodes ) retina  chemical reaction  action
potential ( electrical signal)  brain .
2. Absorption of the light produce heat .
- Use IR in medicine to heat tissue .
- Use laser beam to weld and coagulate small blood
vessels in the retina .
3. Absorption of light might emitted a lower energy
light photon .
This property is known as flurescence .
Example: The basis of the fluorescent light bulb .
- Certain materials fluresce in the presence of uv light
and give off v light .
The amount of fluorescence and the color of the
emitted light depend on:1. The wave length () of the uv light .
2. Chemical comosition of the material .
- On way of flurescence is used in medicine is the
detection of prophyia , a condition in which the
teeth fluorescered when irradiated with uv light .
- In the fluorescent microscopes .
Reflection of the light :
There are two light :
a. diffuse reflection :
occur whe the light incident on the rough surfaces
and scatter the light in many direction .
b. Specular reflection :
Occurs from many smooth surface (Mirrors).
Measuement of the light and it units :
The three general categories of light , uv , v and IR
– are defined in terms of their wavelengh () .
measured in – micron 1= 10 -6 m
- angstroms 1A = 10 -10 m
- nanometer 1nm =10 -9 m
UV – subdivided :
UV-C
 100 to 290 nm .
UV-B
 290 to 320 nm .
UV-A  320 to 400 nm .
Photometric units :
- Visible light is measured in photometric units that
relate to how light is seen by the average human
eye.
- Illuminace : is the quantity of light striking the
surface .
- Luminance : is the intensity of light source .
Radiometric units :
- UV and IR measured in radiometric units .
- Irradiance :
The quantity of light strinking the surface ( W/m2).
- Radiance :
The intensity of light source ( W/m2 / solid angle) .
- Light is a form of energy :
V of energy 2 ev to 4 ev .
UV ? , IR?
X-ray in medicine , Energy 50.000 ev or 50 Kev .
The medical applications of the light :
The medical applications of V light :
1. Transillumination :
is the transmission of light through the tissues of
the body .
a. is used clinicolly in the detection of hydrocephalus
(water head ) in infants .
light  skull  excess of csf (cerebrospinal fluid )
 light is scattered to different parts of the skull

produces
patterns
characteristic
of
hydrocephalus .
b. is used to detect pneumothorax ( collapsed lung) in
infants .
2. The visible light in medicine is used to obtain visual
information about the patient , for example , the
color of the skin and the presence of abnormal
structures in the body .
3. number of iinstructments , called endoscopes are
used for viewing internal body cavities .
– cystoscopes :
are used to examine the bladder .
– proctoscopes :
are used for examining the rectum .
– bronchoscopes :
are used for examining the air passages into the
lungs .
flexible ( fiber optics )
– endoscop
rigid [ light source and lenses
for magnification ]
Flexible endoscope used to obtain information from
regions of the body that can not be examined with rigid
endoscope , such as the small intestine and much of the
large intestine . Flexible endoscope have an opening or
channel that permits the physician to take samples of the
tissues ( biopsies ) for microscopic examination .
– Cold light endoscopy , the light source contains
very little IR radiation by using IR absorbing glass
fliters .
– Opthalmoscope :
Is used for looking into the eyes .
– Otoscope :
Is used for looking into the ears .
– Phototherapy :
Use visible light for treating Jaundice in premature
infants .
UV – Light :
- It is more scattered than visible light because of its
wavelength ().
- More useful than Infrared radiation .
- UV can not seen by the eye, UV – light is invisible )
because :- It is absorbed before it reaches the retina .
a. Cornea absorbs most of the energy at  < 300 nm .
This is causes ( Corneal damages ).
b. Lens : strongly absorbs radiation of wavelength
below 380 nm and cause of Catorocts .
- UV-Light with  < 290 nm ( UV-C) is used to
sterilize medical instruments .
- The absorbption of UV-radiation in macromolecules
is dependent on :a. wavelength of UV – light .
b. amount of protein and DNA .
- UV – light produce reactions in the skin
a. solar UV – light  conversion molecular produce
in the skin to vitamin D.
b. UV – light – improves certain skin condition .
- The percentages of UV – light of different
wavelengths absorbed by the eye .
λ (nm) Absorption
< 280
300
iris
100%
92
6
2
45
16
37
14
12
38
48
52
34
Cornea
1
1
2
rotina
Lens
Infrared radiation :
- It is electromagnetic radiation .
-  between 760 nm – 1 mm (103 m) .
- divided into three regions :
a. IR-A  760 to 1400 nm ( It is most pentrating
radiation ) .
b. IR-B  1400 nm – 3 m pentrates only slightly
into tissue because its heavily absorbed by water .
c. IR-C  3 m - 1000 m is absored very
superficially and does not pentrate the eye or skin .
- half of the energy from the sun is in IR-region .
- IR – radiations are not usually hazards , but looking
at sun through a red filter can cause a burn on the
retina .
- Heat Lamps that produce a large percentage of IR
with   1000-2000 nm are used in physical therapy
for heat tissues .
Medical application :
IR – radiations are used in photography :
There are two types of photography :a. Thermography ( Emissive IR – photography ) it
depend on IR – emitted by the body .
b. Reflective IR – photography :
-It is used to show the changes in venous pattern
just below the skin .
-uses near IR of  ( 700 – 900 nm) . The incidence
IR on the skin pentrates about 3 mm below the
skin  The reflected IR give an image of the
venous pattern just below the skin .
The Laser
Laser :
Light Amplification by Stimulated Emission of
Radiation.
T. H. Maiman (1960) produced a laser beam from a
ruby material .
The properties of the Laser :
1. Spectral Purity :
The Laser ray is monochromatic light ( single
wavelength  )) the spectral purity depend on the
physical quantity , or  .
 ,  is the broadening of emission Line .
Δυ , Δλ
υ1
υ2
υ,λ
2. Directtionality :
The directionality of the laser beam is very
important in the collection of the laser energy at a way
distance .
θ
 = the angle of diverging  10—3- m radian .
3. Coherence :
A laser is a light that emits a narrow beam of a
single () in which each wave is in phase with the others
near it ( Coherent light ).
Temporal
Coherent
full Coherent
Spatial
4. The high bright .
5. The output of the laser :
a. CW / output of laser is constant with time
(continuous wave).
b. P/ pulsing wave , high power at very short time
( m sec ).
P
Power
Power
CW
Time (sec)
Time (Sec)
CW = continueous delivery
of energy
Time (msec)
Time ( mSec)
Pulsed = delivery of
energy in form of disreate
, the decration of pulsed
is less than (mSec)
Intraction the light with material :
Monochromatic light of intensity (I) passes through
medium of its thickness (T) .
I(T)
I
0
T
I (T)  -I (T) T
I (T) = - a I (T) T ………………..(1)
Where:
I(T) = intensity at thickness T .
a = absorption coefficent
dI (T) = - a I (T) dT ……………….(2)
dI (T )
 adT
I (T )
I
T
dI
Io I  a 0 dT
I
Ln I
=-aT
I0
Io at thickness 0
I at thickness T
Ln I – ln Io = - a T  I = Io e – a T …………(3)
I
T
If we consider molecular or atomic system with two
energy state Eo , EI .
Eo is the ground energy state .
No no. of atoms in Eo
E1 excited energy state .
N1 no. of atoms in E1 .
N1 , 1
E1
Photon hν
E0
N0 , 0
absorption
1 mean life of the state E1 .
o mean life of the state Eo .
N1 , No is the population of the state in natural , all
the atoms in Eo and o > 1 , Eo < E1 , No > N1
E1
After the absorption with
photon of its energy > =
E1- Eo
E0
E1
pontaneous emission
E0
- absorption
spontaneous emission
- Emission
Stimulated emission
E1
E1
E0
E0
E1- E0= hν
This is very important for the lasing .
The laser action` ( The lasing ):
The laser production :The basic components of the laser :
1. active medium .
2. resonator .
3. pump source .
Optical cavity
Active medium
100% Total
reflection
Pump Source
Partial or output
reflector
Active medium :
Is a collection of atoms or molecules ( Carbon
dioxide , Argon , Dye , Ruby ,………..) is excited by one
method of pumping to the upper energy state in order to
find the population inversion in the active medium .
When a small percentage of the spontaneous radiation
will travel along the axis of the laser cavity , these
photons will collide with atoms in the excited state ,
thereby stimulating the emission of radiation as further
collisions of photos and excited atoms occurs more inter
cavity energy develops . The intensity of these energy is
amplified by reflections between the parallel mirrors . At
partial reflector apportion of the energy is permitted to
escape out of the cavity this energy is in the form of an
intense beam of Coherent light ( laser ).
Method of pumping :
The method to find the population inversion in the
medium active :
1. optical pumping .
2. electrical pumping .
3. chemical pumping .
4. thermal pumping .
The kind of the laser :
1. Soild state laser :
– Ruby laser [ Al2O3 with Cr+2 ,  = 692.7 nm
(Red) ].
– Nd – laser .
2. Gas laser :
– He – Ne laser ,  =633 nm (Red laser).
– Ar+ laser
3. Liquid laser :
– dye laser .
4. Chemical laser :
– HF laser .
5. Semiconductor laser :
– GaAs laser .
The pumping scheme ( process ) :1. Two – level scheme :
When using electromagnetic radiation with high
intensity in the pumping process , produce the state of
saturation .
 the population inversion are equal in the two state
of energy .
E2
E1
E0
N
E
Energy (E)
2. Three level:
Boltzman distribution with out pumping .
Pumping
E2
E1
Upper Laser Level
Laser Light
E0
N
E
3. Four level laser:
Pumping
E3
3
E2
E1
E0
2
Laser Light
1
0
- the atoms from 0 state to 3 state by pumping .
- select special medium to give fast decay from state
3 to state 2 & from state 1 to state 0 .
- population inversion both state 2 & 1 .
- state 2 is the upper laser level .
- state 1 is the lower .
Ex: Neduim laser :
- the efficiency of population inversion in four level
process is very high than 3 level process .
The medical application of the laser :
The physical characteristics of the laser light in
medicine application :
- Coherent light .
- Monochromatic light .
- A laser beam can be focused to a spot only few
microns in diameter , the mean remains narrow over
a long distance .
- All of the energy of the laser concentrated in a
small area .
 The power density :
Pd = power / area
If area of spot is small
Pd is very large .
 the total energy of the typical laser pulse used in
medicine can be delivered in less than s , and the
resultant instantaneous power may be in megawatts .
The using of laser beam depend on :
1- the power
2- time of the exposure the
power is a number of photons arriving per unit time
(Sec) .
W= J / Sec
- power density ( Pd ) Pd = P / area
laser beam at the area of interaction area ( dia of the
spot size of laser beam ) .
Pd 
P 4P

A D 2
Where D is dia of spot size .
Medical application :
- photocoagulation .
- detachment retinal .
- in surgery .
- interal medicine .
- red spot on the skin of human .
In opthal malogy lasers are primarily used for
photocoagulation of the retina , that is , heating blood
vessel to the point where the blood coagulates and blocks
the vessel .
Disadvantages of the xenon are for photocoagutania:1. the retinal spot size is much larger ( 750 m) than
that formed with a laser beam ( 30 . m).
2. the total amount of energy deposited in the eye is 20
to 50 times greater than that deposited during laser
treatment .
3. Long exposure ( up to 1 sec ) than the laser .
The photocoagulation :
Depend on :
1. The laser energy .
2. Spot size of laser beam .
3. The time of exposure .
The of the treatment
MRD = Minimal reactive dose = the minimum
amount of laser energy that will do observable damage to
the retina ; ex :- MRD for a 50 m spot in the eye is
about 2.4m delivered in 0.25 sec for photo – need to 50
time of MRD.