teorie aplicatii llt in medicina

7/26/2019 TEORIE APLICATII LLT IN MEDICINA

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1. WHAT IS WORTH KNOWING ABOUT LASER PHYSICS AND THEIR

RADIATION

1.1. The basic information on optical radiation

Laser is known as the device generating light of specific extraordinary

properties. To our understanding light is the electromagnetic radiation which emits

both visible and invisible for the human eyes rays. Physicists consider this radiation

as a waveform or as a beam of photons. According to the first mentioned above

theory, the electromagnetic wave is characterised by its length, freuency and

amplitude. !ave amplitude is a measure of emitted power and luminous intensity.

!ave freuency is connected with so"called wave period. This relationship results in

the following euation#

f $ 1 %T

A scheme of &wave& fragment with draw of amplitude A and term T is shown in 'ig. 1.

P e r i o d ( T )

A m p l i t u d e ( A )

T i m e

'ig. 1. (asic parameters characterising the wave.

The oscillation period )T* is usually measured in seconds while oscillation

freuency in units called +ert, abbr. +. The unit of freuency measure, 1 + means

one pulse )a single wave oscillation* emitted within 1 sec. -n other words 1 + is an

inverse measure of wave period i.e. 1%s. !avelength means the distance a wave

disturbance covers within one full period. A wavelength of laser radiation is very often

measured in meter derivative unit called nanometer )abbr. 1 nm*.

anometer is a milliard part of meter. -n practice other derivative units of measure of

wavelength and freuency are used. The most typical ones are presented in Table 1.

/


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Tab. 1. The most ty!"a# $%!ts o& meas$'e $se( to e)'ess *a+e#e%,th a%(

$#se 'eet!t!o% &'e-$e%"y

!avelength 'reuency

1cm $ 10"m $ 0.01m

)1 centimetre*

1k+ $ 102+ $ 1000+

)1 kilohert*

1mm $ 10"2m $ 0.001m

)1milimetr*

13+ $ 10/+ $ 1000000+

)1 megahert*

14m $ 10"/m $ 0.000001m

)1 micrometer*

15+ $ 106+ $ 1000000000+

)1 gigahert*

1nm $ 10"6m $ 0.000000001m

)1 nanometer*

1T+ $ 101+ $ 1000000000000+

)1 tetrahert*

7ome other important relationships between wave parameters are considered below.

-t should be remembered that if a wave moves in a medium at speed )v*, the

relationship between the wavelength and speed )v* is as follows#

$ v t or $ v 1%f

As regards the electromagnetic radiation, the above euation can be modified by

replacing the radiation speed with the speed of light, which is usually indicated by

symbol c. -n case of vacuum the euation looks as follows#c $ c0 $ f

-t should be remembered that the speed of light in vacuum is 66869:9 m%s i.e.

about 2x10;m%s. The ratio of speed of light in vacuum )c 0* to speed in material

medium )c* has been called the index of refraction of this medium )n $ c0%c*.


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U l t r a v i o l e t

V i o l e t

B l u e

G r e e n

Y e l l o w

O r a n g e

R e d

I n f r a r e dr a d i a t i o n

V

is

i

le

lig

!

t

T

!

e

w

a

v

e

le

n

g

t!

o

f

la

s

e

r

o

p

e

ra

tio

n

R a d i o w a v e s

I n f r a r e dr a d i a t i o n

V i s i l e r a d i a t i o n

U l t r a v i o l e tr a d i a t i o n

" # r a d i a t i o n

G a m m a r a d i a t i o n

$ o s m i %r a d i a t i o n

& ' ' ' T ( )

& ' T ( )

& ' ' G ( )

& G ( )

& ' * ( )

+ , - n m

. ' / n m

, ' ' n m

0 ' ' n m

- ' ' n m

/ ' ' n m

& . ' m

+ ' n m-

f 1 2 1 n m 2

'ig. . -llustration of wavelength and freuency range for known kinds of radiation >

from radio waves through visible light up to cosmic radiation ?efinition of

radiation ranges including laser light

The 'ig. shows distinctly that wave freuency is inversely proportional to the

wavelength. As freuency increases the wave becomes shorter. The visible range

covers only a small part of wide field of the electromagnetic radiation spectrum

known in the nature. The light wavelengths visible to the human eye are in the range

2;0%900"800%8/0 nm. The human eye perceives a change of wavelength in this field

as the change of colour. The wavelengths exceeding 8/0 nm )more often 800 nm*

belong to the invisible infrared area )-@*, whereas the wavelengths less than 2;0 nm

)rarely 900 nm* are in the invisible ultraviolet area )


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Tab. . T*o *ays o& ty!"a# se"t'$m (!+!s!o%

Physical Photobiological

< distant ultraviolet

)1"10nm to 1;0nm*


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P $ C%t

The radiant power is measured and specified in watts )!*. The basic units derived

from 1 ! are as follows#

1 m! $ 0.001! $ 10"2!

1 k! $ 1000! $ 102!

The relationship between the radiant power and energy is therefore obvious#

1! $ 1D%1s or 1D $ 1! 1s

The most important task of laser therapy is to deliver adeuate energy dose to the

proper tissue area.

(elow there are three examples of calculating the same energy dose eual to 1 D for

three different power values of laser radiation#

1 D $ 10 m! x 100 sec

1 D $ 0 m! x :0 sec

1 D $ 90 m! x : sec

The lasers most commonly used in the present therapy have the following radiation

power# 10 m!, 0 m! and 90 m!. The calculation of radiation power density )P?*

and radiation energy density )C?* are more important criteria for physicians using

lasers in their practice. Power density is defined as a power density in a laser beam

incident to a treated tissue. -t can be calculated by dividing the initial power )P* by

laser radiation delivered to the surface 7 of volume < of the tissue.P?s $ P%7 or P?v $ P%

teorie aplicatii llt in medicina

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