Test 15  (Laser)

1. The following are properties of laser:

a. it is entirely monochromatic

b. all the photons are in phase

c. all the photons have the same wavelengths

d. the waves of light are parallel

e. the distance between the mirrors within a laser tube is a 
    multiple of the wavelength of the light emitted.

Answera. F   b.T   c.T   d.T  e.T
Laser is virtually monochromatic but not entirely so. 
It is coherent i.e. photons have the same wavelengths 
and in phase and collimated ie. the light waves are parallel. The laser tube contains two mirrors and the distance between them is a multiple of the wavelength of the light emitted.

2. The following are true about the modes of laser output:

a. both Q-switched and mode-locked mode increases the 
    laser energy by compressing the energy in time.

b. mode-locked produces more power than either 
    continuous or Q-switched mode

c. the duration of a pulse of laser produced by 
    mode-locked mode is shorter than that of Q-switched 
    mode

d. in Q-switched mode, a shutter is placed inside the laser 
    tube to limit energy loss.

e. laser from continuous mode has a constant power and 
    is measured in watts.

Answera.T   b.T c.T  d.T e.T
There are three modes of laser: continuous, Q-switched and mode-locked. Laser from continuous mode has a constant power and is measured in watts. Q-switched and mode-locked increases the energy by compressing the energy in time and the energy is best measured in joules. Mode-locked laser compresses the laser more than Q-switched laser and therefore produces more energy. 

3. Regarding the Nd-YAG laser:

a. it emits infrared radiation

b. it is invisible

c. doubling its frequency makes it suitable for retinal 
    photocoagulation

d. it is usually mode locked when used to treat the eye

e. it is used to create iridotomy
 

Answera.T   b.T   c.T   d.F   e.T
Nd-YAG produces an infrared radiation with a wavelength of 1064nm. Another laser He-Ne is used to produce the red aiming beam. Doubling its frequency makes it suitable for photocoagulation. It is usually Q-switched when used to treat the eye. It is used to treat posterior capsular thickening following cataract surgery and iridotomy for narrow angle glaucoma. 

4. Regarding argon laser:

a. it produces a wavelength of between 488-514nm.

b. argon blue laser is the preferred choice during 
    photocoagulation.

c. the xanthophylls in the macular absorbs argon green 
    better than argon blue laser.

d. a white burn is the end point during panretinal 
    photocoagulation.

e. a higher energy is required for lighter than pigmented 
    fundus.

Answera. T   b.F   c.F   d.F   e.T
Argon laser can produce both blue and green light.
The new argon laser limits the emission to green light which is not absorbed by the macular xanthophylls and therefore less damaging to the macula of both the patients and the doctors. During photocoagulation, the laser should just blanch the retina instead of heavy or white burn which increases the risk of visual field loss and reduced dark vision. Argon laser is well-absorbed by melanin and a pigmented fundus requires less power than a light fundus.

5. The following laser investigations can be used to monitor the 
     progression of glaucoma:

a. confocal microscopy

b. laser interferometry

c. confocal scanning laser tomography

d. scanning laser polarimetry

e. laser Doppler flowmetry

Answera. F   b.F   c.T   d.T   e.F
Confocal microscopy is used to study the cornea. Laser interferometry tests the potential visual acuity of a patient with dense cataract. Laser Doppler flowmetry measures retinal capillary blood flow.
Laser interferometry measures the thickness of the retinal fibre layer and can be used to monitor the progression of glaucoma. Confocal scanning laser tomography gives the topographic map of the optic nerve head and can be used to monitor glaucoma damage. 

6. True statements about laser safety include:

a. laser with output energy of 5mW or more is damaging 
    to the eye.

b. lasers of class 3a and above are detrimental to human
    eye.

c. argon laser used for retinal photocoagulation is class 3b
    laser.

d. the eye is more sensitive to laser damage than the skin

e. safety goggles are not necessary for visitors during 
    YAG capsulotomy.

Answera.T   b.F   c.T   d.T   e.F
The International Safety Classification of Lasers divides the lasers into 4 groups. Group 3 is subdivided into 3a and 3b. Class 3b and above is damaging to the eye and their powers are 5mW and above. All lasers used in ophthalmology are classed as 3b and above. Safety goggles should always be worn by people in the vicinity.

 

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