Lasers

Question 1

Key laser properties

Answer 1

One wavelength (monochromatic)
One direction (small diversion)
One phase (coherent)

Question 2

Range of lasers used clinically

Answer 2

193nm (UV) - 10600nm (IR)
Wavelength is critical as the effects and penetration depths will be different

Question 3

Alternatives

Answer 3

Intense Pulse Light Source - used for hair removal. Broad band, only pulsed, non-coherent.
LEDs: broadband, pseudo continuous, non-coherent
Both chepear and can do larger area

Question 4

Photon path length

Answer 4

l = 1/a
a is absorption coefficient

Question 5

CO2 laser

Answer 5

10600nm
penetration depth <0.1mm
Creates thinner band of tissue damage than scalpel

Question 6

Nd-YAG laser

Answer 6

1064nm
PD in soft tissue ~3mm
Used in cutting and coagulation. Gum surgery one application

Question 7

Pulse dye laser

Answer 7

In theory use at different wavelengths eg 595nm
Penetration depth in blood 0.1mm Used in treatment of rosacea

Question 8

KTP laser

Answer 8

532nm
Penetration depth in blood 0.1mm
Used in retinal surgery - focuses onto structures at back of eye and passes through front

Question 9

Excimer laser

Answer 9

193nm
PD corneal tissue 0.1mm
Used in corneal resurfacing.

Question 10

Laser testing - what do we want to test

Answer 10

Most important - power, pulse energy, pulse length
Alignment, emergency stop, uniformity

Question 11

How do we measure power of laser

Answer 11

Thermopile: bank of thermocouples has hot and cold end, heat gradient generates voltage gradient and indicates heat rise.

Question 12

How do we check shape/alignment of laser

Answer 12

Thermal paper

Question 13

Why is eye particularly sensitive to laser light

Answer 13

Focuses light on retina with magnification factor of 10^5

Question 14

Order of things in eye

Answer 14

Cornea
Lens
Retina
(going deeper

Question 15

What UVs are dangerous for what part of eye

Answer 15

UVC/UVB interact with cornea
UVA with lens

Question 16

Order in skin

Answer 16

Epidermis
Dermis
Subcutaneous tissue
(going deeper)

Question 17

How does UV interact with skin

Answer 17

UVC/B penetrate to epidermis
UVA penetrates to dermis

Question 18

Health effects of UV/V/IR

Answer 18

UV: burn, incrased risk cancer, skin aging, photosensitive reactions but production of vit. D.
Visible: photosensitive reactions, skin burn, gives vision.
IR: thermal burn, retina and cornea. Warmth

Question 19

Continuous vs pulsed

Answer 19

Pulsed laser may have same output but higher peak
Continuous: watts (J/s), irradiance (W/m^2)

Pulsed: joules (J), radiant exposure (J/m^2)

Question 20

Laser classes

Answer 20

1, 1M, 1C, 2, 2M, 3R, 3B, 4

Question 21

MPE

Answer 21

maximum permissible exposure: level of laser radiation one can receive repeatedly without injury

Question 22

NOHD

Answer 22

nominal ocular hazard distance, distance to laser at which exposure is equal to MPE_eye. Can still have temporary hazards beyond this.

Question 23

NOHD eqs

Answer 23

NOHD = (2/theta).sqrt(P/pi.MPE)
theta is beam divergence - laser

NOHD = (2/AA).sqrt(P/pi.MPE)
AA = acceptance angle - fibre

Question 24

Main legislation

Answer 24

The Control of Artificial Optical Radiation at work Regulations 2010.

Ensures risk assessments done, HSE responsible.

Question 25

Hazard vs risk

Answer 25

Hazard: something that can cause harm
Risk: how likely it is that hazard is realised

Question 26

Risk assessment steps

Answer 26

1. Identify hazards
2. Decide who might be harmed and how
3. Evaluate risks and decide on precautions and controls
4. Record findings and implement
5. Review assessment and update as necessary

Question 27

Identifying hazards and grading risks

Answer 27

What is laser device, beam deliver, laser process, environment and people
Risk grading: likelihood and consequence

Question 28

Where do you take someone who has been exposed

Answer 28

A&E - used to be opthamologist but now A&E as they could be in shock