Lasers Flashcards
Key laser properties
One wavelength (monochromatic)
One direction (small diversion)
One phase (coherent)
Range of lasers used clinically
193nm (UV) - 10600nm (IR)
Wavelength is critical as the effects and penetration depths will be different
Alternatives
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
Photon path length
l = 1/a
a is absorption coefficient
CO2 laser
10600nm
penetration depth <0.1mm
Creates thinner band of tissue damage than scalpel
Nd-YAG laser
1064nm
PD in soft tissue ~3mm
Used in cutting and coagulation. Gum surgery one application
Pulse dye laser
In theory use at different wavelengths eg 595nm
Penetration depth in blood 0.1mm Used in treatment of rosacea
KTP laser
532nm
Penetration depth in blood 0.1mm
Used in retinal surgery - focuses onto structures at back of eye and passes through front
Excimer laser
193nm
PD corneal tissue 0.1mm
Used in corneal resurfacing.
Laser testing - what do we want to test
Most important - power, pulse energy, pulse length
Alignment, emergency stop, uniformity
How do we measure power of laser
Thermopile: bank of thermocouples has hot and cold end, heat gradient generates voltage gradient and indicates heat rise.
How do we check shape/alignment of laser
Thermal paper
Why is eye particularly sensitive to laser light
Focuses light on retina with magnification factor of 10^5
Order of things in eye
Cornea
Lens
Retina
(going deeper
What UVs are dangerous for what part of eye
UVC/UVB interact with cornea
UVA with lens
Order in skin
Epidermis
Dermis
Subcutaneous tissue
(going deeper)
How does UV interact with skin
UVC/B penetrate to epidermis
UVA penetrates to dermis
Health effects of UV/V/IR
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
Continuous vs pulsed
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)
Laser classes
1, 1M, 1C, 2, 2M, 3R, 3B, 4
MPE
maximum permissible exposure: level of laser radiation one can receive repeatedly without injury
NOHD
nominal ocular hazard distance, distance to laser at which exposure is equal to MPE_eye. Can still have temporary hazards beyond this.
NOHD eqs
NOHD = (2/theta).sqrt(P/pi.MPE)
theta is beam divergence - laser
NOHD = (2/AA).sqrt(P/pi.MPE)
AA = acceptance angle - fibre
Main legislation
The Control of Artificial Optical Radiation at work Regulations 2010.
Ensures risk assessments done, HSE responsible.
Hazard vs risk
Hazard: something that can cause harm
Risk: how likely it is that hazard is realised
Risk assessment steps
- Identify hazards
- Decide who might be harmed and how
- Evaluate risks and decide on precautions and controls
- Record findings and implement
- Review assessment and update as necessary
Identifying hazards and grading risks
What is laser device, beam deliver, laser process, environment and people
Risk grading: likelihood and consequence
Where do you take someone who has been exposed
A&E - used to be opthamologist but now A&E as they could be in shock
