LASERs: Visible and IR

Question 1

What are the essential properties of a LASER?

Answer 1

Monochromatic
Small Divergence
Coherent.

Question 2

What LASER is used in LASIK surgery and why?

Answer 2

EXCIMER LASER
UVC - 193nm
Penetrates to a depth of 0.1mm into the cornea, allowing the cornea to be reshaped but does not penetrate into other parts of the eye.

Question 3

What LASER is used in the treatment of diabetic retinopathy surgery and why?

Answer 3

Green LASER (e.g. Argon 584nm)
Passes through the eye but it is absorbed by the blood sealing the leaks.

Question 4

What LASER is used in LASER Hair Removal and why?

Answer 4

Red/IR LASER (e.g. Alexandrite 700-800nm)

Passes through dermis and tissue, absorbed by hair follicle.

Question 5

What LASER is used in LASER surgery (e.g. ENT or Gynae) and why?

Answer 5

Visible or IR (e.g. CO2 10,600nm)
Delicate excisions with minimal collateral damage.
Excision depth of 0.1mm.

Question 6

What LASER is used in Endoscopic LASER surgery and why?

Answer 6

Ho-YAG 2080nm
Cuts and coagulates with minimal bleeding.
Can destroy urinary stones.

Question 7

What are the different classes of LASER?

Answer 7

Class 1 - Safe: enclosed or low power (<1mW)
Class 1M - Safe: except with viewing aids.

Class 2 - Safe for accidental viewing: 1mW max in the visible spectrum.
Class 2M - Use of collimating optics may be hazardous.

Class 3R - Intentional intra-beam viewing may be hazardous (Power <5mW)
Class 3B - Eye hazard but no skin Hazard (Power <500mW)

Class 4 - Eye, Skin, and Fire Hazard

Question 8

Where is data for the calculation of MPE found?

Answer 8

IEC 60825-1

Question 9

What is the Nominal Ocular Hazard Distance (NOHD)?

Answer 9

The distance within which the MPR may be exceeded, including both specular and diffuse reflection.

Question 10

What is the equation for NOHD from a LASER?

Answer 10

NOHD = 1000SQRT[Power/(piMPE)]

Question 11

What is the equation for NOHD from a Fibre?

Answer 11

NOHD = (1/AA)*SQRT[Power/(pi*MPE)]
AA = Acceptance angle (0.1-0.4 radians)

Question 12

What is the equation for the Acceptance angle of a fibre?

Answer 12

AA= = arcsin(NA)
NA = numerical aperture

Question 13

What measures are required for safe working with LASERs?

Answer 13

Classification of the LASER
Risk Assessment
Designation of a controlled area
Adequate Personal Protection.
LASER local rules
LASER protection Supervisor
Contingency plans

Question 14

What duty holders are required for working with class 3B and class 4 LASERs?

Answer 14

Responsible body (e.g. Trust) (delegates responsibility to LPA/Head of Med Phys)
Laser Protection Supervisor
Equipment Management (EBME)

Question 15

What are the duties of the LASER Protection Advisor?

Answer 15

Advises the employer on LASER Saftey matters.

Audits LASER Safety.

Question 16

What are the duties of the Clinical LASER Expert?

Answer 16

Assesses competency of operators.

Oversees Training of Operators.

Question 17

What are the duties of the LASER Protection Supervisor?

Answer 17

Carries out LASER Risk assessment
Writes Local Rules
Supervises Training and Safe Working.

Question 18

Give some examples of Pre-Use checks.

Answer 18

Condition of fibres, cables etc.
Safe initial power setting
Doors locked or controlled
Warning signs
Windows covered
Staff instructed.

Question 19

Give some examples of shutdown/handover checks.

Answer 19

Power setting to zero
Filters to safe
Reporting of untoward incidents
Returned to usual power settings
Return Key.

Question 20

What are common faults resulting in reported incidents to staff involving a Medical LASERs ?

Answer 20

No/Incorrect eye protection
Unitended ootsiwtch operation
Non-standard procedure
Incorrect replacement parts.

Question 21

What are common faults resulting in reported incidents to patients involving a Medical LASERs ?

Answer 21

Incorrect eye-protection
Internal electrical fire
Electrical shock
Failure of fibre
Non-standard parts/procedure.

Question 22

What two types of measurement device are used for the with a LASER power meter?

Answer 22

Thermopiles and Photodiodes.

Question 23

How does a thermopile measure LASER power?

Answer 23

A Thermopile is a bank of thermocouples that use the Seebeck effect* to measure temperature that then indicates power.
The cool end must be kept at an ambient temperature.
*Two dissimilar metals joined together that creates a voltage when heated or cooled. Voltage indicates temp.

Question 24

What are the characteristics of a thermopile?

Answer 24

10mm diameter aperture.
Low reflection (~2% as matte black)
Wavelength response of a few percent 200-20,000nm.
Can handle 10W max power or 10kW/cm^2 max power intensity.
Damage threshold 10GW/cm^2.
Response time of 1-2s.

Question 25

How does a (Si) Photodiode measure LASER power?

Answer 25

Currents in diode proportional to LASER power.

Question 26

What are the characteristics of a photodiode?

Answer 26

Wavelength specific
Quick response time
Limited to a few mW continuous pulse.

Question 27

What is the purpose of a spectra-radiometer?

Answer 27

LASER power meters are designed for single wavelength sources, and so will give inconsistent results if used with LEDs of Xenon flash lamps.
Power output is therefore measured using a spectra-radiometer.