LASER: Visible and IR

Question 1

What are the duties of the Clinical LASER Expert?

Answer 1

Assesses competency of operators

Oversses training of operators

Question 2

What are the duties of the LASER Protection Supervisor?

Answer 2

Carries out LASER risk assesment
Writes local rules
Supervises training and safe working

Question 3

Give some examples of Pre-Use checks.

Answer 3

Condition of fibres, cables, etc
Safe intial power setting
Door locked/controlled warning lights
Windows covered
Staff instructed

Question 4

Give some examples of shutdown/handover checks.

Answer 4

Power setting to zero
Reporting of incidents
Returned to usual power settings
Return key

Question 5

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

Answer 5

No/incorrect eye protection
Unintended off switch operation
Non-standard procedure
Incorrect replacement parts

Question 6

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

Answer 6

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

Question 7

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

Answer 7

Thermopiles and photodiodes

Question 8

How does a thermopile measure LASER power?

Answer 8

A thermopile is a bank of thermocouples that use the Seeback 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 9

What are the characteristics of a thermopile?

Answer 9

10mm diameter aperture
low reflection (~2% as matte black)
wavelength responce of a few percent 200-20,000nm
can handle 10W max power or 10

Question 10

What are the characteristics of a thermopile?

Answer 10

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

Question 11

How does a (Si) Photodiode measure LASER power?

Answer 11

Currents in diode proportional to LASER power

Question 12

What are the characteristics of a photodiode?

Answer 12

Wavelength specific
Quick response time
Limited to a few mW continous pulse

Question 13

What is the purpose of a spectra-radiometer?

Answer 13

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

Question 14

What are the essential properties of a LASER?

Answer 14

Monochromatic
Small Divergence
Coherent

Question 15

What LASER is used in LASIK surgery and why?

Answer 15

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 16

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

Answer 16

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

Question 17

What LASER is used in LASER Hair Removal and why?

Answer 17

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

Passes through dermis and tissue, absorbed by hair follicle.

Question 18

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

Answer 18

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

Question 19

What LASER is used in Endoscopic LASER surgery and why?

Answer 19

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

Question 20

What are the different classes of LASER?

Answer 20

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 21

Where is data for the calculation of MPE found?

Answer 21

IEC 60825-1

Question 22

What is the Nominal Ocular Hazard Distance (NOHD)?

Answer 22

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

Question 23

What is the equation for NOHD from a LASER?

Answer 23

NOHD = 1000SQRT[Power/(piMPE)]

Question 24

What is the equation for NOHD from a Fibre?

Answer 24

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

Question 25

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

Answer 25

AA= = arcsin(NA)
NA = numerical aperture

Question 26

What measures are required for safe working with LASERs?

Answer 26

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

Question 27

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

Answer 27

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