LASERs: Visible and IR Flashcards

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1
Q

What are the essential properties of a LASER?

A

Monochromatic
Small Divergence
Coherent.

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2
Q

What LASER is used in LASIK surgery and why?

A

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.

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3
Q

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

A
Green LASER (e.g. Argon 584nm)
Passes through the eye but it is absorbed by the blood sealing the leaks.
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4
Q

What LASER is used in LASER Hair Removal and why?

A

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

Passes through dermis and tissue, absorbed by hair follicle.

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5
Q

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

A

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

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6
Q

What LASER is used in Endoscopic LASER surgery and why?

A

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

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7
Q

What are the different classes of LASER?

A

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

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8
Q

Where is data for the calculation of MPE found?

A

IEC 60825-1

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9
Q

What is the Nominal Ocular Hazard Distance (NOHD)?

A

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

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10
Q

What is the equation for NOHD from a LASER?

A

NOHD = 1000SQRT[Power/(piMPE)]

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11
Q

What is the equation for NOHD from a Fibre?

A
NOHD = (1/AA)*SQRT[Power/(pi*MPE)]
AA = Acceptance angle (0.1-0.4 radians)
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12
Q

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

A
AA= = arcsin(NA)
NA = numerical aperture
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13
Q

What measures are required for safe working with LASERs?

A
Classification of the LASER
Risk Assessment
Designation of a controlled area
Adequate Personal Protection.
LASER local rules
LASER protection Supervisor
Contingency plans
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14
Q

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

A
Responsible body (e.g. Trust) (delegates responsibility to LPA/Head of Med Phys)
Laser Protection Supervisor
Equipment Management (EBME)
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15
Q

What are the duties of the LASER Protection Advisor?

A

Advises the employer on LASER Saftey matters.

Audits LASER Safety.

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16
Q

What are the duties of the Clinical LASER Expert?

A

Assesses competency of operators.

Oversees Training of Operators.

17
Q

What are the duties of the LASER Protection Supervisor?

A

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

18
Q

Give some examples of Pre-Use checks.

A
Condition of fibres, cables etc.
Safe initial power setting
Doors locked or controlled
Warning signs
Windows covered
Staff instructed.
19
Q

Give some examples of shutdown/handover checks.

A
Power setting to zero
Filters to safe
Reporting of untoward incidents
Returned to usual power settings
Return Key.
20
Q

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

A

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

21
Q

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

A
Incorrect eye-protection
Internal electrical fire
Electrical shock
Failure of fibre
Non-standard parts/procedure.
22
Q

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

A

Thermopiles and Photodiodes.

23
Q

How does a thermopile measure LASER power?

A

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.

24
Q

What are the characteristics of a thermopile?

A

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.

25
Q

How does a (Si) Photodiode measure LASER power?

A

Currents in diode proportional to LASER power.

26
Q

What are the characteristics of a photodiode?

A

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

27
Q

What is the purpose of a spectra-radiometer?

A

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.