# Laser Science (Terminology and Definitions)

### Laser Wavelength (nm)

Laser light can be thought of as periodic waves of energy traveling through space (wavelength refers to the physical distance between crests of successive waves in the laser beam). Typical medical laser wavelengths are ; 1064 nm (near infra red), 2940 nm (mid infra red) etc. Only laser wavelengths between 400 nm and 700 nm are visible.

### Laser Pulse Energy (J)

If the laser is working in pulsed mode, the energy of laser pulse is a more reliable parameter than laser power. The energy is measured in Joules.

### Laser Power (W)

Laser power refers to the rate at which energy is generated by the laser. Laser power of 1 watt means that 1 joule of energy is in one second.

### Pulse Duration (ms) or Pulse Width

This refers to the temporal length of laser pulse; that is, the time during which the laser actually emits energy.

### Peak Power

Peak power refers to the power level during an individual laser pulse. Peak Power = Pulse energy/ Pulse duration

### Laser Beam Spot Size (mm)

Laser beam spot size refers to the diameter of the laser beam on the target. By changing the laser beam spot size while keeping

### Fluence (J/cms2)

Fluence refers to the amount of energy (J) delivered to the treated area (in square centimeters). It is called the dose of energy or energy density. Fluence = Energy/Area

### Repetition Rate (Frequency) (Hz)

Medical lasers are usually operated in a repetitive pulse mode. Laser pulses are emitted periodically at a pulse rate, such as 10 pulses per second. A commonly used term for pulses per second is Hertz (Hz)

### Absorption coefficient

One of the most important optical features of target tissue is its ability to absorb the laser light. The amount of absorbed energy versus the total used energy is called absorption coefficient. Absorption Coefficient = Absorbed energy / Laser energy.

### Thermal Relaxation Time (TRT)

TRT can be defined as the time it takes for an object to cool down from 100º C to 50ºC. It is a rule that a smaller object cools faster than a larger object of the same material and shape which means that the smaller target has a shorter thermal relaxation time. This fact is important when the tissue needs to be heated to a desired temperature at a certain fluence setting. If the pulse width is too long, the tissue will start cooling itself via thermal conduction prior to the completion of a pulse causing a negative clinical effect. The second parameter that should be taken into consideration when estimating the TRT is the shape of the target tissue. A sphere (skin cells) having 360 degrees of cooling surface area cools faster than a cylinder (hair follicles). This allows hair follicle to retain its heat while the skin cells can cool itself much more efficiently. For this reason, parameters can be selected to destroy the follicle without causing damage to the skin. For targeting smaller structures, a shorter pulse duration and higher fluence are recommended.

## Laser safety

The following steps should be adhered to:

• EYE PROTECTION – All persons in operating room must wear safety euewear. Light from the laser can cause severe corneal and retinal damage to the unprotected eye. Eyewear must have side shields and be worn over prescription glasses.
• REFLECTION – Laser light is easily reflected and care must be taken to ensure the beam is not directed toward shiny surfaces.
• ELECTRICAL HAZARD – The interior of the laser machine contain high voltage and exposed invisible laser radiation. Only technicians trained in electrical and laser safety are authorized to perform internal maintenance.