• Coherent-referring to the wave nature of light, the peaks and troughs of the waves occur synchronously in time (ie, a fixed phase relationship between the electric fields of the electromagnetic field)
• Collimated-exhibiting minimal divergence (increase in the beam diameter) as the beam propagates
• Monochromatic-of a single or very limited spectral linewidth, ie, a single color
• High intensity-displaying a high optical power per unit area for a given amount of energy compared to broadband sources

Monochromaticity and high optical power are the most important properties when considering the interaction of laser light with tissue for medical applications.

Wavelength-When laser energy hits a target tissue, it may be

• Transmitted
• Reflected
• Absorbed
• Scattered

For there to be a biologic effect on a target tissue, the energy must be absorbed.

Each tissue has specific absorption characteristics base on its composition and chromophore content. The principal chromophores present in mammalian tissue are:

• Hemoglobin
• Melanin
• Water
• Protein

Infrared light is absorbed primarily by water, while visible and ultraviolet light are primarily absorbed by hemoglobin and melanin, respectively. As wavelength decreases toward the violet and ultraviolet, scatter or absorption from covalent bonds in protein limits penetration depth in this range.

In order to target a specific tissue, one should select a wavelength which is strongly absorbed by a chromophore present in that tissue.

Exposure time: Most medical laser applications depend on the absorption of laser light to heat the target tissue. To prevent undesirable thermal injury to adjacent tissue, light can be applied in suitably timed pulses related to the size of the target structure, according to the priniciple of Selective Photothermolysis.

The Thermal Relaxation Time ( t_r) of a given structure is the time needed for 50% of the heat generated by absorption of a laser pulse to diffuse into the surrounding tissue, and is approximately equal to the square of the diameter of the target structure. The Thermal Containment Time (t_c) is the pulse width in which all of the heat is confined to the target and is approximately 25% of the the t_r.

The proper pulse width for targeting a structure will be in this range-larger structures will be best treated with a longer pulse, smaller structures by shorter pulses. Too long a pulse, and adjacent structures may sustain thermal injury; too short, and insufficient energy is delivered for a biologic effect on the target.

With proper selection of the wavelength, exposure time, and intensity of the incident laser energy, the biologic effect on the target tissue can be optimized and undesirable collateral effect on adjacent tissue can be minimized.

• Photomechanical effects: Extremely short (nanosecond domain) pulses strongly absorbed by a chromophore can induce extremely rapid heating and formation of an expanding thermal plasma. As the plasma collapses, the shock wave causes mechanical disruption of the target. The dwell time on tissue is too short for significant thermal effects on adjacent tissue. This photomechanical effect is exploited by Q-Switched medical lasers for treatment of tattoos and certain pigmented lesions.
• Photothermal effects: Most medical applications involve the selective absorption of light energy using a longer (micro to millisecond domain) pulse width to cause rapid but selective heating of the target tissue with thermal injury. The biologic effect of tissue coagulation or ablation is exploited for laser resurfacing, treatment of vascular lesions, and laser hair removal.
• Photochemical effects: Laser energy can interact directly or indirectly with chemical structures within tissue. Noble gas-halide, or Excimer lasers for LASIK refractive surgery exploit ultraviolet laser energy's ability to disrupt covalent bonds non-thermally in corneal protein. In Photodynamic Therapy (PDT), laser or narrowband light energy can trigger a chemical reaction directly by interacting with endogenous photosensitizing compounds in cells.

In photodynamic therapy, a photosensitizer drug precursor is administered before treatment and taken up selectively by target cells. During an incubation period, the cell produce a photosensitizer, typically a porphyrin, which in the presence of molecular oxygen will generate cytotoxic singlet oxygen which destroys the target cells. In certain instances, cellular or bacterial porphyrins are targeted directly, as in the laser treatment of acne.

• Photobiomodulation (laser biostimulation, "cold laser" therapy): Low level laser or narrowband light has benn used with varying success to modulate cellular activity to achieve a biological effect such as stimulation of hair growth, collagen remodeling, accellerated wound healing, etc. In most cases the mechanism of action remains unclear, although changes in mitochondrial activity or cell membrane permeability may be responsible. Accepted medical applications include collagen remodeling for photoaged skin, anti-inflammatory treatments , and blue light therapy for acne treatments.

Absorption, Scatter, and Reflection: The quantity of energy that can be applied to the target must be sufficient to achieve the desired effect, but not enough to cause collateral damage on adjacent tissues. Assuming proper selection of wavelength and pulse width, absorption from competing chromophores, scattering of light in tissue, and surface reflection must be considered.

• Absorption from competing chromophores can be managed by cooling the structure containing the competing chromophore to minimize collateral thermal injury. A common clinical situation is protecting melanin-bearing epidermis while targeting melanin-bearing hair follicles during laser hair removal. The shorter epidermal thermal relaxation time allows heat to diffuse more quickly, and the lower initial temperature increases the threshold of epidermal injury, allowing higher energies to be safely applied to the hair follicle.

• Light scattering broadens the incident beam. Increasing the spot size keeps scattered photons in the beam path to the target area, increasing the energy density in the target volume and making them available to engage the desired target structures. Doubling the spot diameter increases the treatment volume eight times, so a lower applied energy can be used to achieve an effective energy density at the target.