INTRODUCTION

Lasers are devices that emit a single, coherent wavelength of electromagnetic radiation that is used to cut, coagulate or ablate tissue for a variety of clinical applications. Laser systems produce a variety of wavelengths of varying pulse duration and energy levels. Computer-based imaging and guidance systems allow procedures to be performed precisely, quickly and with greater control. Although lasers are commonly used superficially for cutaneous and ocular applications, smaller, efficient laser delivery systems are available for minimally-invasive applications including endoscopy, bronchoscopy, laparoscopy and endovenous ablation.

The safe and appropriate use of lasers requires a trained clinician with a working knowledge of laser delivery systems and laser-tissue interactions to achieve the desired clinical effect while minimizing complications.

The basic principles of medical lasers will be reviewed here. The use and effectiveness of lasers for specific clinical indications are discussed in separate topic reviews. (See 'Clinical utility of lasers' below.)

ELECTROMAGNETIC SPECTRUM

Light is electromagnetic radiation within the range of wavelength that is visible to the human eye. Medical lasers produce photons of electromagnetic energy that can be within, above or below this range (figure 1). The ranges of frequency for each region of the electromagnetic spectrum are as follows:

• Gamma rays: <0.1 nm
• X-Rays: 0.1 to 10 nm
• Ultraviolet: 10 to 400 nm
• Visible: 440 to 760 nm
• Near-infrared: 700 to 1400 nm
• Mid-infrared: 1400 to 20,000 nm
• Far-infrared: 20,000 to 100,000 nm
• Microwaves: >100,000 nm