Medical Laser Dosimetry

Diomed Ltd (now Angio Dynamics) manufactures lasers used in medicine. Diomed’s PDT 630 is a diode laser system used in photodynamic therapy (PDT), a treatment for oesophageal and endobroncial cancers. In PDT, a cytotoxic chemical is selectively delivered to cancerous tissue and ‘activated’ by the application of light of a particular wavelength. The output from the Diomed laser (up to 2W at 630nm) is coupled into an optical fibre, the distal end of which is fitted with a cylindrical, diffusing tip which serves to deliver the laser energy uniformly at the treatment site. For accurate treatment dosimetry, the output power from the fibre must be measured.

Measuring the output of a diffusing tip is impossible using a regular laser power meter. Diomed turned to light measurement specialists Labsphere who developed an integrating sphere solution. To measure the Diomed laser output, the fibre is simply inserted into the centre of the integrating sphere. The total power emitted from the fibre is collected in the sphere and recorded using a photodiode mounted at the sphere wall.

An integrating sphere is a hollow, spherical chamber coated internally with a high reflectance coating that exhibits diffuse reflectance. Spheres are used as directionally-insensitive collectors of light when combined with photodetectors. Light sources that are difficult to measure (that is, those with a combination of high power, extended source size and high divergence) present a challenge. An integrating sphere provides a simple solution to testing sources as diverse as laser diodes, LEDs, lamps, optical fibres, and projectors.

To learn more about integrating spheres, click here.