Construction, quality assurance and calibration of spherical isotropic fibre optic light diffusers

Spherical isotropic fibre optic light diffusers are used in photodynamic therapy either as a light source or as a light detector. The construction of light diffusers using different materials is described, viz. an optical method involving local polymerization of a dental fissure sealant, which is referred to as the Henderson method, and a second method using plastic or ceramic pre-fabricated spheres. Quality tests necessary for reliable clinical use are presented for the mechanical strength, output power and isotropy. The maximum pull-off force and blow-off output power for the different kinds of diffusers were determined. The calibration procedures are given for measurement of the output power and wavelength of the light emitted by a diffuser and for measurement of the fluence rate by a light-detecting diffuser, using a compact integrating sphere device. With all types of diffusers described, an isotropy can be obtained of better than ± 20% measured over a 320° angle for spheres as small as ≈ 1 mm. Larger ceramic diffusers are particularly suitable for delivering high output powers. A 3-mm-diameter ceramic diffuser mounted on a 600-μm-core fibre can emit up to ≈ 5 W of continuous wave (CW) visible light in air. Diffusers used for light detection can measure the light fluence rate in tissue with ≈15% accuracy or better if calibration factors are determined for each individual probe.