## 4 Discussion  
This study aimed at exploring concepts often overlooked in the context of PDT, in particular in intravascular PDT, namely the accurate and complete description of the light delivery to the target tissue by the light distributor. The overall objective of the exploration was to fully characterize the angular distribution of the radiance at the surface of light distributors along two directions, in order to better master the desired therapeutic results. The radiance imaging setup used for this purpose is of great interest since, in spite of the concern potentially associated with a limited control of light distribution profile compared to what is expected, few light delivery systems are optically characterized in great detail [31; 35; 36]. This setup is original in the sense that it enables measuring the spatial and angular distribution of the radiance, which is a directional radiometric quantity, of cylindrical light distributors with a minimal number of moving parts. Indeed, the analysis of the images provided by this setup enables us to determine the value of the radiance according to one angle, the angle \(\varphi\) in the present study, in \"real time\" (potentially in a fraction of second), whereas fully goniometer-based setups are much less convenient and are more subject to bias [11]. Indeed, one critical problem faced by the conventional fully goniometer-based setups is in relation with their pointing stability while measuring the angular distribution of the radiance emitted by a \"small\" surface element of the distributor. It should be noted that a cheap camera, optimized in terms of sensitivity, linearity and resolution for specific cylindrical light distributors, could be used to record the image, enabling us to determine the angular distribution of the radiance along one of the angles without moving parts. The radiance measurements show that the diffuser emits a stable, almost ideal Lambertian profile at 652 nm, a wavelength of interest in PDT. Additional comparisons with its profiles at 635 (4 watts diode laser Ceralas(r) PDT, 635 \(\pm\) 3 nm/400 \(\upmu\)m, CeramOptec GmbH, Bonn, Germany) and 730 nm (Diode laser LTL7305 730 nm, TechLaser, Shanghai, China) showed that the diffusers' radiance profile was remarkably similar at these three wavelengths (data not shown). In addition, the profile is almost perfectly rectangular when measured longitudinally in air. This observation, combined with the almost perfectly Lambertian emission, is of importance to fully master the light dosimetry, and, as far as the longitudinal profile is concerned, is in good agreement with the claims of the manufacturer providing the cylindrical light distributor. This is also a positive result for possible clinical applications since it means that the diffuser can be aligned with the lesion to emit light where it is needed, with a minimal dependence on the vessel's diameter and on the chosen PS/illumination wavelength. The deviations from ideal longitudinal and Lambertian profiles probably have different origins. The longitudinal variations are likely to be due to a heterogeneous processing of the fiber core to decouple the light, whereas the deviations from a Lambertian profile are probably due to the Snell's law effect which takes place at the diffuser-air interface. Since the angular dispersion of the light exiting the diffuser is minimal for nearly orthogonal rays, this effect generates a forward-peaked emission, even if the light is isotropic in the distributor wall. It should be noted that this effect is probably significantly reduced if the distributor is surrounded by tissues or blood instead of air, since the refractive index step is reduced. The implications of these limited deviations on the treatment outcome are certainly minimal since the lengths of the longitudinal heterogeneities are smaller than the propagation length of red light in soft tissues [37].