## 1 Introduction  
The versatility of photodynamic therapy (PDT) as a treatment method for different diseases in various organs is well established. However, apart from the general principles underpinning the method, which are unchanged from organ to organ, the technical aspects of the treatment can vary greatly (including, but not limited to, choice of photosensitizer (PS), light dose and fluence rate, drug-light interval (DLI), PS dose and administration mode, and excitation wavelength). One such aspect pertains to light delivery, which is crucially impacted by the geometry, access and sterility properties of the target location. This aspect is particularly central for vascular PDT applications. In organs with difficult geometries, care must be taken to use or develop suitable diffusers. This is the case for arterial applications of PDT, which have recently regained attention for plaque progression prevention [1; 2; 3; 4; 5]. Some studies have reported certain drawbacks (skin photosensitivity [6; 7; 8], long DLIs [9; 10]; heterogenous illumination leading to irregular effects [11], plaque stabilization instead of ablation or prevention of restenosis; use of PDT for vascular treatments limited to invasive procedures; limited use for acute events; long illumination times sometimes incompatible with clinical and physiological realities [12]), but local PS delivery and homogenous intra-arterial illumination often allow for circumventing them [2]. In all instances, the goal is to achieve a controlled and as homogenous as possible fluence rate in the lesion without temperature increase in order to maximize the photodynamic effects whilst avoiding possibly deleterious side effects (thermal or otherwise) [13]. Although some studies positively describe external illuminations [14; 15], pragmatism indicates that the best way to achieve suitable illumination of the coronaries is through endovascular light delivery, using a cylindrical (radial) diffuser such as the one described in Figure 1b or in [16; 17; 18]. As small (core diameters in the several hundreds of \(\upmu\)m) optical fibers are usually used in these applications to deliver up to several hundreds of mW of light following an endovascular route, the natural choice is to use laser sources as light emitters [19]. The main advantage of laser sources for these applications is that they deliver relatively powerful and bright light beams, which can easily be coupled into optical fibers. It should be noted, however, that the nature of this coupling as well as the laser beam specifications have an important influence on the spatial and angular distributions of the intensity emitted by light distributors that are not equipped with mode scramblers. Generally, radial/cylindrical diffusers offer an interesting way to deliver intravascular illumination for the treatment of acute coronary syndromes and regional atherosclerosis [5; 20], especially for plaque stabilization, as opposed to plaque ablation [12]. In the specific case of diffusers used for the treatment of atherosclerotic plaque, numerous groups active in the field use fiber-based diffusers passed through catheters. Caution must be exercised in that case to ensure that the illumination profile of the 10-40 mm-long light-emitting windows is not modified by the catheter's material [21; 22]. Longitudinal, azimuthal or radial uncontrolled variations of the light radiance by the catheter can lead to localized irregular light distribution and hence to undesirable effects due to local under- or over-treatments. Similar caution must be exercised when the light distributor is centered within the vessel's lumen with an inflatable balloon. This approach is particularly appropriate when the fiber diameter is much smaller than the lumen diameter. Balloon catheters provide the benefit to interrupt blood flow to center the fiber in the vessel and to remove blood from the illuminated surface, features that are sometimes used intermittently [23; 24].