## 1 Introduction  
Easier access to intra-operative near infrared (NIR) imaging has resulted in widespread use of this technology, allowing surgeons to see beyond the visual spectrum [1]. As an emerging optical imaging technique, NIR imaging has already shown clinical benefits in various surgical practices. NIR camera systems have been deployed to visualize intravenous injected indocyanine green (ICG) spreading through the vascular bed, thereby creating the ability to assess tissue perfusion, demarcate tumor tissue, and visualize vital structures [2; 3]. When bound to blood plasma, ICG has a peak spectral absorption of around 800 nm and emits fluorescence at longer wavelengths [4]. In gastrointestinal surgery, indocyanine green fluorescence angiography (ICG-FA) has mainly been used to tackle perfusion-related complications, such as anastomotic leakage [5; 6]. However, several studies report variable results in reducing the anastomotic leakage rates by using ICG-FA [5; 6; 7; 8]. This may be due to the inter-user variation in interpretation of the FA signal, which remains mainly subjective using visual assessment only and is therefore associated with a considerable learning curve [9; 10]. However, while clinical use of fluorescence imaging systems is rapidly growing, the appropriate knowledge for reliable image interpretation is lagging. Essential in overcoming these challenges for broad effective implementation of ICG-FA, is standardization of fluorescence assessments to make them more comparable and reproducible. Standardization and protocolization enable the possibility of quantifying the fluorescence signal, which is an important focus of research in this field [11]. When standardizing the fluorescence measurement, tracer administration (dosage, volume, infusion rate), working distance, and ambient light are often taken into consideration [12]. To accurately interpret and quantify the fluorescence signal being displayed, a thorough understanding of the underlying physics is necessary. Factors such as distance, movement, and the relationship between the center and periphery can all have a significant effect on the intensity of the signal. These factors have received little attention by clinicians while it directly influences the fluorescence signal and may hamper correct interpretation. In clinical practice, for instance, the bowel located in the center may show more fluorescent intensity than a more proximal segment located in the periphery of the field of view (Figure 1) [13]. Figure 1: A segment of colon during a transanal total mesorectal excision. The top image is the white light image and the image below, the corresponding fluorescent image, demonstrating less fluorescent enhancement at a more proximal bowel segment (see blue arrow). For other medical imaging modalities, universal standards are described to benchmark their performance. In order to do so for NIR imaging, solid tissue-mimicking phantoms have been developed to characterize NIR imaging systems [14; 15], however, these papers have not yet been translated into applicable solutions for interpretation by clinicians. Moreover, so far, parameters such as the illumination homogeneity, the resolution, or the dependency of fluorescence intensity on tissue optical properties are not generally comprehensively addressed in the phantoms built. The studies that do address inhomogeneities of light distribution plot the intensity measurements only for a few locations (i.e., five reflective wells placed in the corners and center of the phantom), therefore, it is not possible to perform a correction of the recorded pixel data [16]. To better understand this periphery-center effect, which is observed in clinical practice, and promote awareness among clinicians and the potential consequences on fluorescent parameters, we investigated the light distribution within the field of view of five different commercial NIR imaging devices using a fluorescent phantom.