## 1 Introduction  
The ocular surface is susceptible to infection, inflammation, trauma and other ocular surface diseases that strongly affect people's health and quality of life [1; 2; 3]. Various clinical symptoms may exist for most of the ocular surface diseases. For example, mainly caused by the inflammation of the cornea, keratitis typically manifests as a defect of the corneal epithelium and even stromal infiltration, which could cause conjunctival vessel dilation, distortion and congestion, and even lead to abnormal blood vessel growth [4; 5; 6]. To establish complementary information for the diagnosis and treatment evaluation of ocular surface diseases, different imaging modalities are usually applied in ophthalmic clinics to provide multi-modal and multi-dimensional images [7; 8]. Slit-lamp biomicroscopy (SLB) is the gold standard for routine ophthalmic examinations. By shining a thin sheet of light into the eye, an in vivo stereoscopic evaluation of the pathological changes of the cornea, conjunctiva and other anterior segment tissues can be performed by SLB [9]. However, the limited magnification and poor spatial resolution of traditional SLB make it difficult to visualize more detailed pathological structures and perform accurate biometric measurements [10; 11; 12]. Fluorescein stain could stain stroma while leaving the intact epithelium with a specific fluorescein dye, thereby demarcating the location and area of the epithelial loss in ocular surface diseases, on a relative macro scale as well [13]. Developed in the 1990s, optical coherence tomography (OCT) has revolutionized the field of ophthalmology and vision science with its non-contact, non-invasive and high-resolution nature and become one of the prominent diagnostic tools in offering cross-sectional tissue imaging [14; 15; 16]. Applications of OCT for in vivo human eye imaging have been widely demonstrated in the literature [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. With ever-improving techniques in broadband light sources, ultra-high resolution OCT (UHR-OCT) has achieved micron-scale axial resolution [21; 22]. UHR-OCT imaging of the ocular surface can now resolve and even measure the thickness of tear film and the major structures, such as the individual corneal layers: the epithelium, the Bowman's membrane, the Stroma, Descemet's membrane and the endothelium [23; 24; 25; 26], which helps clinicians be better in the diagnosis and management of ocular surface pathologies and diseases, including keratitis [23; 26], keratoconus [27], ocular surface neoplasia [28], etc. For ocular surface diseases, apart from the physiological structural changes, the microvascular system would also change when the eye is inflamed, as inflammatory mediators would induce a series of vascular changes, including vasodilation, increased permeability and increased blood flow [5; 6], which correlates with the severity of the lesion. Techniques like OCT angiography [29; 30] have been applied to image the vessels of the anterior segment, but they could only provide morphological information and lack of the function of providing quantitative functional parameters, such as the blood flow velocity. To measure the blood flow velocity in bulbar conjunctival vessels and also evaluate their network complexity, our team has developed functional slit-lamp biomicroscopy to achieve conjunctival vessel network imaging as well as high-resolution conjunctival blood flow videography [10; 31]. With customized software, images and videos can be processed to achieve morphological parameters such as vessel diameter and complexity, as well as functional parameters of blood flow velocity. These methods have been demonstrated in clinical studies of healthy subjects [32; 33] and ocular surface defects and inflammatory diseases [34; 31; 35], and proved to be a considerably easier and more cost-effective alternative to image and evaluate the bulbar conjunctival microvasculature system.