## 1 Introduction  
According to the World Health Organization (WHO), around 1.3 billion people suffered from visual impairment globally due to ocular diseases in 2018 [1]. Among various ocular diseases, glaucoma is the second leading cause of blindness after cataracts worldwide, affecting more than 70 million people worldwide [2, 3]. Because there are usually no noticeable symptoms of glaucoma at an early stage, the process may develop gradually to an advanced stage without the awareness of the patients, leading to subsequent visual loss [4; 5]. The two most common types of glaucoma--primary open-angle glaucoma (POAG) and primary angle-closure (PACG) glaucoma--have different risk factors [6]. In the region of drainage pathways, aqueous humor outflow through trabecular meshwork brings increasing resistance, which causes POAG. However, for PACG, drainage pathways in human eyes are blocked [7]. Studies have discovered that glaucoma is associated with-optic nerve degeneration and the death of retinal ganglion cells (RGCs) [8; 9; 10]. Because the level of intraocular pressure (IOP) corresponds to the death of RGCs, both POAG and PACG can be characterized by the raised IOP. Although IOP, age and genetics have been proved to be the factors in glaucoma, the exact mechanism of glaucoma is still not fully understood [11; 12; 13]. Currently, gonioscopic examination, ultrasound bio-microscopy and optical coherence tomography are commonly used to observe anterior chamber structures in clinical diagnosis of glaucoma. However, the detection process is commonly complicated, and POAG and PACG are difficult to distinguish with a single indicator. Human tears are the body fluids secreted by lacrimal glands. The functions of tears include protecting the eyes from infection, forming the water-based surface layer on the cornea for light penetration, flushing waste and small particles, and maintaining lubrication and moisture of the eye [14; 15]. Tear fluid is a complex aqueous mixture that consists of more than 95% water and some other important proteins, electrolytes, lipids, etc., which correspond to the state of the ocular pathophysiology [16; 17]. Several specific components in tears were measured with different techniques. For instance, lactoferrin levels and secretory immunoglobulin A concentrations were evaluated through immunoassaysays [18; 19]. In addition, mass spectrometry was used to analyze proteins and peptides in tear fluid [20; 21]. Previous studies proposed that brain-derived neurotrophic factor [22], some differential proteins [23] and autoantibly profiles [24] in tears may provide efficient biomarkers for glaucoma diagnosis. Raman spectroscopy (RS) is a non-invasive optical technique to determine the existence of certain molecules that can be used in ophthalmology [16; 25]. RS can reveal a frequency shift at a specific wavelength caused by the specific vibration mode of the molecular bond, providing a chemical \"fingerprint\" of the detecting material [26; 27]. So far, RS has been used to detect retinal tissues for glaucoma characterization [28; 29]. Tears have not been considered as samples with RS for glaucoma detection. However, RS can be utilized to discriminate various diseases via different body fluids, such as classification of glioma with blood serum [30], oral cancer detection with saliva [31] and prostate cancer detection with urine [32]. The total concentration of proteins in tears is about 10 mg/mL, which is much lower than that of blood serum [33]. Therefore, drop-coating deposition Raman spectroscopy (DCDRS) is involved in tear sample detection with RS. The principle of DCDRS is based on the \"coffee-ring effect\". When a drop of solution material evaporates on a hydrophobic surface, a ring-shaped deposit forms spontaneously [34]. As the edges of the droplets on the contact surface are fixed and the solvent begins to evaporate, the coffee-ring formation occurs with a net flow of the solution from the center to the edge.