Retinal photoreceptors are cones and rods. Cones are responsible for color vision and best function relatively in bright light. They allow the perception of color, perceive fine details and more rapid changes in images. Cone cells are densely packed in the fovea centralis, their number six to seven million. (Valberg, 2007(Cones are three types, namely: S-cones, Mcones and L-cones. Each cone is therefore sensitive to visible wavelength of light that correspond to short-wavelength, mediumwavelength and long-wavelength light.(Labin et al., 2014) S-cones represent a minority of the cone photoreceptors in the human retina about 10% of cones. They are more vulnerable to damage by certain retinal disease than L and M-cones. (Hunt & Peichl, 2014)
The eye is one of the body organs affected in diabetes mellitus. More than 90% of patients with type 1 diabetes of 15 years and longer duration show features of retinopathy. (Keech et al., 2007)Electroretinography measures the electrical 
responses of various cell types in the retina, including the photoreceptors (rods and cones), inner retinal cells (bipolar and amacrine cells), and the ganglion cells. It is used for the diagnosis of various retinal diseases and is extensively used in eye research, as it provides information about retinal function. (Marmor et al., 2004) Optical coherence tomography angiography (OCTA) is a non-invasive technique that allows visualization of retinal and choroidal vasculature via motion contrast imaging. This relatively new imaging technique maps erythrocyte movement over time by comparing 
sequential OCT B-scans at a given cross-section.(De Carlo et al., 2015)