Glaucoma is the second leading cause of blindness among seniors in Canada. It is often associated with an elevated intraocular pressure (IOP), but its exact mechanism is still largely unknown. Some studies have shown a link between glaucoma and changes in the amount of oxygen in the veins of the eye. The study aims to compare the amount of oxygen in ocular veins among three different groups using a spectrophotometer. This instrument is linked to a camera and can measure the quantity of oxygen in the veins using different characteristics of the blood inside.