Overall, it is cheap, easy, and quick to implement the distributed backbone network. Collapsed backbone. This type of backbone uses single, powerful router as the central connection point for multiple subnetworks. As Figure 4 shows, the central device is the highest level of the backbone.arallel backbone. Parallel backbone is a variation of the collapsed backbone, where devices are having more than one connection between them. As Figure 5 shows, there are multiple connections between the high level routers and the network segments.
The most common method to measure visual acuity is the Snellen eye chart. Like all eye charts, the Snellen chart tests a patient's ability to resolve high-contrast letters and is satisfactory if their vision is good. Unfortunately, the chart does not have enough sensitivity for patients with poor vision.
Acuity is tested with vision charts of letters or images. Changes in luminous intensity are perceived as flicker, and the difference in luminous intensity from one object to another is perceived as contrast [1]. The visual field encompasses the entire region of space seen while gaze is directed at any central object. This tutorial explains visual field testing.