The horizontal peripheral prisms extend the visual field straight out to the sides by more than 30 degrees. For mobility, this maximizes field expansion. When you’re walking, the natural movement of your head up and down creates an awareness of the field between the prisms. There are a couple of issues that one may encounter with the peripheral prisms:
- Some visual field legislation states that the visual field be continuous along the horizontal plane. Because the horizontal prisms expand field above and below the midline, they do not satisfy this requirement. But Oblique prisms do!
- The horizontal prisms rely on head motion to fill in the awareness between the prisms. What can you do when there’s no up and down head motion, like when you’re in a car? Oblique prisms!
For the Peli Oblique prisms, the base of each prism is tilted towards the midline. The amount of tilt varies can vary based on vertex distance and prism separation, but is generally around 25 degrees. This tilt redirects field expansion so it comes from the midline rather than coming from the top and bottom. This can help someone pass a visual field test requiring continuous horizontal field. It can also help someone see expanded field out of a windshield when they’re in a car.