• Clinical success: the proportion of participants fitted with each type of prism glasses for whom the clinical decision is to continue using the glasses (i.e. participants who find the glasses helpful for obstacle avoidance when walking). [ Time Frame: Evaluated after 4 weeks of wearing each type of prism glasses ] [ Designated as safety issue: No ]
  
• Rating of how helpful each type of prism glasses are in avoidance of obstacles when walking [ Time Frame: Evaluated after 4 weeks of wearing each type of prism glasses ] [ Designated as safety issue: No ]
  

• Ratings of mobility difficulties in various situations without and with each type of prism glasses [ Time Frame: Evaluated at baseline and after 4 weeks of wearing each type of prism glasses ] [ Designated as safety issue: No ]
  
• Long-term clinical success: proportion of participants fitted with each type of prism glasses who continue to use and find them helpful for mobility after 6 months [ Time Frame: Evaluated 6 months after the clinical decision to continue with the prism glasses ] [ Designated as safety issue: No ]
  

Patients with hemianopic field loss are unaware of objects in their blind (non-seeing) hemi-field and often experience difficulties with mobility and navigation, such as walking into obstacles on the side of the field loss. In 2000, Peli (2000) described a new peripheral-prism design of prismatic correction for hemianopia, which addresses many of the inadequacies of existing designs of hemianopic visual aids, and produces true field expansion (i.e. the simultaneously seen field is larger with the device than without). In collaboration with the Schepens Eye Research Institute (Boston, MA), Chadwick Optical Inc (White River Junction, VT) has developed a permanent form of Fresnel prism segments, which are made from an acrylic material and can be embedded in a plastic spectacle lens. These permanent prisms offer better cosmesis, optical quality and durability than the temporary 40 prism-diopter press-on Fresnel prism segments used in previous evaluations of the peripheral prism system.

In this study we will evaluate new high-power (57 prism diopter) permanent peripheral prism glasses. Two designs will be evaluated - the "oblique" design and the "horizontal" design. Each design provides expansion in a different area of the visual field. The functional utility of the two designs for general mobility will be assessed.

Each participant will wear both designs of glasses in a cross-over fashion; one pair will contain real prisms and the other pair will contain sham, low power prisms, that provide little field expansion. Real prisms and sham prisms will appear similar. Participants will have either the "oblique" or the "horizontal" design in their real prism glasses and the other design in their sham prism glasses. Prism glasses will be fitted by Low Vision Practitioners at community-based Vision Rehabilitation Clinics.

After wearing the first pair of glasses for 4 weeks, participants will return for an in-office follow up visit, at which a questionnaire will be administered to record their experiences of wearing the glasses. The second pair of prism glasses will then be fitted. Another questionnaire will be administered 4 weeks later to record the experiences of wearing the second set of glasses.

At the end of the period of wearing the second pair of prism glasses, a clinical decision will be made as to whether the participant should continue to use the prism glasses (e.g. if a participant finds the prism glasses helpful for obstacle avoidance when walking). For participants who continue with the prism glasses, a final telephone follow-up interview will be conducted after about 6 months.