Field Expansion for Acquired Monocular Vision Using a Multiplexing Prism

Abstract

Significance: Acquired monocular vision (AMV) is a common visual field loss. Patients report mobility difficulties in walking due to collisions with objects or other pedestrians on the blind side.

Purpose: The visual field of people with AMV extends more than 90° temporally on the side of the seeing eye but is restricted to approximately 55° nasally. We developed a novel field expansion device using a multiplexing prism (MxP) that superimposes the see-through and shifted views for true field expansion without apical scotoma. We present various designs of the device that enable customized fitting and improved cosmetics.

Methods: A partial MxP segment is attached (base-in) near the nose bridge. To avoid total internal reflection due to the high angle of incidence at nasal field end (55°), we fit the MxP with serrations facing the eye and tilt the prism base toward the nose. We calculated the width of the MxP (the apex location) needed to prevent apical scotoma and monocular diplopia. We also consider the effect of spectacle prescriptions on these settings. The results are verified perimetrically.

Results: We documented the effectivity of various prototype glasses designs with perimetric measurements. With the prototypes, all patients with AMV had field-of-view expansions up to 90° nasally without any loss of seeing field.

Conclusions: The novel and properly mounted MxP in glasses has the potential for meaningful field-of-view expansion up to the size of normal binocular vision in cosmetically acceptable form.

Figures

Figure A1

Critical angle of incidence with 50% transmittance. Note negative sign in angle of incidence is toward the base side. (A) Transmittance and (B) effective prism power vary with angle of incidence in 57Δ prisms. Total internal reflection starts at −5.3° angle of incidence with 0% transmittance and maximizes the effective prism power. However, the transmittance is below 50% with angles of incidence higher than 4.7° toward the base side, which cannot be used for the field expansion due to the low visibility of the shifted view. Therefore, we set the critical angle of incidence for field expansion to −4.7°. At this angle of incidence, the effective prism power is 39° (≈81Δ). (C) Critical angle of incidence with 50% transmittance in various prism powers of OPS and EPS prisms. Dashed line indicates the maximum prism power of OPS prisms without TIR at 55° nasal field (Fig. 1B).

Figure 1

Partial-field segment of outward prism serrations (OPS) Fresnel prisms (base-in) for left acquired monocular vision. Note that the prisms span the nasal field from 30° to 55° eccentricity (VN). (A) Ray diagram (left) and calculated field diagram (right) with a 57Δ OPS Fresnel prism. There is no prism shift into the blind side due to the total internal reflection (TIR). Note the TIR area in the field diagram marks the expected shifted field if there is no TIR. (B) 7Δ OPS Fresnel prism. This is the maximum prism power possible without TIR (Fig. A1C in Appendix, available at [LWW insert link]). The critical angle of incidence (iC) determines the nasal visual eccentricity where TIR starts (VC). Although the rated prism power as measured at the normal incidence is only 7Δ (8° apex angle in a PMMA prism), the effective prism power increases to ~31Δ (≈ 17° calculated by Eq. A1 in Appendix, available at [LWW insert link]) at the end of the nasal field (VN = 55°), expanding the nasal field of view (FoV) up to 72°. The size of field expansion into the blind side (~17°) is 11° wider than the apical scotoma (~6°), which results in true field expansion. Note the corresponding colors of the rays as marked on the fields on the right.

Figure 2

Partial-field segments of eyeward prism serrations (EPS) fitted base-in for left acquired monocular vision. Note that the prisms span the nasal field from eccentricity 30° to 55° (VN). The critical angle of incidence (iC) determines the nasal visual eccentricity where total internal reflection (TIR) starts (VC). Note the corresponding colors of the rays marked on the fields on the right. (A) 57Δ EPS Fresnel prism. Reduced angle of incidence of the EPS configuration partially avoids TIR, but the field from 44° (VC) to VN is still blocked by TIR. Yet, due to the high effective prism power at VC, up to 82° of the nasal field can be seen (albeit with a minified view). However, the size of the shifted (expanded) field into the blind side is only slightly wider (~4°) than the size of the apical scotoma, resulting in a very small net field expansion while the main effect is field substitution. (B) EPS right angle prism mounted over the nose bridge (Type-2 Cros-Vision). Despite the higher prism power, TIR starts more centrally (VC ≈ 40°) than in (A) and the expansion of the nasal field of view (FoV) is also limited to 82°. However, with an apical scotoma slightly wider (~2°) than that of the shifted view, the main effect is still field substitution.

Figure 3

Optimized prism field expansion for left acquired monocular vision using a tilted eyeward prism serrations (EPS) Fresnel prism segment to avoid total internal reflection (TIR) within the seeing field. Since the current highest prism power available in Fresnel prisms is 57Δ, an additional 11° tilt of the base of the prism segment toward the eye per Eq. 2 (t =55°-44°=55°-5°-39°) is required to move TIR onset (VC = 44° in Fig. 2A) to the end of the seeing nasal field (55°). This expands the field of view (FoV) into the blind side out to 96°, because the effective deflection power is increased to 87Δ (~41°). With the extended shift of the field larger than the apical scotoma, this configuration results in a small net field expansion.

Figure 4

Tilt angle (blue dotted line) required to prevent total internal reflection within the seeing nasal field of 55° and allow for the maximum nasal field of view (FoV) expansion into the blind side (red solid line), as a function of the rated power of eyeward prism serrations (EPS) prisms. For EPS prism configurations, the advantage of the higher power prism with a tilt saturates over 40Δ. Restoring the size of normal visual field (180°) requires a higher than 33Δ prism (with 8° tilt angle) for 90° nasal FoV expansion into the blind side (black marker with dashed lines).

Figure 5

The impact of the multiplexing prism (MxP) segment width (apex location, VA) on the field of view. Assuming that the location of the MxP base position and the tilt angle were optimally determined based on the end of the nasal field (VN) and considered the total internal reflection (Fig. 4). Depending on the location of the MxP apex (VA), the MxP results in a volume scotoma (with a too short segment) or monocular diplopia (with a too long segment) between the see-through view (solid arrows) and the shifted view (dotted arrows). (A) With an optimal width of the MxP segment, the see-through view is exactly the same width as the apical scotoma (the apex end of the shifted view is parallel to the end of the nasal field), and thus there is no apical scotoma. There is a slight tunnel diplopia that diminishes rapidly with distance. (B) With a shorter MxP segment, there is a gap (volume scotoma) between two views (with small volume diplopia closest to the MxP segment). For practical purposes, this volume scotoma effect is not different from a regular apical scotoma. (C) With a longer MxP segment, there is an overlap (monocular diplopia) between the two views. The crosschecked overlapped area is perceived in two different directions. Note the field expansion in the MxP, except where monocular diplopia or volume scotoma is noted, is provided via monocular confusion.

Figure 6

The angular direction of the apex of the multiplexing prism (MxP) segment (VA) that avoids both monocular diplopia and apical scotoma (red solid line). When the base end of see-through view (the end of the nasal field, VN) is fixed at 55° (blue dotted line) and the tilt angle maximizes field of view (FoV) (Fig. 4), VA for higher power MxPs extend more centrally than lower power MxPs. The optimal visual angular width of the MxP as a function of prism power is the distance between the two curves.

Figure 7

Interaction of eye scanning with the width of the multiplexing prism (MxP) segment. Calculated perimetry diagram in (A–C) no diplopia design and (D–F) no apical scotoma design, for (A&D) 15° eye scanning away from the blind side, (B&E) at primary position of gaze, and (C&F) 15° eye scanning toward the blind side. Head position remains fixed, with the field diagrams centered on the primary position of gaze. The cross marks and the black arrows indicate the fixation target and the direction of the eye scan, respectively; note the corresponding shift of the blind spot. (A–C) When the MxP segment span between eccentricity VA (33°) and VN (55°), there is no monocular diplopia even when the eye scans. However, eye scans away from the blind side (left scanning) result in an apical scotoma that is smaller in width than the magnitude of the eye scans. (D–F) To avoid the apical scotoma caused by eye scans away from the blind side (left scanning), a wider MxP segment (VA = 23°) can be used. Although this eliminates the apical scotoma in (D), it creates monocular diplopia in (E) and (F).

Figure 8

The impact of spectacle prescription lenses on the multiplexing prism (MxP) field expansion. (A) Effect of myopic correction. The end of the nasal field (solid arrow) is shifted by the prismatic effect (p > 0). To reach the critical angle of incidence at the base end of the corrected nasal field (expanded by the myopic prescription), the MxP segment should be tilted farther than the tilt required without the spectacle correction shown in Fig. 4. The field expansion (dashed arrow) is also wider than the result without correction. (B) Effect of hyperopia prescription. The end of the nasal field is shifted by the prismatic effect centrally (p < 0). The MxP segment should be tilted less, which results in smaller field expansion.

Figure 9

Multiplexing prism (MxP) field expansion glasses for left acquired monocular vision. (A) Front and back views of the 12° tilted 57Δ base-right, eyeward prism serration MxP, for the patient’s 56° nasal field. The prism was attached over the nose bridge inside the wrap-around sunglasses. (B) Goldman perimetry demonstrating field expansion 85° nasally with the MxP without apical scotoma. Dashed lines indicate the visual field measured without the MxP.

Figure 10

Multiplexing prism (MxP) prescription glasses for field expansion of a patient with right acquired monocular vision with sliding MxP segment. (A) Prescription field expansion glasses with a titled MxP segment mounted on a sliding mounting support, shown from the front (top) and from above (bottom). (B) Goldmann field diagram with a tilted 57Δ base-in eyeward prism serration MxP for a 57° corrected nasal field frame (RX −5.00D). The perimetry result shows the field expansion (solid line) to 89° achieved with the MxP glasses, with measured monocular diplopia shown in the hatched area. The dashed line indicates the FoV measured with only the prescription glasses. After sliding the MxP segment nasally to the end of the range, as shown in (A), the monocular diplopia was eliminated with no change in the field expansion.

Figure 11

Multiplexing prism (MxP) field expansion glasses for a right acquired monocular vision patient who needed spectacle correction. (A) MxP glasses with the segment fitted on a plastic frame with a narrow nose bridge (16mm). An 11° tilted 57Δ base-in eyeward prism serration MxP was used for the 55° corrected nasal field (measured with −1.25D). The base end of MxP segment was trimmed to fit it to subject’s nose. (B) Goldman field diagram. The dashed line indicates the field of view (FoV) with the prescription spectacles but without the MxP. The FoV was expanded to 84° by the MxP glasses at the primary position of gaze. To reduce the apical scotoma during eye scans away from the blind side (Fig. 13B), we used a slightly larger MxP segment which resulted in a narrow diplopic area at the primary position of gaze (hatched area).

Figure 12

Panoramic scenes captured (A) without and (B) with multiplexing prism glasses for right acquired monocular vision. The far-left blind field (see a man and a boat between the red dashed lines in A) is visible with the multiplexing prism glasses and is shifted to the seeing nasal field. It is shown through the multiplexing prism as monocular confusion with the see-through view (note the sky around tree crown above the excavator), but not diplopia. Due to the multiplexing prism, the contrast of both parts of the scene is reduced. The camera entrance pupil was located 17 mm from the prism to match the distance between the back surface of the spectacle lens and the entrance pupil of the human eye. The vertical dashed lines indicate horizontal eccentricities, with 0° representing the direction of foveation. The left and right side of the scene are trimmed off.

Figure 13

The effects of 15° eye shifts toward and away from the blind side with the field expansion glasses of Fig. 11A. The tilted arrows and the red cross marks indicate the fixation of the shifted eye, respectively, which is confirmed by the shift of the mapped physiological blind spot. (A) Goldmann perimetry with gaze shifted 15° (left) toward the blind side. Due to blocking by the nose and the nasal edge of the spectacles eye wire, field expansion to 87° and monocular diplopia (hatched area) are not different from the results at primary position of gaze, but the temporal field is reduced by ~10° due to the gaze shift. (B) Goldmann perimetry with gaze shifted 15° (right) toward the seeing side. This resulted in an apical scotoma, but it was narrower than the size of the gaze shift due to the slightly wider segment used. The field of view is slightly expanded on the temporal right side due to the gaze shift.

Figure 14

Magnetic clip-on multiplexing prism (MxP) glasses for field expansion of right acquired monocular vision ametropic patient. (A) Front and top views of the magnetic clip-on MxP mounted on the prescription glasses. Note the natural tilt of the clip-on lens at the position of the MxP. (B) Goldmann perimetry result with and without the magnetic clip-on MxP. The dashed line indicates the visual field without the magnetic clip-on MxP. The MxP clip-on (6° tilted 57Δ base-in EPS MxP) can be effective for users who have narrow nasal field (<50° in this case) as derived from Eq. 2. With the MxP clip-on, the field of view is expanded to about 75°.

Figure 15

Factors affecting visibility with the tilted MxP at the apex end and the base end of the shifted views as a function of prism rated power. Since the base end of the shifted view is near the critical angle of incidence, the farthest expanded field has the highest compression (minification), lowest transmittance, and lowest contrast. (A) Magnification factor (in log scale). Note that the reciprocal of magnification factor is compression factor. While the base end of the shifted view is compressed (magnification factor

Figure 16

The aperture ratio of 57Δ multiplexing prism (MxP) at each eccentricity required for uniform 50% contrast. If the aperture ratio were constant across the MxP, as eccentricities approach the base (nasal) end contrast of the shifted view would be greatly reduced relative to the see-through view. Increasing the aperture ratio toward the base end, as shown, can maintain a constant contrast ratio at all visual eccentricities.