In-the-Spectacle-Lens Telescopic Device for Low Vision

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eye due to pupil coupling 40 . For the same example, assuming an eye pupil of 3mm, themaximum light efficiency is 90% (Fig. 16a) but is reduced to 80% with a square 8×8mmentrance pupil (Fig. 16c). With an eye pupil of 4mm, the light efficiency with the rectangleentrance pupil is reduced to 72% (Fig. 16b), but with the square pupil it will fall further to 58%(Fig. 16d). Thus extending the horizontal dimension of the objective (which is possible) wouldimprove the light collection of the telescope when the pupil is enlarged (e.g., in dim lightconditions), compared with a square objective.Ø 3mm90%Ø 4mm72%abØ 3mm80%Ø 4mm58%cdFigure 16: improvements in light efficiency of the effective pupil of the eye-telescopesystem resulting from extending the exit pupil horizontally. Dashed squares andrectangles represent the exit pupil sizes for with 2 different eye pupil diameters (3 and4 mm, respectively) using both a square and horizontally extended entrance pupils.The light gain (or actual loss) is equal to the percentage area of the eye pupil coveredby the exit pupil, as shown in the diagrams. The benefit of a rectangular objective interms of light efficiency is shown to increase with increased eye pupil diameter.The overlap between the exit pupil and the eye’s pupil may also be reduced by scanningeye movements that take the pupil away from the center of the exit pupil. Since most eyemovements, especially with a wider horizontal FOV, will be in a lateral direction, the wider26
horizontal extent of the exit pupil will protect the light efficiency during such eye scanning.Thus a wider objective is desirable, even if the wider FOV is not necessary.We computed the effects of the carrier lens thickness on the maximal possible lightefficiency due to the exit pupil coupling. Fig. 17 shows the results for an eye centered on the exitpupil in a telescope of magnification 2.8×, as a function of eye pupil diameters, for two levels ofcarrier lens thickness (5 and 8mm respectively), and for different format factors representing theratio of vertical to horizontal dimensions of the entrance pupil. Older people would present smallpupil diameters (about 3mm in diameter), while larger pupils would be expected for youngwearers under dim light conditions. As can be seen expanding the horizontal by a factor of 2substantially increases the maximum light efficiency for both levels of lens thickness. Expandingthe entrance pupil further horizontally results in minimal additional benefit (expanding the 8mmvertical entrance pupil to 8:24 results in no additional benefit over the 8:16 case (not shown)).Maximum Light Efficiency100%80%60%40%20%0%Format (mm)5:55:105:208:88:16Magnification 2.8×1 2 3 4 5 6 7Eye Pupil Diameter (mm)Figure 17: The influence of the carrier lens thickness, entrance pupil format factor, andeye’s pupil diameter on the light loss due to exit pupil coupling in a Keplerian telescopeof magnification 2.8×. For the 5mm lens thickness expanding the entrance pupilhorizontally to 10 mm substantially improves light efficiency for all pupil diameters.Further expansion results in modest effect and only for large eye pupils. For the 8mm27
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In-the-Spectacle-Lens Telescopic Device for Low Vision

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