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A.R. Bowers, G. Luo, E. Peli; Functionally Relevant Illumination Levels for Evaluation of a New Night Vision Device . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2772.
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Purpose: We are developing a new type of night vision device (incorporating a novel vision display) to aid outdoor night mobility specifically for patients with night blindness, but good daylight visual acuity. As part of our preliminary evaluations, we determined the functionally relevant range of lighting levels at which the device should operate to provide maximum benefit for outdoor mobility in a range of environments and at which visual function, mobility and device performance should then be evaluated during clinical trials. Methods: Detailed surveys of lighting levels on busy and quiet city center, residential and rural streets were carried out in the Boston area. Visual performance of 3 retinitis pigmentosa (RP) subjects and 2 control subjects was assessed with and without a commercially available night vision device (Visys) at a range of illumination levels found in the street lighting surveys. Independent night-time outdoor mobility with habitual mobility aids (long canes) was assessed for 3 RP subjects under a range of street lighting conditions with and without the Visys device. Results: Street lighting ranged from a median of 13 (range 1.0 - 694) lux on busy city center streets to a median of 3.2 (range 0.3 – 22) lux on quiet residential streets and a median of 0.5 (range 0 – 17) lux on rural streets. Two of the RP subjects had good VA (20/50 or better in daylight). For these 2 subjects, visual functions, walking speed and subjective confidence to carry out independent night-time mobility were reduced when outdoor illumination levels were less than 5 lux; there was a marked improvement in visual function and walking speed with the Visys device at these light levels. The third RP subject had reduced VA (20/400). His mobility performance showed less illuminance dependence than the 2 RP subjects with good daylight VA. Conclusions: Although our survey indicated that a night vision device should operate across a wide illuminance range, particular attention should be given to device performance below 5 lux as this was the illuminance level below which our target population (night blindness, but good visual acuity) felt unsafe to undertake independent outdoor night mobility.
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