<title>Visual acuity versus field of view and light level for night vision goggles (NVGs)</title>

Abstract

Visual acuity (resolution) and field of view are two significant parameters used to characterize night vision goggles (NVG5). Most users of NVGs surveyed within the US Air Force put these two parameters at the head of the list of characteristics that they would like to see improved. Unfortunately, it is well established that these two parameters are coupled together in an inverse relationship: an increase in field of view results in a reduction in visual acuity and vice versa. However, there are at least two ways to model the relationship between these two parameters: one based simply on geometric considerations and one based on the concept of the modulation transfer function (MTF) of the image intensifier tube and the contrast threshold function of the human visual system.To investigate these relationships, an experiment was conducted to determine how visual acuity through night vision goggles (NVGs) changes as a function of NVG field of view and ambient scene illumination level. A total of three trained observers were used for this study who ranged in age from 33 to 42 years of age. The NVGs used in the study had fields of view of 40, 47, and 52 degrees, respectively. Five levels of ambient scene illumination (corresponding to NVG output luminance levels of 0.01, 0.03, 0.08, 0.26, and 1.9 fL) were provided by a 2856K light source which ranged from overcast starlight to quarter moon. The targets used in the study were approximately 95+% contrast square wave targets ranging in size from 45 cycles! degree to 5 cycles per degree. The method of adjustment was employed by having the trained observer start at a distance of 30 feet and determine the highest spatial frequency target which was clearly discemable. The subject was then directed to walk back slowly from the target until it was just out of focus, and then walk forward until the target was barely discernable. The distance from the target was recorded and used to calculate the angular spatial frequency (and equivalent Snellen acuity).The results indicate that the simple geometric model of the inverse relationship between resolution and field of view is adequate for characterizing this design trade-off for the quality of image intensifier tubes currently available. INTRODUCTION AND BACKGROUNDTwo key parameters used to characterize night vision goggles (NVG5) are visual acuity (resolution) and field-of-view. An increase in field-of-view is accomplished by providing higher magnification of the image intensifier tube. However, since the image intensifier tube has a fixed linear resolution at its output screen, increased magnification means that the "picture elements" (pixels) will subtend a larger angle with respect to the O-8194-1522-7/94/$6.00 SPIE Vol. 2218 / 71 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Depth and Collision Judgment Using Night Vision Goggles

“…We note that an increase in FOV results in a reduction in resolution (Donohue-Perry, Task, & Dixon, 1994;Genco, 1985). Next, we measured several tasks involving depth perception, and varied FOV.…”

Development and Evaluation of the Panoramic Night Vision Goggle

“…(Figure 3) (AN/AVS-6 and AN/AVS-9). One major design characteristic of these NVGs is that increased FOV could only be obtained at the expense of resolution [3], [4] since each ocular uses only a single image intensifier tube. The image intensifier tube has a fixed number of pixels (picture elements).…”

Theoretical and applied aspects of night vision goggle resolution and visual acuity assessment

“…The competing concepts of fixed contrast criteria versus contrast threshold function criteria for resolution (NVG-aided visual acuity assessment) was addressed by Donohue-Perry et al 24 when investigating the field of view effects on NVGaided visual acuity. Their results indicated that the fixed modulation level criteria approach provided an excellent fit to their data.…”

“…The field of view of the NVGs also can affect the NVG-aided visual acuity obtained because larger fields of view are obtained by shorter focal length lenses (objective and eyepiece lenses), which spread the fixed number of pixels of the image intensifier tube over a larger angular subtense. In general, the larger the field of view the worse the visual acuity level 24 . In addition, the eyepiece diopter setting can interact with the observer's ability to accommodate to the image distance, which in turn can affect the NVG-aided visual acuity obtained 29,30 .…”

“…The Hoffman Engineering ANV-20/20 TM device 18 was a later development using square-wave patterns to pre-flight NVGs. In an effort to refine the square-wave grating pattern to obtain smaller step sizes between resolutions (visual acuities), a variation was developed and constructed containing six pairs of vertically and horizontally oriented square-wave gratings 24 . While looking through the NVGs at the pattern from a distance of 30 ft, the observer selected the smallest resolvable target pair.…”