Measuring ObserversÕ Visual Acuity Through Night Vision Goggles

Use of night vision goggles (NVGs) for military applications has grown steadily over the past 30 years. Each successive NVG model represents some kind of improvement in terms of size, weight, ruggedness, gain, noise, spectral sensitivity, field-of-view or resolution. The primary focus of this paper is the determination of NVG resolution. Many methods have been devised to measure the resolving power of NVGs and each method has with it an associated variance or accuracy of measurement. This variance is most likely caused by several sources including observer visual capability (since most methods involve visual observations and judgement to assess NVG resolution). The main purpose of this paper is to present the different methods that have been used to assess NVG resolution and to determine to what extent observer visual capability limits the accuracy of NVG resolution measurement. This study uses a methodology that measures an observerÕs psychometric function when viewing through NVGs (percent correct detection as a function of spatial separation) to determine their visual acuity using probit analysis. INTRODUCTION and BACKGROUND Night vision goggles (NVGs) allow an observer to see objects that are illuminated by very low amounts of light energy by greatly amplifying the light level. Present generation NVGs have a gain (as measured by the Hoffman ANV-120) of 6000 or more which means that for an object illuminated by a 2856K color temperature light source the NVGs present a luminance that is 6000 times brighter than the object viewed directly. However, the image intensifier tubes that are the heart of the NVGs also have an automatic brightness control which limits the output luminance. For present generation NVGs, this maximum average output luminance is on the order of 2 to 4 foot-Lamberts. Since visual acuity depends on light level, it is apparent that the level of detail that can be seen through the NVGs depends on the illumination level on the target scene. This becomes a factor in determining the resolution of NVGs. The term "resolution" is defined (the definition of interest to this topic) by Webster's Ninth New Collegiate Dictionary as "the process or capability of making distinguishable the individual parts of an object, closely adjacent optical images, or sources of light." The same dictionary defines "visual acuity" as "the relative ability of the visual organ to resolve detail that is usually expressed as the reciprocal of the minimum angular separation in minutes of two lines just resolvable as separate and that forms in the average human eye an angle of one minute." It is apparent from these two definitions that "resolution" and "visual acuity" are somewhat connected but are not quite the same thing, especially when we refer to the "resolution" of the NVGs. The primary reason for having a parameter such as resolution is to try to describe the capability of the NVG. However, all current widely used methods of measuring NVG resolution involve the use of human observers and vision. This has both good and bad points. The good point is that the NVGs are intended to be used with human vision in operation; so using vision as the means to assess NVG resolution seems to make sense. The bad point is that when one uses human visual capability as an integral part of a measurement procedure one may end up with increased variance due to individual differences or dynamic shifting of human visual threshold. The purpose of the research described herein is to determine the extent of human visual acuity variance when viewing through NVGs by using "frequency of seeing" curves. This is a time-consuming approach and is not suitable as a routine method for characterizing NVG resolution; but it does provide some insight into limitations of other methods used to measure NVG resolution. There is a subtle but very real difference between "NVG resolution" and "visual acuity through NVGs." This can be demonstrated by the following example. Suppose that some day advanced technology produces a "super" NVG capable of presenting details down to a tenth of a minute of arc. If vision is used to assess these "super" NVGs we would get a reading of about 1 minute of arc since that is the limit of visual capability; even though the NVGs were presenting details one tenth of this size. Thus, in this case, what is being measured is Report Documentation Page Form Approved