A Novel Method for Optical Distortion Quantification

Abstract

SIGNIFICANCE 

Optical distortion is the image degradation of a visual target induced by a transparent material. Current Air Force evaluation of distortion is an entirely qualitative assessment of the acceptability of image distortion. The novel, quantitative technique described here is capable of identifying 0.1% distortion across an array of optical samples.

PURPOSE 

Optical distortion is the effect by which a transparent object spatially warps the perception of a visual target. All U.S. Air Force visors are required to pass military standards outlined in MIL-DTL-43511D (2006). Although specifications for the optical distortion setup and critical areas of vision are outlined, the evaluation technique is entirely qualitative, with a panel of several human evaluators assessing the distortion acceptability. The evaluation is not explicitly tied to a visual acceptability rating and has variable levels of consistency over time or across evaluators and a fabrication tolerance limit of 3% distortion.

METHODS 

The technique proposed in this article is a modification to the recommended optical tester used to analyze distortion patterns. An image-processing algorithm was developed to analyze patterns of Ronchi grid distortion mathematically to provide a quantitative approach that can subsequently be tied to visual metrics.

RESULTS 

This effort developed and refined an algorithm that allowed for a standardized assessment creation of high-resolution distortion maps from digital images. A 1-inch-diameter region imaged through ophthalmic material allowed for two-dimensional median filtering down to 15-pixel areas with enhanced contrast between grid lines leading to possible resolution capabilities of 0.10% distortion.

CONCLUSIONS 

Quantification of the standard for measuring optical distortion is the initial step toward determining the effects of distortions on human visual performance metrics. The future goal for this effort will focus on obtaining empirical results from human experimental efforts and relating the distortion location and magnitude to effects on visual performance activities.