A Flight by Periscope and Where It Landed

Abstract

Objective: This study defines display design factors linking visual accommodation and the perceived size of distant objects. Background: In 1947, in anticipation of augmented contact and sensor-relayed contact displays, a periscope was installed in an airplane to serve as a sensor-based contact display simulator. To achieve normal landing performance, however, the unity image had to be magnified. This successful intervention, first published in 1966 in Human Factors, implicated oculomotor mechanisms and higher perceptual functions and became the observational basis for a series of investigative hypotheses. Method: Observers registered the perceived size of the collimated image of a “moon” by adjusting a disk of light while alternatively providing optometric measurements of accommodative distance. Results: Various investigators found high correlations between focal distances and perceived moon sizes. Conclusion: The simulated moon provided a superior vehicle for revealing the relationship between focal distance and perceived size and the factors affecting both. The operational display design implications and the possibility of a partial explanation for the moon illusion provided the motivation for an important doctoral research project involving eight factors that affect both focal distance and perceived size. Application: The investigation reaffirmed that virtual images, as found in head-up and head-mounted displays (HUDs and HMDs, respectively), do not consistently draw focus to optical infinity and that a variety of factors necessarily manipulated by display designers and present in many operational systems can affect visual performance partially through the mediation of accommodation.