In six experiments we demonstrate that the vertical-horizontal illusion that is evoked when viewing photographs and line drawings is relatively small, whereas the magnitude of this illusion when large objects are viewed is at least twice as great. Furthermore, we show that the illusion is due more to vertical overestimation than horizontal underestimation. The lack of a difference in vertical overestimation between pictures and line drawings suggests that vertical overestimation in pictures depends solely on the perceived physical size of the projection on the picture surface, rather than on what is apparent about an object's represented size. The vertical-horizontal illusion is influenced by perceived physical size. It is greater when viewing large objects than small pictures of these same objects, even when visual angles are equated.

Eye-height (EH) scaling of absolute height was investigated in three experiments. In Experiment 1, standing observers viewed cubes in an immersive virtual environment. Observers' center of projection was placed at actual EH and at 0.7 times actual EH. Observers' size judgments revealed that the EH manipulation was 76.8% effective. In Experiment 2, seated observers viewed the same cubes on an interactive desktop display; however, no effect of EH was found in response to the simulated EH manipulation. Experiment 3 tested standing observers in the immersive environment with the field of view reduced to match that of the desktop. Comparable to Experiment 1, the effect of EH was 77%. These results suggest that EH scaling is not generally used when people view an interactive desktop display because the altitude of the center of projection is indeterminate. EH scaling is spontaneously evoked, however, in immersive environments.

One important aspect of the pictorial representation of a scene is the depiction of object proportions. Yang, Dixon, and Proffitt (1999 Perception 28 445-467) recently reported that the magnitude of the vertical-horizontal illusion was greater for vertical extents presented in three-dimensional (3-D) environments compared to two-dimensional (2-D) displays. However, because all of the 3-D environments were large and all of the 2-D displays were small, the question remains whether the observed magnitude differences were due solely to the dimensionality of the displays (2-D versus 3-D) or to the perceived distal size of the extents (small versus large). We investigated this question by comparing observers' judgments of vertical relative to horizontal extents on a large but 2-D display compared to the large 3-D and the small 2-D displays used by Yang et al (1999). The results confirmed that the magnitude differences for vertical overestimation between display media are influenced more by the perceived distal object size rather than by the dimensionality of the display.

Aviation security checkpoints present a great challenge for designers. Paradoxically, they should not be a burden to innocent passengers, and yet they must be impenetrable by terrorists. Therefore, human factors engineers, security experts, and architects need to work together to design checkpoints to meet stakeholders' needs, as well as the competing demands of deterrence, detection, and throughput.When flying from their origin to their destination, air passengers have three basic desires: to obtain a ticket for an economical price, to arrive (with their luggage) safely, and to arrive on time. Yes, it is nice to get a tasty meal to fill the void in our stomachs or a refreshment to quench our thirsts, but we usually can do without such options, as long as our three main needs are fulfilled.Airport security checkpoints have an effect on all of three of these desires. First, they are one of the most crucial aspects of safety. While flight crew skills and aircraft maintenance are vital to a safe arrival, checkpoints are the only means by which the Transportation Security Administration (TSA) can ensure that no weapons or dangerous goods are brought into the sterile area and subsequently on board an aircraft.Second, checkpoints can have an impact on passengers arriving at their destination on time. Passengers are advised to arrive two hours early for domestic flights and three hours early for international flights. Generally, passengers who miss their flights do not arrive at the airport within the requested time prior to departure. Screening must continue to be conducted in a relatively short amount of time since required "early arrival" times have an impact on travel convenience and passenger decisions about how, and if, they should travel.Lastly, checkpoints have a monetary cost associated with them. Nothing in life is free, and this is especially true for security. Passengers pay for the cost of screening individuals and their belongings through a "security service fee" of $2.50 per enplanement, not to exceed $5.00 per one-way trip. These fees pay for the salaries and benefits of checkpoint personnel; training; the acquisition, operation, and maintenance of security equipment; background investigations; Federal Air Marshals; security operations; inspectors; and civil aviation security research and development.

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