The use of a head-up display (HUD) system has become popular recently, as it can provide feedback information at a position easily seen by the driver. However, the outline of the HUD bezel often reflects on the windshield of a HUD equipped vehicle. This phenomenon occurs when the sun is at a high position and reflects off the top of the instrument panel and the front view is dark. For this reason, it can occur when driving on asphalt paved roads, causing annoyance to the driver.Under fixed environmental conditions, the vehicle based factors that influence the annoyance caused by reflected boundary lines are the position of the reflection, line thickness, and the contrast of the reflected boundary line. These can be represented by the conspicuity of a striped pattern (contrast sensitivity function).In previous research in 1991, M. S. Banks et al. studied a contrast sensitivity function that included the factors stated above. However, they did not use the bright conditions that would cause reflection on a windshield. Therefore, the current study simulated conditions in which the sunlight comes from the sun at a high position, in order to obtain a new contrast sensitivity function that includes these factors.In the experiment, simulated sunlight was cast on the top of the instrument panel, and then a Campbell Chart of various spatial frequencies and contrasts was reflected on the windshield. Eight people participated in the experiment, and contrast was measured at positions corresponding to central vision (depression angle of 0 deg) and peripheral vision (depression angle of 2 to 5 deg). The resulting data approximately paralleled that of M. S. Banks's chart.In order to allow this to be treated as a value representing the conspicuity of the border line, this study employed a value of centroid frequency that was weighted by the contrast sensitivity obtained in the experiments (by convolutional integration), and verified experimentally that this value closely correlates with the sensed annoyance. These results allow the contribution levels of annoyance factors to be used for the creation of effective measures, rather than having to perform sensory determination of the design through consideration of all of the relevant complicated factors at the time of vehicle development.