# An Analysis of Visibility Requirements and Reaction Times of Near-Field Projections
## 2 Materials and Methods
### Study Design  
To answer the three research questions of this subject study, the detection probability and the resulting visibility level of a projected light stimulus and the corresponding reaction time of each subject served as the dependent parameters of the study, while the different contents, the greyscales and the resulting contrast, and the distance served as the independent parameters. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Age Group in Years** & **20–29** & **30–39** & **40–49** & **50–59** & **60–69** \\ \hline Quantity & 13 & 13 & 9 & 10 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Age distribution of the study subjects. Figure 1: Setup of the study with the mounted projector on the vehicle. Figure 2: Setup of the study with the position of the ground projection (grey rectangle) oriented to the vehicle and the different viewing distances of the subjects in 1 and 5 m (orange dots) distance to the projected area. The projection content's influence on the detection probability was investigated using two simple images as content for the projection (see Figure 3). The images used in Figure 3 represent typical use cases in the automotive sector. One text-based image, 'Welcome to Munich,' includes just three words for easy understanding and represents a welcoming scenario. The English language was selected for the text-based content to ensure that all participants could understand the wording. Furthermore, it encompassed a diverse range of letters and characters. The symbol-based content consisted of a battery, representing an everyday use case in future vehicle communication, particularly for electric vehicles, indicating the battery status. Consequently, the size of the two images was similar at 61 cm \(\times\) 50 cm. The subject viewed the displayed contents (text, symbol) in different greyscales at a distance of 1 and 5 m to the center of the projection area, respectively; see Figure 2. The two distances studied represent different real-world scenarios. The 1 m viewing distance represents entering or exiting the vehicle (welcome/goodbye scenario for the vehicle owner), and the 5 m viewing distance relates to a safety-relevant scenario in urban areas (e.g., vehicle communicating with surrounding road users). Furthermore, the viewing angles of the two images were greater than 100 angular minute at both distances (1 and 5 m). Therefore, the Weber range is extensive enough and assumes a constant behaviour to the corresponding luminance [31]. The design used for the subject study was a constant stimulus method with two-alternative forced-choice implementations. In all subtests (distance x content), the brightness of the projected content varied, while the subject had to press a key ('y'--'Yes' or 'n'--'No') on a keyboard upon detecting the displayed content as a light stimulus or not. The brightness of the projected content was adjusted by showing eight different greyscale levels for the text- and symbol-based content. These greyscale levels were presented randomly, one after the other, in the consecutive sub-trials. Each greyscale level was repeated three times to avoid any potential learning effect. Per one experimental part, the subjects thus had to evaluate 24 stimuli (1 distance (1 or 5 m) \(\times\) 1 content (text or symbol) \(\times\) 8 greyscale \(\times\) 3 repetitions). In addition, the different scenarios (distance of 1 or 5 m) and projected content (text or symbol) used different greyscales to present subthreshold and suprathreshold stimuli in all conditions. These thresholds were determined in advance with a smaller sample of subjects. The exact definition of detection was explained to the subjects beforehand. According to  
Figure 3: Projected images: symbol—battery; text—Welcome to Munich. [PERSON] and [PERSON] [32], detection is the mere perception of the presence of an object, as opposed to recognition, which is the discrimination of an object.