# An Analysis of Visibility Requirements and Reaction Times of Near-Field Projections
## 5 Conclusions  
This study aimed to investigate whether the symbol- or text-based projection influences the detection contrast and the reaction time in various distances and, therefore,  
Figure 10: Logistic regression curves of detection probability for 1 m (blue line) and the corresponding data points (blue crosses) as well as of detection probability for 5 m (orange line) with the data points (black circles) over visibility level (VL) averaged over all subjects. raises the question: Does content matter? Accordingly, a study with 47 test persons was analyzed, which examined both the projection content and the viewing distances using a psychophysical two-alternative forced-choice subject study. The analyses suggest that the projected content, whether text- or symbol-based, and the viewing distance of the subject influence the detection probability. Therefore, the study results lead to the conclusion that content matters. A symbol-based projection can be detected in the testing at significantly lower contrast levels than a text-based projection by approximately 25%. The findings of the visibility level underline these effects. There were no differences in reaction times to symbol or text-based projection. Due to the lower necessary contrast for symbol-based projections at the same reaction time, a lower greyscale input image for the projector is needed. This results in advantages for the projection system, as it can save energy due to lower required illuminance levels and benefits regarding the comprehensibility of symbols in different countries and cultures. This is beneficial for the electrically driven vehicle, which, e.g., is already dominant in urban traffic nowadays, since it is crucial to design an energy-saving driving assistance system with near-field projections instead of following the approach by [PERSON] and [PERSON] [22], who state maximal luminance is essential for low reaction times. Furthermore, the assumption is that symbols are understood independently of other languages. Compared to text-based messages, the easier understandability of symbols correlates with and confirms the fundamental perception process by [PERSON] and [PERSON], where recognition demands a higher cognitive load than detection [32]. Further research should focus on the clarity and comprehensibility of these symbols because this subject study only investigated detection and not recognition. Regarding safety-critical situations, other studies should address the aspect of dynamic field studies and more complex cases, as this study has dealt with the fundamental elements of the detection of simplified images in a static test environment. Nevertheless, it will be challenging to put native participants into safety-critical situations. Therefore, a study design in virtual reality can be beneficial to elaborate safety-relevant projection content more realistically, as used already by some research institutes [2; 3; 4]. Another important aspect is the positioning of the projection on or next to the vehicle, as already shown in the first explorations by [PERSON] et al., who state that projected exterior displays are a beneficial feature for urban mobility [39]. It is crucial to verify if the proposed results for near-field projections on the road are adaptable to projected exterior displays on the vehicle surface. Another essential point to consider is a projection in a photonic environment. The question is whether projectors with the technology used in this study are sufficient for this application to achieve the required brightness levels or whether another technology, such as a laser scanner, needs to be used. With the fundamental research of the detection probability of simple text and symbol-based contents, future studies for projection content for automated vehicle communication in safety-relevant situations can be derived. Conceptualization, T.S., A.S., A.E. and T.Q.K. ; methodology, T.S., A.S., A.E. and T.Q.K. ; software, T.S. and A.S.; validation, T.S., A.S. and A.E. ; formal analysis, T.S. and A.S.; investigation, T.S., A.S., A.E.