# An Analysis of Visibility Requirements and Reaction Times of Near-Field Projections
## 4 Discussion  
Current research concentrates on the development of external human-machine interfaces (eHMI), such as displays or projections for interactions of a highly automated vehicle with a vulnerable road user, such as a pedestrian or a cyclist [2; 3; 4; 5; 6; 7; 8; 9; 11; 12; 19]. This research has yet to focus on the visibility factor correlating with comprehensibility relevant to developing eHMI for the automotive sector. Therefore, a projection-based eHMI is examined in detail, focusing on projections in the near-field of a vehicle. The subject study states that using a projection-based illustration, the Weber contrast of a symbol-based message is approximately 25% lower than a text-based content independent of the observation distance, even though the reaction times saturate for both contents in the 600 to 650 ms range. The saturation of the reaction time appears before the 90% contrast threshold level, which implies that an arbitrary increase, in contrast, does not lead to a reduction of the reaction time.  
The reaction time saturation also ensures a feasible contrast for detecting the projection for both projection contents. Consideration of the statistical test of detection illuminance underlines the results of the Weber contrast. Higher illuminance levels are also necessary for the detection of the text. In addition, a higher illuminance is required for greater distances. Since the projection system receives specific digital values as input signals, the illuminance is the quantity that must be considered for an efficient projection system. The lower necessary contrast for symbol-based projections empowers using symbol-based over text-based content. This preference for symbol-based content has an advantage over text-based content worldwide, not only because of the lower contrast but also because, for example, Chinese characters are more complex than English characters and, therefore, could lead to even a higher necessary contrast [24; 25]. Therefore, text- or language-based eHMI must be examined for each specific language for the visual requirements, when implementing different languages with the corresponding complexity of the languages.  
Additionally, text-based messages can have cultural barriers and are not as understandable as symbols, which are known and used in daily traffic. Moreover, the usage of symbols nowadays is preferred in the regulations for homologating some projections, which support the driver [6; 15; 16; 18]. In addition, research into symbol-based content for eHMIs needs to be targeted.  
This subject study was conducted in a mesopic lighting scenario of 20 lx to represent a traffic use case in a parking lot. However, in real urban scenarios, dynamic environmental lighting conditions such as street lighting and additional lighting from oncoming vehicles are present. Current near-field projection systems around the vehicle still need to be visible in photonic lighting environments but should be considered in further research. These scenarios are becoming increasingly crucial for safety-related situations that must be visible in any lighting environment.