# Influence of Headlight Level on Object Detection in Urban Traffic at Night
## 3 Results
### Photometric Results  
First, the luminances and the corresponding Weber contrasts \(C_{\mathrm{W}}\) are considered, which can be calculated according to Equation (3), where \(L_{\mathrm{O}}\) represents the object luminance and \(L_{\mathrm{B}}\) the background luminance [67]. \[C_{\mathrm{W}}=\frac{L_{\mathrm{O}}-L_{\mathrm{B}}}{L_{\mathrm{B}}} \tag{3}\]  
Using these data, the influence of the low beam intensity on the present contrast can be described in terms of polarity (positive or negative contrast) and value. The contrast curves obtained for the three test roads and vehicle position 1 are shown in Figures 6-8. The contrast curves for vehicle position 2 are not shown here due to analogous results. From Figures 6-8, it is clear that the low beam intensity has a considerable influence on the value and polarity of the Weber contrast. At all object positions, there is initially a negative contrast between the object and its background created by the existing street lighting. If the low beam intensity is increased, a transition point from the negative contrast to the positive contrast is reached in the majority of the object positions considered on the three test roads, in which the negative contrast generated by the street lighting and the positive contrast generated by the vehicle low beam neutralize each other and no more contrast can be perceived between the object and its background. In this case, the detection object is no longer visible, and disappears for the driver. It is noticeable that the intensity of the low beam has to be increased to different extents depending on the distance and the object position in order to reach this transition point. Figure 5: Example of data evaluation using the psychometric function according to [PERSON] [68]. Figure 6: Contrast curves on test road 1 (0.91 cd m\({}^{-2}\)) and vehicle position 1 depending on the intensity percentage from 0 % (low beam off) to 100 % (low beam fully switched on). Figure 7: Contrast curves on test road 2 (0.53 cd m\({}^{-2}\)) and vehicle position 1 depending on the intensity percentage from 0 % (low beam off) to 100 % (low beam fully switched on). A further increase in the low beam intensity leads to the positive contrast gaining the upper hand on most object positions, which can make the object visible again. Again, a closer look reveals that the required increase in intensity of the low beam depends on the distance and the object position. In addition to the transition point between negative and positive contrast, the positive contrast that is exactly as large in absolute value as the initial negative contrast represents an important threshold, as between these two points the additional low beam light merely leads to a reduction in the contrast in absolute value, which worsens the visibility conditions. This condition becomes particularly critical on test road 1 with an average roadway luminance of 0.91 cd m\({}^{-2}\), as here this second threshold for object positions 5 to 16 cannot be reached at all with the existing headlight system. On test roads 2 (0.53 cd m\({}^{-2}\)) and 3 (0.38 cd m\({}^{-2}\)), the same applies to the objects at a distance of 65 m. In these cases, the low beam function merely causes a downgrading of the visibility conditions, and achieves the opposite of its actual function of improving visibility. This transition from negative to positive contrast is illustrated by the luminance images in Figure 9.  
Figure 8: Contrast curves on test road 3 (0.38 cd m\({}^{-2}\)) and vehicle position 1 depending on the intensity percentage from 0 % (low beam off) to 100 % (low beam fully switched on). Figure 9: Illustration of the transition from negative contrast to positive contrast; the two objects in the middle are clearly visible with the negative contrast (**left**), while an increase in the low beam intensity leads to a neutralization of the contrast and makes the objects disappear (**center**) and a further increase in the low beam intensity makes the objects visible again (**right**).