Pupil Dilation Under Different Lighting Conditions

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terested reader is referred to the original publications on the topics general eye tracking in automotive use: [138,139], gaze distribution on motorways [140], orientation in curve driving [141–144], adaptation luminance in dependence of gaze behaviour [145] but the presented se-lection of research on eye tracking should showcase the different topics that can be tackled using eye tracking.

One very important fact that arises when comparing the different findings from previous research is, that there are areas where all previous studies come to the same conclusion. This is for example the case for the dependence of the orientation behaviour between experienced and inexperienced drivers or the general orientation behaviour when driving through curves.

Other very basic findings, the difference between day- and night-time driving however, lead to different results depending on which research one reads. While Brückmannand Damasky found a wider gaze distribution during the day, Diemrecorded the exact opposite.

Another remark is, that many of the presented studies were performed by a very limited amount of test subjects ranging from 3 subjects (Stahl), 4 subjects (Cengiz), 6 subjects (Shibata), 12 subjects (Winter), 20 subjects Schulz, to a maximum of 23 subjects (Winter), since eye tracking studies involve a time consuming calibration and (at least used to) include an even more time consuming evaluation of the generated data. However, due to the large variance shown in all data sets, it is inevitable, that a large number of subjects participates in eye tracking studies for statistical relevance.

The conclusion from the presented available data is, that due to the new and improved technology available for todays eye tracking systems, new data sets should be recorded and evaluated to today’s standards to compare them to the findings presented here. This is not only necessary due to the new technology in terms of eye tracking but new and improved headlamp technology as well. Furthermore, the traffic space in general has changed over the last couple of years with a much higher traffic density a lot more traffic happening in urban areas.

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(a) (b)

Figure4.19– Summarized pupil diameter over the adaptation luminance for(a)binocular viewing with a 60° adaptation field and(b)monocular viewing with a 10° adaptation field for the unified pupil diameter by Watson[153].

The data shows large differences in all regions starting from the highest pupil diameter ranging from 6.8 mm (Winn, Crawford and DeGroot andGebhard) to 7.8 mm (Watson, Bartenand Moon andSpencer) over the general behaviour in the intermediate range to the smallest calculated pupil size that ranges from 3.6 mm by Winn down to less than 2.0 mm (Holladay,deGroot andGebhard, Backie andHowland). The interested reader can re-fer to either to the summary of Watson([153]) or to the original publications as mentioned previously.

While the presented thesis analyses the measured pupil diameter during real life driving tests and correlates the pupil diameter with the recorded photometric value, the illuminance, a significant portion of this work is dedicated to the estimatimation of the discomfort glare by measuring the pupil contraction as a result of short illuminance pulses. However, first studies regarding the correlation between the psychological glare and the measured pupil diameter show, that the pupil diameter does not correlate with the psychological glare, but that the main correlating value is indeed the illumination and the size of the glare source [154].

However, newer research indicates, that due to the correlation between the illuminance of a glare source and the discomfort glare as well as the obvious relation between the illumi-nance and the pupil diameter, a correlation between the pupil diameter and the discomfort glare exists.

Fryfound a correlation between theBCDvalues and pupillary movement for different photo-metric values [78]. However, since the research of Fryrequires very controlled environment parameters with no other light intereference to measure the exact pupil movement in corre-lation with the light source.

Similar results are found by Ohbawho investigated the pupil adaptation to light flashes [155].

In this study, a 50 W ^HIDlight source was used to produce glare pulses. His findings show a very good correlation for the change in pupil diameter in relation to the flash intensity in dependence of the background luminance. It can also be shown, that for intermediate flashes, the pupil diameter contraction is nearly linear to the flash intensity and only for very low

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and very high intensities, this behaviour changes. The results for one investigated participant are shown in figure4.20where the change in pupil diameter is shown over the relative flash intensity, as the fraction of the brightest available flash, for different background luminances.

Figure4.20– Relative pupil size correlated with the relative flash intensity over different background lumi-nances as measured by Ohba[155].

This shows, that depending on the background, and therefore the adaptation of the pupil, the absolute contraction of the pupil diameter decreases as well. The highest adaptation lu-minance, indicated by squares, show the least pupil movement, while the lowest background luminance, indicated by circles, show the largest movement in the pupil diameter. Further-more, the data shows, that for an increase in flash intensity, the change in the pupil size always increases. However, no information is provided on the absolute flash intensity or the flash duration.

Lintook the approach one step further and measured both, the eye movement and the pupil diameter in correlation to glare pulses [85]. The setup for this test consists of a dimmable ambient light source (8000 K) at the ceiling and the glare source (3300 K and 5700 K) located 10° above the fixation angle. The ambient light was set to values of 0.0 lx, 10 lx and 200 lx vertically and the glare source was set to 20 lx, 50 lx, 125 lx and 300 lx at the test subjects eye.

The horizontal viewing angle was additionally varied between 2°, 4°, 8° and 16°. The glare source was switched on for a duration of 3.0 s and during this time, both eye movement and pupillary reaction were recorded. The results show correlations for both variables but since the eye movement was only tracked usingEOGand thereby mainly delivers if there is movement. Therefore, no definitive answer about the movement is given by Lin and only the findings for the pupil diameter are discussed. Findings regarding the de Boer rating correlating with the illuminance or the background luminance are consistent to the findings presented before and will also not be discussed further here. While Lindoes not provide any data on the measured pupil diameter over the given illuminance, the findings regarding the correlation between the relative pupil diameter and the glare rating on thede Boerscale is shown in figure4.21.