## 4 Discussion  
To the best of our knowledge, this is the first time that a nonwearable technology has been applied to evaluate user-VDT interactions, distinguishing the times in which the user simply faces the screen from the times of active fixation on the screen. The specific design of the nonwearable device does not interfere with the usual activity, granting optimal compliance to the tested workers. Other studies have used eye-tracking technologies to investigate different types of workers, such as construction workers [36], medical practitioners [37], drivers [38] and offshore operators [39]. To the best of our knowledge, this work represents one of the first attempts to evaluate real-life visual activity in VDT users. We used an updated version of a system that we described in a previous paper [17] and successfully demonstrated that the monitoring system can be adapted to the specific context of the working environment, potentially measuring different parameters on the basis of the technical demands for working tasks and organizations. On a random working day, VDT users at the University of Brescia performed very different tasks, leading to different screen fixation patterns. Importantly, despite such significant differences among the users that the proposed monitoring system was capable of spotting, all the workers were generically classified as \"VDT users\" in the risk assessment document, which means that they are all supposed to use the video display terminal for more than 20 h weekly [2]. The results of the experiments showed that the visual activity performed by the employees could vary greatly on a daily basis due to the enormous differences between scheduled and unscheduled tasks (from 3% to 81% screen fixation, breaks included). The screen fixation and face detection patterns of the recordings overlapped the activities of the workers, as the diagrams showed perfect matching with recorded breaks as they were included in the tests. A more precise knowledge of the visual activity level during working tasks in VDT users could help occupational medicine physicians to better understand and correlate eye strain conditions with job duties. To confirm this, the results shown in Figures 4 and 5 allowed us to assess the reliability of the software to describe and distinguish different visual activity patterns (active fixation vs. no-screen tasks) during daily job routines. The VDT monitoring system and its software developments could help objectively assess health risks in office jobs, potentially leading managers to plan different working task schedules throughout the week. A possible evolution of this experiment could include a paired clinical evaluation of the tested workers by an ophthalmologist to correlate video terminal use time patterns with eye strain, dry eye disease and meibomian gland dysfunction. The test could be repeated after a working schedule change to improve the health conditions of VDT users, not only from an ophthalmic point of view but also in terms of global perceived well-being and psychological stress exposure. As we can see, there were no correlations in this study between the shift duration and face detection or fixation on the screen. A possible explanation for this is that the tasks performed during the daily routine were very different among the VDT workers and that the number of breaks from PC use could vary greatly due to these working schedules. The scatter graph of facial detection and eye fixation shows a positive correlation, despite some interindividual differences. This demonstrates that the system was able to detect the visual activity of the workers and separate it from the passive presence of the workers in front of the screen, creating a better description of possible eye distress between single tasks. These measurements represent a starting point, not only for the description and quantification of the actual work of the operator at the video terminal but also for an embryonic definition of risk profiles related to the operator-screen interaction profile.