This implies that a rule-based approach to EEG fatigue measurements would be impractical, as each physiological variation would require a specific rule applicable to a specific person. To overcome this problem, scientists developed the Universal Fatigue Algorithm based on a data-driven approach. Drowsiness is a state determined by independent non-EEG measures. The Oxford Sleep Resistance Test (OSLER test) and the Psychomotor Vigilance Test (PVT) are the most commonly used measures in sleep research. Both tests were used to establish the sample dataset for development of the Universal Fatigue Algorithm. The algorithm was developed from real EEG of a large number of individuals. Artificial intelligence techniques were then used to map the multitude of individual relationships. The implication is that the result gets progressively universal and significant as more data from a wider range of individuals are included in the algorithm. In addition to an unseen-blinded experiment approach, testing of the algorithm is also subject to independent external parties. PERCLOS is a drowsiness detection measure, referred to as the percentage of eyelid closure over the pupil over time and reflects slow eyelid closures or droops rather than blinks. Various real-time operator drowsiness detection systems use PERCLOS assessment and propriety developed software to determine the onset of fatigue. Each technology developer use a unique set-up and combination of hardware to improve the accuracy and ability to track eye movement, eyelid behaviour, head and face poses under all possible circumstances. Some systems rely on a camera module on a rotating base that is mounted on the dashboard inside the cab. The device has a large field of view to accommodate operator head movements. The equipment uses eye-tracking software with a structured illumination approach that depends on the high contrast between the pupils and the face to identify and track the operator's pupils. Alternatively, flexible and mobile tracking systems provide head and face tracking which include eye, eyelid and gaze tracking. These systems now provide real time feedback without the use of wire, magnets or headgear. Although studies confirmed a correlation between PERCLOS and impairment, some experts are concerned by the influence which eye-behaviour unrelated to fatigue levels may have on the accuracy of measurements. Dust, insufficient lighting, glare and changes in humidity are non-fatigue related factors that may influence operator eye-behaviour. This system may therefore be prone to higher rates of false alarms and missed instances of impairment. The computer vision system utilises an unobtrusive dashboard mounted camera and two infra-red illumination sources to detect and track the facial features of the operator.