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“It was counter intuitive. We were not expecting to see the face getting colder. With this accurate way to estimate workload, we can develop methods that will assist the operator at times of maximum stress.”

The team are hoping that the technique could be used to remotely monitor whether employees are working too hard, without interfering with their tasks. They suggest, for example, that thermal imaging cameras could be placed in aircraft cockpits to check the pressure on pilots. The team is working alongside Airbus on the project.

Pilots suffering elevated levels of stress or tiredness have become a major concern in recent years, particularly since the 2015 Germanwings crash caused by co-pilot Andreas Lubitz. Lubitz had been treated for suicidal tendencies and been declared unfit to fly by his doctor, but had kept the information from his employer.

Herve Morvan, director of the Integration of Novel Aerospace Technologies (INNOVATIVE), said: “Pilot fatigue has received greater exposure recently. Developing an advanced system to monitor it is a significant step in a sector where safety is paramount.”

Results show that when people are fully focused on a task, their breathing rate is changed by the nervous system, diverting blood flow into the brain.

Sarah Sharples, professor of Human Factors, said: “Specialists have been challenged for some years on how to measure workload without needing to interrupt people to ask how busy they are. We have developed a much better understanding of how physical changes associated with workloads manifest themselves as physiological symptoms, and how these symptoms translate into the parameters that we can measure.”

Adrian Marinescu, who is studying physiological responses to workload at Nottingham, added: “Our goal is to estimate workload using non-invasive and non-intrusive physiological monitoring. Of all the approaches, facial thermography is the least intrusive, and has proved to be an excellent method. It is convenient, gives real-time data. The cameras have been getting smaller, lighter and more affordable,” he said.

The research was published in the journal Human Factors.