Basic Information on Thermal Imaging and Fever Screening
Thermal imaging is a non-contact technology that uses cameras with thermal imaging sensors to measure body temperature. They provide a fast and safe way of carrying out temperature screening without the need for close contact.
A thermal camera is typically positioned between a distance of 0.5m and 9m away from the screening point where people walk towards the camera so that their face is visible. Often a person may not be aware they are having their body temperature taken.
The cameras are capable of measuring the temperatures of multiple people walking towards the camera at once – with some cameras capable of up to 30 faces per second.
Different types of fever screening system are available with the most common and fastest systems taking a measurement from the face and forehead as a person walks towards the camera or through an archway, meaning they will work with glasses and hats (but not full face coverings). While other systems require the person to stand still in front of a screening terminal or thermal camera allowing a clear view of the corner of the inner eye where body temperature is closest to inner core temperature.
Thermal imaging technology is not new, it has been used for many years within the security and safety industry. In the UK, up until now typical applications were for security surveillance and detection in complete darkness, or as early fire detection – often used in waste centres or industrial plants where a temperature alert is used to indicate a possible fire source before a fire actually starts.
Following the SARS (Severe Acute Respiratory Syndrome) epidemic in 2003, major airports in Asia (Singapore, Japan and Hong Kong airports were amongst the first) started using thermal cameras to carry out non-invasive screening of passengers’ body temperatures to detect a possible fever and a tell-tale sign of the disease. Those showing a high temperature of over 38°C were taken to one side for examination by a nurse and were required to be certified by a doctor as not having SARS before they were allowed to fly1.
Thermal cameras measure thermal radiation from an object or body, called mid-wavelength infrared, which is invisible to the human eye. All objects above absolute zero emit this radiation. It is caused by thermal motion within a body’s molecules. Thermal imaging technology captures and displays heat intensity information in artificial colours.
Infrared radiation, also referred to as infrared or IR, is an electromagnetic wave in the spectrum between visible light and radio waves. Infrared is grouped into three bands called near, mid and far. As well as thermal imaging applications infrared is also used in fibre optic cables and most commonly in remote controls for home entertainment devices such as TV’s.
In terms of the number of pixels in thermal camera, the resolution is considerably lower than that for optical cameras – the latest iPhone has a 12MP (megapixel) camera, in contrast some of the higher resolution thermal cameras used for fever screening have a resolution of 110k (110,000 pixels) which is 0.1MP or a resolution that is 120 times lower than the camera in an iPhone 11.
Temperature screening has to be carried out internally within a controlled environment. Just like light is reflected off of object such as walls, floors or tables – so is the infrared radiation measured by a thermal camera.
The accuracy of thermal cameras can vary greatly. Cameras used for industrial applications will usually have an accuracy of ±2°C, which is clearly no good when trying to measure a temperature of 38°C.
For fever screening applications a camera will typically have an accuracy of between ±0.2°C and ±0.5°C. In order to achieve the most accurate results a calibration device that emits a constant infrared radiation, called a blackbody calibration source, is placed in the view of the thermal cameras to provide a temperature reference.
Furthermore, there are different types of thermal technology available which greatly influence the effectiveness of a thermal camera when used for a fever screening application. These two technologies are called Thermographic and Thermopile, with the latter being less effective for body thermometry.
A thermographic camera is a more advanced (but also more expensive) method of temperature measuring than solutions using thermopile. The main difference is that a thermographic sensor is capable of taking a measurement from individual pixels. In contrast, devices using thermopile sensors can only measure the highest temperature from a fixed window (typically in the middle of the image area) making them less effective.
Anyone looking to invest in this technology should be sure that they consult with an expert to ensure they achieve the best results and do not end up being miss sold equipment designed for industrial thermal applications.
Part of a managed solution
An early WHO (World Health Organisation) joint mission report in February on a study of COVID-19 stated that 88% of laboratory confirmed cases reported symptoms that included a fever2.
Temperature screening for fever is an important first step but fever-detection systems are not foolproof and they are not medical devices meaning they are not designed to diagnose and detect medical conditions.
A raised facial skin temperature does not always present with a raised internal core body temperature. Conversely, a raised facial skin temperature is not by itself an indicator that a person is infected with a fever or COVID-19.
Additionally, we know that some carriers of the disease will be asymptomatic and that during the early infection period (5-6 days) symptoms are not displayed2.
Fever detection systems should be deployed as the initial step of a screening programme to identify a raised body temperature where secondary screening then takes place, which could include the use of medical grade thermometer, medical questionnaire or assessment, or testing.
How common are they likely become?
With the recent coronavirus pandemic and the tragic loss of life caused by COVID-19 it is understandable that Western governments, businesses and organisations in Europe and America are now looking at ways in which they can protect their customers and employees.
News media in April reported that Amazon, Intel and Walmart were all investing in and trialling body temperature measuring cameras3. More recently, trials in UK airports and temperature measuring of passengers by Air France have been announced.
Due to the need to carry out fever screening in a controlled environment it is likely that we will also start to see ‘fever screening cabins’ or ‘fever screening pods’ located outside the entrances to buildings or sites, whereby a person will be required to walk through the pod to undergo temperature screening before being allowed entry.
Companies and organisations are already starting to install this technology as an important first step against the fight against coronavirus and it is likely that it will become much more common place at the entrances of the places we work, educate, travel, shop and socialise in.
About the Author
Lee Staff is Managing Director of Custom Fire & Security (Custom Security Services Ltd) and has 23 years’ experience working in the fire and security sector where his company has provided security and safety solutions to a variety of commercial companies, private residential customers, high security critical national infrastructure sites, and public bodies including NHS trust, police, local authority, social care and schools.
1.New Scientist: 24/04/2003 article by Shaoni Bhattacharya
3.Reuters: 09/04/2020 article by Stephen Nellis
2. World Health Organisation Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf