Fever screening recommendations for our thermoIMAGER cameras

Introduction

Viral epidemics like the current coronavirus / COVID-19 has created a worldwide demand for infrared cameras that are able to screen for a fever condition in humans.

The basis for the effectiveness of thermal cameras as a screening tool lies in the correlation of outside skin surface temperature with that of the internal or core body temperature.

Thermal cameras are looking to detect elevated body temperature (EBT) that would indicate a possible fever and would require further enhanced screening by a professional. They are not detecting any specific virus or condition!

Not all of our cameras are suited to this task and care should be taken in ensuring you have the correct models and associated equipment.

Where to measure?

IR temperature measurement at the Medial Canthus is generally accepted as a region for measurement that offers a close correlation to the core body temperature. 

You need sufficient pixels from the imager to cover this region. For our imagers and associated optics this means that the MFOV (measuring field of view), which is 3x3 pixels must be no larger than 4mm.

Basic Thermal Camera Requirements

1. Use a camera with a spatial resolution of 382 x 288 pixels or better and with a NETD of 80 mK or better

Within our current product range you could use the TIM40, TIM QVGA (previously sold as TIM400), TIM QVGA-HD (previosuly sold as TIM450) and TIM640.

In addition for new application requests we have introduced a special infrared camera ideally suited for this application:

TIM QVGA-HD with T100 range (-20…100 °C & 40mK NETD)

This imager is sold with certificate of calibration validating the measurement made against a traceable 35 °C temperature reference source.

2. Select the right optics using our optics calculator.

Check the Field of View (FOV) and measurement Field of View (MFOV) which defines the smallest spot size that can be measured accurately.  Essentially the medial cathus (tear duct) of the eye is the target and this should have at least 3x3 pixels covering this region. We suggest a 29° lens with target around 750-1000mm away.

3. Set the emissivity for temperature measurement on human tissue to 0.98 

The default camera emissivity is set as 1.000 so make sure you change it in the software

Absolute temperature measurement accuracy

Most IR cameras with 8-14 µm spectral response are specified with an accuracy of +/- 2°C or 2% of reading whichever is greater based on deployments in industrial environments in a wide variety of ambient conditions from 0 °C to 50 °C. Many IR cameras with microbolometer sensors are promoted today with accuracies of +/- 0.5 °C or better. These accuracies cannot be achieved without the use of a black body reference source.
 

Our TM-BR20AR-TIM ambient referencing source can be combined with our TIM QVGA-HD-T100. The black body is equipped with a 16 bit digital temperature sensor with +/- 0.1 °C accuracy. By integrating this highly accurate reference signal to our TIM Connect software, we can reduce camera uncertainties resulting from device adjustment, ambient temperature drift and short term stability down to a system accuracy of +/- 0.5 °C with a confidence interval of 95%.

 The source needs to be stable, have high emissivity and positioned in the scene approximate to the subject to be scanned.

Important Information to Consider

• Eyewear and sunglasses are opaque in the 8-14 µm infrared spectral range. Therefore they should be removed before the individual screening. Contact lenses need not be removed as these do not cover the tear duct.
• Medications including aspirin, acetaminophen/ paracetamol and ibuprofen or other antipyretics will reduce the human core and also skin temperature and make it impossible for screening a fever condition.
  
• The evaporative cooling effect from perspiration will decrease outside skin temperatures particularly when a subject is positioned below air flow vents.

• Subjects visibly perspiring will not deliver temperature measurements useful for fever screening with an IR camera or any remote infrared device.
• Vascular dilatation can occur after alcohol consumption increasing skin temperatures.
• High blood pressure, pregnancy and other physical conditions can also result in increased skin temperature.
• Influences from extremes in ambient temperature such as a long walk through a cold parking area will impact measurements possibly masking a fever and reporting a false negative reading.

If you would like a call back to discuss a requirement for using our Thermal Imaging cameras then you can send an email request to info@micro-epsilon.co.uk.

Please ensure to include your contact details and phone number so we can get back to you.