“Necessity is the mother of invention. A need or problem encourages creative efforts to meet the need or solve the problem” – Plato, Republic dialogue.
Covid-19, once it hooked its claws into our modern world, certainly proved this to be as true as when Plato penned the words in 376 BC. Just about everything had to become touchless and, either by government dictate, edict, regulation or mandate, required that anyone who visited any governmental, medical, retail or commercial premise – for whatever reason – had to have their temperature measured and logged for future reference.
Handheld infrared temperature measurement devices were imported wholesale and they sold like hot cakes. Every retail outlet stocked some or other variant of these devices whether the equipment was reputable or not, whether it worked or not. The process of measuring every visitor, however, is a burdensome manual operation often resulting in incorrect information being logged due to a variety of reasons.
To the rescue come manufacturers and vendors of touchless access control systems. Out of nowhere a variety of facial recognition biometric systems became available that featured temperature measurement modules that make use of infrared radiation. Two birds with one stone, so to speak. Facial recognition access control that would grant or deny access on not only a user’s face, but also his or her body temperature while logging the results as per requirement.
This is all fine and well, but do you know how this temperature module works? I am of the opinion very few people, both providers and users of these systems alike, researched the subject and even fewer know the difference between the technologies or what the application is for them.
Two temperature technologies
Facial recognition with temperature measurement is, for the most part, available in one of two technologies – thermopile and thermography/IRT. There are similarities between the two, but there is also a myriad of differences.
Both technologies require emissivity to measure temperature, both technologies use the infrared spectrum, both use electrical current in their sensors to determine a reading, both use equations (or algorithms) to convert current into temperature and both technologies are – dependant on quality of the sensor manufacture – accurate to between 0,2°C and 0,3°C.
What is emissivity and why is it important?
“The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that may include both visible radiation and infrared radiation, which is not visible to human eyes. Emitted energy indicates the temperature of the object. Emissivity can have a value from 0 (shiny mirror) to 1,0 (blackbody). Most organic, painted, or oxidised surfaces have emissivity values close to 0,95.” (source: Wikipedia).
The human body (or more particularly, human skin) has an emissivity in the range of 0,95 to 0,98. Without emissivity, it would be impossible to determine the temperature of an object without physical contact with that object.
What is thermopile and how does it work?
A thermopile is an array of sensors called thermocouples. Thermocouples consist of two different thermoelectric materials joined to create a simplified electrical circuit that causes a differential in electron movement when exposed to heat. This differential is called the Seebeck effect and is measured in millivolts or mV.
Thermocouples are then connected in series (parallel connections are rare) to form the thermopile. As the current is measured, an equation is used to convert the flow of electricity to temperature. This allows the temperature of a body to be measured on a single area only. If temperature needs to be measured on multiple areas, multiple measurements need to be taken.
The subject being measured also needs to be stationary and in operating proximity to the sensor. Thermopile is mostly used on handheld temperature measurement devices and often called laser guns as a laser pointer will indicate to the operator where the measurement is being taken. Thermopile based facial recognition terminals generally take the temperature of an individual from the plane of the face due to mounting requirements which could provide erroneous temperature readings.
What is thermography (IRT) and how does it work?
Thermal imaging and thermal video are examples of the infrared imaging science known as infrared thermography or IRT. The premise behind the temperature measurement is that a thermal imaging device, such as focal plane array sensor – manufactured from either ferroelectric or pyroelectric materials – or a microbolometer, is used to measure the temperature of an object. This sensor operates at ambient temperature or is stabilised at a temperature close to ambient using small temperature control elements.
These sensors operate on the change of resistance, voltage or current when heated by infrared radiation. These changes are then measured and compared to the values of the operating temperature of the sensor. These changes are then converted via equation to reflect the temperature of a body. IRT is often used in imaging applications, such as CCTV, and can detect the temperature of a body on multiple locations at once or from bodies in motion and at a distance.
IRT based facial recognition terminals can take the temperature of a subject from multiple locations of the face but ideally the forehead is used as it provides more accurate temperature measurement.
A word of caution at this junction: Many vendors and manufacturers use the acronym IRT when describing the temperature measurement device integrated into their touchless biometric terminals, but the technology being used is not IRT but rather thermopile. When queried regarding this discrepancy, many would reply that the acronym is for infrared temperature. Make sure which technology your device uses.
Where to use thermopile and where to use IRT?
As stated previously, thermopile can only detect a temperature reading from a single area at any one time. It accordingly requires the user to be stationary, even if only momentarily, and in operating proximity in front of the sensor for a correct reading of the temperature. Most facial recognition devices also require the user to be stationary in front of the device. Facial recognition with thermopile is ideal for low-traffic areas, where ease of access or speed of access is not an issue.
IRT on the other hand can measure temperature from a moving object and at a distance. When coupled with ‘walkthrough’ facial recognition scanners – these are scanners that detect, extract and match facial features at a distance before the user gets to the barrier– they are ideal for high-traffic areas and applications where the ease of access and speed of access is of importance.
Virdi has a reputation for providing high-quality products that offer performance and quality. We believe that any investment by end users in either facial recognition or temperature measurement technology should mirror this reputation. Therefore, we have chosen to marry our UBio-X Pro2 walkthrough facial recognition terminal with an uncooled microbolometer IRT imaging device for fast, efficient, and accurate access control and temperature measurement. Both facial recognition and temperature measurement is up to 3 metres from the device while recognition speed is 1:20 000 in under 1 second and temperature measurement is under 300 ms.
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