Maximising effectiveness of thermal cameras for temperature screening
Thermal cameras can be used for rapid and safe initial temperature screening of staff, visitors and customers. Used the right way, the cameras can help prevent unnecessary spread of viruses like the novel coronavirus. During the global pandemic, use of thermal cameras has increased, but they have not always been used correctly, and therefore, not effectively.
Hikvision’s temperature screening thermal products are currently assisting users in initial temperature screening across the global market. During 2020, demand increased in most markets, and the company highly recommends that Hikvision’s thermographic cameras be used in accordance with local laws and regulations. Limitations of the technology include throughput and the impact of ambient conditions.
Detect viruses and fever
Hikvision releases a video that illustrates how skin temperature measurements are normalised within minutes
Thermal cameras cannot detect viruses and fever and should only be used as a first line of screening before using secondary measures to confirm, says Stefan Li, Thermal Product Director at Hikvision. “We also believe it is important for businesses and authorities to use [thermal cameras] alongside a full programme of additional health and safety procedures, which includes handwashing, regular disinfection of surfaces, wearing protective clothing such as masks, and social distancing.”
Hikvision has released a video that illustrates how skin temperature measurements are normalised within minutes after someone emerges from the cold. Mr. Li says the video demonstrates the accuracy of forehead measurement under difficult circumstances when people come inside from a cold outdoor environment.
Temperature screening facilities
“There have been some claims that measuring the forehead temperature is not as accurate as measuring the inner canthus, and we believe this video demonstrates the accuracy of forehead measurement very well,” he says. “We also illustrate how the skin temperature will experience a process of recovery (warming up), no matter if it is measured by a thermal camera or a thermometer.”
Mr. Li adds that people should wait five minutes in such circumstances before starting a temperature measurement. “We hope that stakeholders who are involved in the design of temperature screening facilities and associated health and safety procedures will recognise how important it is to consider the skin temperature recovery time, and that forehead measurement can provide accurate test results,” says Mr. Li.
Thermal imaging manufacturers
The algorithm is based on a large number of test results to obtain a value that tends to be dynamically balanced
The temperature measurement principle of thermal imaging is to detect the heat radiation emitted by the human body. The detected heat value often does not reflect the true internal body temperature of an individual. Furthermore, the temperature varies among different parts of the human, such as the forehead, ears, underarms, etc. A temperature compensation algorithm can be used to adjust the measured skin temperature to align with the internal body temperature.
The algorithm is based on a large number of test results to obtain a value that tends to be dynamically balanced. At present, thermal imaging manufacturers in the market, and even forehead thermometer manufacturers, have developed their own algorithms to map the skin temperature measured by the camera to the internal body temperature, so as to compensate the skin temperature to the internal body temperature.
This is also why Hikvision recommends that the "actual body temperature" should be checked with a secondary device for confirmation. The calibration work for a thermal camera is completed in the production process at the factory, including calibration of reference values and detection point and so on. At the same time, the equipment parameters should be adjusted before on-site use to ensure accurate temperature reads.
Hikvision does not deny the accuracy of temperature measurement at the inner canthus but prefers forehead temperature measurement and algorithms based on actual use scenarios, says Mr. Li. A large amount of test data and practical results indicates that the forehead is a correct and easy-to-use temperature measurement area, says the company. There are advantages and disadvantages of choosing different facial areas for temperature measurement.
Default compensation temperature
Two main approaches direct the measurement area and how compensation algorithms are applied:
Forehead area + default forehead compensation algorithm value
Upper half face (forehead + canthus) + default inner canthus compensation algorithm value.
Both methods deploy compensation algorithms, but the default compensation temperature of the inner canthus will be less than the default compensation temperature of the forehead, generally speaking. The reason is that the temperature of the inner canthus of most people is higher than their forehead, so the temperature compensation is relatively low (i.e., closer to the actual temperature inside the body.)
Upper face area
Hikvision found that selecting the upper face area plus the default compensation value for the inner canthus resulted in situations when the calculated temperature is lower than the actual temperature.
For the Hikvision solution, the forehead is a relatively obvious and easy-to-capture area on an entire face
Mr. Li explains: “The reason is that when the camera cannot capture the position of the inner canthus (for example, when a person is walking, or the face is not facing the camera), the camera will automatically capture the temperature of the forehead. Then the result that appears is the sum of the forehead temperature plus the default compensation temperature of the inner canthus, which is lower than the actual temperature of the person being measured. Therefore, errors are prone to occur.”
Thermal imaging products
But for the Hikvision solution, the forehead is a relatively obvious and easy-to-capture area on an entire face. Also, the default forehead compensation temperature is based on rigorous testing and can also correctly mimic the actual temperature of the person being measured, says Mr. Li.
After many test comparisons, considering that the results of forehead temperature measurement are relatively more stable, and in order to avoid the false results from inner canthus temperature measurement, Hikvision chose the forehead temperature measurement approach. “We look forward to bringing thermal imaging products from a niche market where there is a relatively high-end industry application to a mass market and serving more users,” says Mr. Li.
Facial recognition terminals
Additional application parameters can maximise effectiveness of thermal cameras for measuring body temperature:
Positioning and height - All cameras must be mounted appropriately to avoid loss of accuracy and performance. The installation height of each camera must be adjusted according to camera resolution and focal length, and stable installation is needed to avoid errors caused by shaking.
Ensuring a ‘one-direction path’ - The detection area must ensure that cameras capture the full faces of all those passing by or stopping, and obstacles should be avoided in the field of view, such as glass doors that block the camera.
Adequate start-up and usage - A waiting time of more than 90 minutes is required for preheating, after the initial start-up. Before conducting a thermal scan, people should be given three to five minutes to allow their body temperature to stabilise. When Hikvision MinMoe facial recognition terminals are used, people must stand at a fixed distance, pass one by one, make a short stop, and face the camera directly. Hikvision cameras support efficient group screening, but one-by-one screening is suggested for more accurate results, says Mr. Li.
Unstable environmental condition
An unstable environmental condition may affect the accuracy of thermal camera systems
Environmental factors can impact the accuracy of thermal cameras, and the idea of using a black body is to provide the camera with a reference point that has a stable temperature. The black body is heated to a specific temperature and helps the thermal camera to know how much error is caused by environmental factors in the room, and how the camera should calibrate itself in real time to improve its accuracy.
A black body can help increase the temperature measurement accuracy, and the most common improvement is from ±0.5 degrees to ±0.3 degrees. However, it also increases the cost of the installation. In some markets, customers may require black bodies in order to comply with regulatory accuracy requirements. An unstable environmental condition may affect the accuracy of thermal camera systems for measuring temperature.
Medical temperature measurement
Therefore, Hikvision suggests that the ambient conditions should be met for installation and use. First of all, users should avoid installing devices in hot or changeable environments. All cameras require indoor environments with calm air, consistent temperature and no direct sunlight. Installation should also be avoided in semi-open locations that may be prone to changes in ambient conditions, such as doorways, and there should be enough stable, visible light.
All devices should be installed to avoid backlighting, high temperature targets, and reflections in the field of view as far as possible. “We often see the misconception that thermal cameras can replace medical temperature measurement equipment, which is not the case,” says Mr. Li.
Rapid preliminary screening
“Temperature screening thermographic cameras are designed for the detection of skin-surface temperatures, and the measurement should be conducted to achieve rapid preliminary screening in public areas. It is really important that actual core body temperatures are measured subsequently with clinical measurement devices.”