Principle and Development of Medical Infrared Thermal Imager

Principle and Development of Medical Infrared Thermal Imager

   Body temperature is an important indicator of health. Since 400 BC, body temperature has been used in clinical diagnosis. As a thermostatic animal, human beings can maintain a constant body temperature different from the ambient temperature. Thermometers were produced around the 17th century. Thermometers are often used to measure the diurnal variation of body temperature of normal people. In 1868, Wunderlich first systematically studied the body temperature of patients with fever and compared it with that of normal people. He believes that the body temperature between 36.3 and 37.5 ℃ is normal, beyond which the disease may occur. In 1800, sir Herschel discovered infrared radiation, and his son John Herschel's first infrared thermal imaging opened up a new space in the field of temperature measurement. In 1934, hardy team elaborated the physiological function of human body infrared radiation, and proposed that human skin can be regarded as a blackbody radiator. He established the diagnostic significance of infrared technology for temperature measurement, paving the way for the application of infrared thermography (IRT) technology in the medical field.

   Infrared thermal imaging technology converts the invisible infrared energy radiated by the target into visible pseudo color thermal images, in which different colors represent different temperature levels. The main components of the infrared thermal imager are: the optical devices that focus the infrared energy on the detector, the infrared detector array that converts the infrared energy into electrical signals, the shutter system that performs image correction when the automatic adjustment button is pressed, and a digital signal processing unit that processes electrical signals to generate radiation images and perform temperature calculation. The accuracy of temperature measurement depends on emissivity, ambient temperature and humidity, air flow and distance from the target.

Figure 1. Medical infrared thermal imager

   So far, infrared thermal imager has experienced three generations of development. The first generation of cameras used a single element detector and two scanning mirrors to generate images. They're experiencing a problem with high saturation. In the second generation camera, two scanning mirrors and a large linear array or a small two-dimensional array are used as detectors, and the delay integral algorithm is used for image enhancement. The third generation camera has no mirror, but has a large focal plane array (FPA) detector and image processing chip, which improves the reliability and sensitivity of the device.

   Detector is the core of infrared camera. Infrared detectors are divided into two types: refrigeration type and uncooled type. The development of solid-state system paves the way for the production of new detectors, which have better precision and resolution. At present, the thermal sensitivity of uncooled cameras is about 0.05 ℃, while that of refrigerated cameras is 0.01 ℃. Uncooled camera has many advantages, such as high spatial resolution, high temperature resolution, compactness and portability. Frederickson reports that the FPA based camera has a spatial resolution of less than 2 mm in working distance and field of view (200 mm working distance over 1 m and working distance and field of view from 500 mm). In addition, they are light in weight and manufactured with silicon wafer technology and cost less than refrigeration detectors. This kind of modern digital uncooled infrared camera has greatly improved the medical infrared thermal imager, making the research and application of infrared thermal imaging technology in the medical field rejuvenated.