5.6.1　Principles of photothermal imaging

Every object above the absolute zero temperature（-273.15℃）emits an electromagnetic radiation.Human body is a natural biological infrared radiator，which continuously emits infrared radiation.Besides，infrared radiation from human body is closely related with metabolism，body heat balance，body temperature regulation and tissue structures.Human is warm-blooded animal，and skin is the main infrared radiator of human body.When room temperature slightly lower than body temperature，according to the radiation wavelength of human body，an infrared detector with a sensitive wavelength of 8-14μmis usually selected.The infrared signal emitted by human body is collected through remote sensing technique and is converted into an electrical signal，followed by converting to digital signals with analogs.The computer displays a visible and quantitative infrared thermal image via the pseudo-color temperature distribution field（Figure 5-13）.When physiological condition changes somewhere in human body，inevitably deviation of lesion temperature appears due to changes in blood flow and metabolism.As a result，the infrared radiation changes.Therefore，based on this principle，photothermal imaging can accurately monitor changes in human body temperature，and even exhibits positive scan results before clinical symptoms or histological changes appear in the lesion.(https://www.daowen.com)