Infrared thermography can be used as a measuring technology that records the thermal reaction of body to an external effect. In this case, the external impact is low-intensity optical radiation (LOR). Temperature is measured using a short-wave (1.5 to 5.1  μm) infrared camera. It is known that LOR produces a therapeutic effect. At the same time, the reason for this action remains unclear. The effect of LOR of the human forearm with λ  =  640  ±  10  nm with dose 5.04, 8.4  J  /  cm² on the temperature of the palm of the irradiated hand is studied. Temperature measurements continue during and after irradiation for 20 min. To interpret the results, a change in the temperature of the palm of volunteers who drank hot water is also investigated. It is found that the temperature after exposure to LOR is definitely established at a new level, depending on the radiation dose. Changes in surface temperature are associated with stimulation of blood flow. Literature data suggest that the observed response of the body to radiation may be caused by the work of the vascular relaxation factor, photoreactivation of superoxide dismutase, and the death of blood cells during irradiation.