Why don't far-infrared rays light up?

The customer reported that the infrared lamp he used before would light up, but why doesn't our claimed far-infrared product light up
In fact, far-infrared is indeed invisible light. Its wavelength range is between 4 and 16 micrometers, which is the same as that of the human body. It is generated by carbon fiber heat imported from Japan, which is characterized by mild heat without stimulation, sufficient heat without stinging spicy, suitable for long-term physical therapy as well as the prevention of human metabolism, rheumatism, blockage of several types of sub-health problems.