Normally only coldblooded animals such as snakes can see infrared. John Bell used modern infrared technologies to visualize our heat signatures, transforming them into life sized ghost appearances that turn out to be a reproduction of the visitor. The measurement of this thermal energy is called Enthalpy, thus the name of his interactive artwork.
Lydia Fraaije states that Bell his artwork illustrates that humans are only a small fraction of nature and highlights that we can learn more about our environment for new technologies. Not all information available is visible to the human eye.