In a different approach to optical tunneling, Balcou and Dutriaux measured the group delay associated with light transport across a small gap between two prisms. When a light beam travelling through a prism impinges upon the glass-air interface at an angle greater than a certain critical angle, it undergoes total internal reflection and no energy is transmitted into the air. However, when another prism is brought very close (within a wavelength) to the first prism, light can tunnel across the gap and carry energy into the second prism. This phenomenon is known as frustrated total internal reflection (FTIR) and is an optical analog of quantum tunneling. Balcou and Dutriaux obtained the group delay from a measurement of the beam shift (known as the Goos–Hänchen shift) during FTIR. They found that the group delay saturates with the separation between the prisms, thus confirming the Hartman effect. They also found that the group delays were equal for both transmitted and reflected beams, a result that is predicted for symmetric barriers.