Energy transfer upconversion: a lanthanide ion called the sensitizer absorbs a photon and is excited to E1, then transfers its energy to an ion called the activator. The sensitizer then decays back to G. This occurs twice, causing the activator to beexcited to E1 and then to E2. When the electron decays from E2 all the way to G, this large change in energy again causes a photon with a shorter wavelength to be emitted. Some of the most efficient UCNPs used for biomedical purposes operate by energy transfer upconversion and involve sensitizer/activator ion pairs of Yb/Tm, Yb/Er, or Yb/Howith excitation wavelengths of about 975 nm. The Ybion does not have an E2 energy level, it can only be excited from G to E1. This makes the upconversion process more efficient since the two energy levels of Ybcannot cause cross-relaxations (a phenomenon which has a deleterious effect on upconversion). In addition to Yb-based UCNPs that work by energy transfer upconverion, some single-lanthanide UCNPs have also been developed. In these, the single type of lanthanide ion acts as a sensitizer and an activator.