The Shockley–Queisser limit (also known as the detailed balance limit, Shockley Queisser Efficiency Limit or SQ Limit, or in physical terms the radiative efficiency limit) refers to the maximum theoretical efficiency of a solar cell using a single p-n junction to collect power from the cell where the only loss mechanism is radiative recombination in the solar cell. It was first calculated by William Shockley and Hans-Joachim Queisser at Shockley Semiconductor in 1961, giving a maximum efficiency of 30% at 1.1 eV. This first calculation used the 6000K black-body spectrum as an approximation to the solar spectrum. Subsequent calculations have used measured global solar spectra (AM1.5G) and included a back surface mirror which increases the maximum efficiency to 33.7% for a solar cell with a bandgap of 1.34 eV. The limit is one of the most fundamental to solar energy production with photovoltaic cells, and is considered to be one of the most important contributions in the field. The limit is that the maximum solar conversion efficiency is around 33.7% for a single p-n junction photovoltaic cell, assuming typical sunlight conditions (unconcentrated, AM 1.5 solar spectrum), and subject to other caveats and assumptions. This maximum occurs at a band gap of 1.34 eV. That is, of all the power contained in sunlight (about 1000 W/m²) falling on an ideal solar cell, only 33.7% of that could ever be turned into electricity (337 W/m²). The most popular solar cell material, silicon, has a less favorable band gap of 1.1 eV, resulting in a maximum efficiency of about 32%. Modern commercial mono-crystalline solar cells produce about 24% conversion efficiency, the losses due largely to practical concerns like reflection off the front of the cell and light blockage from the thin wires on the cell surface. The Shockley–Queisser limit only applies to conventional solar cells with a single p-n junction; tandem solar cells with multiple layers can (and do) outperform this limit, and so can solar thermal and certain other solar energy systems. In the extreme limit, for a tandem solar cell with an infinite number of layers, the corresponding limit is 86.8% using concentrated sunlight.