Arena

An arena is an enclosed area, often circular or oval-shaped, designed to showcase theater, musical performances, or sporting events. The word derives from Latin harena, a particularly fine/smooth sand used to absorb blood in ancient arenas such as the Colosseum in Rome. It is composed of a large open space surrounded on most or all sides by tiered seating for spectators. The key feature of an arena is that the event space is the lowest point, allowing for maximum visibility. Arenas are usually designed to accommodate a large number of spectators.

The term arena is sometimes used as a synonym for a very large venue such as Pasadena's Rose Bowl, but such a facility is typically called a stadium, especially if it does not have a roof. The use of one term over the other has mostly to do with the type of event. Football (be it association, rugby, or gridiron) is typically played in a stadium while basketball and ice hockey are typically played in an arena, although many of the larger arenas hold more spectators than do the stadiums of smaller colleges or high schools. There are exceptions. The home of the Duke University basketball team would qualify as an arena, but the facility is called Cameron Indoor Stadium. Domed stadiums, which, like arenas, are enclosed but have the larger playing surfaces and seating capacities found in stadiums, are generally not referred to as arenas in North America. There is also the sport of indoor American football (one variant of which is explicitly known as arena football), a variant of the gridiron-based game that is designed for the usual smaller playing surface of most arenas; variants of other traditionally outdoor sports, including box lacrosse and futsal/indoor soccer, also exist.

Radiance

In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area, and spectral radiance is the radiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. These are directional quantities. The SI unit of radiance is the watt per steradian per square metre (W·sr⁻¹·m⁻²), while that of spectral radiance in frequency is the watt per steradian per square metre per hertz (W·sr⁻¹·m⁻²·Hz⁻¹) and that of spectral radiance in wavelength is the watt per steradian per square metre, per metre (W·sr⁻¹·m⁻³)—commonly the watt per steradian per square metre per nanometre (W·sr⁻¹·m⁻²·nm⁻¹). The microflick is also used to measure spectral radiance in some fields. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiation, or to quantify emission of neutrinos and other particles. Historically, radiance is called "intensity" and spectral radiance is called "specific intensity". Many fields still use this nomenclature. It is especially dominant in heat transfer, astrophysics and astronomy. "Intensity" has many other meanings in physics, with the most common being power per unit area.