Every physical body spontaneously and continuously emits electromagnetic radiation and the spectral radiance of a body, B ν, describes the spectral emissive power per unit area, per unit solid angle and per unit frequency for particular radiation frequencies. The relationship given by Planck's radiation law, given below, shows that with increasing temperature, the total radiated energy of a body increases and the peak of the emitted spectrum shifts to shorter wavelengths. According to Planck's distribution law, the spectral energy density (energy per unit volume per unit frequency) at given temperature is given by (SI units): u ν ν ( ν ν , T ) = 8 π π h ν ν 3 c 3 1 exp ⁡ ⁡ ( h ν ν k B T ) − − 1 alternatively, the law can be expressed for the spectral radiance of a body for frequency ν at absolute temperature T (in the cgs units) given as: B ν ν ( ν ν , T ) = 2 h ν ν 3 c 2 1 exp ⁡ ⁡ ( h ν ν k B T ) − − 1 where k B is the Boltzmann constant, h is the Planck constant, and c is the speed of light in the medium, whether material or vacuum. The cgs units of spectral radiance B ν are erg · s · sr · cm · Hz. The terms B and u are related to each other by a factor of ⁠ 4π / c ⁠ since B is independent of direction and radiation travels at speed c. The spectral radiance can also be expressed per unit wavelength λ instead of per unit frequency. In addition, the law may be expressed in other terms, such as the number of photons emitted at a certain wavelength, or the energy density in a volume of radiation.