where N ˙ ˙ ph is the number of photons per second in the beam, σ σ x and σ σ y are the root mean square values for the size of the beam in the axes perpendicular to the beam direction, σ σ x ′ and σ σ y ′ are the RMS values for the beam solid angle in the x and y dimensions, and d ω ω ω ω is the relative bandwidth, or spread in beam frequency around the central frequency. The customary value for bandwidth is 0.1%.