where M (λ) is the spectral irradiance (or exitance) of the light (SI units: W /m = kg ·m · s); Φ is the radiant flux of the source (SI unit: watt, W); A is the area over which the radiant flux is integrated (SI unit: square meter, m); and λ is the wavelength (SI unit: meter, m). (Note that it is more convenient to express the wavelength of light in terms of nanometers ; spectral exitance would then be expressed in units of W·m ·nm.) The approximation is valid when the area and wavelength interval are small.