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.