The transmission coefficient, TL, commonly used to propagate the upwelling nadir radiance, just below the ocean surface, to above the surface has been assumed to be a constant value of 0.543 in seawater. Because the index of refraction of seawater varies with wavelength, salinity, and temperature, the variation of TL with these parameters should be taken into account, especially if low uncertainty is required for the quantities derived using TL. In particular the wavelength dependence of this factor is important. For example at a salinity of 35 g/kg and a temperature of 26° C, TL will be 1.3% lower at 380 nm and 1.1 % higher at 700 nm than the constant value (0.543) and should be taken into account when calculating the water leaving radiance and normalized water leaving radiance from in-water measurements.