Multiwavelength infrared radiometers used for determining the heat flux through the ocean surface are generally calibrated by using a near-ambient temperature reference radiation source. Typically these sources have spectral emissivities that are less than unity and wavelength dependent. Analysis of the error produced by using a reference source which only approximates an ideal blackbody indicates that significant errors in the heat flux determination can arise unless the emissive properties of the source are well-known and accounted for.