The invention provides a passive two-stage multiwavelength approach for measuring temperature, emissivity and stray-light levels. The first stage comprises the steps of, (1) acquiring spectral intensity measurements over a predetermined spectral width of a thermal radiation source radiating at a true effective spectral emissivity and with a true source temperature, (2) forming a composite function that relates said spectral intensity measurements to the true effective spectral emissivity and the true source temperature, (3) providing emissivity estimating means for approximately determining how the true effective emissivity affects the color temperature of the thermal radiation source, (4) substituting an estimated effective spectral emissivity for the true effective spectral emissivity within the composite function such that the estimated emissivity approximately accounts for the effects of the true effective emissivity on the color temperature, (5) substituting a source temperature projection for the true source temperature within the composite function, and (6) utilizing the composite function to provide a best-fit correlation between the spectral intensity measurements, the estimated emissivity, and the projected source temperature such that when the projected source temperature equals the true source temperature the composite function attains an extremum, thereby obtaining an approximation of said source temperature. The second stage comprises the steps of (1) utilizing spectral acquisition means to acquire and measure a set of multiple spectral intensity distributions of a thermal radiation source radiating at multiple source temperatures at an effective spectral emissivity, wherein each of the spectral intensity distributions is associated with a particular source temperature, and (2) forming a first function of at least two of the measured spectral intensity distributions and of a set of temperature variables that represent the temperatures of the spectral intensity distributions used, such that the first function attains an extremum when the temperature variables equal the corresponding source temperatures, thereby calculating the source temperatures.