Abstract: A method and apparatus for checking the calibration of analyzers employing radiant energy is disclosed. The invention embodies changing the radiant energy source color temperature by a precise increment and adjusting the span to reflect the predetermined amount of differential absorbance between reference and analytical interference filters which would result from said increment of change in source color temperature. The invention is particularly useful when employing radiant energy in the ultraviolet, visible and infrared spectra. The invention eliminates the need for standard liquid or gaseous calibration solutions or special optical calibration filters.