The absolute spectral intensity calibration of optical detectors has always been difficult. In the past, this was only possible using expensive sources, which had been cross-calibrated against national standards. We describe a simple theoretical approach to absolute optical calibrations using any ordinary Tungsten light bulb. A key element of the theory is transforming Tungsten into an equivalent black body radiator. This permits direct application of Stefan- Boltzmann’s and Planck’s formulas of radiation.