INSTRUCTIONS: Enter a temperature with the slidebar at the bottom and click "New" to plot a Planck radiation function. "Wien Law" illustrates Wien's law for this temperature, "SB Law" illustrates the Steffan- Boltzmann Law, "B-V" calculates the blue-visual color index, and "U-B" calculates the UV-blue color index. Enter a new T by moving the slidebar and clicking on "New". "Regions" chooses if divisions of the spectrum are displayed in the Planck Law plot.
Color indices are only approximate because intensities are being evaluated at a single point corresponding to the maximum transmission of the filter. In reality, the filters have an acceptance width of about 1000 angstroms and a skewed gaussian shape. Thus a correct calculation would integrate with these acceptances as weights. Nevertheless the color indices calculated with the simple method are approximately correct.