News

Color temperature of the radiator

Standard Illuminant 4 (Sa) is the most commonly used standard in color temperature determination and is also the calibrated light source in spectral power distribution determination. This is a gas-filled tungsten filament lamp, originally specified at a color temperature of 2848K (according to the 1927 International Practical Temperature Scale), and the spectral power distribution of a perfect radiator at 2848K is assumed. Due to the change of the constant C2 in Planck's law, the color temperature was changed to 2854K in 1948 and has been used until recently. Later, according to the 1968 International Practical Temperature Scale, the C value changed again, and the gold (freezing) point also changed slightly.

If the lamp is recalibrated at 2858.7K, its spectral power distribution will be the same as when it was previously calibrated at 2854K (more accurately 2853.95K), and if T/C2 is a constant, the spectrum calculated by Planck's formula will also be the same. In order to obtain the same spectrum, when C2 changes from 1.435 (T27) to 1.438 (Tt8) and then to 1.4388 (T68), T must also change from 2848K to 2854K and then to 2855.5K. These values can be calculated from the first term of equation 4.1. If the lamp is recalibrated according to the new temperature standard, each change will bring a slight difference to the spectral power distribution, but it will be closer to the original Sa distribution.