Second radiation constant

The second radiation constant lives in the exponent of Planck's law, e^(c₂/λT) − 1. It is hc/k: the photon energy at a given wavelength divided by the thermal energy scale. When λT is much smaller than c₂ the exponential dominates and emission collapses — that is why a 300 K object emits essentially nothing visible.

Historically c₂ mattered far beyond spectroscopy, because optical pyrometry above the silver point realises the temperature scale through Planck's law. Every revision of the international temperature scale in the twentieth century carried its own conventional value of c₂ — 1.432 cm·K in ITS-27, 1.438 in IPTS-48 and after — and changing it visibly shifted tabulated high temperatures. Since 2019 there is nothing left to revise: h, c and k are all defined, so c₂ is exact.