 Microstate multiplier from photon absorption

A single photon thermalized in a macroscopic body adds energy $hc/\lambda$. If its heat capacity is sufficiently large that the temperature change is negligible, the entropy increase is $hc/(\lambda T)$. The Boltzmann entropy then gives the displayed ratio of accessible microstates. The exponential can be enormous even though the deposited energy is macroscopically tiny.