Blackbody radiation among various types of photon emission employs the photon gas model with thermalized energy distribution without interphoton interaction. From the linear dispersion relation (i.e., dispersionless), phase and group speeds are equal (u ph = d ω ph / dκ = ω ph / κ, u ph : photon speed) and the Debye (used for dispersionless photon) density of states is D ph,b,ω dω = ω ph dω ph / π u ph. With D ph,b,ω and equilibrium distribution f ph, photon energy spectral distribution dI b,ω or dI b,λ (λ ph : wavelength) and total emissive power E b are derived as d I b , ω ω = D p h , b , ω ω f p h u p h d ω ω p h 4 π π = ℏ ℏ ω ω p h 3 4 π π 3 u p h 2 1 e ℏ ℏ ω ω p h / k B T − − 1 d ω ω p h or d I b , λ λ = 4 π π ℏ ℏ u p h 2 d λ λ p h λ λ p h 5 ( e 2 π π ℏ ℏ u p h / λ λ p h k B T − − 1 ) (Planck law), E b = ∫ ∫ 0 ∞ ∞ d E b , λ λ = σ σ S B T 4 , where σ σ S B = π π 2 k B 4 60 ℏ ℏ 3 u p h 2 (Stefan–Boltzmann law).