sentences:  Two-time correlation functions are generally shown to be independent of formula_58, and instead rely on formula_35 as formula_24.These functions can be used to find the spectral density formula_61 by computing the transform
  
  formula_62,
  
  where K is a constant.The spectral density can be viewed as the rate of photon emission of photons of frequency formula_63 at the given time formula_58, which is useful in determining the power output of a system at a given time.