Graphical model of the hierarchical Bayesian network introduced in Sect. 2. Shown are the model parameters α, q, σ, η and β, in rectangular boxes, as they have to be specified by the user. The logarithmic spectral parameters and the diffuse signal field φ, and the expected number of photons λ, are inferred by the algorithm and shown in black solid circles. The observed photon count data, d, is marked by a dashed circle at the bottom.