  This paper shows that the capacity region of the continuous-time Poisson
broadcast channel is achieved via superposition coding for most channel
parameter values. Interestingly, the channel in some subset of these parameter
values does not belong to any of the existing classes of broadcast channels for
which superposition coding is optimal (e.g., degraded, less noisy, more
capable). In particular, we introduce the notion of effectively less noisy
broadcast channel and show that it implies less noisy but is not in general
implied by more capable. For the rest of the channel parameter values, we show
that there is a gap between Marton's inner bound and the UV outer bound.
