  In this paper, we study two types of optical wireless channels under
average-intensity constraints. One is called the Gaussian optical intensity
channel, where the channel output models the converted electrical current
corrupted by the additive white Gaussian noise. The other one is the Poisson
optical intensity channel, where the channel output models the number of
received photons corrupted by the positive dark current. When the average input
intensity $\mathcal{E}$ is small, the capacity of the Gaussian optical
intensity channel is shown to scale as
$\mathcal{E}\sqrt{\frac{\log\frac{1}{\mathcal{E}}}{2}}$, and the capacity of
the Poisson optical intensity channel as
$\mathcal{E}\log\log\frac{1}{\mathcal{E}}$, which close the existing capacity
gaps in these two channels.
