In the vertebrate eye, light increments lead to hyperpolarization of the photoreceptors (decreased transmitter released at its terinals) and decrements to depolarization (whereas in the invertebrate eye, it is the opposite). This may seem strange if we consider photoreceptors (as well as neurons in the cortex) as light detectors which should turn on or get excited (instead of off or inhibited) when there is more light.
However, the main concern of the visual system is not whether there is light or not, but the visual scene of the external world. It is not necessary for the photoreptors to turn on (depolarized) to respond to the presence of certain visual stimulus. It is the change (either depolarization or hyperpolarization) of the photoreceptors that indicates the visual stimuli.